The Kc-effect

Abstract

This thesis examines an experimental and theoretical investigation into the dynamics of the reaction HZCO (v, J, K8, K) + hv -—> HCO (v, J, N, K3, K) + H (2H). Formaldehyde molecules were cooled in a supersonic free-jet expansion, dissociated with a laser and the ensuing formyl (HCO) fragments were probed by laser induced fluorescence spectroscopy.

The initial experiments were carried out near the energetic threshold where very little energy is available to the product fragments. Subsequent experiments were carried out in three higher vibrational bands in the A( IAz) state. At these energies the dissociative surface is the formaldehyde ground state So, and the formyl and H fragments are formed in their ground electronic and vibrational states. Formaldehyde was prepared in numerous rotational states, characterised by J = 0 to 5, K8 = 0 to 3 and Kc = 0 to 5. The ensuing distribution of rotational energy in the HCO fragment was measured as a function of the N, K“, K and J = N i S quantum numbers of the fragment, and also the initial v, J, K“, Kr quantum numbers of the parent. The results found a dependence on both N and K2, in the fragment that could be modelled well using statistical analysis (phase space theory). This is consistent with statistical dynamics on a bound, barrierless surface. Within le cm‘1 of the energetic threshold, a centrifugal barrier affected the populations by inhibiting product states that require large orbital angular momentum. No dependence on the spin states, S = il/z, has been observed.

The results also show a strong dependence on the quantum number, Kc, of both the parent and the product. When the Kc is resolved in both the parent and product, there are large deviations from the phase space theory model. We find that in the HCO K,l = l manifold there is always a preference (up to 5:1) for HCO to be produced in either the higher energy Kr state (NW—1) or the lower energy state (N1,N)- This preference is consistent over all N for any particular initial HZCO state, but may vary for different initial states. A correlation between this Kr preference and the initial state was observed: odd Kr formaldehyde states produce preferentially KC (lower) of HC0 and vice versa for initially even Kr states.

A “Kr—effect” has been observed previously in the photodissociation of acetaldehyde and propionaldehyde to produce HCO. These experiments saw a strong preference for the upper energy component of the KC doublet, irrespective of the initial parent rotational state. The other system where Kc has been seen to play a role is the photodissociation of water. When H20 is photodissociated from a variety of rotational states with full KC resolution, different Kr states produced different A doublet states in the OH fragment. The similarities with these previous studies and the “Kc—effect” in formaldehyde are discussed and used as the starting point to develop a theory to explain the “Kr-effect” observations.