Photodissociation studies on nitrosyl bromide: I. Photofragment spectroscopy and electronic structure

The UV-visible absorption spectrum of nitrosyl bromide (BrNO) was recorded. The spectrum exhibits several broad features in the 200–740 nm region. To aid the assignment of the spectrum, photodissociation experiments were carried out at several selected excitation wavelengths. Resonance-enhanced multiphoton-ionization spectra of both the NO and Br fragments as well as photofragment time-of-flight profiles were recorded. The electronic transitions were assigned based on the observed spatial anisotropy parameters (β), the Br spin orbit state population, and NO rotational, vibrational, spin-orbit, Λ-doublet state populations, as well as comparisons with isovalent ClNO system. The strongest UV band peaked around 210 nm was attributed to the parallel S5←S0 transition (σBr–N*←σBr–N) while all other features at longer wavelengths were assigned to transitions to excited triplet states. The intensity borrowing from the S5 state is responsible for the apparent parallel nature of these singlet–triplet transitions. Implications of this assignment on the electronic structure and the non-adiabatic dissociation dynamics are discussed.