Vacuum ultraviolet laser/time-of-flight mass spectroscopy: Ion-pair spectra of ⁷⁹Br³⁵Cl

A newly built vacuum ultraviolet (VUV) laser/time-of-flight mass spectrometer is described, which has been used to record the first vibrationally and rotationally resoved spectra of the E0⁺ ion-pair state ←X0⁺ ground state transition of jet-cooled bromine monochloride BrCl, near 145 nm. The mass resolution of the instrument (m/Δm≈1000) was sufficient to obtain single isotopomer data. Preliminary results for ⁷⁹Br³⁵Cl are reported. Like other interhalogens, BrCl vibronic band intensities vary widely in the VUV, presumably as a result of excited state perturbations. This scenario has been corroborated by the observation of an irregularly spaced vibrational progression, and the irregular behavior of the excited state rotational constants as a function of v′.