Ultrathin water films made to flow - A flat ray for X-ray spectroscopy

An important advance in the spectroscopy of soft X-ray liquid samples by a new flat-beam system paves the way for novel stationary and time-resolved experiments.

Element-specific X-ray methods play a key role in the study of the atomic structure and composition of functional materials. X-ray spectroscopy can be used to determine oxidation states, distances, coordination numbers and the nature of the nearest neighbors of the selected element. With a wide variety of X-ray spectroscopic methods, many gaseous, liquid and solid samples or molecular systems at interfaces have been studied. Stationary and time-dependent material properties were determined predominantly at synchrotron radiation sources and more recently at X-ray free-electron lasers.

The investigation of liquid samples with absorption spectroscopy in the soft X-ray range (in the energy range of about 0.2 to 1.5 keV) presents a particular challenge. First, the experiments must be carried out under ultrahigh-vacuum conditions, in an environment that seems to be incompatible with the high vapor pressure of water. In addition, due to the large absorption cross sections in the soft X-ray range, measurement of the transmission requires difficult-to-implement sample thicknesses in the range of one micron and less (1 micron = 10-6 m = one millionth of a meter). In contrast, absorption spectrum measurements based on detection of secondary decay signals, such as X-ray fluorescence, are limited to relatively high concentration samples.

Fig. Flat jet system for liquids with the two nozzles, the two colliding laminar liquid jets and the 1 mm wide and 5 mm long sheet-shaped water film with a thickness of 1 - 2 microns. The thickness of the film was determined from transmission measurements at the oxygen K absorption edge (left). The flat-jet system allows absorption measurements in the soft X-ray range in transmission, as shown for example by measuring the absorption spectrum at the nitrogen K absorption edge of ammonium chloride (right).

A solution to these problems is the use of cells with thin membrane-based windows for transmission measurements. Although the thickness of the liquid film can be controlled, it can not be used to examine radiation-sensitive molecular samples because the sample is in the X-ray or in a visible laser beam In laser excitation and X-ray interrogation measurements is destroyed. This radiation damage is avoided by continuously replacing the sample in a liquid jet. With such liquid jets, however, wherein the liquid is forced through a nozzle into the high vacuum chamber, it is difficult or even impossible to realize sample thicknesses in the range of one micron or less.

In a collaboration, scientists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI), the Helmholtz Center Berlin (HZB) and the Max Planck Institute for Dynamics and Self-Organization (MPIDS) have now successfully implemented a novel flat fan system for Transmittance measurements of liquid samples in the soft X-ray range shown. A well-known phenomenon of fluid dynamics was exploited: When two identical laminar liquid jets meet at a well-defined angle, the liquid spreads radially, resulting in the formation of a thin sheet-like liquid film perpendicular to the plane of the two jets. This film is stabilized by a likewise formed from the liquid edge lip.

The innovation here is that an over-hours stable flat jet in a vacuum (at pressures less than 10-3 mbar) with a thickness in the range of one to two micrometers was realized and applied. For the first time, absorption spectra of liquid samples in transmission with photon energies in the soft X-ray region and completely without membrane-based windows could be measured. The X-ray spectroscopic measurements were carried out at the synchrotron radiation source for soft X-rays BESSYII of the Helmholtz Center Berlin. This technological breakthrough opens up completely new possibilities for the stationary and time-resolved spectroscopy of liquid samples with soft X-radiation.

Original publication

A liquid flatjet system for solution phase soft-X-ray spectroscopy

M. Ekimova, W. Quevedo, M. Faubel, P. Wernet, E. T. J. Nibbering

Structural Dynamics 2 (2015) 054301/1-13