An ab initio study of the photodissociation of HN3 molecules following excitation in the Ã 1A″ ←  1A′ absorption system

Ab initio calculations of two-dimensional cuts through the full six-dimensional potential energy surfaces of the ground ( 1A′) and first excited (Ã 1A″) states of HN3 are reported. Specifically, we have investigated the variation of the potential energy surfaces with respect to simultaneous changes of the H–NNN and HN–NN bond lengths, and to changes of the H–NNN bond length and ∠N–N–N angle. These pairs of co-ordinates were chosen in the light of the deduced importance of each of these motions in interpreting the experimentally observed ultraviolet photochemistry of this molecule. Two-dimensional quantum mechanical wavepacket calculations of the photodissociation dynamics have been carried out using each of these two-dimensional surfaces. These offer strong support to our earlier interpretations (Cook et al., Phys. Chem. Chem. Phys., 1999, 1, 45) regarding the form of the vibrational energy disposal in the N3() products arising via H–NNN bond fission on the à state potential energy surface. They also allow investigation of factors influencing the relative branching into the competing H–NNN and HN–NN bond fission channels on the excited state potential energy surface, and predict relative branching ratios for these two channels, as a function of excitation energy, in reasonable accord with the limited available experimental data.