Decay dynamics of the vibrationally activated OH–CO reactant complex

Abstract

The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.