Chemical Phystcs 173 (1993) 479-489 North-Holland High-resolution threshold photoelectron spectrum of molecular oxygen F. Merkt ‘, P.M. Guyon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA LCAM Bbtunent 35 I, Umversltk Parrs XI, 91405 Orsay, France and LURE Bbment 209d, Umversib! Paris Xi, 91405 Orsay, France and J. Hepburn Department of Chemrstry, Umversrty of Ontarro, W aterloo, Canada Received 4 January 1993 The threshold TPE photoelectron spectrum of molecular oxygen has been reinvestigated using a partially cooled effustve mo- lecular jet and the monochromatised VUV synchrotron radiation from super-AC0 in Orsay. Three vibrational progressions are tdentttied. One of them corresponds to and confirms that observed recently by Baltzer et al. (Phys. Rev. A 45 ( 1992) 4374), the two other ones are observed for the first ttme m TPE spectroscopy. Possible assignments for these progressions are discussed. The vibrational progression in the X ‘Us state has been seen up to u+ ~26 and the two spin-orbit components (2TJ1,2 and 211s,2) are for the first ttme fully resolved in TPES. High-lying vtbratlonal levels wrth v+ > 20 of the X state are seen to overlap with the a state levels. The relative contribution of both states is obtained through a deconvolutton procedure. The vtbrational progression m the b 4x, is extended to u+ = 18. The threshold photoelectron spectrum around 20 eV shows a particularly high denstty oflines. Possible assignments of these lines to new progressions are discussed with the help of a series of time-of-flight photoelectron spectra (TOF-PES) measured at a series of excitation energies between 18 and 20 eV. 1. Introduction The purpose of this article is to present a high-res- olution (40 cm- ’ ) threshold (TPE) photoelectron spectrum of oxygen measured between 12 and 22 eV at the synchrotron storage ring super-AC0 in Orsay. In the past 25 years, there have been numerous stud- ies of O2 by conventional photoelectron spectroscopy (see for example refs. [ l-51 ) and time-of-flight photoelectron (TOF-PES) studies using tunable syn- chrotron radiation [ 6,7]. In addition, several low- resolution threshold photoelectron spectroscopic studies have been reported [8-l 0] and yet another report requires some justifications. The low-energy electron lines are usually not reported in conven- tional PES since they are embedded in a large back- ’ Present address: Physical Chemistry Laboratory, Universtty of Oxford, OX 1 3QZ, UK. ground of low-energy scattered electrons. The actual origin of these electrons is not yet understood and our TPES studies show that at least some of them result from photoionisation of the gas under investigation. Indeed the low-energy electrons can well be sepa- rated by their time of flight according to their veloc- ity and are detected with great efficiency, an ideal sit- uation for TOF-PES studies. This type of spectros- copy, which implies a pulsed light source, has been developed around pulsed synchrotron radiation sources. The power of the method is illustrated in a recent study of autoionisation in O2 [ 71 in which a resolution of 5 meV is obtained for low-energy elec- trons, i.e. 25 to 50 meV. Threshold photoelectron spectroscopy (TPES) differs from PES in that the energy of the detected electrons is kept equal to zero while scanning the ex- citing photon energy. This is made possible by the use of tunable VUV light sources. As the frequency is O301-0104/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.