The pioneering works of Raman and Mizushima in the area of vibrational spectroscopy have been widely exploited by physical chemists for obtaining structural information about molecules. This lecture, dedicated to the two great scientists, reviews our work on low-frequency vibronic spectra of jet-cooled large organic molecules.

    A chemical change involves hopping of the system from one well in the multi-dimensional potential energy surface (PES) to another. A simplified model for this process which lends itself to further detailed treatment is large-amplitude vibration along the soft mode of a floppy molecule. It is therefore pertinent that we collect more information about the ground and/or excited state PES corresponding to motions like aryl rotation, ring-puckering, H-bond stretching and intermolecular dissociation.

    In the condensed phase the perturbation by the surrounding environment is large enough to cause deformation and blurring of conformer identities. It is necessary, therefore, to work with a set of collisionless, cold, uniformly moving molecules as provided in a jet. The vibronic features of the laser-induced fluorescence and fluorescence excitation spectra of the cold molecular beam are often amenable to analysis in terms of a fundamental frequency and its higher harmonics, which in turn can be correlated with the nature of excited and ground PESs. Many spectroscopic examples are given. Furthermore, the power of two-colour IR-UV double resonance technique in determining structures of clusters is illustrated with five hydrates of tetrahydroisoquinoline.