Vibrationally resolved photoelectron spectroscopy using synchrotron radiation: studies of the 5σ ionisation of CO between 20.936 and 21.417 eV

Abstract

Vibrationally resolved photoelectron spectroscopy has been used to study the photoionisation from the 5 sigma orbital of the CO molecule. Synchrotron radiation has been employed and spectra have been recorded at seven different ionisation energies in the energy range between 20.936 and 21.417 eV. Five of the selected energies coincide with highly excited discrete states of the neutral molecule and the two remaining energies are half-way between adjacent discrete states. Vibrational excitations in the final ionic state corresponding to vibrational quantum numbers 2-7 could be observed at all photon energies with an intensity of about 1% of the v=0 component but with substantial variations with vibrational quantum number and photon energy. These excitations are attributed to autoionisation from intermediate Rydberg states and the presence of a shape resonance in the same energy region. The highly excited neutral states were found to have an equilibrium bond distance of 1.24+or-0.02 AA.