Rototranslational Spectra Research Articles

Effect of stable and metastable dimers on collision-induced rototranslational spectra: Carbon dioxide – rare gas mixtures

The role of stable and metastable dimers as well as of free collisions in the collision-induced rototranslational absorption by the compressed CO2–Ar and CO2–Xe gas mixtures is elucidated using the classical three-dimensional trajectories method. The contribution from the stable dimers is obtained via Fourier transform of the dipole correlation function. The spectral bandshape due to the unbound trajectories (metastable dimers and free collisions) is calculated as an averaged Fourier spectrum of the collision-induced dipole moment. The mean lifetimes of metastable dimers have been estimated as 3.8 ps for CO2–Ar and 5.9 ps for CO2–Xe pairs. Trajectory computations are complemented by calculations of zero spectral moments using pair distribution functions. The stable and metastable dimer contribution to the zero spectral moment is shown to be comparable with that from free collisions.

Line mixing in Raman scattering spectra of CO2 modelled by a non-Markovian Energy-Corrected Sudden approach

Non-Markovian Energy-Corrected Sudden modelling of room-temperature CO2 Raman scattering spectra up to the far wing is revisited and completed for gas densities between 1 and 45 amagat. The model parameters are fitted on an extended set of low-pressure isolated line widths including very high values of the rotational quantum number J and well reproduce the decreasing of line broadening for the experimentally studied J-values. From calculations of the absolute-intensity rototranslational spectrum at 23 amagat, it is shown that the traditional adiabaticity factor of De Pristo gives a more coherent description of low-pressure line widths and high-pressure wing intensities than the two-parameter factor of Birnbaum–Cohen. A simple qualitative correction of the ECS-modelled intensities for the induced and cross allowed-induced scattering appears to be insufficient to match the experimental data in the spectral wing. Theoretical spectral shapes are also calculated for the O-Q-S branches of vibrotational ν1 and 2ν2 Raman bands and compared to experimental results in the density range 1–45 amagat.

Molecular dynamics simulations for CO2 spectra. II. The far infrared collision-induced absorption band

Classical molecular dynamics simulations have been carried out for gaseous CO(2) starting from various anisotropic intermolecular potential energy surfaces. Through calculations for a large number of molecules treated as rigid rotors, the time evolution of the interaction-induced electric dipole vector is obtained and the Laplace transform of its autocorrelation function gives the collision-induced absorption rototranslational spectrum. The results are successfully compared with those of previous similar calculations before studies of the influences of the intermolecular potential and induced-dipole components are made. The calculated spectra show a significant sensitivity to anisotropic forces consistently with previous analyses limited to the spectral moments. The present results also demonstrate the importance of vibrational and back-induction contributions to the induced dipole. Comparisons between measured far infrared (0-250 cm(-1)) spectra at different temperatures and results calculated without the use of any adjustable parameter are made. When the best and more complete input data are used, the quality of our predictions is similar to that obtained by Gruszka et al. [Mol. Phys. 93, 1007 (1998)] after the introduction of ad hoc short-range overlap contributions. Our results thus largely obviate the need for such contributions the magnitudes of which remain questioned. Nevertheless, problems remain since, whereas good agreements with measurements are obtained above 50 cm(-1), the calculations significantly underestimate the absorption below, a problem which is discussed in terms of various possible error sources.

Collision-induced absorption by H2-He pairs in the fundamental band: Rotational states dependence.

Previously, we have computed the induced-dipole-moment surface of collisional ${\mathrm{H}}_{2}$-He complexes from first principles using highly correlated wave functions. These data were used to calculate the collision-induced absorption spectrum of the fundamental band [L. Frommhold and W. Meyer, Phys. Rev. A 35, 632 (1987)]. The present study examines the dependence of radial induced-dipole transition moments and the associated spectra on the rotational states involved, a dependence that was suppressed in the previous study. Whereas the neglect of the rotational quantum numbers j,j' of the initial and final state of ${\mathrm{H}}_{2}$ is justified for the purely rototranslational spectra of ${\mathrm{H}}_{2}$-He, we find significant variations with j,j' of the radial induced-dipole transition elements and the induced spectra for the fundamental band of ${\mathrm{H}}_{2}$. Similar effects are expected for other molecular bands and systems of interest, especially those involving hydrogen molecules.

Collision-induced absorption for H 2-H 2 and H 2-He interactions at 5 μm

Absorption coefficients for collision-induced spectra of H 2 have been measured at 299 K between 1830 and 2260 cm -1. The weak absorption of H 2 in this spectral range has been detected by using a 3-m long cell with pressures ranging from 80 to 215 bar. The rototranslational spectrum of H 2 has been computed in the high-frequency wing from an extended Birnbaum-Cohen model that fits the results of quantum calculations accurately. The theoretical results for the binary absorption coefficient are generally 3–13% larger than the experimental values. The very weak absorption coefficients for H 2-He interactions have also been estimated between 1830 and 2200 cm -1 by using pressures of the gas mixtures up to 346 bar. A calculation based on two simple line-shape models, the parameters of which were recently obtained from spectral moments computed with a quantum formalism, yields binary coefficients for H 2-He that are in satisfactory agreement with our measurements.

Measurements and analysis of rototranslational absorption spectra of low density H2–Ar mixtures

We present new measurements of the rototranslational absorption of H2–Ar mixtures at 195 and 298 K. Measurements were made at densities as low as possible. The first accurate (5% error) rototranslational spectrum of the H2–Ar pair was thus derived. We performed a line-shape analysis of these spectra; as a result, we found a low-frequency component associated with an isotropic overlap induced dipole. We also found an important anisotropic overlap contribution associated with the dominant quadrupolar induction. Through a spectral-moment analysis we determined the intensity of isotropic (λ0 = 2.35 ± 0.15 × 10−3ea0) and anisotropic (λ1 = 0.9 ± 0.1 × 10−3ea0) overlap induced dipoles. The range of the isotropic overlap induced dipole was found to be ρ0 = (0.105 ± 0.010)σ, in very good agreement with the value 0.11 σ frequently assumed in the literature for the range of overlap induced dipoles. Our measurements and the results of our analysis were in good agreement with results of recent computations.

Concerning the rototranslational absorption spectra of He[sbnd]CH4 pairs

Existing measurements of the collision-induced rototranslational absorption spectrum of gaseous mixtures of helium and methane are analyzed using quantum profiles computed from recent interaction potentials and various assumptions concerning the interaction-induced dipole components. The computed profiles are expressed in terms of simple functions that reproduce closely both the computed spectra at temperatures from 40 to 350 K and the existing measurements at temperatures from 150 to 353 K. Such a simple analytical representation of the HeCH4 rototranslational spectra facilitates accurate modeling of planetary atmospheres in the far-infrared region of the spectrum.

Raman spectra of water in the translational and librational region

Polarized and depolarized Raman spectra in the translational and librational region of liquid water and heavy water have been measured as a function of temperature. A detailed analysis of the isotropic spectrum is performed. This component, which cannot arise from any ‘allowed’ molecular contribution in this frequency region, shows two distinct spectral features which can be obtained from an analysis of temperature dependent lineshapes. The first one, which dominates the spectrum up to 500 cm-1, shows on the basis of a H2O-D2O comparison a translational behaviour at very low frequency and a librational one at higher frequencies. It is assigned on the basis of its frequency profile, of its absolute intensity and of its temperature dependence to rototranslational spectra due to long range induction mechanisms already found in many molecular liquids. The second one, which is peaked at roughly 800 cm-1 in water and extends up to the bending mode region suggests the existence of a short range interaction induced polarizability.

Dipoles induced by long-range interactions between centrosymmetric linear molecules: Theory and numerical results for H2⋅⋅⋅H2, H2⋅⋅⋅N2, and N2⋅⋅⋅N2

We have derived symmetry-adapted expressions for the dipole moments of pairs of D∞h molecules interacting at long range, in a form useful for line shape analyses of collision-induced rototranslational spectra. Our results are complete to order R−7 in the intermolecular separation R. In addition to quadrupolar and hexadecapolar induction effects, results to this order include induction due to nonuniformities in the local field acting on a molecule (E-tensor induction), back induction, and polarization due to dispersion forces. The dispersion terms are computed within our recently developed reaction field model, from which we have obtained accurate integral expressions for the dipole coefficients, and approximations in terms of static susceptibilities and dispersion energy coefficients. For H2⋅⋅⋅H2, H2⋅⋅⋅N2, and N2⋅⋅⋅N2, numerical results for the dipole coefficients are tabulated. While quadrupolar induction dominates the long-range dipole, other induction effects are evident in the far-infrared collision-induced spectra. Over the range of validity of the model, E-tensor induction, back induction, and dispersion effects are generally smaller than hexadecapolar induction, but appreciable. The magnitudes of the dipole coefficients and the orientation dependence of each polarization mechanism determine its contributions to the observed collision-induced absorption spectra.

Dipoles induced by van der Waals interactions during collisions of atoms with heteroatoms or with centrosymmetric linear molecules

Using a reaction field model, we have derived symmetry-adapted expressions for the van der Waals dipoles of atom–heteroatom and atom-D∞h molecule pairs, with nonoverlapping charge distributions.The leading van der Waals contributions vary as R−7 in the intermolecular separation R and depend upon products of the polarizability ααβ(iω) of one molecule with a dipole–quadrupole hyperpolarizability Bαβ,γδ (0,iω) of the other, integrated over imaginary frequencies. We have developed new approximations for these integrals in terms of the static polarizabilities ααβ, the hyperpolarizabilities Bαβ,γδ, and the van der Waals energy coefficients C6 and C8 (both isotropic and anisotropic components for atom–molecule pairs). The approximations agree well with accurate perturbation results for two model systems. Applied to He⋅⋅⋅H2, our approximations give the first direct results for the leading van der Waals contributions to the dipole. In two symmetry components of the He⋅⋅⋅H2 dipole at long range, van der Waals effects are larger than induction effects; both should be included in fitting collision-induced roto-translational spectra.

Effects of the anisotropic interaction on collision-induced rototranslational spectra of H2-He pairs.

The anisotropy of the interaction potential of the ${\mathrm{H}}_{2}$-He collisional complex is treated as a small perturbation in a computation of the zeroth and first spectral moment of the collision-induced rototranslational spectra in the far-infrared region. The moments of the rototranslational spectra are separated into sums of moments of rotational lines, characterized by rotational transitions j\ensuremath{\rightarrow}j' with j\ensuremath{\ne}j', and translational components (j=j', for j=0,1,. . .). The quantum expressions thus obtained consist of a leading term computable from the isotropic component of the interaction potential and dipole operator, plus an anisotropic ``correction term'' accounting for the lowest-order (${P}_{2}$) anisotropy. These are evaluated for temperatures between 77 and 195 K. The results provide a much needed basis for assessing the validity of line-shape computations based on the isotropic potential, distorted-wave, or infinite-order sudden approximations often employed to avoid the enormous cost and complexity of close-coupling computations. The spectroscopic effects of the anisotropy of the ${\mathrm{H}}_{2}$-He system are briefly discussed.

Collision-induced rototranslational spectra of H2-He: Accounting for the anisotropic interaction.

In a previous paper [G. Birnbaum et al., Phys. Rev. A 29, 595 (1984)] the collision-induced rototranslational absorption spectra of ${\mathrm{H}}_{2}$-He were computed from first principles, with the assumption of isotropic interaction. Here, we study the effects of the anisotropy on the basis of a distorted-wave approximation. Computed spectral intensities and shapes of the rotational lines differ from the previous results by less than the numerical uncertainties of the computations (\ensuremath{\sim}1%), except in the far wings where intensities are increasingly greater than comparable results based on isotropic interaction.

Density behaviour of rototranslational spectrum of gaseous N2

(1979). Density behaviour of rototranslational spectrum of gaseous N2. Molecular Physics: Vol. 38, No. 6, pp. 2111-2115.