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High-Resolution Laser-Induced Fluorescence Study of a Cage Molecule, 1,4-Diazabicyclo[2,2,2]Octane, DABCO

Rotationally resolved laser-induced fluorescence spectra of the caged amine DABCO (1,4-diazabicyclo[2,2,2] octane) seeded in a supersonic He beam are recorded using a high-resolution pulsed dye laser system. Both one-photon and two-photon spectra of the S1(A1') <-- S0(A1') transition are measured. In the one-photon spectra we observe electric quadrupole transitions to the S1 origin and to a1' vibrational levels and vibrationally induced electric dipole transitions to a2'' and e' vibrational levels. For the first time parallel one-photon transitions with their characteristic rotational structure have been observed. Rotational constants are extracted from both the two-photon and one-photon spectra. The quantum mechanical model previously applied to explain the occurrence of the ''memory effect'' in fluorescence spectra emanating from the S, (e') levels is extended here for the S1(a2'') levels. Despite the presence of long progressions in several vibrational modes for the S1(A1') <-- S0(A1') transition, the difference in S0 and S1 rotational constants are remarkably small, suggesting that compensating geometry changes take place on excitation to S1.

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