Knowledge Bank

The $A~\leftarrow X~(n,\pi \ast )$ absorption system of pyrimidine vapor has been shown to be an allowed transition, namely $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1$ $\leftarrow$ $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$. We have obtained sharp richly structured fluorescence spectra by using tuned, narrow-band excitation to pump selected absorption bands, including the 0,0 band of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1$ $\leftarrow$ $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1$ system. No phosphorescence can be detected, even at high pressures of added gas. Our fluorescence spectra confirm that little, if any, induced vibronic activity is present. However, inconsistencies occur between the present absorption assignments and evidence available from fluorescence. In fluorescence from the zero-point level, the totally symmetric in-plane ring bending mode, $v12\prime$ ($1065cm-1$) appears as a strong progression, 12 and is prolific in forming other progressions of the type $X0112n0$, $X10Y1012n0$. In absorption, it has been proposed that the ring stretching mode, $\nu 8a$ is Largely responsible for the upper state progressions, whereas we find little activity in fluorescence for $\nu 8a$. SVL fluorescence generated by pumping the absorption band at $0,0+1012cm-1$ shows that this absorption is $1201$ rather than the previously proposed 8a$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow><mrow\; data-mjx-texclass="ORD">1</mrow>}$. Spectra obtained by exciting other absorption bands comment on further uncertainties in absorption assignments. Vapor fluorescence has now been characterised for all the known azabenzenes excepting pyridine. We have been unsuccessful in our own attempts co detect luminescence from pryidine vapor.