Predissociation model for formaldehyde

Abstract

The collisionless nonradiative decay of single vibronic levels of the first excited singlet state of formaldehyde (1A2) is explained in terms of coupling with vibronic levels of the ground (1A1) state. Interaction is provided by the breakdown of the Born-Oppenheimer approximation through the nuclear kinetic energy operator. A full, Franck-Condon-factor-weighted summation of states calculation is undertaken for both H2CO and D2CO. Harmonic oscillator wavefunctions derived from published spectroscopic studies are used throughout. Anharmonic mixing of the ground state vibrational levels is artificially accounted for by averaging the rate over a range of final state energies. The bound-to-bound state calculation is consistent with a two-step dissociation process, in which the excited state is less strongly coupled to the ground state than the ground state is coupled to the continuum. Direct coupling to the continuum is neglected. The results are in good agreement with experiment as regards the dependence of rate on total energy and on vibrational modes, the absolute magnitude of the rates, and the effect of isotopic substitution. Rates for levels with CH stretches excited are underestimated. The largest contributions to the rates come from coupling to ground state levels with the CO stretch and the out-of-plane bend highly excited.