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

Abstract

Long known as a fully polarized band with a near vanishing depolarization ratio [ηs = 0.05, W. Holzer and R. Ouillon, Chem. Phys. Lett. 24, 589 (1974)], the 2ν5 Raman overtone of SF6 has so far been considered as of having a prohibitively weak anisotropic spectrum [D. P. Shelton and L. Ulivi, J. Chem. Phys. 89, 149 (1988)]. Here, we report the first anisotropic spectrum of this overtone, at room temperature and for 13 gas densities ranging between 2 and 27 amagat. This spectrum is 10 times broader and 50 times weaker than the isotropic counterpart of the overtone [D. Kremer, F. Rachet, and M. Chrysos, J. Chem. Phys. 138, 174308 (2013)] and its profile much more sensitive to pressure effects than the profile of the isotropic spectrum. From our measurements an accurate value for the anisotropy matrix-element |⟨000020|Δα|000000⟩| was derived and this value was found to be comparable to that of the mean-polarizability 000020α¯000000. Among other conclusions our study offers compelling evidence that, in Raman spectroscopy, highly polarized bands or tiny depolarization ratios are not necessarily incompatible with large polarizability anisotropy transition matrix-elements. Our findings and the way to analyze them suggest that new strategies should be developed on the basis of the complementarity inherent in independent incoherent Raman experiments that run with two different incident-beam polarizations, and on concerted efforts to ab initio calculate accurate data for first and second polarizability derivatives. Values for these derivatives are still rarities in the literature of SF6.

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

“…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.…”

“…27 The 2ν 3 overtone, though still qualified as a "polarized band," is definitely far less polarized than the 2ν 5 overtone. 26 Isotropic and anisotropic zeroth-order spectral moments, M 0 , were also calculated, and their values (in cm 6 ) are gathered in Table II along with the M 0 values from previous studies. Note that in none of the past studies was the incidentbeam polarization oriented in other than the ⊥ direction.…”

“…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.…”