HIGH-RESOLUTION LASER-INDUCED FLUORESCENCE STUDY OF A CAGE MOLECULE, 1,4-DIAZABICYCLO2,2,2!OCTANE, DABCO

Rotationally resolved laser-induced fluorescence spectra of the caged amine DABCO (1,4-diazabicyclo2,2,2! octane) seeded in a supersonic He beam are recorded using a high-resolution pulsed dye laser system. Bo th one-photon and two-photon spectra of the S1(A1') <-- S0(A1') transi tion are measured. In the one-photon spectra we observe electric quadr upole transitions to the S1 origin and to a1' vibrational levels and v ibrationally induced electric dipole transitions to a2'' and e' vibrat ional levels. For the first time parallel one-photon transitions with their characteristic rotational structure have been observed. Rotation al constants are extracted from both the two-photon and one-photon spe ctra. The quantum mechanical model previously applied to explain the o ccurrence of the ''memory effect'' in fluorescence spectra emanating f rom the S, (e') levels is extended here for the S1(a2'') levels. Despi te the presence of long progressions in several vibrational modes for the S1(A1') <-- S0(A1') transition, the difference in S0 and S1 rotati onal constants are remarkably small, suggesting that compensating geom etry changes take place on excitation to S1.