VUV Spectroscopy of Atoms, Molecules and Surfaces

82 Chapter 5. Femtosecond VUV core-level spectroscopy ...
the technique of pico- or femtosecond X-ray diffraction, using X-rays emitted
from fs laser-produced plasmas or obtained from ”sliced” or time-gated
synchrotron-radiation pulses (cf. chapter 1.4). The non-thermal melting process
is characterized by a disordering of the crystal lattice accomplished by
the sudden excitation of a large (∼10 %) fraction of the electrons from the
valence band to the conduction band of the semiconductor. The lattice instability
occurs before a significant amount of the electronic energy has been
transferred to the lattice by electron-phonon interactions and is thus purely
electronic in nature. The disordering has been observed for different laser
fluences encompassing the damage threshold [19, 20, 22], below which it is
found to be reversible and accompanied by a band-gap collapse for fluences
near the damage threshold [23].
5.2 Laser-induced desorption
The field of photo-induced desorption of molecules from surfaces using pulsed
laser light of nanosecond (ns) or femtosecond (fs) duration has undergone a
rapid development within the last ten years as reflected by the large amount of
review literature available on the subject [24, 25, 26, 27, 28, 29, 30, 31, 32, 33].
A chemical reaction taking place at a surface can be very different from that
of the gas-phase, since the molecules are confined to aligned geometries and
their chemical properties are modified by the presence of the surface [29].
In addition, a non-insulating surface acts as a heat-reservoir with an almost
infinte number of degrees of freedom, interacting with the adsorbate and
representing a challenge to theorists. Since the heating- or exciting laser
light penetrates several atomic layers into the substrate supporting the adsorbate
layer, the desorption process is much more likely the result of an
adsorbate-substrate coupling than a direct laser-adsorbate interaction. The
emphasis of the present chapter is on the laser-induced desorption of diatomic
molecules from metal surfaces, which are the systems that have been most
intensively investigated with ns–fs laser pulses. In particular, the interest
has concentrated on the catalytically- and theoretically relevant systems of
CO, NO or O2 molecules adsorbed on Cu, Ru, Pd or Pt surfaces. The focus
of the following discussion will therefore mainly be on the information obtained
from previous all-optical studies of such systems, representing a good
starting point for the application of the new technique of fs VUV core-level
spectroscopy. In most of the all-optical approaches the desorbed molecules
have been detected in a mass spectrometer [34, 35] or by the technique of
Resonantly Enhanced Multi-Photon Ionization (REMPI) [36, 37, 38]. In this
manner information was obtained about the translational-, vibrational- and
rotational distributions of the desorbed molecules and the desorption prob-