ANALYSIS OF SF-6 QUASI-CONTINUUM STATES .2.

The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelie, J. Chem. Phys. %, 8072 (1992)], are analyzed in comparison with IR multiphoton absorption (IRMPA) data, either in the nanosecond or in the picosecond regimes. A general description of the QC transition matrix elements is first derived. All the following features must be taken into account: multiple rovibrational transitions, perturbative redistribution onto secondary resonances, dissipative redistribution onto the whole energy shell, and intramolecular mode inhomogeneity producing a frequency dispersion. The intramolecular couplings obey a hierarchy V(k) versus the number k of quanta exchanged, with an effective density of couplings rho(k). A model without arbitrary parameters settles that the parameter gamma = pirho[V2] can be shared between a dissipative part gamma(d) approximately 1-1.5 cm-1 (for E approximately 10 000-15 000 cm-1), corresponding to the orders k greater-than-or-equal-to 5, and a perturbative part gamma(p) approximately 2-15 cm-1, corresponding to the lowest orders k congruent-to 3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E approximately 4000 cm-1, as demonstrated by Raman experiments, with a half-width gamma(d) approximately 0.21 cm-1, in excellent agreement with picosecond spectroscopy giving a T1 relaxation time of approximately 11 ps. The parameter gamma is also extracted from IRMPA cross sections. It is found from these data that gamma(E) increases from 0.25 to 15 cm-1 when E increases from 4000 to 34000 cm-1. Finally, picosecond data, showing a spectacular enhancement of the number of photons absorbed for short pulses of duration tau(L) approximately 30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the nu3 mode, producing a dramatic enhancement of the Rabi width. Then, all known data on the SF6 QC can be incorporated in a unique framework, likely generalizable to other molecules.