State-selective photodissociation dynamics of formaldehyde: Near threshold studies of the H+HCO product channel

The laser-induced photodissociation of formaldehyde in the wavelength range 309<λ<330 nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the A(tilde sign) 1A2←X(tilde sign) 1A1 201403, 202401, 202403, 203401, and 201501 bands of H2CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different NKa levels in the A(tilde sign) 1A2 2243 and 2341 states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X(tilde sign) 1A1 and the a(tilde sign) 3A2 potential energy surfaces. Most rovibrational states of H2CO(A(tilde sign) 1A2) investigated in this work produceH+HCO(X(tilde sign) 2A') photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the A(tilde sign) 1A2 2243 11,1 (and 11,0) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the a(tilde sign) 3A2 state. The results also suggest the need for further careful measurements of the H+HCO quantum yield from H2CO photolysis at energies approaching, and above, the barrier to C-H bond fission on the a(tilde sign) 3A2 potential energy surface