14 - Photoelectron spectra of CO₂, OCS, and CS₂ in a molecular beam

Summary

Concepts illustrated: supersonic expansion cooling; adiabatic and vertical ionization energies; vibrational structure in the spectra of triatomic molecules; Franck–Condon principle; link between photoelectron spectra and molecular orbital diagrams.

A severe restriction of conventional photoelectron spectroscopy is its low resolution. The main limitation is instrumental resolution, particularly that caused by the electron energy analyser, as was discussed in Chapter 12. Resolving rotational structure is not a realistic prospect for conventional photoelectron spectroscopy but even vibrational structure may be difficult to resolve. In addition to the instrumental resolution must be added other factors such as rotational and Doppler broadening which, if they could be dramatically reduced, might make a sufficient difference to improve many photoelectron spectra. A potential solution is to combine conventional photoelectron spectroscopy with supersonic molecular beams. Supersonic expansions can produce dramatic cooling of rotational degrees of freedom and, if part of the expansion is skimmed into a second vacuum chamber, can be converted to a beam with a very narrow range of velocities. This is precisely the approach adopted by Wang et al. [1], the molecular beam being crossed at right angles by HeI VUV radiation (58.4 nm) to produce a near Doppler-free photoelectron spectrum. The resolution achieved is in the region of 12 meV (100 cm^-1).

The ultraviolet photoelectron spectra of CO₂, OCS, and CS₂ in molecular beams are discussed here. These illustrate some of the important concepts involved in the interpretation of the photoelectron spectra of polyatomic molecules.