15min:

HONGMING YIN, KLAAS NAUTA AND SCOTT H. KABLE, School of Chemistry, University of Sydney, NSW, 2006, Australia.

The photodissociation dynamics of the reaction H₂CO + h H + HCO have been investigated in the range 60-400 cm^-1 above the reaction threshold. Supersonically-cooled formaldehyde was excited into 15 specific J, K_a, K_c rotational states in two vibrational levels 2¹4¹6¹ and 2²4¹ in the \textrmA(¹A₂) state. The laser induced fluorescence spectra of the nascent HCO fragment provided detailed product state distributions (PSDs), resolved by N, K_a, K_c and J. When just the overall molecular rotation, N, is considered the PSDs are in remarkable agreement with calculations based on phase space theory (PST). However, when the projection of N onto the molecular frame (K_a, K_c) is included the distributions show consistent deviations from PST. In particular, there is a tendency to preserve the initial parent rotational motion about the a and b axes. The effect is that states with higher initial K_a in H₂CO produce higher final K_a in the HCO fragment. There is also a tendency for the upper/lower members of the asymmetry doublets in H₂CO to map onto the same upper/lower set of product state asymmetry doublets. Finally, there are oscillations in some of the detailed PSDs that remain unexplained.