n,pgr;ast; fluorescence from selected vibronic levels of pyrimidine vapor: Franckndash;Condon factors and excited state anharmonic coupling

摘要

An analysis is presented of fluorescence from various selected vibrational levels in the1B1(S1) state of pyrimidine vapor. The intensity distribution in fluorescence from the zero point level establishes with security the general pattern of vibrational activity in the1B1ndash;1A1transition, and the Franckndash;Condon intensities in this fluorescence set the molecular parameters required for calculation of Franckndash;Condon intensities in fluorescence from other levels. The fluorescence from every higher level so far reached shows marked deviations from the predicted Franckndash;Condon intensities. These deviations can be attributed to strong (ca., 30 cmminus;1) anharmonic coupling in the excited state. Every observed totally symmetric fundamental in the1B1state is perturbed by this mixing. Calculations which include the coupling provide a successful account of the perturbed Franckndash;Condon intensities. The fluorescence analyses secure the assignment of a number of absorption bands as well as several excited state fundamentals. Specifically, ngr;6aprime;, ngr;9aprime;, and ngr;16aprime; are confirmed and the fundamentals ngr;1prime;=941 cmminus;1, ngr;6bprime;=335 cmminus;1, ngr;12prime;=1012 cmminus;1, and ngr;16bprime;=366 cmminus;1are newly established. Fluorescence studies at various pressures show that vibrational relaxation is a poor competitor to a very efficient collisionhyphen;induced electronic relaxation of the1B1state. Vapor phosphorescence cannot be detected under any conditions.