Radiative lifetimes of trapped molecular ions: HCl+ and HBr+

Excitation spectra in the A 2Σ+−X 2Π system of HCl+(0,0) and HBr+(1,0) as well as radiative lifetimes of selectively excited rotational levels in the A 2Σ+ electronic states have been measured in a RF ion trap using laser induced fluorescence and time-resolved single-photon counting techniques. Coherent ultraviolet radiation was generated by frequency mixing of a Nd:YAG/dye laser system. For HBr+ the lifetimes of four rotational levels adjacent to the v′=1 predissociation limit at J′=25/2 were measured. No statistically significant variation with J was found. The average of these four values (3.89±0.15 μs) is therefore reported as the effective lifetime of the v′=1 level of A 2Σ+ HBr+. For HCl+ the lifetime measurements likewise indicate no strong variation with J. The average lifetime of the v′=0 level of the A 2Σ+ state of HCl+ is 3.4±0.4 μs. Measurements of radiative lifetimes of molecular ions by the RF ion trap technique eliminates the determinate error that results from drift of ions out of the detection volume in conventional experiments.