From light-scattering measurements to polarizability derivatives in vibrational Raman spectroscopy: The 2ν5 overtone of SF6

Abstract

The room-temperature isotropic spectrum of SF6 was recorded at the frequency of the 2ν5 overtone by running high-sensitivity incoherent Raman experiments for two independent polarizations of the incident beam and for gas densities varying from 2 to 27 amagat. Weak yet observable pressure effects were found. A transparent analysis of the Raman cross-section problem along with the first-ever prediction of the value of the mean polarizability second derivative ∂(2)α/∂q5(2) are made and the hitherto underestimated role of the hot bands of SF6 is brought to the wider public. The emergence of an analytic hotband factor is shown whose magnitude is dramatically increased with the order of the overtone and the gas temperature and all the more so upon considering low-frequency molecular vibrations. Our formulas, which in the harmonic approximation are exact, are still applicable to real situations provided certain conditions are fulfilled. For nondegenerated modes, generalization to higher order overtones is made, an issue addressing the much challenging problem of the IR-allowed second overtone bands. The content of this paper is also an invitation towards ab initio derivative-calculations for sulfur hexafluoride, especially given the today's needs in interpreting spectra of significance for greenhouse atmospheric issues.

The 2ν3 Raman overtone of sulfur hexafluoride: Absolute spectra, pressure effects, and polarizability properties

“…As a last remark about this spectrum, note that its shape closely resembles that of the recently studied 2ν 5 overtone, 5 in which the hot-band transition ν 6 + 2ν 5 − ν 6 had again a significant role both in shape and in integrated intensity. At room temperature, the state (000001) of SF 6 is the second most populated level after the ground level.…”

“…The description of our experimental setup has been given in recent 5,26 and other publications 31-33 and will be skipped. We only recall (i) that the scattering intensities were recorded at an angle of 90 • from the incident beam, (ii) that the polarization of this beam was chosen to be perpendicular (⊥) or parallel ( ) to the scattering plane, at will, and (iii) that, for either polarizations, the scattering signal (counts/s) was recorded at a fixed temperature, T = 294.5 ± 1 K, and over a wide range of Raman wave numbers ν and gas densities ρ.…”

“…Recently, a prediction of ∂ 2ᾱ /∂q 2 5 was made by our group for the "scissoring" mode of that molecule on the basis of high-sensitivity Raman experiments on the 2ν 5 overtone. 5 Yet the poor accuracy of the existing data for cubic force constants involving bending modes 6, 7 of SF 6 prevented us from giving an unambiguous value for that derivative (even though a formula involving k 155 as a free parameter was also given therein, 5 which will allow for a more reliable estimate of the derivative whenever a more reliable input for k 155 comes out).…”

“…24 Since few weeks, SF 6 is again in the forefront after the use of new cryogenic multipass cells, in high-resolution absorption experiments, which allowed for resolving its ever-challenging ν 6 forbidden band. 25 Here, prompted by our recent studies on the 2ν 5 spectra, 5,26 we employ incoherent Raman spectroscopy to detect the first overtone of the ν 3 vibration and to circumvent the handicap of the Raman inactivity of the ν 3 fundamental transition. Our work is a threefold advancement on an issue of vibrational Raman spectroscopy on which much has been accomplished 27, 28 but on which the last is yet to be heard.…”

More light on the 2ν5 Raman overtone of SF6: Can a weak anisotropic spectrum be due to a strong transition anisotropy?

“…21,22 In a recent article, highly accurate isotropic spectra have been reported for this overtone and a systematic procedure to determine second derivatives for the mean dipole polarizability of this vibration was developed. 23 Such quantities are of a significant physical content and their contribution to the understanding of the "greenhouse mechanisms" can be substantial. Yet owing to the difficulty of making ab initio computations for those quantities, values of derivatives other than for the "breathing" ν 1 mode 16 or reliable values for the cubic force constants, especially for the bending modes of SF 6 , are rarities in the literature of this molecule.…”