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

Photodissociation of the gas-phase tri-iodide anion, I-3(-), 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-(S-1)+I(P-2(3/2))+I(P-2(3/2)) with the yield of approximate to 30%-40%, where the excess charge is localized on the end atoms of the dissociating I-3(-), and that (2) two-body dissociation via the (3)Pi(u)(0(u)(+))<-(1)Sigma(+)(g)(0(g)(+)) excitation proceeds as I-(S-1)+I-2(X (1)Sigma(+)(g))/I-2(A (3)Pi(1u)) or I(P-2(3/2))+I-2(-)(X (2)Sigma(+)(u)) with the yield of approximate to 60%, while that via the (1)Sigma(+)(u)(0(u)(+))<-(1)Sigma(+)(g)(0(g)(+)) excitation alternatively as I-*(P-2(1/2))+I-2(-)(X (2)Sigma(+)(u)) or I-(S-1)+I-2(B (3)Pi(u)) with the yield of approximate to 60%. Ab initio calculations including spin-orbit configuration interactions were also performed to gain precise information on the potential energy surfaces relevant to the I-3(-) 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 I-3(-), which play key roles for the two-body and the three-body product branching. The nonadiabatic nature of the I-3(-) photodissociation dynamics is discussed by combining the experimental findings and the ab initio results. (c) 2007 American Institute of Physics.