Zero kinetic energy electron spectroscopy of molecules: Rotational symmetry selection rules and intensities

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Abstract
The rotationally resolved ‘‘zero kinetic energy (ZEKE)’’ photoelectron spectra for linear and planar molecules are interpreted as transitions into highly excited Rydberg states corresponding to Hund’s coupling case (d). The general case (d) is described for planar and linear molecules. The observed ZEKE transitions can be understood in terms of near-symmetry selection rules for the one-electron states involved. The proposed model consists of two steps: photon absorption leads to a state where the angular momentum of the excited electron is still coupled to the figure axis (the internuclear axis for diatomics and the highest symmetry axis for planar molecules). For planar molecules this leads to a selection rule for the projection quantum number K. The second step is the decoupling of the excited electron from the figure axis and its recoupling to the total angular momentum vector of the ion core. The decoupling process is assumed to occur adiabatically and hence with preservation of the electronic symmetry of the excited electron state. This leads to a selection rule for the total angular momentum N and explains the occurrence of only a single peak with v+2=1, N+=4, K+=3 in the ZEKE spectrum of the B̃ state of ortho-NH3 with v2=2, JB=3, KB=1. Also, analytical formulas are presented for the intensities of 1+1 and 2+1 photoionization processes including the alignment of the intermediate state.