Photodissociation of gas-phase I₃ ^-: Comprehensive understanding of nonadiabatic dissociation dynamics

Photodissociation of the gas-phase tri-iodide anion, I3-, was investigated using photofragment time of flight (TOF) mass spectrometry combined with the core extraction method. An analysis of the TOF profiles provided the kinetic energy and angular distributions of photofragment ions and photoneutrals, from which the photoproduct branching fractions were determined in the excitation energy range of 3.26-4.27 eV. The measurement has revealed that (1) in the entire energy range investigated, three-body dissociation occurs preferentially as the "charge-asymmetric" process I- (S1) +I (P 32 2) +I (P 32 2) with the yield of ≈30%-40%, where the excess charge is localized on the end atoms of the dissociating I3-, and that (2) two-body dissociation via the Πu3 (0u+) g+1 (0g+) excitation proceeds as I- (S1) + I2 (X g+1) I2 (A Π 1u 3) or I (P 32 2) + I2- (X u+2) with the yield of ≈60%, while that via the u+1 (0u+) g+1 (0g+) excitation alternatively as I* (P 12 2) + I2- (X u+2) or I- (S1) + I2 (B Πu3) with the yield of ≈60%. Ab initio calculations including spin-orbit configuration interactions were also performed to gain precise information on the potential energy surfaces relevant to the I3- photodissociation. The calculations have shown the presence of conical intersections and avoided crossings located along the symmetric stretch coordinate near the ground-state equilibrium geometry of I3-, which play key roles for the two-body and the three-body product branching. The nonadiabatic nature of the I3- photodissociation dynamics is discussed by combining the experimental findings and the ab initio results.