The photoelectron spectra of methane, silane, germane and stannane

Abstract

The photoelectron spectra of the group IV hydrides, CH$_{4}$, SiH$_{4}$, GeH$_{4}$ and SnH$_{4}$ are shown to be due to ionization from valence a$_{1}$ and t$_{2}$ orbitals analogous to the s and p orbitals of the corresponding inert gases. The associated vibrational pattern of the a$_{1}$ bands is a simple progression of the breathing vibration. That of the t$_{2}$ systems shows the effect of Jahn-Teller splitting part of which converts to spin-orbit splitting for the heavier hydrides. The bond lengths and angles of the ionized states are evaluated approximately from the structures and intensities of the band patterns. The experimental results give some support for theoretical calculations by Dixon which indicate that the triple degeneracy of the (t$_{2})^{-1}$ state is removed by distortion to a D$_{2\text{d}}$ conformation, its low energy $^{2}$B$_{2}$ component involving a fairly long progression in the $\nu _{2}$ vibration and, in the case of methane, progressions in $\nu _{1}$ containing several members. This agrees with our observations but our analysis of the spectra indicates that this state of CH$_{4}^{+}$ takes up a conformation that is almost square coplanar with an extremely low inversion barrier. The $^{2}$B$_{2}$ ionized states of SiH$_{4}$, GeH$_{4}$ and SnH$_{4}$ though considerably distorted do not become coplanar.