Signatures of H₂CO Photodissociation from Two Electronic States

Even in small molecules, the influence of electronic state on rotational and vibrational product energies is not well understood. Here, we use experiments and theory to address this issue in photodissociation of formaldehyde, H₂CO, to the radical products H + HCO. These products result from dissociation from the singlet ground electronic state or the first excited triplet state (T₁) of H₂CO. Fluorescence spectra reveal a sudden decrease in the HCO rotational energy with increasing photolysis energy accompanied by substantial HCO vibrational excitation. Calculations of the rotational distribution using an ab initio potential energy surface for the T₁ state are in very good agreement with experiment and strongly support dominance of the T₁ state in the dynamics at the higher photolysis energies.