Berlin 2005 – wissenschaftliches Programm
SYPE 2.1: Hauptvortrag
Freitag, 4. März 2005, 16:30–17:00, HU Senatssaal
From Einstein’s Photoelectric Effect to ZEKE Rydberg States and a New Molecular Orbital Description — •Klaus Müller-Dethlefs — York Centre for Laser Spectroscopy, Department of Chemistry — The University of York, York YO10 5DD, United Kingdom
From Einstein’s discovery of the photo-electric effect, one of the pillars of modern quantum theory, it has taken nearly fifty years until the first practical application of the photoelectric effect to molecules was demonstrated in the 1960’s by David Turner at Imperial College. The main advance came from the discovery of a very high intensity He(I) light source. Another twenty years later, we developed rotationally resolved ZEKE (Zero Electron Kinetic Energy) photoelectron spectroscopy, based on the pulsed field ionization of long-lived, very high n, Rydberg states and, over the last two decades, refined it as a valuable tool for the study of molecular ions and clusters. Here we present a new useful definition for molecular orbitals that is based on the analysis of rotationally resolved ZEKE spectra. The selection rules for ZEKE transitions from a neutral molecule of total angular momentum (J) to an ion core (J+) and a Rydberg electron (l) are more complicated than for neutral transitions since angular momentum is transferred to the final state ZEKE Rydberg electron. Hence for the rotational band contour analysis of ZEKE spectra a suitable model must be employed to account for the ionisation dynamics. In the model presented here we find that the overlap of the ion ground state wave-function with the neutral state wave-function leads to an unambiguous definition of the HOMO of the neutral state. We call this orbital a spectator orbital since it is coupled to a fully electronically relaxed ion wave-function. The orbital, which is related to the "Dyson" orbital, is well defined for any level of theory used to compute the electronic wavefunctions of the neutral or of the ion.