Collisionless nonradiative decay rates of single rotational levels of S1 formaldehyde

Abstract

Fluorescence lifetimes of single rotational levels of the lowest vibrational level of the first excited singlet state of H₂CO and D₂CO have been measured under collision‐free conditions following excitation by a pulsed dye laser. For H₂CO, the lifetimes range from 66 ns to 4.2 μs with a median of about 160 ns. Individual lifetimes show no systematic variation with J′, K′, or E_rot. K‐doublet levels split by as little as 8×10⁻⁴cm⁻¹ in S₁ are observed to have different lifetimes. The H₂CO results are interpreted in terms of a sequential coupling model (S₁→S₀→continuum) in which the final states are those of the H₂+CO dissociation continuum. For D₂CO, the lifetimes vary between 5.5 and 8.1 μs and are nearly radiative lifetimes.