Collisionless nonradiative decay rates of single rotational levels of S1 formaldehyde

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 Eatsub rotat. K-doublet levels split by as little as 8 x 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