THE DYNAMICS OF CORE-HOLE EXCITATION AND DECAY IN ADSORBED AND CONDENSED MOLECULES ON SOLID-SURFACES

In isolated molecules, the decay of neutral and ionic core-excited spe cies leads to distinctly different decay electron spectra which are us ually termed autoionization and Auger spectra, respectively. If a mole cule is coupled to a substrate in an adsorbate state, or to other surr ounding species in a condensed phase, this coupling can modify or, in the extreme, eliminate this difference; also, satellite structure can be influenced. This results from charge and/or energy transfer between the target molecule and its surroundings before the decay event, and can therefore be used to obtain information about such processes on ti me scales on the order of the core hole life time or shorter. Addition al information can be derived from the registration of fragmentation o f the molecule as a consequence of the primary excitation and/or the f inal states of decay. In this case, a third time scale, that of motion of atoms or groups of atoms, comes into play which in extreme cases c an compete with those of hole decay and electronic coupling to the sur roundings. Such coupling to nuclear motion is also at work in the inte rference of the electronic evolution with vibrational motion in the mo lecule which can be investigated with sufficient resolution in the pri mary excitation. Again, the change induced here into the behavior of t he free molecule by the presence of a coupled medium (substrate or con densate) can be used to extract information on the relative time scale s. At present much of this information is of a qualitative nature; ade quate theories would be very helpful for more detailed understanding.