Photodissociation dynamics of formaldehyde: H2 rotational distributions and product quantum state correlations

Abstract

The alignment and rotational state populations of ortho and para H2 (v=1) and H2 (v=3) produced from formaldehyde photolysis near the threshold for dissociation are measured with Doppler-resolved laser-induced fluorescence spectroscopy in the vacuum ultraviolet (VUV). A single rovibronic level of the first excited singlet state of H2 CO in a molecular jet is excited by a pulsed UV laser and the H2 is probed under collisionless conditions with VUV generated by third harmonic generation in Kr or Xe. The rotation of the excited H2 CO during the 10−7–10−8 s before dissociation does not completely wash out the alignment: e.g., photolysis on the 2141 rR0(0)e H2 CO line produces H2 (v=3,J=2) with A(2)0=−0.31±0.06. The ortho and para H2 rotational distributions lie on a single smooth curve peaked at J≊3. The H2 CO(S1 ) vibrational state has little influence on the JH2 distributions, but there is evidence that photolysis of higher rotational states of H2 CO(S1 ) results in warmer JH2 distributions. The Doppler profiles enable the measurement of the H2 (v,J) average velocities. Conservation of energy and linear momentum permit the corresponding CO J state to be calculated. H2 formed in high vibrational states is correlated with CO formed in low rotational states. A three-parameter, semiclassical model which combines an impulsive force with the momenta of zero-point vibrations of the parent near or at the transition state geometry reproduces all of the general features of the H2 (v,J) and CO(v,J) distributions from H2 CO photolysis near the barrier to dissociation.