Knowledge Bank

Pyrazine, as noted above, exhibits a fluorescence decay which is biexponential in character. Studies of this decay under collision-free conditions reveal a strong rotational state dependence of the $A+/A-$ ratio, as first reported two years ago at this conference. We now describe further experiments which show that, under nanosecond excitation conditions, $A+/A-$ scales as $2J\prime +1$but $\tau +$ and $\tau -$ are $J\prime$ independent. By considering the spin-orbit properties of triplet pyrazine in the condensed phase, and transforming the problem into the gas phase, we then demonstrate that this behavior is to be expected whenever J (the total angular momentum quantum number), but not P (the projection of J on the top axis), is conserved in the intersystem crossing process. Some possible origins of the breakdown in the $\Delta P=0$ selection rule will be discussed. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$Work supported by the National Science Foundation. $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$G. ter Horst, J. Kommandeur, and D. W. Pratt, 36th Symposium on Molecular Spectroscopy, June, 1981, Paper FB10.