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

Abstract

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 H 2 CO in S 0. We recently proposed a novel mechanism [M. Araujo, 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. © 2009 American Institute of Physics.