Molecular dynamics and quantum chaos in small polyatomic molecules (CS_2, C_2H_2) through stimulated-emission pumping experiments and statistical Fourier-transform analysis

The experimental study of the quantum properties of polyatomic molecules at high vibrational energy when classical chaos is present (quantum chaos) is discussed. Such a study requires the improvement of both experimental and analytical techniques. Methods by which stimulated-emission pumping spectroscopy and statistical Fourier-transform analysis of spectra can be successfully used to achieve this goal are emphasized. Stimulated-emission pumping and the statistical Fourier transform share the same concept of wave-packet preparation and evolution. This dynamical approach leads to interesting observations in molecular spectra. A numerical model of two coupled Morse oscillators provides information about the physical significance of the correlation hole observed in the Fourier transform of spectra of chaotic molecules. A relevant example in the experimental spectrum of CS2 shows the destabilization of given periodic orbits in a regime of soft chaos. In such a case the destabilizing process starts with Fermi resonances. In CS2 the chaos appears first in a portion of the degree of freedom (symmetric stretching and bending). In C2H2, in contrast, a new periodic orbit appears, corresponding to a new low-frequency mode that emerges from the chaotic bath of C2H2 and that could induce specific chemical reactions.