VUV Spectroscopy of Atoms, Molecules and Surfaces

5.6 Future prospects 117
core-ionized photoelectrons from the optimum ∼50 eV. Since the growth of
the inelastic mean-free path is faster towards lower kinetic energies when
going away from this value (staying below 10 ˚A in the 20–250 eV region),
one should probably concentrate on systems with more loosely bound corelevels
than has been done here. The 4f core-levels of the metals to the left
of Pt in the third transition-metal group, i.e. Ta [148], W [149], Re [150],
Os, and Ir [151] are all in the 20–60 eV range—increasing from the left of
the periodic table—and have been clearly resolved with ≤100 eV photons.
These systems could like-wise act as important model systems for desorption
dynamics, although catalytically not quite as relevant as Pt.
The semiconductor elements Ga, Ge, As, Se, In and Sb have 3d or 4d corelevels
in the 20–50 eV range which are known to behave properly for ≤100 eV
incident photons [152, 153, 154, 155]. In addition, the photo-induced reaction
dynamics of adsorbates on semiconductors—in particular Si, Ge and
GaAs—is technologically highly relevant and has been the subject of numerous
studies using either continuous-wave light [156, 157], synchrotron radiation
[158] or ns laser pulses [159, 160, 161]. These studies have included
the passivating oxidation-, nitridation- and sulfidication reactions using O2
[157], NO [156], NH3 [161] and H2S [158] adsorbates, and the photo-assisted
etching by adsorbed halogen molecules (Cl2) [160]. All of these systems have
been the subjects of core-level spectroscopic studies and pronounced chemical
shifts induced by the presence of the adsorbate have been observed (see, e.g.,
[162, 163, 164, 165]).
Departing from the dynamics associated with the desorption or reaction
of adsorbed species, one could follow the foot steps of the fs X-ray diffraction
experiments mentioned in the introduction and study the evolution in a
semiconductor core-level spectrum following the excitation of the bare surface
with a visible pump pulse. The use of a completely different technique might
shed some new light on, e.g., the non-thermal melting process observed in
InSb [19, 21], Ge [20] and GaAs [22, 23], and the nature of the band-structure
collapse (equivalent to a semiconductor-to-metal transition) which has been
observed in GaAs but apparently not in InSb [166]. Another experiment could
address the amorphous-to-crystalline phase-transition in GeSb of relevance
for optical data storage [167] or a non-specified phase transition recently observed
in fs laser-excited Al in an all-optical reflectivity measurement [168].