Chapter 5 - Polyatomic Molecules

Abstract

Polyatomic molecules are electrically neutral groups of three or more atoms held together by covalent bonds. Absolute partial cross-sections of polyatomic molecules (NH3, CH4, C2H2, C2H4, C2H6, CH3OH, SiH4) are studied experimentally and theoretically from various sources and assignments, taking their ground state electronic configuration into consideration. An overview of the valence shell photoelectron spectrum (PES) of each polyatomic molecule has been provided in the beginning, and adiabatic ionization potentials (AIP) have been obtained from various sources for this study. The data of geometries, AIPs, and molecular parameters of outer and inner valence and core orbitals have been listed and tabulated and used for further studies. Absolute partial cross-section of polyatomic molecules for outer and inner valence shells, parent and fragment ions, and core orbitals have been studied, drawing conclusions from various sources. For NH3 and CH4, we have combined the branching ratios with the absolute total photoionization cross-sections of Samson et al. (Sam 89) to arrive at the absolute partial orbital cross-sections. At low resolution, step-like features are observed indicating direct ionization. At higher resolution, weak autoionization features are superimposed. Double-to-single photoionization ratio has been obtained from PIPICO studies. Pseudophoton (e, e + ion) coincidence spectroscopic study from threshold to 80 eV provided absolute partial cross-sections in ethylene. We have adopted the newer total absorption cross-sections and utilized the quantum yield of ionization and ion branching ratios from sources to deduce absolute partial cross-sections for ions in silane. Several pieces of evidence have been collected from statistical RRKM theory and synchrotron-based experiments to correlate and draw results. The absolute photoionization cross-sections of polyatomic molecules and ions have been studied in the vicinity of the k-edge. Decay mechanisms have been studied by resonant and normal Auger spectra and the results have been plotted. At and above the K-edge, double ionization ensues. At higher energy states, there is a sudden change in the direction from participator to spectator decay, where low-energy states indicate participator decay.