Unrivaled accuracy in measuring rotational transitions of greenhouse gases: THz CRDS of CF4.

Tetrafluoromethane CF4 is the most abundant perfluorocarbon in the atmosphere, where it is designated as PFC-14. This greenhouse gas is very stable, has an atmospheric lifetime of 50 000 years, and a high greenhouse warming potential 6500 times that of CO2. Over the last 15 years, its atmospheric concentration has increased at a rate of 0.8 ppt per year. The accurate quantification of CF4 is key to understanding the contribution of its emissions to the radiative forcing budget, and the most precise spectroscopic parameters possible are hence required. In this study, a novel high finesse THz cavity, providing an interaction length in excess of 1 km, has enabled highly resolved spectra, and quantification of the weak transitions of CF4 by cavity ring-down spectroscopy (CRDS). More than 50 pure rotational P6 - P6 : ν3 - ν3 lines of CF4 have been measured, yielding both position and intensity with unequalled precision. Several tetrahedral splittings are fully resolved and measured with sub-MHz accuracy. Moreover, CRDS-THz allows determining absolute intensities and, using a global fit of the ν2 polyad series, a CF4 dipole parameter, namely 3,3, has been fitted to 106.38(53) mD. This value is in very good agreement with that of the ab initio-based parameter deduced from a dipole moment surface. For the first time, a set of ab initio effective dipole moment parameters is derived for the computation of the transitions of the type Pn - Pn (n = 0,…, 8) and the resulting line list composed of 25 863 transitions can be used to model the whole CF4 rotational spectrum. Finally, the TFMeCaSDa database is updated and is available for future spectroscopic and monitoring activities.

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy.

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

Nitrogen-Broadening Parameters for Atmospheric Spectra Modelling of the ν3 Band of SF6.

The infrared absorption of the ν3 band region of SF6, at temperatures spanning the 130 to 297 K range, has been reexamined using improved instrumentation with one goal: to estimate the broadening of parameters by nitrogen gas. These parameters are compared to previous literature predictions and an extended set of IR cross-sections is proposed and compared to other existing datasets.

New investigation of the ν3 C-H stretching region of 12CH4 through the analysis of high temperature infrared emission spectra.

The ν3 C-H stretching region of methane was reinvestigated in this work using high temperature (620-1715 K) emission spectra recorded in Rennes at Doppler limited resolution. This work follows our recent global analysis of the Dyad system Δn = ±1 (1000-1500 cm-1), with n being the polyad number [B. Amyay et al., J. Chem. Phys. 144, 24312 (2016)]. Thanks to the high temperature, new assignments of vibration-rotation methane line positions have been achieved successfully in the Pentad system and some associated hot bands (Δn = ±2) observed in the spectral region 2600-3300 cm-1. In particular, rotational assignments in the cold band [Pentad-ground state (GS)] and in the first related hot band (Octad-Dyad) were extended up to J = 30 and 27, respectively. In addition, 1525 new transitions belonging to the Tetradecad-Pentad hot band system were assigned for the first time, up to J = 20. The effective global model used to deal with the new assignments was developed to the 6th order for the first three polyads (Monad, Dyad, and Pentad), and to the 5th order for both the Octad and the Tetradecad. 1306 effective parameters were fitted with a dimensionless standard deviation σ = 2.64. The root mean square deviations dRMS obtained are 4.18 × 10-3 cm-1 for the Pentad-GS cold band, 2.48 × 10-3 cm-1 for the Octad-Dyad, and 1.43 × 10-3 cm-1 for the Tetradecad-Pentad hot bands.

Conformational landscape of the SF6 dimer as revealed by high resolution infrared spectroscopy and complexation with rare gas atoms.

Taking advantage of a versatile set-up, combining pulsed pin hole or slit nozzle supersonic expansion with an external cavity quantum cascade laser, the rovibrational absorption spectrum of the SF6 dimer in the ν3 mode region has been revisited at high resolution under various experimental conditions in SF6:He mixtures. Two new rotationally resolved spectral bands have been identified in the range of the parallel band of the dimer spectrum in addition to that previously reported. Among these three spectral features, two of them are assigned to conformations of the dimer (noted #1 and #2), clearly distinguished from their different S-S interatomic distances, i.e. 474 and 480 pm respectively. The third one is assigned to a (SF6)2-He complex, from comparison with additional experiments in which (SF6)2-Rg heterotrimers (Rg = Ne, Ar, Kr, Xe) are observed. A schematic picture of the potential energy landscape of the SF6 dimer in terms of a nearly flat surface is proposed to account for the conformational relaxation observed in the expansions and for the structure of the (SF6)2-Rg heterotrimers, which are exclusively formed from the conformer #2 dimer. Although modelling qualitatively supports this picture, much effort has still to be achieved from a theoretical point of view to reach a quantitative agreement with the present benchmark experimental data both in terms of structure and energetics.

Global analysis of the high temperature infrared emission spectrum of (12)CH4 in the dyad (ν2/ν4) region.

We report new assignments of vibration-rotation line positions of methane ((12)CH4) in the so-called dyad (ν2/ν4) region (1100-1500 cm(-1)), and the resulting update of the vibration-rotation effective model of methane, previously reported by Nikitin et al. [Phys. Chem. Chem. Phys. 15, 10071 (2013)], up to and including the tetradecad. High resolution (0.01 cm(-1)) emission spectra of methane have been recorded up to about 1400 K using the high-enthalpy source developed at Institut de Physique de Rennes associated with the Fourier transform spectrometer of the SOLEIL synchrotron facility (AILES beamline). Analysis of these spectra allowed extending rotational assignments in the well-known cold band (dyad-ground state (GS)) and related hot bands in the pentad-dyad system (3000 cm(-1)) up to Jmax = 30 and 29, respectively. In addition, 8512 new transitions belonging to the octad-pentad (up to J = 28) and tetradecad-octad (up to J = 21) hot band systems were successfully identified. As a result, the MeCaSDa database of methane was significantly improved. The line positions assigned in this work, together with the information available in the literature, were fitted using 1096 effective parameters with a dimensionless standard deviation σ = 2.09. The root mean square deviations dRMS are 3.60 × 10(-3) cm(-1) for dyad-GS cold band, 4.47 ×10(-3) cm(-1) for the pentad-dyad, 5.43 × 10(-3) cm(-1) for the octad-pentad, and 4.70 × 10(-3) cm(-1) for the tetradecad-octad hot bands. The resulting new line list will contribute to improve opacity and radiative transfer models for hot atmospheres, such as those of hot-Jupiter type exoplanets.

High-resolution spectroscopy and structure of osmium tetroxide. A benchmark study on 192OsO4.

Osmium tetroxide (OsO(4)) is a heavy tetrahedral molecule that constitutes a benchmark for quantum chemistry calculations. Its favorable spin statistics (due to the zero nuclear spin of oxygen atoms) is such that only A(1) and A(2) (T(d) symmetry) rovibrational levels are allowed, leading to a dense but quite easily resolvable spectrum. We reinvestigate here the ν(1)/ν(3) stretching fundamental (940-980 cm(-1)) dyad region and perform new assignments and effective Hamiltonian parameter fits for the main isotopologue ((192)OsO(4)). We also investigate the ν(2)/ν(4) bending fundamental dyad (300-360 cm(-1)) for the first time and perform a preliminary analysis. New experimental data have been obtained at 0.001 cm(-1) resolution using an isotopically pure (192)OsO(4) sample and the Synchrotron SOLEIL light source. Assignments and analyses were performed using SPVIEW and XTDS software, respectively. We provide precise effective Hamiltonian parameters, including the band centers for all of the fundamental levels and rotational constants for the ground state and for all four fundamental levels. We discuss isotopic shifts, estimate the equilibrium rotational constant B(e), and derive a precise value for the equilibrium bond length r(e)(Os-O) = 1.70919(16) Å. We also performed experiments to measure for the first time the IR integrated intensities for the ν(2)/ν(4) bending fundamental dyad. These new data are compared to current ab initio predictions.

Stark spectrum simulation for X2Y4 molecules: application to the ν12 band of ethylene in a high-silica zeolite.

The influence of an electric field of silicalite-1-zeolite on the FTIR vibrational absorption spectrum of ethylene has been simulated and compared to experimental spectra. The presence of silicalite-1 produces a global shift and a change of the structure of vibrational bands. To explain the global shift of the ν(12) band (CH(2) scissor mode) and therefore to estimate an effective average field produced by silicalite-1, Stark calculations were performed. These calculations were based on a tensorial formalism implemented in the D(2h)TDS-ST package [M. Sanzharov, M. Rotger, C. Wenger, M. Loëte, V. Boudon, and A. Rouzée, J. Quant. Spectrosc. Radiat. Transf. 112, 41 (2011)]. The value of the field obtained using tensorial formalism (8-11 GV/m) is compared with values obtained using ab initio calculations. A theory of the molecular alignment in the electric field using tensorial formalism is also developed to model the interaction of ethylene in contact with a zeolite environment.

Theoretical investigation of the ethylene dimer: interaction energy and dipole moment.

The interaction potential energy and the interaction-induced dipole moment surfaces of the van der Waals C(2)H(4)-C(2)H(4) complex has been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules. The calculations have been carried out using high-level ab initio theory with the aug-cc-pVTZ basis set and within the framework of the analytical description of long-range interactions between ethylene molecules. Binding energy for the most stable configuration of the C(2)H(4)-C(2)H(4) complex was calculated at the CCSD(T)/CBS level of theory. The harmonic fundamental vibrational frequencies for this complex were calculated at the MP2 level of theory.

Dipole moment surface of the van der Waals complex CH4-N2.

The interaction-induced dipole moment surface of the van der Waals CH(4)-N(2) complex has been calculated for a broad range of intermolecular separations R and configurations in the approximation of the rigid interacting molecules at the MP2 and CCSD(T) levels of theory using the correlation-consistent aug-cc-pVTZ basis set with the basis set superposition error correction. The simple model to account for the exchange effects in the range of small overlap of the electron shells of interacting molecules and the induction and dispersion interactions for large R has been suggested. This model allows describing the dipole moment of van der Waals complexes in analytical form both for large R, where induction and dispersion have the key role, and for smaller R including whole ranges of their potential wells, where the exchange effects are important. The proposed model was tested on a number of configurations of the CH(4)-N(2) complex and was applied for the analytical description of the dipole moment surface for the family of the most stable configurations of the CH(4)-N(2) complex.

Static polarizability surfaces of the van der Waals complex CH4-N2.

The static polarizability surfaces of the van der Waals complex CH(4)-N(2) have been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules. The calculations have been carried out at the CCSD(T) and MP2 levels of the theory using the aug-cc-pVTZ basis set with the BSSE correction and within the framework of the classical long-range multipolar induction and dispersion interactions. It was shown that the results of analytical polarizability calculations for the CH(4)-N(2) complex are in a good agreement with the ab initio polarizabilities in the outer part of the van der Waals well on the complex potential surface. Ab initio calculations of the polarizability tensor invariants for the complex being in the most stable configurations were carried out. The change in the polarizability of CH(4)-N(2) due to the deformation of the CH(4) and N(2) monomers at the formation of the complex was estimated. In the framework of the analytical approach the polarizability functions alpha(ii)(R) of the free oriented interacting molecules CH(4) and N(2) were calculated.

Theoretical investigation of the potential energy surface of the van der Waals complex CH4-N2.

The interaction potential energy surface of the van der Waals CH(4)-N(2) complex has been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules at the CCSD(T) and MP2 levels of theory using the correlation consistent aug-cc-pVTZ basis set. The BSSE correction was taken into account for all the calculations. The most stable configurations of the complex were found. Binding energies were calculated in the CBS limit with accounting for the molecular deformations. The harmonic and anharmonic fundamental vibrational frequencies and rotational constants for the ground and first excited vibrational states were calculated for the most stable configurations at the MP2 level of theory with BSSE correction. Fitting parameters were found for the most stable configuration for the Lennard-Jones and Esposti-Werner potentials.