High resolution angle resolved photoelectron spectrum of co excited with polarized resonance radiation

Abstract

The photoelectron spectrum of CO involving ionization from the three outermost valence orbitals has been studied by He I, He II, Ne I and Ne II excitation. Angular distributions of the electrons have been obtained and β-parameter spectra (βPS) showing the value of β recorded in small energy steps at high resolution are presented. The vibrational structure of the ionic states has been resolved in detail. In the spectrum excited by He Iα radiation, long progressions have been observed, the one of the X state extending up to υ' = 33. By using the energies of this progression, the coefficient of the third order term ħω ey e(υ+1/2) 3 in the power series in υ+1/2 for the vibrational energy has been determined. The value is ħω ey e = −(12±3) × 10 −3cm −1. The intensity distribution in the vibrational progressions of both the Ne I and He I excited spectra indicates that the ionization proceeds partly via autoionization from a vibrationally excited level. For these excitation sources, large variations in the β -value have also been observed. They are associated with both autoionization and shape resonances. At the higher excitation energies of Ne II and He II, no such variations are present. A similar study of N 2 gave no indications of autoionization contributions to the He I excited spectrum.