High resolution ion imaging studies of the photodissociation of the Br2+cation

Abstract

We demonstrate the potential of velocity map ion imaging methods for high resolution studies of the photofragmentation of a state selected molecular ion, Br₂⁺. ⁷⁹Br₂⁺ cations were prepared in the v⁺ = 1 levels of both spin–orbit components of the X ²Π_g ground state by 2 + 1 resonance enhanced multiphoton ionisation (REMPI) of jet-cooled, isotopomer-selected ⁷⁹Br₂ molecules at an excitation wavelength of 263.012 nm. The subsequent photolysis of these state selected ions was then investigated in the wavelength range 372 < λ_phot < 432 nm by imaging the resulting ⁷⁹Br⁺ ions. Use of a home-built Pockels cell assembly allowed measurement of images with and without λ_phot present, on successive laser shots. The resulting difference image allows clear visualisation of the parent ion fragmentation, free from unintended fragment ion signal induced by the REMPI preparation step. The images reveal participation of dissociation channels yielding Br⁺ ions in both their ground (³P₂) and first excited (³P₁) spin–orbit states, together with ground state Br atoms. No fragmentation channels involving formation of spin–orbit excited Br(²P_1/2) atoms are found to be active in this range of photolysis wavelengths. Image analysis yields a precise value for the bond strength of the Br₂⁺ cation, in both spin–orbit components of its ground electronic state: D₀[⁷⁹Br₂⁺(X ²Π_1/2,g)] = 23528.1 ± 0.6 cm⁻¹ and D₀[⁷⁹Br₂⁺(X ²Π_3/2,g)] = 26345 ± 2 cm⁻¹, and thus a refined measure of the spin–orbit splitting in the ground state Br₂⁺ cation: A = 2817 ± 3 cm⁻¹.