Shape resonance effects in the rotationally resolved photoelectron spectra of O2

We report the results of theoretical and experimental studies of the rotationally resolved photoelectron spectra of O2 at low temperature leading to the v+=0, 1, and 2 levels of the X 2Πg state of O+2. A delayed, pulsed field ionization technique is used in conjunction with a coherent VUV radiation source to obtain high resolution spectra near threshold. The data are compared with theoretical results obtained using static-exchange photoelectron orbitals and a full description of the mixed Hund’s case (a)–(b) ionic ground state. Agreement with experiment is good, especially for the v+=1 and v+=2 levels. Analysis of the rotational branch intensities yields detailed information on the angular momentum composition of the shape resonance near threshold. We also show that the dependence of the electronic transition moment on internuclear distance caused by the shape resonance leads to a significant dependence of the rotational branch intensity on ion vibrational level.