The photodissociation dynamics of NO_2 at 308 nm and of NO_2 and N_2O_4 at 226 nm

摘要

Velocity-map ion imaging has been applied to the photodissociation of NO_2 via the first absorption band at 308 nm using (2+1) resonantly enhanced multiphoton ionization detection of the atomic O(~3P_J) products.The resulting ion images have been analyzed to provide information about the speed distribution of the O(~3P_J) products,the translational anisotropy,and the electronic angular momentum alignment.The atomic speed distributions were used to provide information about the internal quantum-state distribution in the NO coproducts.The data were found to be consistent with an inverted NO vibrational quantum-state distribution,and thereby point to a dynamical,as opposed to a statistical dissociation mechanism subsequent to photodissociation at 308 nm.Surprisingly,at this wavelength the O-atom electronic angular momentum alignment was found to be small.Probe-only ion images obtained under a variety of molecular-beam backing-pressure conditions,and corresponding to 0 atoms generated in the photodissociation of either the monomer,NO_2,or the dimer,N_2O_4,at 226 nm,are also reported.For the monomer,where 226 nm corresponds to excitation into the second absorption band,the kinetic-energy release distributions are also found to indicate a strong population inversion in the NO cofragment,and are shown to be remarkably similar to those previously observed in the wavelength range of 193-248 nm.Mechanistic implications of this result are discussed.At 226 nm it has also been possible to observe directly O atoms from the photodissociation of the dimer.The O-atom velocity distribution has been analyzed to provide information about its production mechanism.