Photoionization of gas-phase bromotrifluoromethane and its complexes with methanol: State dependence of intracluster reactions

Weakly bound complexes of CF3Br molecules or of CF3Br and CH3OH were prepared in supersonic jet expansions and studied by photoionization mass spectrometry. Argon was present in the jet expansion to promote cooling of the complexes. Measured ionization potentials are CF3Br, 11.404±0.014 eV; (CF3Br)2, 11.10±0.02 eV; (CF3Br⋅CH3OH), 10.76±0.05 eV. The appearance potential of CF+3 from CF3Br was found to be 11.56±0.02 eV, and evidence is presented that this value is adiabatic. From this appearance potential, ΔH0f0(CF+3)=86.6±1.1 kcal mol−1. The magnitude of the spin–orbit splitting in X̃ 2E CF3Br+ into E1/2 and E3/2 states dominates any Jahn–Teller distortion. The effects of these phenomena on the stability of X̃ 2E CF3Br+ are discussed. The following dissociation channels were observed: CF+3 from X̃ 2E CF3Br+, CF2Br+ from à 2A1 CF3Br+, Br+ from B̃ 2A2 CF3Br+, and CF+2 from D̃ 2E CF3Br+. (CF3⋅CH3OH)+ is not produced from neutral dimers, but is created when a CF3Br moiety embedded in a trimer or larger cluster is ionized to à 2A1 CF3Br+. In contrast, (Br⋅CH3OH)+ does come from dimers as well as larger clusters. The cross sections for the production of (CF3Br)+2 and (CF3Br⋅CH3OH)+ from trimers or larger progenitors are enhanced when intracluster CF3Br is ionized to CF3Br+ in the B̃ or higher electronic states. Possible reasons for these state-specific reactions are given. Finally, a new method is presented for determining neutral cluster distributions in a molecular beam, and the method is shown to be in quantitative agreement with the near-threshold technique previously published by Grover et al. [J. Phys. Chem. 95, 6473 (1991)].