13 - Ultraviolet photoelectron spectrum of CO

Summary

Concepts illustrated: vibrational structure and Franck–Condon principle; adiabatic and vertical ionization energies; Koopmans's theorem; link between photoelectron spectra and molecular orbital diagrams; Morse potentials.

Carbon monoxide was one of the first molecules studied by ultraviolet photoelectron spectroscopy [1]. A typical HeI spectrum is shown in Figure 13.1. The spectrum appears to be clustered into three band systems. The starting point for interpreting this spectrum is to consider the molecular orbitals of CO and the possible electronic states of the cation formed when an electron is removed.

Electronic structures of CO and CO⁺

Any student familiar with chemical bonding will almost certainly be able to construct a qualitative molecular orbital diagram for a diatomic molecule composed of first row atoms. Such a diagram is shown for CO in Figure 13.2. The orbital occupancy corresponds to the ground electronic configuration 1σ²2σ²3σ²4σ²1π⁴5σ². The σ MOs actually have σ⁺ symmetry but it is not uncommon to see the superscript omitted. Since all occupied orbitals are fully occupied, the ground state is therefore a ¹Σ⁺ state and, since it is the lowest electronic state of CO, it is given the prefix X, i.e. X¹Σ⁺, to distinguish it from higher energy ¹Σ⁺ states of CO.

Consider the electronic states of the cation formed by removing an electron. If the electron is removed from the highest occupied molecular orbital (HOMO), the 5 orbital, then the cation will be in a ²Σ⁺ state.