LOW-FREQUENCY VIBRATIONS IN S-1 STATES OF TETRAHYDROQUINOLINE AND N-DEUTEROTETRAHYDROQUINOLINE IN SUPERSONIC JET

The laser-induced fluorescence excitation (FE) and multiphoton ionisat ion (MPI) spectra of tetrahydroquinoline (THQ) and N-deuterotetrahydro quinoline (D-THQ), cooled by seeding in a supersonic jet expansion of He, have been investigated in the wavenumber range 32400 to 33200 cm(- 1). The vibronic origin for the S-0 --> S-1 (pi pi) is found at 32450 cm(-1) in THQ and 32466 cm(-1) in D-THQ. The spectra show a prominent progression of a low-frequency vibration of 100 cm(-1) which is assig ned to the ring puckering ''twist'' motion in the excited state. There is another frequency close to the puckering ''twist'' frequency, whic h has been assigned to the puckering ''bend'' mode. Comparison of freq uencies and intensities with corresponding values calculated from mode l potential shows that the molecules are twisted in both ground and ex cited states. An isotope shift of similar to 16 cm(-1) in the 0-0 band of D-THQ is observed and explained on the basis of zero-point energie s.