NASCENT STATE DISTRIBUTION OF THE HCO PHOTOPRODUCT ARISING FROM THE 309 NM PHOTOLYSIS OF PROPIONALDEHYDE

The photodissociation dynamics of jet-cooled propionaldehyde have been investigated at a wavelength of 309.1 nm by monitoring the resultant nascent HCO fragments by laser induced fluorescence spectroscopy. HCO was formed only in the X(0,0,0) state. The population distribution of different rotational states characterized by N and K-a is reasonably d escribed by a Boltzmann distribution at a temperature of 480 +/- 50 K, which corresponds to an average energy in rotation of 6.0 +/- 0.6 kJ mol(-1). Careful measurement of the width of individual K-a = 0 lines in the LIF spectrum revealed that the average translational energy of the fragments is 23 +/- 4 kJ mol(-1) of HCO. These measurements have a llowed us to estimate that the ethyl radical sibling fragment is born with almost no internal energy. The observed energy partitioning in th e fragments is consistent with a model in which the HCO rotational and translational excitation is determined mostly by the fixed energy in the exit channel. By analogy with acetaldehyde and considering the lac k of vibrational excitation, the barrier to dissociation is predicted to lie around 15 kJ mol(-1) below the photon energy.