Vibrationally mediated dissociation dynamics of H2O in the vOH=2 polyad

Abstract

Vibrationally mediated photodissociation dynamics of jet-cooled H2O in the vOH=2 polyad is studied in a supersonic slit jet expansion. Single rotational states within |02〉− (≡ν1+ν3 in normal mode notation), |02〉+(≡2ν1), |11〉+(≡2ν3), and |01−2〉(≡ν3+2ν2) vibrational states of H2O are selectively prepared with near IR overtone pumping, photodissociated at 193 nm, and the resulting nascent internal state distribution of OH fragments probed via laser induced fluorescence. Strong oscillations in rotational, spin–orbit, and lambda-doublet distributions are observed, often in remarkably close agreement with H2O state-to-state photodissociation studies from both higher and lower vOH polyads. The influence of initially excited bending and JKaKc levels of H2O on spin–orbit, Λ-doublet, and rotational distributions of OH is examined in detail. Several new dynamical trends are identified, for example, a clear propensity at high N for a strong Λ+ versus Λ− inversion in the Π3/2 spin–orbit manifold, which reverses in the Π1/2 manifold, suggesting spin–orbit sensitive stereodynamics in the ejection process. Furthermore, the results highlight significant differences in photodissociation dynamics from gerade (e.g., |02〉+) versus ungerade (e.g., |02〉−) vibrational states, specifically with respect to OH(v=1)/OH(v=0) branching ratios, and signaling a breakdown of the “spectator” model at low vibrational excitation.