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.

Electron attachment to SF6 and lifetimes of SF6- negative ions

“…In the past, the IVR in highly vibrationally excited SF 6 molecules was studied in multiphoton laser excitation experiments (see, e.g., [46] and references therein). These and other studies [47] indicate that the IVR rate of the strong infrared-active ν 3 mode is noticeably smaller than our estimate. However, the two situations are quite different.…”

Transition Spectra in the Vibrational Quasicontinuum of Polyatomic Molecules. IR Multiple-Photon Absorption in SF6. 2. Theoretical Simulation and Comparison with Experiment

“…The average level of preliminary excitation into the QC is taken to be n 1 ) 6, which corresponds to a vibrational temperature of T vib ) 960 K reached by the instant the probe pulse arrives. The filled symbols in Figure 5a indicate the experimental N 2 values, and the solid and dashed curves represent the theoretical N 2 values calculated for γ L defined by (13) and γ L ) 0, respectively. As noted earlier, consideration of the SIB effect alone provides for qualitative agreement between theory and experiment, there being quantitative differences increasing at the ends of the spectrum with increasing Φ 2 .…”

“…The functions Z(n) are presented for three characteristic values of the frequency ω 2 : approximately in the center (ω 2 ) 903.75 cm -1 ), at the "red" end (ω 2 ) 884.18 cm -1 ), and the "blue" end (ω 2 ) 936.8 cm -1 ) of the IR MP spectrum (see Figure 5b). Presented in Figure 6 is the initial Boltzmann distribution (the curve with circles) and also the resultant distribution functions for a purely Gaussian transition profile (the solid curve), and the convolution of the Gaussian and a Lorentzian profiles (the dashed curve), parameters (13) of the latter being found in this work. Note that the aVerage absorbed energy values for the given φ 2 and n 1 values at the frequencies ω 2 ) 884.18 and ω 2 ) 936.8 cm -1 are approximately the same.…”

Statistical Inhomogeneous Broadening of Infrared and Raman Transitions in Highly Vibrationally Excited XY6 Molecules

“…So, a natural way to make things clearer is to calculate the spectral shapes due to the inhomogeneous contribution; then, this should enable us to estimate how fast IVR rates cannot be ignored. Our further consideration is limited to the systems like those where the dominant role of the inhomogeneous broadening has been assumed (and probably proved) previously …”

“…mean vibrational energy of about 2000 cm -1 (see the experimental results in ref and related discussion of these results in ref ). Eventually, inhomogeneity is considered as the principal effect in the width, at high vibrational energies, for the IR spectra of mode ν 4 of CF 3 I in ref and mode ν 3 of SF 6 in ref . We, as said above, will concentrate in this paper on the latter, just the statistical inhomogeneous broadening (in particular, for the ν 3 of SF 6 ).…”