Abdessamad Benidar

Infrared spectroscopy of (CO2)N nanoparticles (30<N<14500) flowing in a uniform supersonic expansion

The infrared signature of carbon dioxide clusters of nanometric size is discussed both in the bending (ν2 mode at 15 μm) and in the asymmetric stretching (ν3 mode at 4.2 μm) spectral region of the monomer. The carbon dioxide nanoparticles were formed using a capillary tube injection inserted upstream of a uniform supersonic flow of argon generated by a Laval nozzle. The size of the formed clusters was varied by changing the stagnation pressure P0 of the capillary. The empirical power law connecting P0 to the number N of monomers per cluster: N∝P02.2 was verified in this work. The cluster mean size was estimated using a Rayleigh scattering experiment showing the formation of nanometric clusters whose radii are in the range 0.7 nm<r<5.3 nm, corresponding to 30<N<14 500. The thermodynamic and kinetic parameters of the flow were determined from the rovibrational absorption lines of the monomer and from a time-of-flight experiment. The measured flow velocity and flow temperature show that CO2 condensation is r...

High-resolution spectroscopy and analysis of the ν3/2ν4 dyad of CF4

CF4 is a strong greenhouse gas of both anthropogenic and natural origin [D.R. Worton et al., Environ. Sci. Technol. 41, 2184 (2007)]. However, high-resolution infrared spectroscopy of this molecule has received only a limited interest up to now. Until very recently, the public databases only contained cross-sections for this species, but no detailed line list. We reinvestigate here the strongly absorbing ν3 region around 7.8 μm. New Fourier transform infrared (FTIR) spectra up to a maximal resolution of 0.0025 cm−1 have been recorded: (i) room-temperature spectra in a static cell and (ii) a supersonic expansion jet spectrum at a 23 K estimated temperature. Following the work of Gabard et al. [Mol. Phys. 85, 735 (1995)], we perform a simultaneous analysis of both the ν3 and 2ν4 bands since a strong Coriolis interaction occurs between them, perturbing the ν3 R-branch rotational clusters around J = 20. Similarly to Gabard et al. , we also include ν4 FTIR data and microwave data in the fit. The analysis is performed thanks to the XTDS and SPVIEW programs [Ch. Wenger et al., J. Mol. Spectrosc., 251 102 (2008)]. Compared to Gabard et al. , the present work extends the analysis up to higher J values (56 instead of 32). Absorption intensities are estimated thanks to the dipole moment derivative from D. Papoušek et al. [J. Phys. Chem. 99, 15387 (1995)] and compare well with the experiment. We have produced a synthetic linelist that is included in the HITRAN 2008 and GEISA 2009 public databases. The rotational energy surfaces for the dyad are also examined in detail in order to understand the distribution and clustering patterns of rovibrational levels.

High-resolution spectroscopy and analysis of the nu3/2nu4 dyad of CF4

CF4 is a strong greenhouse gas of both anthropogenic and natural origin [D.R. Worton et al., Environ. Sci. Technol. 41, 2184 (2007)]. However, high-resolution infrared spectroscopy of this molecule has received only a limited interest up to now. Until very recently, the public databases only contained cross-sections for this species, but no detailed line list. We reinvestigate here the strongly absorbing ν3 region around 7.8 μm. New Fourier transform infrared (FTIR) spectra up to a maximal resolution of 0.0025 cm−1 have been recorded: (i) room-temperature spectra in a static cell and (ii) a supersonic expansion jet spectrum at a 23 K estimated temperature. Following the work of Gabard et al. [Mol. Phys. 85, 735 (1995)], we perform a simultaneous analysis of both the ν3 and 2ν4 bands since a strong Coriolis interaction occurs between them, perturbing the ν3 R-branch rotational clusters around J = 20. Similarly to Gabard et al. , we also include ν4 FTIR data and microwave data in the fit. The analysis is performed thanks to the XTDS and SPVIEW programs [Ch. Wenger et al., J. Mol. Spectrosc., 251 102 (2008)]. Compared to Gabard et al. , the present work extends the analysis up to higher J values (56 instead of 32). Absorption intensities are estimated thanks to the dipole moment derivative from D. Papoušek et al. [J. Phys. Chem. 99, 15387 (1995)] and compare well with the experiment. We have produced a synthetic linelist that is included in the HITRAN 2008 and GEISA 2009 public databases. The rotational energy surfaces for the dyad are also examined in detail in order to understand the distribution and clustering patterns of rovibrational levels.

Line-mixing effects in the ν3 parallel absorption band of CH3F perturbed by rare gases

Abstract We have measured the absorption profiles of CH3F mixed with argon and helium at room temperature in the interval 940–1110 cm−1 of the ν3 band. The spectra measured at pressures of 0.25–3 bar were used for evaluation of the line-broadening parameters. At higher pressures up to 20 bar significant deviations from the Lorentz bandshape were detected. At the maximum of the Q branch the absorption is super-Lorentzian, the differences reaching up to 60%. In the lower frequency wing of the P branch the observed absorption is up to five times lower than the Lorentzian one. The discrepancies observed are sensitive to the perturbing molecule, being more pronounced in the case of helium. They are explained by line-mixing effects and interpreted by means of a simple model with a single adjustable parameter.

Anharmonic treatment of vibrational resonance polyads—the diborane: a critical case for numerical methods

Based on accurate computational results, the IR spectra of diborane B2H6 and its deuterated derivative B2D6 were experimentally revisited to reconsider or complete their band assignments. A pure, variational approach, developed in both mechanical and electrical anharmonicities, was applied to study the diborane molecule for which many uncertainties remain in the spectral IR assignment. This work, together with all the experiments on this system over recent decades, shows the difficulty of interpreting the spectral data, making it a “benchmark” ideal for testing the mathematical approaches for the implementation of vibrational codes.

Gas-Phase Infrared Spectroscopy of Substituted Cyanobutadiynes: Roles of the Bromine Atom and Methyl Group as Substituents.

The IR spectra of 5-bromo-2,4-pentadiynenitrile (Br-C≡C-C≡C-CN) and 2,4-hexadiynenitrile (CH3 -C≡C-C≡C-CN), a compound of interstellar interest, have been recorded within the 4000-500 cm(-1) spectral region and calculated by means of high-level ab initio and density functional calculations. Although the calculated structures of both compounds are rather similar, there are very subtle differences, mainly in the strength of the C≡C bond not directly bound to the substituent. These subtle bonding differences are reflected in small, but not negligible, differences in the electron density at the corresponding bond critical points, and, more importantly, are reflected in the IR spectra. Indeed, the IR spectrum for the bromine derivative presents two well-differentiated strong bands around 2250 cm(-1) , whereas for the methyl derivative both absorptions coalesce in a single band. These bands correspond in both cases to the coupling between C≡C and C≡N stretching displacements. A third, very weak, band also associated with C≡C and C≡N coupled stretches is observed for the bromine derivative, but not for the methyl one, owing to its extremely low intensity.

Infrared spectra of a species of potential prebiotic and astrochemical interest: cyanoethenethiol (NC-CH=CH-SH).

Cyanoethenethiol (NC-CH=CH-SH) was obtained in a 8:1 Z:E ratio by flash vacuum thermolysis of the t-butylsulfide derivative. Density functional theory (DFT) and G3 ab initio calculations predict the existence of Z-and E-isomers, each of which exhibits two rotamers as a function of the relative position of the SH group. All these rotameric forms are planar (C(s) symmetry) and correspond to synperiplanar and antiperiplanar conformations. Calculations indicate that the synperiplanar Z-isomer is the more stable. In pure form, the cyanoethenethiol rapidly decomposes at room temperature, even at low pressure and partially condenses on the wall of the cell. To record its spectrum, a long optical path of 136 m was necessary, and several successive fillings of the cell were required. On the basis of the calculated harmonic and anharmonic vibrational frequencies, a complete and unambiguous assignment of the experimental spectrum has been carried out.

Gas-Phase Infrared Spectra of Vinyl Selenol and Vinyl Tellurol

The infrared spectra (3500-500 cm(-1)) of gaseous vinyl selenol and vinyl tellurol have been recorded at 0.1 cm(-1) resolution. For the latter the spectra were obtained at room temperature, but for the former a temperature of -40 degrees C was required because of the chemical instability of vinyl selenol at room temperature. To compensate the very weak vapor pressure of vinyl tellurol at room temperature, a long optical path up to 136 m was necessary to record its spectrum. B3LYP density functional theory (DFT) calculations have been performed to assign the different absorption bands. Since an unambiguous assignment of the absorption bands requires a precise knowledge on the relative abundance of the syn and gauche rotamers of these compounds, their relative energies and their anharmonic vibrational frequencies were obtained using a very extended Def2-QZVP basis set. Two rotamers, the syn, which is planar, and a nonplanar gauche, were found to be local minima for both compounds. The gauche rotamer presents two degenerate conformers, which differ by the position of the SeH (TeH) hydrogen atom above or below the molecular plane. Our theoretical results are in good agreement with the main features of the experimental spectra. Fundamental bands and some combination bands of vinyl selenol and vinyl tellurol were assigned and compared with those of vinyl alcohol and vinyl thiol, whose spectra had been reported previously in the literature.

Simple modeling of line‐mixing effects in IR bands. II. Nonlinear molecules applications to O3 and CHClF2

A simple semiempirical approach is developed in order to model the shape of infrared absorption bands. It is based on use of the strong collision model and of a classical representation of rotational levels. The absorption coefficient then has a simple analytical expression whose wavenumber and pressure dependencies are computed by using eleven parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities, which are band‐averaged values of the detailed spectroscopic and collisional parameters of the molecular system, can be deduced from direct fits of measured spectra. The model thus requires no previous knowledge of the characteristics of the molecules and is thus applicable to complex systems; in particular it seems a promising approach for very dense molecular spectra for which only absorption cross sections are now available. Tests are presented in the case of O3 and CHClF2 bands perturbed by N2 at room temperature for which n...

Infrared Spectra of Cyanoacetaldehyde (NCCH2CHO): A Potential Prebiotic Compound of Astrochemical Interest

Cyanoacetaldehyde (NC-CH2CH=O) and its isomer, cyanovinylalcohol (NC-CH=CH-OH), as possible components of the interstellar medium, comets, or planetary atmospheres, exist in equilibrium in the gas phase, although the latter compound is very much in the minority (2%). The recording and analysis of the gas-phase infrared spectrum of the former compound within the 4000-500 cm(-1) spectroscopic range and the potential presence of the latter isomer, which could be vital for their detection in these media, are reported. CCSD(T) and G4 high-level ab initio methods, as well as density functional theory calculations, predict the existence of two stable rotamers of cyanoacetaldehyde. The global minimum has a structure with an unusual O-C-C-C dihedral angle (150°) that falls between the antiperiplanar (180°) and anticlinal forms (120°). The second rotamer, which is about 4.0 kJ mol(-1) less stable in terms of free energy, has a planar structure that corresponds to the synperiplanar form (O-C-C-C dihedral angle: 0°). The absorption vibrational bands of the two aldehyde rotamers that are present in the mixture lead to a spectrum with a very complex structure in the region of deformation movements, in which several low-intensity bands overlap. A complete and unambiguous assignment of the experimental spectrum has been achieved by using the calculated harmonic and anharmonic vibrational frequencies.