VUV Spectroscopy of Atoms, Molecules and Surfaces

5.1 Introduction 81
difference in optical path length of rays diffracted by the different groves
of the grating [10]. In order to avoid this effect, a multilayer mirror will
be applied for the selection of a single harmonic order corresponding to a
∼100 eV photon energy in our experiments.
The construction of the present setup is not complete yet, and my main
contributions have been the development of the VUV light source of highorder
harmonics and reflectivity tests of the multilayer mirror that will be
used for the wavelength selection. In addition, I have performed the initial
(static) tests at an ASTRID synchrotron-radiation beamline of systems suitable
for studies with the relatively low photon energy of ∼100 eV, achievable
with the high-order harmonics. These have included the CO/Pt(111) system
which was found to be inappropriate for studies with ∼100 eV photons,
and the O/Al(111) system which was considered a more ”safe” candidate on
account of its well-characterized core-level properties. Despite a large number
of investigations of this technologically relevant system, the initial stages
of the oxidation process are still not fully understood. A secondary aim of
the O/Al study was to shed some more light on the oxidation mechanism,
in particular the influence of the pre-treatment of the Al surface prior to
adsorption.
During these preparative investigations, similar but less ambitious setups
have been constructed by other groups, using either a grating for the wavelength
selection or lower photon energies, created by high-harmonic generation.
Karlsson and Karlsson have studied electron dynamics at a laser-excited
InSb(110) surface using 9.55 eV photons [11], the group of Zacharias has recently
presented a photoelectron spectrum of the CO/Ni(111) system [12]
and the group of Palmer has completed a setup using 11–42 eV photons,
aimed at studies of the H2O/graphite and SF6/graphite systems [13]. All of
these groups use gratings for the wavelength selection and are thus limited to
a ∼ps time resolution. Very recently, setups based on multilayer mirrors have
been presented by the groups of Kapteyn [14] and Heinzmann [15], applying
partially monochromatized light around 44 eV and fully monochromatized
light in the 66–73 eV range, respectively. The setup of Kapteyn et al. was
very recently applied for time-resolved measurements of valence-band photoemission
spectra of the laser-excited O2/Pt(111) system. A transient peak
present in the spectra was ascribed to the dissociation of an excited superoxo-
(O − 2 )orsuperperoxo(O2− 2 ) state while a non-transient change was attributed
to desorption.
In connection with these experiments one should also mention the recent
studies of anharmonic lattice dynamics [16, 17, 18] and fs laser-induced
non-thermal melting [16, 19, 20, 21] of semiconductor surfaces by the groups
of Falcone, Rousse, Shank and Wilson. All of these studies have applied