Non-adiabatic dissociation dynamics of Ar⋯I₂ (E, v) intermolecular vibrational levels probed using velocity-map imaging

Abstract

Ion time-of-flight velocity-map imaging was used to measure the kinetic-energy distributions of the I₂ ion-pair fragments formed after photoexcitation of Ar⋯I₂ complexes to intermolecular vibrational levels bound within the Ar + I₂ (E, v_E = 0–2) potential energy surfaces. The kinetic-energy distributions of the I₂ products indicate that complexes in the Ar⋯I₂ (E, v_E) levels preferentially dissociate into I₂ in the D and β ion-pair states with no change in I₂ vibrational excitation. The energetics of the levels prepared suggest that there is a non-adiabatic coupling of the initially prepared levels with the continuum of states lying above the Ar + I₂ (D, v_D = v_E) and Ar + I₂ (β, v_β = v_E) dissociation limits. The angular anisotropies of the I₂ product signals collected for many of the Ar⋯I₂ (E, v_E) levels have maxima parallel to the laser polarization axis. This contradicts expectations for the prompt dissociation of complexes with T-shaped geometries, which would result in images with maxima perpendicular to the polarization axis. These anisotropies suggest that there is a perturbation of the transition moment in these clusters or there are additional intermolecular interactions, likely those sampled while traversing above the attractive wells of the lower-energy potentials during dissociation. I₂ (D′, v_D′) products are also identified when preparing several of the low-lying levels localized in the T-shaped well of the Ar + I₂ (E, v_E = 0–2) potentials, and they are formed in multiple ν_D′ vibrational levels spanning energy ranges up to 500 cm⁻¹.

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