The work of Rothhardt et al. proposes a very interesting variant of pump-probe spectroscopy, where the probe pulse ionizes the target molecule and the resulting photoproducts (electrons and ions produced by the ionization event) are collected in a highly specialized spectrometer. This detector is able to determine 3-dimensional momentum vectors of all of the photoproducts created by the ionizing pulse. Such information about the photoproducts allows experimenters to ask very specific questions about the molecule at the time when it was ionized, such as, what was the orientation of the molecule in the laboratory frame, and in what directions did the electrons leave relative to this molecular axis? This type of information is key to understanding the dynamics that is taking place in the excited molecular system. However, the ability to ask these questions comes at a great cost, the more specific (or differential) the experimenters want to be, the more data they need to collect.
In order to record the large amount of data required to ask very specific questions of the molecular system Rothhardt et al. use a very unique laser system. This laser system produces extreme ultraviolet (XUV) photons. XUV photons are great probes of chemical dynamics because the interaction of the XUV probe pulse with a target molecule can be described quite easily through the well understood method of first-order time-dependent perturbation theory. Appealing to this well-understood theory allows for easy interpretation of the recorded momentum vectors. In order to generate this XUV radiation, the authors employ the method of strong-field driven high harmonic generation (HHG). In this process, a strong laser field, usually with an infrared wavelength, undergoes a non-linear frequency down-conversion process to produce radiation at odd-integer multiples of the drive laser frequency. This radiation can stretch deep into the XUV regime of the spectrum. The unique property of the source developed by Rothhardt et al., which enables the application of such highly differential measurement techniques, is the level of XUV flux, or the number of XUV photons created per second. They produce ~10¹¹ photons/sec, at a repetition rate of 100 kHz, which is extremely high for these types of sources.
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