Photodissociation Dynamics of H2O via the Ẽ′ (1B2) Electronic State

Photodissociation dynamics of H2O via the Ẽ′1B2 state were studied using the high-resolution H atom photofragment translational spectroscopy method, in combination with the tunable vacuum ultraviolet free electron laser (VUV FEL). The measured translational energy spectra allow us to determine the respective quantum state population distributions for the nascent OH(X2Π) and OH(A2ς+) photofragments. Analyses of the quantum state population distributions show both the ground and electronically excited OH fragments to be formed with moderate vibrational excitation but with highly rotational excitation. Unlike the dissociation via the lower-lying electronic states, where OH(X) is the major fragment, the OH(A) products are predominant via the Ẽ′ state. These products are mainly ascribed to a fast dissociation on the B1A1 state surface after nonadiabatic transitions from the initial excited E state to the B state. Meanwhile, another dissociation pathway from the E state to the 1B2 3pb2 state, followed by coupling to the 1A2 3pb2 state, is also observed, which yields the OH(X) + H and O(3P) + 2H products.