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

Multiple Gas Detection by Cavity-Enhanced Raman Spectroscopy with Sub-ppm Sensitivity

Accurate and sensitive detection of multicomponent trace gases below the parts-per-million (ppm) level is needed in a variety of medical, industrial, and environmental applications. Raman spectroscopy can identify multiple molecules in the sample simultaneously and has excellent potential for fast diagnosis of various samples, but applications are often limited by its sensitivity. In this contribution, we report the development of a cavity-enhanced Raman spectroscopy instrument using a narrow-line width 532 nm laser locked with a high-finesse cavity through a Pound-Drever-Hall locking servo, which allows continuous measurement in a broad spectral range. An intracavity laser power of up to 1 kW was achieved with an incident laser power of about 240 mW, resulting in a significant enhancement of the Raman signal in the range of 200-5000 cm^-1 and a sub-ppm sensitivity for various molecules. The technique is applied in the detection of different samples, including ambient air, natural gas, and reference gas of sulfur hexafluoride, demonstrating its capability for the quantitative measurement of various trace components.

Effect of helium nanoclusters on the spectroscopic properties of embedded SF6: Ionization, excitation and vibration

Ionization and excitation energies, IR and Raman spectra of sulfur hexafluoride (SF₆), located inside helium (He) nanoclusters with different sizes (SF₆@He_n; n=20, 40, 60), were calculated. The effect of the cluster size on the spectroscopic properties of the SF₆ was investigated and found that the He_n-SF₆ interaction in the He clusters with large number of atoms is small so that the ionization and absorption energies of SF₆ are not affected while for small He nanoclusters the He_n-SF₆ interaction is more important. The effect of He_n-SF₆ interaction and deformation of the fragments on the photoelectron and absorption spectra of SF₆@He_n were separated theoretically and discussed in details. It was deduced that the effect of the cluster size on the IR and Raman vibrational frequencies of the SF₆ is negligible for the cluster size range considered in this work. Density functional theory (DFT) employing M06-2X functional and 6-31+G(df) basis set were used for optimizing the structures of SF₆@He_n. Symmetry adapted cluster-configuration interaction (SAC-CI) methodology, with the same basis set, were used to calculate the ionization and excitation energies of the SF₆@He_n structures. Using the calculated ionization and absorption energies and their intensities, the photoelectron and absorption spectra of the considered SF₆@He_n structures were simulated and compared with the experiment.

The 2ν(3) Raman overtone of sulfur hexafluoride: absolute spectra, pressure effects, and polarizability properties

Of the six normal vibrations of SF6, ν3 has a key role in the mechanisms of radiative forcing. This vibration, though inactive in Raman, shows up through the transition 2ν3 allowing for a complementary view on the asymmetric stretch of the molecule. Here, we look back into this topic, which has already caught some interest in the past but with some points been left out. We make a systematic incoherent-light-scattering analysis of the overtone with the use of different gas pressures and polarization orientations for the incident beam. Absolute-scale isotropic and anisotropic spectra are reported along with natural and pressure-induced widths and shifts, and other spectral features such as the peaks corresponding to the (experimentally indistinguishable) interfering channels Eg and F2g hitherto seen solely as two-photon IR-absorption features. We make the first-ever prediction of the SF6 polarizability second derivative with respect to the ν3-mode coordinate and we develop a heuristic argument to explain why the superposition of the three degenerate stretching motions that are related to the ν3 mode cannot but generate a polarized Raman band.