Photofragmentation dynamics of formaldehyde: H2(v, J) distributions

Coherent anti‐Stokes Raman Scattering (CARS) is used to monitor the rovibrational population distribution of prompt H2 produced from H2CO+hν→H2+CO. Photolysis is performed on K=1 lines of the 2141 S1←S0 transition. Scanning CARS spectroscopy is employed, with suppression of the nonresonant background of the H2CO. The detection sensitivity is ∼1012 cm−3 per quantum state. Helium buffer is added to cool the product translation to near room temperature. States v=0–4, J=1–9 are detected. The vibrational distribution peaks at v=1. The rotational distributions in the vibrational states are somewhat Boltzmann with ‘‘temperatures’’ ranging from 1700 to 2500 K for v=3–0. The total energy, 29 500 cm−1, is chosen near the threshold for dissociation. The internal energy of H2(v, J) sums with that found earlier for CO(v, J) and for translation to this total. An average impact parameter of 0.9 A with a spread of a few tenths A is found. H2 pushes away from a point about 0.3 A beyond the C nucleus of CO. The distributio...