Molecular square dancing: correlated product motion in photodissociation

Abstract

Since the advent of laser spectroscopy and molecular beam techniques, the photodissociation of small molecules has been intensely studied. Emphasis has been placed on investigating the applicability of statistical theories, in particular for barrierless unimolecular dissociation . Various statistical theories have been devised in order to explain reaction rates and product distributions of unimolecular reactions, especially those in which there is no potential energy barrier to products.In phase space theory (PST), all accessible product states have equal probability to be populated; the only restrictions are those of energy and angular momentum conservation. Recently, an extension of both classical and quantum PST was proposed in order to calculate v*j correlations in the products of a photodissociation reaction. Separate statistical ensembles (SSE) imposes further restrictions on the phase space by constraining energy flow between vibration and overall rotation. For photolysis energies that do not allow for the formation of vibrationally excited products, PST and SSE are identical. Another type of constraint on the phase space can be achieved by using the statistical adiabatic channel model which assumes that each vibrational state of the parent molecular adiabatically correlates to a specific rovibrational state of the fragments. This model has been sued with success on the photodissociation of HOOH. Ketene is a molecule whose photochemistry has been extensively studied both experimentally and theoretically. The two lowest electronic states of methylene are energetically accessible in the 308 nm photolysis of ketene.