Each of these sources has distinct characteristics for instance, HHG produces relatively lower photon fluxes but benefits from greater flexibility of implementation. In this context, scientists were faced with an interesting prospect: can core-level probing provided by X-rays be combined with the exceptional temporal resolution of femtosecond pulses? This goal has turned into a resounding success, with the availability of complementary ultrafast X-ray sources, such as free-electron lasers, femtosecond slicing of the electron bunch at synchrotrons or high-harmonic generation (HHG). These advances in the field of physical chemistry were of major importance and have been recognized by the 1999 chemistry Nobel Prize awarded to Ahmed Zewail. At much longer wavelengths, ultrafast spectroscopy has, on the other hand, allowed for observation of chemical dynamics with time resolutions on the same order as nuclear motion.
Synchrotron sources alone are therefore limited in the time resolution they provide, prohibiting the study of many non-equilibrium dynamics. The temporal structure of X-rays emitted from synchrotron sources mirrors that of the electrons used to produce them: typically, a train of 10–100 ps long pulses separated by a few nanoseconds. Accessing these transitions, especially with synchrotron radiation, gave chemists and physicists a new tool of X-ray spectroscopy that is now commonly used to study properties of atoms, molecules and solids at equilibrium, both for fundamental enquiries and applied systems characterization. X-rays are characterized by very short wavelengths, typically 0.01–10 nm, with enough energy per photon to access transitions from the core levels of atoms. The discovery of these new-found X-rays earned him the first Nobel Prize in Physics, which opened a new era of research in a great number of scientific areas. Röntgen in 1895, who named them using the commonly used letter for unknowns, X. A discovery of particular importance is that of a ‘new kind of rays’ by W.C. During the past century, our understanding has drastically widened owing to the ability to control light in numerous ways. The knowledge of properties of matter comes hand in hand with our ability to measure it beyond our natural senses.
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