Analysis of SF6 quasicontinuum states. II

Abstract

The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelié, J. Chem. Phys. 96, 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 Vk versus the number k of quanta exchanged, with an effective density of couplings ρk. A model without arbitrary parameters settles that the parameter γ=πρ<V2≳ can be shared between a dissipative part γd∼1–1.5 cm−1 (for E∼10 000–15 000 cm−1), corresponding to the orders k≥5, and a perturbative part γp∼2–15 cm−1, corresponding to the lowest orders k≂3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E∼4000 cm−1, as demonstrated by Raman experiments, with a half-width γd∼0.21 cm−1, in excellent agreement with picosecond spectroscopy giving a T1 relaxation time of ∼11 ps. The parameter γ is also extracted from IRMPA cross sections. It is found from these data that γ(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 τL∼30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the ν3 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.