Collisionless nonradiative decay rates of single rotational levels of S1 formaldehyde

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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 Erot. 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.