To understand an atom or a molecule, physicists must not only know their internal structure, but also be able to describe the motion of the electrons. Due to the extremely high speed, this was not possible until now. Now, a European research team has developed such a measurement method. They report about it in Physical Review Letters 105, 053001.
At the level of atoms and molecules, our everyday conception of the world no longer works. An electron is usually thought of as a small particle. "That's it, too," says Prof. Marc Vrakking, Director at the Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin. "But to understand it, sometimes we have to look at it from the quantum mechanical point of view and imagine it as a wave packet." Physicists can use this abstract idea to explain phenomena that later coincide with our everyday ideas.
Since one can not directly observe the motion of an electron because it is too fast, the European research team has measured the properties of the electron as a wave packet. Once they knew all the properties of this wave packet, they were able to derive from it the complete motion of the electron.
For the experiment, the researchers have used the principle of superposition of waves, the so-called interference. They have followed the same procedure as in experiments with light rays, in which regular light falls through two slits and on the screen behind light and dark stripes are visible. The rays of light behave like waves - meet two wave mountains on each other, resulting in a bright strip, a wave crest and a wave trough up and appear as a dark strip.
In order to characterize a wave packet, as the physicists consider the electron, they have first generated a second wave packet, analogous to the second slot for the light beam: With an attosecond laser pulse, they have an electron dissolved out of the investigated atom. An attosecond laser pulse lasts one billionth of a billionth of a second. Since the researchers control this laser pulse, they now know the properties of the released electron - and thus of the wave packet, as they imagine it. If you now overlay this generated wave packet with the unknown wave packet, you can deduce the unknown properties from the interference pattern.
The method explains Matthias Kling from the Laboratory for Attosecond Physics at the Max Planck Institute for Quantum Optics: "For a meaningful interference pattern, we first had to raise the unknown wave packet to the same energy level as the well-known wave packet previously generated by us, which is much affected by the attosecond laser pulse has more energy than the unknown wave packet in its original state, and we used an infrared laser pulse to create this interference. " In the case of very large differences in energy, there is no real interference pattern - that would be as if one could still recognize by the light rays through which slit the light has fallen. By superimposing the two equivalent wave packets, the researchers were able to calculate the known pattern and thus obtained the unknown pattern.
To characterize a wave packet, the physicists must know its different states and how large the shares of these states are in the wave packet. "We call that the population of states," says Vrakking. In addition, the phases of the waves must be known, ie the temporal shift against each other. When they know these factors, the scientists return to our normal imagination and describe the complete movement of the electrons, which one may then think of as particles again.