Effects of clustering of rare-gas atoms on the rate of S1T2 intersystem crossing for 9-methylanthracene

Laser induced fluorescence excitation spectra for 9-methylanthracene (MEA) in a supersonic free jet were measured as a function of the stagnation pressure of Ar, Kr and Xe, used as carrier gas. The 0-0 excitation bands for complexes of type MEA · Arn were observed successively corresponding to an increase in coordination number n with increasing stagnation pressure. On the other hand, excitation bands associated with small clusters of Kr and Xe were scarcely observed, although strong excitation bands associated with larger clusters were observed at largely red-shifted wavelengths. These observations combined with the measurement of fluorescence lifetimes were interpreted in terms of two effects of the clustering of rare-gas atoms on the intersystem crossing rate; energy shift of S1 relative to T2 and heavy atom effect.