Photodissociation dynamics of gas-phase BrICl^- and IBr₂^- anions

This work is the first study of mixed trihalide anions in the gas phase. We report the photochemistry and photodissociation dynamics of BrICl^- and IBr₂^- anions near 400 nm. For BrICl^-, two major photodissociation channels yielding IBr^- and ICl^- products are observed. The photodissociation of IBr₂^- yields primarily IBr^- anion products. No BrCl^- and Br₂^- fragments are formed from BrICl^- and IBr₂^-, respectively, and the parent anions are ascribed linear [Br-I-Cl]^- and [Br-I-Br]^- structures, in agreement with ab initio calculations. The IBr^- and ICl^- products are produced in highly vibrationally excited states. We observe fragment vibrational coherence in femtosecond pump-probe measurements by probing nascent IBr^- via its photodissociation to I^- + Br. The IBr^- vibrational periods observed in the photodissociation of BrICl^- correspond to excitations of ∼80% of the IBr^- dissociation energy (D_o). In IBr₂^- dissociation, the average vibrational excitation of IBr^- is 84-91% of D_o. The high degrees of vibrational excitation raise questions about the effect of the proximity to the dissociation threshold on the dynamics of coherent wave packets. We analyze these dynamics analytically and by means of wave packet simulations and observe smearing of expected wave packet revivals.