Low-frequency vibrations in S1 states of tetrahydroquinoline and N-deuterotetrahydroquinoline in supersonic jet Original Research Article

The laser-induced fluorescence excitation (FE) and multiphoton ionisation (MPI) spectra of tetrahydroquinoline (THQ) and N-deuterotetrahydroquinoline (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 S0 → S1 (ππ∗) is found at 32450 cm− 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 assigned to the ring puckering “twist” motion in the excited state. There is another frequency close to the puckering “twist” frequency, which has been assigned to the puckering “bend” mode. Comparison of frequencies and intensities with corresponding values calculated from model potential shows that the molecules are twisted in both ground and excited states. An isotope shift of ∼ 16 cm−1 in the 0-0 band of D-THQ is observed and explained on the basis of zero-point energies.