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

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 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.