Constant initial state

The unoccupied electronic states of a solid can be experimentally explored by different techniques. The most commonly used are inverse photoemission, where low-energy electrons impinge on the surface of the solid, and the photon-based techniques ellipsometry, NEXAFS and constant-initial-state spectroscopy. Results derived from inverse photoemission spectroscopy might be questionable unless low-energy electrons (c. 10-20 eV) and low beam currents are used as in LEED... 

Two variables of a PES experiment are readily altered the input photon energy (hv) and the output photoelectron kinetic energy (KE). In a classical energy distribution curve (EDC) operation mode, one scans KE only and obtains information on the energy level manifold. While this is the only mode possible with fixed-energy VUV photon sources, SR permits two further combinations a constant final-state (CFS) mode where one scans hv and a constant initial-state (CIS) mode with both hv and KE scanned in such a way that their difference remains constant. CIS and CFS modes permit separate studies of the initial (ground electronic states, ionization probabilities) and final (photoelectron perturbed by the molecular ion) stages of photoionization events. 

Potentiostatic UPD-OPD transition experiments can be carried out either varying A i at T/f = const or changing rg at A i = const In the first case, the influence of a 2D Meads phase and/or of a 2D Me-S alloy phase on the nucleation and growth kinetics of the 3D Me bulk phase can be analyzed. In the second case, the nucleation and/or growth kinetics as a function of the supersaturation at constant initial state in the UPD range can be studied. 

Constant initial state techniques require the photon energy and electron energy analyser to be scanned synchronously so that hv — E is kept constant. In this way, the photon energy dependent partial yield of electrons in an energy window AE at a fixed initial state energy E = E — hv is measured. Core level to empty surface state transitions are enhanced by selecting the appropriate E corresponding to a minimum in the valence band emission. 

Gunnarsson and Schonhammer (1987) and Gunnarsson and Li (1987) have developed a formalism for resonance photoemission in the l/Vf technique. The constant initial state (CIS) spectrum was studied. It was found that the CIS... 

Fig. 5. The origin of energy distribution curves (EDCs), of constant initial state spectra (CISs), and constant final state spectra (CFSs). For the EDC, the photon energy is fixed and the electron energy is scanned. For the CIS, the photon and the electron energies are scanned synchronously. For the CFS, the electron energy is fixed while the photon energy is scanned. Matrix element and escape effects will distort all these spectra.

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