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Water makes the proton shake - ultrafast motions and fleeting geometries in proton hydration

13. July 2017

Basic processes in chemistry and biology involve protons in a water environment. Water structures accommodating protons and their motions have so far remained elusive. Applying ultrafast vibrational spectroscopy, Dahms et al. map fluctuating proton transfer motions and provide direct evidence that protons in liquid water are predominantly shared by two water molecules. Femtosecond proton elongations within a hydration site are 10 to 50 times faster than proton hopping to a new site, the elementary proton transfer step in chemistry.

The proton, the positively charged nucleus H+ of a hydrogen atom and smallest chemical species, is a key player in chemistry and biology. Acids release protons into a liquid water environment where they are highly mobile and dominate the transport of electric charge. In biology, the gradient of proton concentration across cell membranes is the mechanism driving the respiration and energy storage of cells. Even after decades of research, however, the molecular geometries in which protons are accommodated in water, and the elementary steps of proton dynamics have remained highly controversial.

Protons in water are commonly described with the help of two limiting structures (Fig. 1A). In the Eigen complex (H9O4+) (left), the proton is part of the central H3O+ ion surrounded by three water molecules. In the Zundel cation (H5O2+) (right), the proton forms strong hydrogen bonds with two flanking water molecules. A description at the molecular level employs the potential energy surface of the proton (Fig. 1B) which is markedly different for the two limiting geometries. As shown in Fig. 1B, one expects an anharmonic single-minimum potential for the Eigen species and a double minimum potential for the Zundel species. In liquid water, such potentials are highly dynamic in nature and undergo very fast fluctuations due to thermal motions of surrounding water molecules and the proton.

Researchers from the Max Born Institute in Berlin, Germany, and the Ben Gurion University of the Negev in Beer-Sheva, Israel, have now elucidated the ultrafast motions and structural characteristics of protons in water under ambient conditions. They report experimental and theoretical results in Science which identify the Zundel cation as a predominant species in liquid water. The femtosecond (1 fs = 10-15 s) dynamics of proton motions were mapped via vibrational transitions between proton quantum states (red and blue arrows in Fig. 1B). The sophisticated method of two-dimensional vibrational spectroscopy provides the yellow-red and blue contours in Fig. 2A which mark the energy range covered by the two transitions. The blue contour occurs at higher detection frequencies than the red, giving the first direct evidence for the double-minimum character of the proton potential in the native aqueous environment. In contrast, for a single-minimum potential the blue contour is expected to appear at smaller detection frequencies than the red one.

The orientation of the two contours parallel to the vertical frequency axis demonstrates that the two vibrational transitions explore a huge frequency range within less than 100 fs, a hallmark of ultrafast modulations of the shape of proton potential. In other words, the proton explores all locations between the two water molecules within less than 100 fs and very quickly loses the memory of where it has been before. The modulation of the proton potential is caused by the strong electric field imposed by the water molecules in the environment. Their fast thermal motion results in strong field fluctuations and, thus, potential energy modulations on a sub-100 fs time scale. This picture is supported by benchmark experiments with Zundel cations selectively prepared in another solvent and by detailed theoretical simulations of proton dynamics (Fig. 2B).

A specific Zundel cation in water transforms into new proton accommodating geometries by the breaking and reformation of hydrogen bonds. Such processes are much slower than the dithering proton motion and occur on a time scale of a few picoseconds. This new picture of proton dynamics is highly relevant for proton transport by the infamous von Grotthuss mechanism, and for proton translocation mechanisms in biological systems.


Fig. 1 (click to enlarge)

Fig. 1 Chemical structure of hydrated protons in liquid water. A Schematic of the Eigen cation H9O4+ (left) and the Zundel cation H5O2+ (right). The arrows indicate the O-H bond coordinate r and the (O...H+...O) proton transfer coordinate z. In the Eigen cation a covalent O-H bond localizes the proton whereas in the Zundel cation the proton is delocalized between two water molecules. B Anharmonic vibrational potential (left) and double minimum potential of the Zundel cation along z (right, red). Distortions by the solvent surrounding impose a modulation of the double minimum potential (right, dotted line). Red and blue arrows indicate transitions between particular quantum states of the proton motion, i.e., the ground-state-to-first-excited-state transition (red) and the first-excited-state-to-second-excited-state transition (blue). The modulation of the potentials leads to spectral shifts of the vibrational transitions which are mapped by two-dimensional infrared spectroscopy.

Fig. 2 (click to enlarge)

Fig 2. Femtosecond dynamics of proton motions (1 fs = 10-15 s). A Two-dimensional vibrational spectra with the ground-state-to-first-excited-state transition (red) at lower detection frequency than the first-excited-state-to-second-excited-state transition (blue). The orientation of both contours parallel to the excitation frequency axis is due to ultrafast frequency fluctuations and the loss of memory in the proton position. B Simulated real-time dynamics of the proton motions in the Zundel cation. Within less than 100 fs, the proton displays large amplitude excursions along z, the coordinate linking the two water molecules in the Zundel cation. Due to the ultrafast modulation of the shape of proton potential by surrounding solvent molecules, the proton explores all locations between the two water molecules.


Fig. 3 (click to enlarge)

Fig. 3. Cartoon picture of proton hydration dynamics, visualized with the help of classical physics. The proton Smiley is sitting in the middle of a sofa with two seats. When shaking the sofa with a mechanical force, the shape of the seating changes and the proton moves forth and back on the sofa. Such motions occur on a time scale shorter than 100 fs (10-13 s). After an average time of 1 ps = 1000 fs = 10-12 s, the sofa breaks and the proton moves to a new site/sofa, including the red halve on the right.

Original publication: Science (2017) doi:10.1126//science.aan5144
Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy
Fabian Dahms, Benjamin P. Fingerhut, Erik T. J. Nibbering, Ehud Pines, Thomas Elsaesser


Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400
Dr. Benjamin Fingerhut Tel. 030 6392 1404
Dr. Erik T.J. Nibbering Tel. 030 6392 1477
Prof. Dr. E. Pines Tel. +972 8 6461640


A powerful laser system for driving sophisticated experiments in attosecond science

22 June 2017

Attosecond science has revolutionized the way we look into the time-dependent evolution of the microscopic world, where the behaviour of matter is governed by the rules of quantum mechanics. The technological breakthrough that made possible the development of the field is based on the generation of ultra-short laser pulses that last only a few oscillations of the electric field. These short pulses have a focused intensity where the electric field is comparable to the one electrons experience inside atoms and molecules. It is possible to control both the exact temporal shape and the waveform of these ultra-short pulses. While ultra-short laser pulses have been used in a few laboratories worldwide to study light-induced dynamics in atoms and molecules, many questions remain unanswered, due to the low data rates and inherently low SNR achievable with current state-of-the-art laser systems.

Quelle MBI
Quelle: MBI

At the Max Born Institute, a powerful laser system has now been completed, capable of reproducing the parameters of laser systems typically used in attosecond science experiments, but with a 100 times higher pulse repetition rate. This new laser system enables an entirely new class of experiments in simple atomic and small molecular systems, as well as high fidelity investigations of more complex molecules.


In the last 15-20 years, the availability of light pulses in the extreme ultraviolet (XUV) region of the electromagnetic spectrum, with durations in the order of 100s of attoseconds (1 as = 10-18 s) has enabled the emergence of the field of attosecond science. Utilizing these extremely short pulses scientists have gained unprecedented insight into the time evolution of electrons in atoms, molecules and solids, by taking advantage of the pump-probe technique: The system under investigation is excited by one "pump" laser pulse und after some time delay a second "probe" pulse interrogates the system (e.g. through ionization). The dynamics induced by the pump pulse can be retrieved by repeating the experiment at different delay times. Using the pump-probe technique a number of impressive results have been obtained in the last years addressing topics like light-induced charge migration, multi-electron correlations, and the coupling between electronic and nuclear degrees of freedom. Typically the velocity distributions of ions or electrons generated during the pump-probe sequence is determined experimentally or the transient absorption spectrum of the XUV pulse as a function of the pump-probe delay is detected. Often the light-induced processes are complex and measuring only one observable is not sufficient to fully understand the experimental results. Already several years ago, thanks to the development of the so-called "reaction microscope", a great improvement was achieved. This apparatus enables a measurement of the three-dimensional velocity distribution of all electrons and ions created in the pump-probe process The drawback of this technique is that very low signal rates are necessary, i.e. only 10% to 20% of all laser shots should induce the formation of an electron-ion pair. This leads to very long measurement times using current state of the art laser systems.

Pulses in the XUV with attosecond duration are produced when a strong laser pulse in the VIS-NIR interacts with a gas of atoms in a process called high-order harmonic generation (HHG). In order for a single XUV pulse with attosecond duration to be formed during the HHG process, the laser pulses interacting with the gas should last only a few oscillations of the electromagnetic field, which typically means less than 10 fs (1 fs = 10-15 s), and the exact temporal shape of the pulse must be controlled. The most widely spread way of producing such laser pulses consists in amplifying short pulses with a controlled waveform (Carrier-Envelope Phase- or CEP-controlled) in a Ti:Sapphire laser amplifier and shortening the duration of the pulses via non-linear pulse compression, using e.g. a gas-filled hollow-core capillary. However, the pulse repetition rate of these systems is typically limited to a few (1-3) kHz, and a maximum reported frequency of 10 kHz, due to detrimental thermal effects intrinsic to the laser amplifiers.

Now, researchers at the Max Born Institute in Germany, in collaboration with colleagues at the Norwegian Defence Research Establishment, have designed and built a laser system capable of operating at much higher pulse repetition rates than the typical Ti:Sapphire amplifiers. The newly developed system is perfectly suited for performing pump-probe experiments in attosecond science implementing electron-ion coincidence detection in a reaction microscope.

The system is based on a noncollinear optical parametric amplifier (NOPA). In a parametric amplifier, the energy from a strong pump pulse is transferred to a weak signal pulse in an instantaneous nonlinear interaction in a crystal. The gain and the bandwidth of the process are determined by conditions of phase-matching, that is, by ensuring that all the photons at the signal frequency are emitted in phase and add up coherently as the signal pulse propagates in the crystal. When the pump and the seed pulses enter the crystal subtending a small angle (noncollinear geometry), the bandwidth of the process is maximized and it is possible to amplify ultrashort pulses lasting only a few cycles. Moreover, since the process is instantaneous and there is no absorption of light in the crystal, there is no heat accumulation and thermal problems are almost negligible. Therefore, NOPA amplifiers are well suited for high repetition rates.

In the laser system presented in a recently published article in Optics Letters (https://doi.org/10.1364/OL.42.002495), the researchers amplified ultrashort CEP-stable pulses from a Ti:Sapphire laser oscillator in a NOPA amplifier pumped by a high repetition rate commercial Yb:YAG thin-disk laser. In the parametric amplifier a large fraction (about 20%) of the energy of the pulses from the Yb:YAG system is efficiently transferred to the ultrashort CEP-stable pulses from the Ti:Sapphire laser oscillator. The NOPA system is thus capable of delivering pulses with 0.24 mJ of energy at a repetition rate of 100 kHz, resulting in an average power of 24 W at an approximate central wavelength of 800 nm. After compression, filtering of parasitic second harmonic and a broadband variable attenuator for controlling the power incident into the experiments, CEP-stable pulses with 0.19 mJ (19 W) and 7 fs duration (i.e. 2.6 cycles) are available for experiments. The system will be employed for HHG and isolated attosecond pulse production, and will be the basis of an attosecond pump-probe beamline with coincidence detection capabilities.

Furch NOPA Fig. 1: High power NOPA at the MBI
Fig. (click to enlarge)  

Original publication: Optic Letters 2017, Vol. 42, Issue 13
"CEP-stable few-cycle pulses with more than 190 μJ of energy at 100 kHz from a noncollinear optical parametric amplifier"
Federico J. Furch, Tobias Witting, Achut Giree, Chao Luan, Felix Schell, Gunnar Arisholm, Claus P. Schulz, and Marc J. J. Vrakking


Dr. Federico Furch Tel. (030) 6392 1277
Dr. Tobias Witting Tel. (030) 6392 1228
Dr. Claus-Peter Schulz Tel. (030) 6392 1252
Prof. Dr. Marc Vrakking Tel. (030) 6392 1200


A perfect attosecond experiment

16 June 2017

Attosecond science techniques are currently revolutionizing ultrafast laser physics research, and enable experiments that provide unprecedented insights into the structure and time-dependent dynamics of electrons in atoms, molecules and condensed phase systems. In a new experiment, physicists from Waseda University (Japan), the National Research Council (Canada) and the Max Born Institute (Germany) have used attosecond science techniques to fully characterize the quantum mechanical wave function of an electron that is formed by photoionization. The work, reported in Science , is the first example of a "perfect" experiment using attosecond technology.

Quelle:NRC Ottawa
Detection of the shape of an electronic wave function with a six-fold symmetry.
Source: NRC Ottawa
The development of quantum mechanics in the early part of the last century forced scientists to accept that at the microscopic level matter behaves according to physical laws that are altogether different from the physical laws that apply in our macroscopic world. In the microscopic world concepts like the uncertainty principle play a role, posing limits on the precision with which certain properties of tiny particles, such as their position and speed, can simultaneously be measured. Quantum mechanics furthermore introduced wave-particle duality, meaning that the behavior of tiny particles can sometimes better be understood by considering the particles as waves.

These counterintuitive manifestations of quantum mechanics are due to the fact that every measurement that is performed on a quantum mechanical system only gives one out of a huge range of possible outcomes. The likelihood to measure a certain outcome is determined by a probability distribution that derives from the fundamental entity in quantum mechanics, the wave function. The wave function itself is not directly measurable, although strategies can be devised whereby multiple measurements performed on a quantum system lead to a complete characterization of the wave function.

In a paper published in Science (Villeneuve et al., "Coherent Imaging of an Attosecond Electron Wave Packet"), a novel approach is presented for the complete characterization of an atomic wave function using novel ultrafast lasers that have only been developed in the last few years. In the measurement, the scientists characterize the wave function of an electron that is released from a Neon atom as a result of the interaction of the atom with a series of laser pulses.

Electrons are elementary particles that are responsible for everyday things like electricity. They are characterized by several properties, such as one unit of (negative) charge, and an angular momentum, which is a vector that characterizes the rotation of the electron around the center of the atom. A slow rotation or a rotation close to the positive core of the atom, correspond to a low angular momentum, whereas a fast rotation or a rotation far away from the core imply a high angular momentum. The laws of quantum mechanics dictate that the angular momentum can only have certain distinct magnitudes. Accordingly, angular momentum states are called "s", "p", "d" and "f" for angular momentum quantum numbers l=0-3. In addition to the magnitude of the angular momentum, the length of the projection of the angular momentum vector onto a chosen laboratory frame axis (e.g. the polarization axis of the laser used in the experiment), characterized by the magnetic quantum number m, affects the outcome and interpretation of laboratory experiments.

In their paper, the scientists managed to accomplish a complete characterization of the wave function of the ionized electron, which contains contributions from angular momenta up to a value l=3 (i.e. s, p, d and f-contributions). Each of these angular momentum states is contained in the wave function with a specific amplitude, meaning a magnitude and a phase. In the experiment, these magnitudes and phases are determined by carrying out a series of interference experiments. Interference experiments exploit the wave-like character of quantum mechanical particles. Just like two water waves that cross each other can extinguish or enhance each other, so too can interference between different parts of a quantum mechanical wave function lead to an enhanced or a reduced probability to detect the particle at a particular place or with a particular speed. By performing a series of interference experiments under different conditions, pairwise interferences could be observed between the s- and d-part of the wave function, between the p- and the f-part, and finally, between all four components combined (see Figure 1). Accordingly, an exact and complete mathematical expression was obtained for the wave function of the ionized electron.

A crucial component in the accomplishment of this unique feat was the use of attosecond laser pulses (1 as = 10-18 s). Attosecond pulses are the shortest laser pulses that can be produced in state-of-the-art laser laboratories. They are produced in a process called "high-harmonic generation". Here, an atomic gas is exposed to an intense infrared laser that typically has a duration in the femtosecond (1 fs = 10-15 s) range. If the intensity of the infrared laser is high enough, the laser can pull electrons out of the atoms, which are subsequently accelerated by the oscillatory electric field of the infrared laser. Some of the accelerated electrons collide with the atoms from which they were previously removed. When this happens, the electron may be re-absorbed by the atom. All the energy that has been invested in the ionization and acceleration of the electron is then released in the form of a very energetic light particle (i.e. photon in the extreme ultra-violet (XUV) or soft X-ray part of the wavelength spectrum). Since the different steps in the high-harmonic generation process all occur on a timescale that is short compared to the duration of one optical cycle of the infrared laser (typically, just a few femtoseconds), this XUV/X-ray light appears in the form of a short - i.e. attosecond - pulse.

In the experiment the researchers used attosecond XUV pulses to ionize the Neon atoms. When only the attosecond pulse was fired in the experiment, a combination of s- and d-type electrons were formed, whose amplitude and relative phase could be determined from their angular distribution (see Figure 1A). When the ionization by the attosecond pulse was performed under conditions where beside the attosecond laser a replica of the infrared laser was present in the experiment, the amplitude and relative phase of the p- and f-components could be extracted (see Figure 1B). Finally, when the attosecond pulses were generated using a two-color laser field (both the afore-mentioned infrared laser and a copy of this laser with half the wavelength) the amplitude and relative phase of all four components (s, p, d and f) could be determined. The results of the experiment and the determination of the amplitude and phase of all angular momentum components are shown in Figures 1C and 1D. The clearly visible six-fold structure is caused by the dominant contribution of the f-orbital with m=0, which is produced by XUV+IR ionization. By the coherent addition of a contribution from the totally symmetric s-orbital (produced by XUV-only ionization), and changing the delay between the XUV and the IR pulse, an oscillation up and down along the vertical laser polarization axis is induced, revealing the phase of the f-orbital contribution.

The experiment is what atomic physicists consider a "complete" experiment, yielding a complete mathematical description of the wave function of the ionized electron, and is the latest example of how attosecond science techniques are currently revolutionizing ultrafast laser physics research. With the present work, this research has for the first time reached a state of perfection. (German Translation: Dr. Claus-Peter Schulz)

VRAttosecondExperiement Figure 1: A) XUV-only ionization produces an electron that is in a state characterized by "s" and "d" angular momentum, where a measurement of the electron angular distribution yields a determination of their relative amplitude and phase; B) XUV+IR ionization produces an electron that is in a state characterized by "p" and "f" angular momentum, where a measurement of the electron angular distribution once more yields a determination of their relative amplitude and phase; C) combined XUV-only and XUV+IR ionization produces an electron wave function containing both "s", "p", "d" and "f" contributions. The interference between these angular momentum components evolves with the delay between the XUV pulse and the co-propagating IR pulse. The large contribution of the "f" component is clearly visible in the first and last image; D) Measured electron momentum images at two time delays between the XUV pulse and the co-propagating IR pulse (corresponding to the first and last image in C). The reported experiment provides a complete determination of the relative amplitude and phases of all angular momentum components and thus represents a "perfect" experiment.

Original publication: Science. 2017, Vol. 356, Issue 6343
"Coherent imaging of an attosecond electron wave packet"
D. M. Villeneuve, Paul Hockett, M. J. J. Vrakking, Hiromichi Niikura


Prof. Dr. Marc Vrakking Tel. (030) 6392 1200


Turmoil in sluggish electrons' existence

22 May 2017

An international team of physicists has monitored the scattering behavior of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.
We can refer to electrons in non-conducting materials as 'sluggish'. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence relatively still in a dielectric crystal lattice. This idyll has now been heavily shaken up by a team of physicists from various research institutions, including the Laboratory of Attosecond Physics (LAP) at the Ludwig-Maximilians-Universität Munich (LMU) and the Max Planck Institute of Quantum Optics (MPQ), the Institute of Photonics and Nanotechnologies (IFN-CNR) in Milan, the Institute of Physics at the University of Rostock, the Max Born Institute (MBI), the Center for Free-Electron Laser Science (CFEL) and the University of Hamburg. For the first time, these researchers managed to directly observe the interaction of light and electrons in a dielectric, a non-conducting material, on timescales of attoseconds (billionths of a billionth of a second).

The scientists beamed light flashes lasting only a few hundred attoseconds onto 50 nanometer thick glass particles, which released electrons inside the material. Simultaneously, they irradiated the glass particles with an intense light field, which interacted with the electrons for a few femtoseconds (millionths of a billionth of a second), causing them to oscillate. This resulted, generally, in two different reactions by the electrons. First, they started to move, then collided with atoms within the particle, either elastically or inelastically. Because of the dense crystal lattice, the electrons could move freely between each of the interactions for only a few ångstrom (10-10 meter). "Analogous to billiard, the energy of electrons is conserved in an elastic collision, while their direction can change. For inelastic collisions, atoms are excited and part of the kinetic energy is lost. In our experiments, this energy loss leads to a depletion of the electron signal that we can measure," explains Prof. Francesca Calegari (CNR-IFN Milan and CFEL/University of Hamburg).

Since chance decides whether a collision occurs elastically or inelastically, with time inelastic collisions will eventually take place, reducing the number of electrons that scattered only elastically. Employing precise measurements of the electrons' oscillations within the intense light field, the researchers managed to find out that it takes about 150 attoseconds on average until elastically colliding electrons leave the nanoparticle. "Based on our newly developed theoretical model we could extract an inelastic collision time of 370 attoseconds from the measured time delay. This enabled us to clock this process for the first time," describes Prof. Thomas Fennel from the University of Rostock and Berlin's Max Born Institute in his analysis of the data.

The researchers' findings could benefit medical applications. With these worldwide first ultrafast measurements of electron motions inside non-conducting materials, they have obtained important insight into the interaction of radiation with matter, which shares similarities with human tissue. The energy of released electrons is controlled with the incident light, such that the process can be investigated for a broad range of energies and for various dielectrics. "Every interaction of high-energy radiation with tissue results in the generation of electrons. These in turn transfer their energy via inelastic collisions onto atoms and molecules of the tissue, which can destroy it. Detailed insight about electron scattering is therefore relevant for the treatment of tumors. It can be used in computer simulations to optimize the destruction of tumors in radiotherapy while sparing healthy tissue," highlights Prof. Matthias Kling of the impact of the work. As a next step, the scientists plan to replace the glass nanoparticles with water droplets to study the interaction of electrons with the very substance which makes up the largest part of living tissue.
(Text: Thorsten Naeser)


Fig. (click to enlarge)

Figure: A team of physicists clocked the time it takes electrons to leave a dielectric after their generation with extreme ultraviolet light. The measurement (false color plot) was the first of its kind in a dielectric material and yielded a time of 150 attoseconds (as), from which the physicists determined that inelastic scattering in the dielectric takes about 370 as.

Originalpublication: Nature Physics (2017) doi:10.1038/nphys4129
Attosecond Chronoscopy of Electron Scattering in Dielectric Nanoparticles
L. Seiffert, Q. Liu, S. Zherebtsov, A. Trabattoni, P. Rupp, M. C. Castrovilli, M. Galli, F. Süßmann, K. Wintersperger, J. Stierle, G. Sansone, L. Poletto, F. Frassetto, I. Halfpap, V. Mondes, C. Graf, E. Rühl, F. Krausz, M. Nisoli, T. Fennel, F. Calegari, M. F. Kling.


Prof. Dr. Thomas Fennel Tel. 030 6392 1245


Thomas Fennel started as a Heisenberg fellow at the MBI

12 April 2017


Prof. Thomas Fennel, group leader at the Institute of Physics at the University of Rostock, has been awarded a prestigious Heisenberg Fellowship funded by the Deutsche Forschungsgemeinschaft (DFG).

Prof. Dr. Thomas Fennel - Photo: Julia Tetzke, Uni Rostock  

Prof. Thomas Fennel, group leader at the Institute of Physics at the University of Rostock, has been awarded a prestigious Heisenberg Fellowship funded by the Deutsche Forschungsgemeinschaft (DFG). With the Heisenberg fellowship, which officially started on January 1st 2017, the DFG is supporting a research project to explore new routes for imaging and controlling ultrafast electronic motion in nanostructures. The underlying research will be carried out in a joint effort between Prof. Fennel's team at the University of Rostock and researchers in division A of the Max Born Institute, which is led by Prof. Marc Vrakking and to which Prof. Fennel is affiliated as an associated researcher.

The research activities are devoted to the active manipulation and visualization of ultrafast correlated and collective electron motion in finite systems. On the one hand, routes to the control of electronic processes in clusters, nanoparticles, and jets on the timescale of a single optical cycle of light via its detailed electric waveform or with multi-color fields will be explored, theoretically and experimentally. On the other hand, the technology for characterizing the attosecond electron motion in nanostructures via coherent diffractive imaging experiments using ultrashort intense XUV and x-ray laser pulses from free electrons lasers and lab-based high-harmonic sources will be developed. Finally, both approaches should be combined to trace light-induced electron dynamics with unprecedented spatial and temporal resolution and to reveal its classical and quantum aspects. Prof. Fennel is an expert in numerical many-particle physics and nanophotonics. He aims at the further development of atomistic electromagnetic plasma simulations and the efficient inclusion of the relevant quantum dynamics to tackle the challenging scientific questions of the project. The Max Born Institute is happy to welcome Prof. Fennel and is looking forward to a fruitful collaboration with the local experimental and theoretical groups.


Prof. Dr. Marc Vrakking Tel. (030) 6392 1200
Prof. Dr. Thomas Fennel Tel. (030) 6392 1295


Nanostructures give directions to efficient laser-proton accelerators

14 March 2017

Nanostructured surfaces have manifold applications. Among others they are used to selectively increase aborption of light. You can find them everywhere where light harvesting is the key point, e.g. in photovoltaics. But also in laser proton acceleration this approach attracts a lot of attention as nanostructured targets hold the promise to significantly increase maximum proton energies and proton numbers at a given laser energy. As for any other new technology, a high efficiency is a key for a potential future use. Scientists at the Max-Born-Institute (MBI) in Berlin have now investigated under which conditions the use of nanostructures in laser ion acceleration is beneficial.

If an ultrashort laser pulse (˜30 fs, >1 J) is focused onto a solid target foil, such that relativistic intensities (>1018 W/cm2) are reached, matter is transformed immediately into a plasma by field ionization. Electrons are accelerated to relativistic energies in the laser field. While fastest electrons can leave the target, those with less (but still relativistic) energy are trapped in the coulomb field of the (now) positively charged target and start to oscillate in this field. They form a dynamic sheath that, together with the target surface, generates an electric field of several megavolts per micrometer, in which positive ions (e. g. protons and carbon ions from the surface contamination layer) experience extreme acceleration. This process is called target normal sheath acceleration (TNSA). Fig. 1 shows an image of such a proton bunch.

The idea behind using nanostructured surfaces is now straight forward: Nanostructures increase laser absorption, i. e. more and more energetic electrons are generated which, in turn, can accelerate protons to higher energies.

But there are also alternatives for optimizing the TNSA mechanism - particularly important is the optimization of the plasma gradient, i. e. the density profile of the target. The laser intensities applied are so huge, that ionization of the target does not only happen when the peak of the laser pulse interacts with the target, but already starts during the rising edge of the pulse. The pre-ionized plasma expands, the plasma density decreases. The plasma gradient is therefore essentially determined by the exact temporal pulse structure.

The team of Dr. Matthias Schnuerer from Max-Born-Institute in Berlin has investigated, under which conditions the use of nanostructured targets is beneficial. For this purpose, the physicists have laser structured their targets in-situ. This method of generation of periodic surface structures via a laser (LIPSS) is particularly simple and in principle allows the development of a high repetition rate target system. In a first step, the target surface is nanostructured by applying about 20 strongly attenuated laser pulses. A representative scanning electron microscopy image of such a surface is shown in fig. 2. The structural parameters are similar to those that maximize laser absorption. Structural analysis and simulations show that these structures possess nearly optimal parameters for maximum laser absorption. In the following step, a single fully amplified pulse is focused onto this nanostructured area. Dr. Andrea Lübcke and her co-workers have investigated the influence of those nanostructures on the proton spectrum for different laser intensities. They chose a laser contrast that is optimal at highest intensities. First of all, the scientists could show that nanostructures remain functional even at highest intensities at the present contrast conditions in the sense that they increase the laser absorption as evident from an increase of Kα yield (see. Fig. 2a). For relatively low intensities, nanostructures significantly enhance both the conversion efficiency and proton energies. For example, at 5x1017 W/cm2 the maximum proton energies were increased by a factor of four, the conversion efficiency from laser to proton energy was even enhanced by two orders of magnitude. However, at highest laser intensities with optimal laser plasma parameters no significant benefits from the nanostructures for ion acceleration were measured (Fig. 2b,c). The researchers speculate about fundamental limitations in the energy transfer processes. The scientists were, however, not fully surprised by these results: As in many optimization problems, there are different paths to the optimum and combining them usually does not lead to an even better result. So far, these experiments performed at extreme conditions cannot be theoretically simulated in every respect. It is therefore the merit of this work to have clarified under which conditions the use of nanostructures is beneficial and in which direction new theoretical investigations can be initiiated.


Abb. 1 (click to enlarge)

Fig. 1: Laser accelerated ions, becoming visible in a Wilson chamber.

Abb. 1 (click to enlarge)

Fig. 2: Typical scanning electron microscopy image of nanostructured titanium surface (top). The Kα yield (a) of the nanostructured target is enhanced compared to the plane target over the entire investigated intensity range and indicates that nanostructures are functional even at highest intensity. In contrast, the conversion efficiency (energy transfer into fast protons) (b, logarithmic scale) and the maximum proton energy (c) of the two different targets approach each other at highest intensities.

Original publication: Scientific Reports 7, 44030 (2017) doi:10.1038/srep44030
Prospects of target nanostructuring for laser proton acceleration
Andrea Luebcke, Alexander A. Andreev, Sandra Hoehm, Ruediger Grunwald, Lutz Ehrentraut, Matthias Schnuerer


Dr. Andrea Luebcke Tel. 030 6392 1247
Dr. Matthias Schnuerer Tel. 030 6392 1350


Lattice of nanotraps and line narrowing in Raman gas

8 February 2017

Decreasing the emission linewidth from a molecule is one of the key aims in precision spectroscopy. One approach is based on cooling molecules to near absolute zero. An alternative way is to localize the molecules on subwavelength scale. A novel approach in this direction uses a standing wave in a gas-filled hollow fibre. It creates an array of deep, nanometer-scale traps for Raman-active molecules, resulting inlinewidth narrowing by a factor of 10 000.

The radiation emitted by atoms and molecules is usually spectrally broadened due to the motion of the emitters, which results in the Doppler effect. Overcoming this broadening is a difficult task, in particular for molecules. One possibility to overcome the molecular motion is by building deep potential traps with small dimensions. Previously, this was done e.g. by arranging several counterpropagating beams in a complicated setup, with limited success.

In a cooperation effort of the Max Born Institute (A. Husakou) and Xlim Institute in Limoges, researchers show that subwavelength localization and line narrowing is possible in a very simple arrangement due to self-organization of Raman gas (molecular hydrogen) in a hollow photonic crystal fibre. Due to Raman scattering, the continuous-wave pump light transforms into the so-called Stokes sideband, which travels back and forth in the fibre due to reflections from fibre ends and forms a stationary interference pattern - a standing wave with interchanging regions of high and low field [Fig. 1]. In the high-field regions, the Raman transition is saturated and is not active, and the molecules have high potential energy since they are partially in the excited state. In the low-field region, the molecules are Raman-active, and they have low potential energy since they are close to the ground state. These low-field regions form an array of roughly 40 000 narrow, strong traps, which contain localized Raman-active molecules. The size of these traps is around 100 nm (1 nm = 10-9m), which is much smaller than the light wavelength of 1130 nm. Therefore the emitted Stokes sidebands have a very narrow spectral width of only 15 kHz - this is 10 000 times narrower than the Doppler-broadened sidebands for the same conditions!

The self-organization of the gas manifests also on the macroscopic scale. First, the calculations show that the Raman process mainly happens exactly in the fibre section where the standing wave is formed, as shown in the top panel of Fig. 1. Second, the macroscopic gradient of the potential leads to the gas flow towards the fibre end, which is observed by eye in the experiment. This strong localization and the linewidth narrowing can find various uses, e.g. in spectroscopy. However, it can also be used as well as a method to periodically modulate the density of the gas, which is naturally suited for developing quasi-phase-matching schemes for other nonlinear processes, such as effective generation of high harmonics.


Fig. 1 (click to enlarge)

Fig. 1: On the macroscopic scale, the pump light transforms into forward-propagating Stokes (FS) radiation, which is partially reflected from the fibre end and becomes backward-propagating Stokes radiation (BS) which is also amplified by the pump. In the region where both FS and BS are strong, they form interference pattern of standing wave, which is shown on the microscopic scale. In the low-field regions (denoted by red-color molecules) the molecules are in the ground state and strongly trapped, as shown by the potential in the bottom panel. Exactly these trapped molecules are Raman-active, leading to line narrowing.

Original publication: Nature Communications 7, 12779 (2016) doi:10.1038/ncomms12779
"Raman gas self-organizing into deep nano-trap lattice"
M. Alharbi, A. Husakou, M. Chafer, B. Debord, F. Gérôme and F. Benabid


Dr. A. Husakou Tel. 030 6392 1280


Ultrasmall atom motions recorded with ultrashort x-ray pulses

1st Februar 2017

Periodic motions of atoms over a length of a billionth of a millionth of a meter (10-15 m) are mapped by ultrashort x-ray pulses. In a novel type of experiment, regularly arranged atoms in a crystal are set into vibration by a laser pulse and a sequence of snapshots is generated via changes of x-ray absorption.

A crystal represents a regular and periodic spatial arrangement of atoms or ions which is held together by forces between their electrons. The atomic nuclei in this array can undergo different types of oscillations around their equilibrium positions, the so-called lattice vibrations or phonons. The spatial elongation of nuclei in a vibration is much smaller than the distance between atoms, the latter being determined by the distribution of electrons. Nevertheless, the vibrational motions act back on the electrons, modulate their spatial distribution and change the electric and optical properties of the crystal on a time scale which is shorter than 1 ps (10-12 s). To understand these effects and exploit them for novel, e.g., acoustooptical, devices, one needs to image the delicate interplay of nuclear and electronic motions on a time scale much shorter than 1 ps.

In a recent Rapid Communication in Physical Review B, researchers from the Max Born Institute in Berlin (Germany), the Swiss Federal Laboratories for Materials Science and Technology in Dübendorf (Switzerland), and the National Institute of Standards and Technology, Gaithersburg (USA) apply a novel method of optical pump - soft x-ray probe spectroscopy for generating coherent atomic vibrations in small LiBH4 crystals, and reading them out via changes of x-ray absorption. In their experiments, an optical pump pulse centered at 800 nm excites via impulsive Raman scattering a coherent optical phonon with Ag symmetry [movie]. The atomic motions change the distances between the Li+ und (BH4)- ions. The change in distance modulates the electron distribution in the crystal and, thus, the x-ray absorption spectrum of the Li+ ions. In this way, the atomic motions a mapped into a modulation of soft x-ray absorption on the so-called Li K-edge around 60 eV. Ultrashort x-ray pulses measure the x-ray absorption change at different times. From this series of snapshots the atomic motions are reconstructed.

This novel experimental scheme is highly sensitive and allows for the first time to kick off and detect extremely small amplitudes of atomic vibrations. In our case, the Li+ ions move over a distance of only 3 femtometers = 3 x 10-15 m which is comparable to the diameter of the Li+ nucleus and 100000 times smaller than a distance between the ions in the crystal. The experimental observations are in excellent agreement with in-depth theoretical calculations of transient x-ray absorption. This new type of optical pump-soft x-ray probe spectroscopy on a femtosecond time scale holds strong potential for measuring and understanding the interplay of nuclear and electronic motions in liquid and solid matter, a major prerequisite for theoretical simulations and applications in technology.


Abb. 1 (click to enlarge)

Abb. 1: In an x-ray absorption experiment light excites a strongly bound core electron into a conduction band state. On the left of the figure such a transition is shown. An electron which is strongly bound to a Lithium nucleus (green) is excited into a conduction band state (red) that interacts with both the Lithium nucleus and Borohydride group. This conduction band state is therefore sensitive to a modulation of the distance Q between Lithium nucleus and Borohydride group and as a result the x-ray absorption process is sensitive to such a modulation (cf. Figs. 2(b) and 3(d) in the main article). On the right side of the figure the Lithium K-edge x-ray absorption spectrum for different strongly exaggerated displacements is shown.
Movie Movie: What happens in the unit cell of crystalline LiBH4 after impulsive Raman excitation with a femtosecond laser pulse? Upper panel: measured transient absorption change Δ A(t) (symbols) as we vary the time delay between infrared pump pulses and soft x-ray probe pulses at photon energy of ħω = 61.5 eV [cf. Fig. 3(a) in the main article. The lower box shows the atoms in the unit cell of LiBH4 with red boron atoms, gray hydrogen atoms, and green Li atoms. The moving blue circle in the upper panel is synchronized with the moving atoms in the lower panel. The amplitude of the motion is strongly exaggerated (i.e. times 30000) to visualize the pattern of the motion. The reddish color of the unit cell indicates the intensity of the infrared pump pulse.

Original publication: Physical Review B 95, 081101 (R) (2017)
Ultrafast modulation of electronic structure by coherent phonon excitations
J. Weisshaupt, A. Rouzée, M. Woerner, M. J. J. Vrakking, T. Elsaesser, E. L. Shirley, and A. Borgschulte


Dr. Michael Woerner Tel. 030 6392 1470
Jannick Weisshaupt Tel. 030 6392 1471
Dr. Arnaud Rouzée Tel. 030 6392 1240
Prof. Dr. Marc Vrakking Tel. 030 6392 1200
Prof. Dr. Thomas Elsaesser Tel. 030 6392 1400


Unified time and frequency picture of ultrafast atomic excitation in strong fields

5 January 2017

The insight that light sometimes needs to be treated as an electromagnetic wave and sometimes as a stream of energy quanta called photons is as old as quantum physics. In the case of interaction of strong laser fields with atoms the dualism finds its analogue in the intuitive pictures used to explain ionization and excitation: The multiphoton picture and the tunneling picture. In a combined experimental and theoretical study on ultrafast excitation of atoms in intense short pulse laser fields scientists of the Max Born Institute succeeded to show that the prevailing and seemingly disparate intuitive pictures usually used to describe interaction of atoms with intense laser fields can be ascribed to a single nonlinear process. Moreover, they show how the two pictures can be united. The work appeared in the journal Physical Review Letters and has been chosen to be an Editors' suggestion for its particular importance, innovation and broad appeal. Beside the fundamental aspects the work opens new pathways to determine laser intensities with high precision and to control coherent Rydberg population by the laser intensity.

Although the Keldysh parameter, introduced in the 1960's by the eponymous Russian physicist, clearly distinguishes the multiphoton picture and the tunneling picture, it has remained an open question, particularly in the field of strong field excitation, how to reconcile the two seemingly opposing approaches.

In the multiphoton picture the photon character shines through as resonant enhancement in the excitation yield whenever an integer multiple of the photon energy matches the excitation energy of atomic states. However, the energy of atomic states is shifted upwards with increasing laser intensity. This results in resonant-like enhancements in the excitation yield, even at fixed laser frequency (photon energy). In fact, the enhancement occurs periodically, whenever the energy shift corresponds to an additional photon energy (channel closing).

In the tunneling picture the laser field is considered as an electromagnetic wave, where only the oscillating electric field is retained. Excitation can be viewed as a process, where initially the bound electron is liberated by a tunneling process, when the laser field reaches a cycle maximum. In many cases the electron does not gain enough drift energy from the laser field to escape the Coulomb potential of the parent ion by the end of the laser pulse, which would lead to ionization of the atom. Instead, it remains bound in an excited Rydberg state. In the tunneling picture there is no room for resonances in the excitation since tunneling proceeds in a quasi-static electric field, where the laser frequency is irrelevant.

In the study the excitation yield of Ar and Ne atoms as a function of the laser intensity has been directly measured for the first time, covering both the multiphoton and tunneling regimes. In the multiphoton regime pronounced resonant enhancements in the yield have been observed, particularly in the vicinity of the channel closings, while in the tunneling regime no such resonances appeared. However, here excitation has been observed even in an intensity regime which lies above the threshold for expected complete ionization.

The numerical solution of the time dependent Schrödinger equation for the investigated atoms in a strong laser field provided excellent agreement of the theory with the experimental data in both regimes. A more detailed analysis revealed that both pictures represent a complementary description in the time and frequency domain of the same nonlinear process. If one considers excitation in the time domain one can assume that electron wave packets are created periodically at the field cycle maxima. In the multiphoton regime it can be shown that the wave packets are created predominantly close to the maximum intensity of the pulse and thus interfere constructively only if the intensity is close to a channel closing. With this, regular enhancement in the excitation spectrum results effectively only at the photon energy separation. In the tunneling regime the wavepackets are also created periodically at the field cycle maxima, however, predominantly at the rising edge of the laser pulse which, in turn, leads to an irregular interference pattern and consequently, to irregular variations in the excitation spectrum. These rapid variations are not resolved in the experiment and the detected excitation spectrum is smooth.


Fig. 1 (click to enlarge)

Fig. 1: Yield of excited atoms as a function of the laser intensity. At a laser intensity of 200TW/cm2, in the vicinity of a 6 photon channel closing, a strong resonant enhancement of a factor 100 is visible. For the argon data, the theoretical curve is also displayed (red dashed curve), which is in excellent agreement with the experimental data.

Original publication: Phys. Rev. Lett. 118, 013003 (2017) doi:10.1103/PhysRevLett.118.013003
"Unified Time and Frequency Picture of Ultrafast Atomic Excitation in Strong Laser Fields"
H. Zimmermann, S. Patchkovskii, M. Ivanov, and U. Eichmann


Dr. S. Patchkovskii Tel. 030 6392 1241
Prof. Dr. U. Eichmann Tel. 030 6392 1371


Amplification of relativistic Electron Pulses by Direct Laser Field Acceleration

5 January 2017

Controlled direct acceleration of electrons in very strong laser fields can offer a path towards ultra-compact accelerators. Such a direct acceleration requires rectification and decoupling of the oscillating electromagnetic laser field from the electrons in a suitable way. Researchers worldwide try to tackle this challenge. In experiments at the Max Born Institute, direct laser acceleration of electrons could now be demonstrated and understood in detail theoretically. This concept is an important step towards the creation of relativistic and ultra-short electron pulses within very short acceleration distances below one millimeter. Resulting compact electron and related x-ray sources have a broad spectrum of applications in spectroscopy, structural analysis, biomedical sciences and for nanotechnology.

The way electrons can be accelerated up to relativistic kinetic energies in strong laser fields is a fundamental issue in the physics of light-matter interaction. Although the electromagnetic fields of a laser pulse force a free electron previously at rest to oscillations with extremely high velocities, these oscillations cease again when the light pulse has passed by. A net energy transfer by such a direct acceleration of a charged particle in the laser field cannot take place. This fundamental principle - often discussed in physics exams - is valid for certain boundary conditions of the spatial extent and intensity of the laser pulse. Only for particular, different boundary conditions, electrons can indeed receive a net energy transfer via acceleration from the strong laser field. These conditions can be set e.g. by focusing of the laser pulse or the presence of strong electrostatic fields in a plasma.

Worldwide, scientists are looking for solutions how fast electrons can be extracted from extremely strong laser fields and how one can obtain short electron pulses with a high charge density via ultra-short laser pulses.

In light fields of relativistic intensity (I> 1018 W/cm2) electrons oscillate with velocities close to the speed of light. The corresponding kinetic energy reaches values from MeV to GeV (at I> 1022 W/cm2. Strong light fields are realized by focusing ultra-short laser pulses with high energy down to areas of few micrometers. The resulting spatial intensity distribution does already enable the acceleration of the electrons up to high kinetic energies. This process is known as ponderomotive acceleration. It is an essential process for the interaction between strong light fields and matter. Various theoretical studies, however, have predicted that the number of electrons and their kinetic energy can be further significantly increased by a direct acceleration in the laser field, but only if the electron-light interaction is interrupted in a properly tailored way. These considerations were the starting point for the experiments by Julia Braenzel and her colleagues at the Max Born Institute.

In the experiments at MBI, the electrons were decoupled from the light pulse at a particular moment in time, using a separator foil that is opaque for the laser light but can transmit fast electrons. We could show that this method leads to an increase of the number of electrons with high velocities. At first, a 70 TW Ti:Sapphire laser pulse (2 J @ 35 fs) irradiates an 30 - 100 nm thin target foil consisting of a PVF-polymer. In the laser propagation direction, about 109electrons are accelerated up to several MeV energy via the ponderomotive force. During this interaction the foil is almost fully ionized and transformed into plasma.

For sufficiently thin target foil thicknesses below 100nm a fraction of the incident laser light can be transmitted through the plasma. The transmitted light starts to overtake the electrons already emitted in this direction. This corresponds to a quasi-intrinsically synchronized injection of slow electrons into the transmitted, but still relativistic laser field (<8 x 1018W/cm2). If a second thin separator foil is placed at the correct distance behind the first one, amplification in the electron signal for a particular energy interval is observed. Fig. 1a) shows a schematic of the temporal evolution in the experiment and Fig. 1b) presents a direct comparison of the detected electron spectral distribution for a single foil and a double foil configuration, where the second foil acts as a separator. This foil is opaque for the laser light but is transparent for the fast electrons and hence enables a decoupling of both. The time at which the interaction between electrons and transmitted light is interrupted depends on the distance between the two foils.

The experiments carried out in the group of Matthias Schnürer demonstrate that an amplification of the electron signal can obtained and is maximized for a particular distance. The amplification vanishes for very big distances. Numerous measurements as well as numeric simulations confirmed the hypothesis that electrons with high kinetic energy can indeed be extracted out of the light field if they are decoupled appropriately. If the separator foils is located at an optimized position, slow electrons with kinetic energies below 100keV are accelerated to about ten times higher kinetic energies. This effect leads to a concentration of electrons in a narrow energy interval. In contrast to experiments using the different mechanism of laser wake field acceleration, where the production of GeV electrons has already been demonstrated, the direct laser acceleration demonstrated here can be scaled up to high laser intensities and high plasma densities. Beyond the fundamental insight in laser-matter interactions, the direct laser acceleration demonstrated in this work holds promise for the future realization of compact sources of relativistic electrons.


Fig. 1a (click to enlarge)

Fig. 1a: Schematic of the direct electron acceleration in a laser field and its realization in the experiment.
Steinmeyer Fig. 1b: Detected electrons in the laser propagation direction from a single (F1) and double foil (F1F2) target configuration, where the soncond foils acts as a speparator. The plastic foils used were about F1=35nm and F2=85 nm thick. N e values represent the integrated electron numbers for the whole detection range (0,2-7,5 MeV) with respect to the spectrometer aperture.
Fig. 1b (click to enlarge)  

Original Publication: Phys. Rev. Lett. 118, 014801 (2017) doi:10.1103/PhysRevLett.118.014801
"Amplification of Relativistic Electron Bunches by Acceleration in Laser Fields"
J. Braenzel, A.A. Andreev, F. Abicht, L. Ehrentraut, K. Platonov, and M. Schnürer


Julia Bränzel Tel. 030 6392 1338
Dr. Matthias Schnuerer Tel. 030 6392 1315


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