EXPECTED SIGNIFICANCE OF WEAKLY CHAOTIC VIBRATIONAL MOTIONS IN SINGLE-MOLECULE SPECTROSCOPY

It is known that a new kind of large amplitude motion gives rise to as a very characteristic mode of weak chaos. This is essentially an unpr edictable and intermittent motion taking place in a thin quasiseparatr ix which wanders among several very clear vibrational modes. In this p aper, we study the spectroscopic characterization of the quantum versi on of this large amplitude motion in terms of the dynamics of a wave p acket, which is prepared in a narrow energy-range so that it is locali zed along a thin quasiseparatrix. In particular, we discuss possible s ignificance of the weak chaos in single molecule spectroscopy, for whi ch the spectra are supposed to be averaged neither in ensemble of mole cules nor in time. That this wavepacket state is unusual originates fr om the extremely long-time behavior and the strong sensitivity to the initial condition at which the wave packet is prepared. The weak chaos combined with the statistical nature of quantum mechanics brings abou t a notion of unreproducibility in the spectrum. Conversely, it is ant icipated that one of the distinguished features inherent to a single m olecule spectroscopy manifests itself when weak chaos is observed. (C) 1995 American Institute of Physics.