Prompt release of O 1D products upon UV excitation of CH2OO Criegee intermediates

Abstract

Velocity map imaging has been used to characterize the angular and velocity distributions of O 1D photofragments arising from UV excitation of the simplest Criegee intermediate CH2OO in the long wavelength tail region (364 to 417 nm) of the B1A'-X1A' spectrum. The O 1D images exhibit anisotropic angular distributions indicative of rapid dissociation to H2CO X1A1 + O 1D products, which occurs faster than the rotational period (ps) of CH2OO. As a result, the broad oscillatory structure reported previously in the long wavelength region of the UV absorption spectrum is attributed to short-lived resonances associated with the excited B1A' state of CH2OO, which decay by nonadiabatic coupling to repulsive singlet states. The total kinetic energy distributions show that nearly half of the available energy, on average, is partitioned to product translation. The balance results in significant internal excitation of the H2CO co-fragments. The product anisotropy and energy partitioning are unchanged across the UV spectrum and consistent with previously reported experimental and theoretical findings of the CH2OO B-X transition moment and dissociation energy to H2CO X1A1 + O 1D products.