Photochemistry of single vibronic levels of formaldehyde

Abstract

Predissociation, quenching and energy transfer have been studied for single vibrational levels of the first excited singlet state of formaldehyde ${(}^1{A}_2)$ . A tunable ultraviolet laser, summation of dye plus ruby, provided 7 nsec pulses with less than 1 Å wavelength spread. Fluorescence decay lifetimes were measured as a function of pressure. The decay rates, extrapolated to zero pressure, increase rapidly with increasing vibrational energy. A hundredfold increase in rate is observed for 4000 cm⁻¹ in D₂CO. The variation of rate with the particular combination of normal modes excited in comparatively small. The effect of deuteration is marked; H₂CO decays about twenty times faster than D₂CO. We believe that for the energy range studied the only mechanism of radiationless decay compatible with the energy level structure of formaldehyde and with the lifetime observations is internal conversion to high vibrational levels of the ground state which are broadened by unimolecular dissociation. The rates measured for collision‐induced dissociation exhibit an energy and isotope variation parallel to that for the collision‐free process. The cross sections for higher energy levels are several times larger than hard sphere. In D₂CO a small fraction of collisions resulted in vibrational relaxation within the excited electronic state. For H₂CO and HDCO the rate of energy transfer was too much smaller than the quenching rate to be observed.