The molecular dissociation of formaldehyde at medium photoexcitation energies: A quantum chemistry and direct quantum dynamics study

The mechanisms of radiationless decay involved in the photodissociation of formaldehyde into H-2 and CO have been investigated using complete active space self-consistent field (CASSCF) calculations and direct dynamics variational multiconfiguration Gaussian (DD-vMCG) quantum dynamics in the S-1, T-1, and S-0 states. A commonly accepted scheme involves Fermi Golden Rule internal conversion from S-1 followed by dissociation of vibrationally hot H2CO in S-0. We recently proposed a novel mechanism [M. Araujo et al., J. Phys. Chem. A 112, 7489 (2008)] whereby internal conversion and dissociation take place in concert through a seam of conical intersection between S-1 and S-0 after the system has passed through an S-1 transition barrier. The relevance of this mechanism depends on the efficiency of tunneling in S-1. At lower energy, an alternative scheme to internal conversion involves intersystem crossing via T-1 to regenerate the reactant before the S-0 barrier to dissociation. We propose here a previously unidentified mechanism leading directly to H-2 and CO products via T-1. This channel opens at medium energies, near or above the T-1 barrier to dissociation and still lower than the S-1 barrier, thus making T-1 a possible shortcut to molecular dissociation. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3242082]