SXR Technical Design Report - Stanford University

2. SCIENTIFIC PROGRAM
2.1 Pump-Probe Ultrafast Chemistry
The ultimate goal in chemistry and physical chemistry is to understand on a fundamental level how bonds break
and reform during chemical reactions. In many cases we arrive at simple pictures of electron motion with respect to
electron pair redistributions or electrostatic interactions along a reaction path. For many systems bonding can be
understood in terms of molecular orbitals and reactivity in dynamical rearrangements of different molecular states.
Such knowledge provides the basis for the understanding of chemical trends and prediction of chemical reactivity
for chemical compounds. Since the excitation and probe steps with conventional optical lasers involve valence
electrons that are delocalized over many atomic centers it is difficult to study complex systems. Unprecedented
insight into chemical reaction dynamics would be gained by probing exactly the atomic site involved. X-ray
spectroscopies can directly access molecular orbital changes associated with or even during chemical reactions. In
particular, accessing core levels in the soft x-ray regime with spectroscopy opens up new prospects to study timeresolved
changes in the electronic structure of complex systems containing the essential elements C, O and N or 3dmetal
atoms. Detailed insight into surface reactions, catalysis, hydrogen-bonded systems and aqueous solutions can
be expected.
Figure 0-1. Schematic illustration of X-ray Spectroscopy. Here, the radiative decay of a core-hole in N 2
adsorbed on a Ni surface by x-ray emission spectroscopy is shown. In addition to elemental sensitivity the
method provides specificity to different chemical sites as shown in this example.
X-ray spectroscopy has the unique ability to provide an atom-specific probe of the electronic structure. In x-ray
emission spectroscopy (XES) the atomic or elemental sensitivity arises from the filling of a core hole by valence
electrons from the same atomic site. In addition, core-level energy shifts (often denoted chemical shifts) connected
with different environments allow for selective probing of chemically non-equivalent atoms (Figue 0-1). The final
state of the x-ray emission process is a valence-hole state similar to the final state in valence band photoemission
with the unique feature that the valence electronic structure is projected onto a specific atom. Notably, selection
rules of XES, and similarly of X-ray photoelectron spectroscopy (XPS), in conjunction with variation of polarization
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