Knowledge Bank

Among the anomalies for which the emission of azulene vapor is noted is the fact that $S2\to S0$ fluorescence after atomic line excitation is found generally to be diffuse whereas the corresponding $S0\to S2$ absorption is sharp. We have used narrow band excitation tuned to specific absorption bands or regions in the low pressure vapor. Under these conditions, sharp $S2\to So$ fluorescence spectra are observed. The diffuse fluorescence arising from atomic line excitation is apparently due to congestion. The lines pump regions of absorption which are crowded with overlapping bands Our excitation in the 0,0 absorption band yields $S2\to So$ fluorescence which is sparsely structured, and which displays well developed rotational band contours that mimic those seen in absorption. The vibrationa1 structure is generally consistent with that seen in fluorescence from azu1ene in low temperature crystalline hosts. Both $a1$ and $b1$ vibronica1ly induced structure is present with the former of dominant intensity. The allowed components of fluorescence are relatively week, Pressurization of the emitting sample with benzene produces normal development of $S2$ vibrationa1 equilibrium without inducing appreciable electronic destruction. Other excitations have pumped absorption bands which stand relatively clear of crowding at the high energy edges of band groups lying near 650, 1000, and $1300cm-1$ above the 0,0 position. Each gives fluorescence with sharp structure, but with underlying congestion that becomes marked with the latter. Fluorescence confirms the absorption at $0,0+663cm-1$ as the transition $xo1$ with $\nu x\prime \prime (a1)=671cm-1$.