Dissociation dynamics of H2O(D2O) following photoexcitation at the Lyman-α wavelength (121.6 nm)

Abstract

The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B̃(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three-body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B̃ state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B̃ state surface; their rotational excitation results from the marked angular anisotropy of the B̃ state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower Ã(1B1) or X̃(1A1) surfaces. The wavepacket calculations show that B̃■X̃ transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B̃■Ã radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a-axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.