Knowledge Bank

Measurements of collision-free single rotational level fluorescence lifetimes and relative quantum yields are reported for the $4\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}0$ band of the $S1-S0$ absorption system of thioformaldehyde ($H2CS$). These data are compared to spectroscopic $evidence1$ for internal conversion and intersystem crossing from the $S1$ state. The radiative lifetime is found to be approximately $170\mu$s substantially longer than previously reported values, with a number of levels having even longer lifetimes. These anomalously long lifetimes can be explained by the dilution of the oscillator strength due to coupling with dark levels in the ground and first excited triplet states. Quantum yield measurements indicate that some of the long-lived levels have a nonradiative decay component in the coupled state. The results are interpreted in terms of a qualitative model involving level specific coupling between $s1$ and levels in $T1$ and $s0$.