Volume 92, number 6 CHEMICAL PHYSICS LETTERS 12 November 1982 SHAPE RESONANCE AND ELECTRONIC AUTOIONIZATION IN VIBRATIONALLY RESOLVED PARTIAL PHOTOIONIZATION CROSS SECTIONS OF 0 2 Paul MORIN, Irene NENNER D~parternent de Physico-Chirnie, Centre d'Etudes Nucl~aires de Saclay, 91490 Gif sur Yvette Cedex, France Marie Yvonne ADAM Equipe de Physique Atomique, Universitd de Paris Vll, Place Jussieu, 75005 Paris, France Marie Jeanne HUBIN-FRANSKIN *, Jacques DELWlCHE * lnstinlt de Chimie, Universit~ de Libge, Sart Tilman, 4000 par Libge 1, Belgium and LURE, bat, 209c, Universit~Paris-Sud, 91405 Orsay Cedex, France and H616ne LEFEBVRE-BRION and Annick GIUSTI-SUZOR Laboratoire de Photophysique Mol~culaire,Bdt. 213, Universit~ Paris-Sud, 91405 Orsay Cedex, France Received 20 August 1982 Vibrationally resolved partial photoionization cross sections of 02 leading to the b 4:gg and B 2:gg (3Ogl) ionic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The ou shape resonance is observed around 21.5 eV, in fairly good agreement with "one-electron" theoretical predictions. The series of resonances from 19 to 20.5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing. Calculations using multichannel quantum defect theory reproduce the most important features. 1. Introduction Vibrationally resolved photoelectron spectroscopy has been shown to play a major role in understanding molecular photoionization processes [ 1-5]. Shape resonances have been predicted [6-8] in a large set of molecules including N 2 and CO, and induce large deviations from Franck-Condon (FC) distributions in the vibrationaUy resolved (vibronic) partial cross sections because of the internuclear distance (R) de- pendence of the electronic transition moment over a wide energy range (a few eV). Autoionization struc- tures may also induce deviations from FC distributions [9] but in narrower energy ranges (typically less than 100 meV). * "Chercheur qualifi~" of the Fonds National de la Recherche Scientifique of Belgium. The case of the ionization of a 3trg valence electron in 0 2 presented here provides a clear example of both of these effects. Ionization of this open-sheU molecule with ejection of a 3trg electron leads to two ionic states b 4E~- and B 2Zg with o = 0 thresholds respectively located at 18.17 and 20.29 eV [10]. Recent measurements [11] by fluorescence excita- tion spectroscopy (FES) of the b cross section have suggested, in contrast with an assignment based on prior low-resolution photoelectron spectroscopy (PES) work [12], that the broad peak (fwhm ~0.4 eV) lo- cated at 19 eV could be the predicted shape resonance. This interpretation is however in disagreement with moment theory [13], static exchange [14] and multi- ple scattering calculations [ 15,16], which place the resonance at 21 eV with a 4 eV width. Vibronic b and B cross sections obtained recently 0 009-2614/82/0000-0000/$ 02.75 © 1982 North-Holland 609