Conservation of angular momentum in polyatomic photochemical reactions: H₂CO(v,J,Ka,Kc)→H+HCO(N,Ka,Kc,J)

Abstract

The photodissociation dynamics of the reaction H2CO + hv → H + HCO have been investigated just above the reaction threshold (λdiss ≈ 330 nm). Formaldehyde was excited into specific J, Ka, Kc rotational states of three vibrational levels in the à (1A2) state. Molecules in these states undergo internal conversion back to the X̃ (1A1) ground state on which the radical fragments are formed. The ensuing distribution of rotational energy in the HCO fragment was measured as a function of the N, Ka, Kc and J = N ± S quantum numbers of the fragment, and also the initial v, J, Ka, Kc quantum numbers of the parent. In a previous publication (J. Chem. Phys., in press) we investigated the dynamics of this reaction at low available energy and concluded that when only the N and Ka quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are modelled well by phase space theory (PST). This is consistent with statistical dynamics on a bound, barrierless surface. Within ≈10 cm-1 of the energetic threshold, a centrifugal barrier affected the populations by inhibiting product states that require large orbital angular momentum. Resolution of Kc in the parent and product gave large deviations from the PST model, however little data were available to quantify this observation. In this work we have extended the number of initially excited H2CO levels to explore this "Kc effect" further. We find that in the HCO Ka = 1 manifold there is always a preference (up to 5:1) for HCO to be produced in either the higher energy Kc state (N1,N-1) or the lower energy state (N1,N). This preference is consistent over all N for any particular initial H2CO state but may vary for different initial states. Over the seven initial states probed here, four favoured Kc (lower) and the other three Kc (upper). A correlation between this Kc preference and the initial state was observed: odd Kc formaldehyde states produce Kc (lower) preference in HCO and vice versa for initially even Kc states. A comparison with one previous observation of this effect is presented, however no concrete explanation can be offered at this stage.