Spectroscopic Studies of Transitions to Vibrational Chaotic States of Highly Excited Polyatomic Molecules

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Dispersed fluorescence spectroscopy has been applied to elucidate the vibrational dynamics of highly excited polyatomic molecules. The major concern of this project is to clarify the mechanism of the transition from the 'regular' to 'chaotic' vibrational dynamics in highly excited polyatomic molecules.The observed vibrational level structure of a highly excited polyatomic molecule was found to be of a hierarchical one owing to the anharmonic resonances among certain vibrational modes. The intermode couplings occurs in a stepwise manner as the vibhrational energy increases towards the vibrational chaotic state in the final stage.It was found that the vibrational spectra of excited acetylene in the ground electronic state reflect the above situation, i.e., the trans-CH bending mode loses its physical ground in a high energy region through a strong coupling with the cis-CH bending mode, and that in a more higher energy region further couplings with CC and CH stretching modes occurs.The conclusion derived above is generalized by observing the spectra of other simple polyatomic molecules in their highly excited states.