Photon Stimulated Desorption of Neutrals by Synchrotron Radiation from Chemisorbed, Physisorbed, and Condensed Layers

Abstract

The intention of this contribution is to show that photon stimulated desorption investigations with synchrotron light can increase the understanding of the DIET process, at least for simple adsorbates like diatomic molecules or rare gases on metal surfaces and particularly if desorption of neutral particles is monitored as well. Generally, the desorption rate dN_k/dt for a species k (e.g. CO^·, CO⁺, O⁺, O⁺⁺, etc.) can be written as /1/

$$d{N_k}/dt\, = \,i\;\left( {\hbar w} \right)\; \cdot \;{\sigma _j}\left( {\hbar w} \right)\; \cdot \,{P_{escape}}\left( {k,j} \right),$$

(1)

where i(ħω), σ and P denote the photon flux, the cross section for a primary excitation of type j and the probability that this excitation j leads to desorption of a particle k. Understanding DIET quantitatively requires the ability to assign primary electronic excitations and to calculate the interdependent dynamics of the evolution of the electronic excitation and the nuclear motion of the adsorbate or its fragments /1,2/. In this way, escape probabilities and branching ratios can be obtained. Experimental data are necessary, in particular on energies and symmetries of primary and secondary thresholds and resonant maxima of the PSD yield vs. photon energy, for the assignment of the primary electronic excitations; and also branching ratios, total desorption cross sections as function of coverage, angular distributions, and kinetic as well as internal energies of the desorbing particles, for the evaluation of P.