State resolved photodissociation of vibrationally excited water: Rotations, stretching vibrations, and relative cross sections

Abstract

The state resolved photodissociation of highly vibrationally excited water molecules using laser induced fluorescence detection of the OH product demonstrates the control that the initially selected state exerts over product state populations. These vibrationally mediated photodissociation experiments, in which one photon prepares a highly vibrationally excited molecule and a second photon dissociates it, determine the role of overall rotations and of O–H stretching vibrations as well as measure the relative cross section for the photodissociation of water. The rotational state of the vibrationally excited water molecule governs the rotational state of the OH product of the dissociation, in agreement with ab initio calculations and previous measurements on single rotational states excited in the fundamental asymmetric stretching vibration band. The initially selected vibrational state of the water molecule determines the vibrational energy disposal in the products, which agrees with a simple qualitative model based on the pattern of the initially selected vibrational wave function. Dissociating vibrational states with similar energies but very different nuclear motions produces dramatically different product vibrational state populations. The vibrational energy initially present in the surviving bond primarily appears as vibrational excitation of the product. Dissociation of the ‖04〉− state produces no vibrationally excited OH, but dissociation of the ‖13〉− state produces mostly vibrationally excited products. These qualitative notions agree well with recently detailed ab initio calculations. The relative photodissociatiuib cross section of the highly vibrationally excited molecule shows structure over the wavelength range of 218.5 to 266 nm that reflects the nodal pattern of the intermediate vibrational state in the dissociation and confirms the predictions of theoretical calculations.