Photoelectron spectroscopy of ethylene, isobutylene, trimethylethylene, and tetramethylethylene at variable angles

Abstract

Using a HeI line 58.4 nm source lamp, photoelectron angular distributions were measured for the series of four olefins: ethylene, isobutylene (2‐methyl‐propene), trimethylethylene (2‐methyl‐2‐butene), and tetramethylethylene (2,3‐dimethyl‐2‐butene). From these, the asymmetry parameter β as a function of photoelectron energy was obtained for each of these molecules. The following important effects in the behavior of β are observed: (a) In the π orbital ionization regions of the spectrum of each molecule, β increases with increasing electron energy across the vibrational envelopes. (b) With increasing methyl substitution (and at a fixed photoelectron enegy) β for this band decreases. (c) In the region of the spectra of each of the methyl‐substituted ethylenes involving several 2pσ bands, this energy dependence of β behaves as if they constituted a single band, in spite of the widely differing orbital symmetries. (d) Over most of the 2pσ region of each molecule, β decreases with increasing photoelectron energy, except for the high ionization potential end of this region, where β increases instead. We attribute effects (b) and (d) to σ–π orbital mixing.