Studies of some small transient species by ultraviolet photoelectron spectroscopy

An instrument is described for the detection by Vacuum Ultraviolet Photoelectron Spectroscopy of radical species produced by atom-molecule reactions in the gas-phase. In particular, modifications to allow for the generation of transient molecules by rapid reactions involving fluorine atoms are detailed. The photoelectron spectra so obtained are assigned on the basis of ionization potentials calculated by ab initio molecular orbital methods and by comparison of the observed frequencies with those of the resultant cation estimated using force-field calculations. Alternatively, comparison has been made with the known vibrational frequencies of related isoelectronic species. The assignment is confirmed by deuteriation, where appropriate. The first photoelectron bands of the formyl (HCO, X2A'), fluoroformyl (FCO, X2A'), hydroperoxyl (HO2, X2A") and silyl (SiH3, X2A1) radicals have been recorded, leading to accurate values of the first adiabatic and vertical ionization potentials and knowledge of the frequencies of the vibrations excited on ionization. In addition, a study of the products of the F + HBr and F + DBr reactions has enabled the photoelectron spectrum of the electronicallyexcited bromine atom (Br*(2P;)) to be recorded. The initial proportionof Br*(ZP}) produced in these reactions is also established.