Journal of Electron Spectroscopy and Related Phenomena 108 (2000) 75–88 www.elsevier.nl / locate / elspec Simulation of photoelectron and electronic spectra of small molecules a, a,b a a * Foo-Tim Chau , Edmond Pak-Fai Lee , Daniel Kam-Wah Mok , De-Chao Wang , b John M. Dyke a Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, PR China b Department of Chemistry, Southampton University, Highfield, Southampton SO17 1BJ, UK Abstract In this contribution, a description is given of the application of high-level ab initio calculations and Franck–Condon analysis to study some photoelectron and electronic spectra, where there were uncertainties in their assignments. The spectra chosen are the He I photoelectron spectrum of BrO and the single vibrational level (SVL) dispersed fluorescence spectra of 2 AlCN and AlNC. With suitable ab initio calculations and subsequent spectral simulations, assignments of these spectra can now be made. In addition, the iterative Franck–Condon analysis (IFCA) procedure was also applied to some of these spectra 3 to derive excited and ionic state geometries. In the investigation of the He I photoelectron spectrum observed when O( P) 1 1 reacted with Br ( S ), calculations were performed on the low-lying triplet states of both BrOBr and BrBrO and their 2 g low-lying cationic quartet states for the first time. It was found that there are a number of weakly bound triplet radical-radical states and quartet radical-cation states for both BrOBr and BrBrO.  2000 Elsevier Science B.V. All rights reserved. Keywords: Franck–Condon; ab initio calculation; Fluorescence spectroscopy; Photoelectron spectroscopy 1. Introduction the spectra. When vibrational structure is observed in an electronic band, one of the techniques useful for Photoelectron spectroscopy and electronic spec- spectral analysis is the Franck–Condon (FC) analy- troscopy, notably absorption, emission and fluores- sis method. We have previously applied the tech- cence spectroscopy, are some primary experimental nique of iterative Franck–Condon analysis (IFCA), tools used in studying the electronic structure of which combines ab initio molecular orbital calcula- molecules and radicals in the gas phase. From the tions with Franck–Condon calculations, to a number observed spectra, information on the geometries, of photoelectron and emission spectra, and obtained vibrational frequencies, bonding properties and ther- for the first time, experimentally derived geometrical mochemical values of the species involved in the parameters of the upper states involved in the electronic transitions can be either obtained directly, electronic transitions for a number of species [1–7]. or extracted with the aid of appropriate analyses of However, sometimes, when a complex reaction scheme is involved in producing the species of interest, and / or when a spectrum with complicated vibrational structure is observed, the interpretation of *Corresponding author. Tel.: 1852-276-656-03; fax: 1852-236- the observed spectrum may not be straightforward. 499-32. E-mail address: bcftchau@polyu.edu.hk (F.-T. Chau) In some cases, even the identity of the molecular 0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048(00)00147-X