Collision-induced Dipole Moment Research Articles

Intracollisional interference in the pure rotational spectrum of HD: Determination of the permanent electric dipole moment

The intensity of the pure rotational absorption spectrum of gaseous HD is measured as a function of density for pure HD and HD-He and HD-Ar mixtures. The decrease in the absorption coefficient with increasing density is consistent with a destructive intracollisional interference between the transitions involving the permanent and collision-induced dipole moments. The permanent dipole moment of the molecule in its ground vibrational state is found to be $J$ dependent and in the range (8-10)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ D.

Theoretical study of collision-induced far-infrared absorption of dense rare-gas mixtures

A theory investigating collision-induced absorption in dense rare-gas mixtures is presented. The basic assumption of the theory is that the collision-induced dipole moment of a pair of dissimilar atoms is proportional to the force acting between them. It is then shown that the relevant dynamical variable is the interdiffusion current fluctuation, $\stackrel{\ensuremath{\rightarrow}}{\mathrm{J}}=(1\ensuremath{-}x)\ensuremath{\Sigma}{i}^{{N}_{A}}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{i}\ensuremath{-}x\ensuremath{\Sigma}{{i}^{\ensuremath{'}}}^{{N}_{B}}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{{i}^{\ensuremath{'}}}$, in the fluid mixture. The correlation function and absorption spectrum are generated by the Zwanzig-Mori theory of Brownian motion. Two modes are theoretically predicted. A diffusive mode, due to the mutual diffusion of rare-gas atoms, produces a low-frequency dip in the line shape whereas an oscillatory mode, due to the oscillations of atoms in the local structure of the fluid, generates a caging spike in the wing of the spectrum. Spectral behavior is shown to depend on density, concentration, and atomic masses of the two components of the mixture and is discussed in detail.

Analysis of the translational band measured in Ne–Ar mixtures with a new line-shape profile

The translational absorption spectra of Ne–Ar liquid mixtures, arising from the collision induced dipole moment, have been measured at different temperatures along the coexistence curve. The experimental line-shapes have been fitted with a new general profile suitable to reproduce the intercollisional interference effect. In this way we have derived the frequency ω0 that characterizes the rattling component of the motion of the impurity in the liquid. The self diffusion coefficient of Ne in Ar has also been derived from experimental values of ω0 using the Rice–Allnatt theory.

Lineshapes and dipole moments in collision-induced absorption

Wave mechanical lineshapes of collision-induced absorption spectra are computed for binary mixtures of argon with helium, neon, and krypton using theoretical dipole moments as input. Comparison with measured spectra shows satisfactory agreement except for the neon–argon mixture, for which either theory or measurement is seen to be in substantial error. Empirical models of the collision-induced dipole moment which reproduce the experimental spectra more closely than the fundamental theory are also given. Best agreement between computed and experimental lineshapes is obtained when potential models which are accurate in the repulsive region are used.

The collision-induced translational spectrum of CF4–He gas mixtures

The collision-induced far infrared spectrum of CF4–He gas mixtures was measured at 213 K. It is shown that the observed enhancement of the absorption represents the translational spectrum induced by isotropic overlap forces. The band shape calculations are consistent with the assumption that the collision-induced dipole moment varies exponentially with intermolecular separation, and the parameters of the dipole moment function have been determined.

On nuclear spin statistics in rotational transition intensities in tetrahedral AB4 molecules

A general expression is derived for the integrated intensity of rotational transitions in the vibronic ground state of tetrahedral molecules, taking into account the nuclear spin statistics. It is shown that the ratio of this expression to previously published spin-free integrated intensities depends only on the tensor character N of the operator driving the transition, the appropriate rotational quantum numbers J and J′, and the nuclear spin IB of the identical nuclei. Tables are given for N = 3, 4 and J ≤ 50 which enable the calculation of integrated intensities for octopole and hexadecapole collision induced dipole moment transitions, centrifugal distortion induced dipole moment transitions, and centrifugal distortion induced anisotropic polarizability tensor Raman transitions. It is shown that the relative error of the spin-free integrated intensity calculation is proportional to (2IB + 1)−2.

Collision-induced atomic dipole moments

A multipole expansion procedure is given for obtaining collision-induced atomic dipole moments from the molecular moments of a pair of atoms in a collision governed by the Pauli exclusion principle. Atomic moments are reported for He, Ne, Ar, and H, as determined from homonuclear diatomic wave functions. These are combined, via the distortion model of Smith [Phys. Rev. A 5, 1708 (1972)], to obtain molecular dipole moments for the heteronuclear collision pairs. The molecular dipole moments are in fair agreement with results of molecular orbital calculations. Limitations and extensions of the procedure are discussed.

The influence of intermolecular interactions on the Kerr effect in gases

Expressions are derived for the second and third Kerr virial coefficients, B K and C K, of spherical top molecules in terms of irreducible cluster polarizabilities, and values are calculated using the dipole-induced dipole model for argon, krypton, xenon, methane, tetrafluoromethane, neopentane and sulphur hexafluoride. For mixtures of rare gases it is shown that the collision-induced dipole moment makes a negligible contribution to B K. The effect of the choice of intermolecular potential function on the calculated second Kerr virial coefficients is also demonstrated. It is found that the predominant contributions to C K arise from the pair polarizability, and that the triplet polarizability is only of minor importance.

The influence of intermolecular interactions on the Kerr effect in gases

Expressions are derived for the second and third Kerr virial coefficients, B K and C K, of spherical top molecules in terms of irreducible cluster polarizabilities, and values are calculated using the dipole-induced dipole model for argon, krypton, xenon, methane, tetrafluoromethane, neopentane and sulphur hexafluoride. For mixtures of rare gases it is shown that the collision-induced dipole moment makes a negligible contribution to B K. The effect of the choice of intermolecular potential function on the calculated second Kerr virial coefficients is also demonstrated. It is found that the predominant contributions to C K arise from the pair polarizability, and that the triplet polarizability is only of minor importance.

Analysis of translational band observed in gaseous and liquid Kr–Ar mixtures

The translational absorption spectrum of Kr–Ar gaseous and liquid mixtures, arising from the collision-induced dipole moment, has been measured in the 13–300 cm−1 frequency region. From the translational band obtained in the gaseous phase we have derived the intensity and the range ρo of the induced dipole moment. The high frequency part of the spectrum observed in the gaseous phase is similar in shape to that observed in the liquid phase. The low frequency part of the spectrum obtained in the latter case is dominated by the intercollisional interference effect and is discussed in terms of a simple model for the motion of the impurities in the liquid. We have also derived the value of the mean square force 〈F2〉 acting on the impurity dissolved in the liquid matrix.

Zusätzliche dielektrische Verluste starrer, polarer Moleküle in verdünnter Lösung bei Frequenzen oberhalb des Maximums der Debye-Absorption / Additional Dielectric Loss of Rigid Polar Molecules in Dilute Solutions at Frequencies in the Far Infrared [FIR

Abstract The dielectric loss of ten polar molecules in dilute solutions has been measured in the 0.3 GHz to 300 GHz range. Besides the Debye-absorption of the polar molecules, in all solvents, i.e. heptane, cyclohexane, mesitylene and decalin there could be separated an additional part of dielectric loss in the range of mm-waves. This absorption has been approximated by a Debye-curve to gain the step of dispersion and the time-constant. The dielectric loss of the unpolar solvents themselves, which is due to collision induced dipole moments, has also been investigated. The results are discussed and compared with the FIR-absorption of the polar molecules.

Memory Effects in the Collision-Induced Spectra of Noble Gases in Liquid Argon

We have measured the far infrared absorption spectra of dilute solutions of helium and neon in liquid argon arising from collision-induced electric dipole moments. The two spectra consist of broad bands similar in shape peaked at 70 cm−1 (Ne–A) and 120 cm−1 (He–A). They are characterized by a long high-frequency tail and a rapid fall at low frequency. The reduced line shapes are compared taking into account the difference in the duration of the collision-induced dipole moment. At high frequencies the two curves are similar to those observed in the gas phase. Their departure from the gaseous spectra at low frequencies is attributed to a negative correlation of the dipole moment induced in successive repulsive interactions.

Quantum Theory of Collision-Induced Absorption in Rare-Gas Mixtures

A theoretical calculation of the collision-induced far-infrared absorption, which has been observed in rare-gas mixtures, is carried out using the quantum-mechanical expression for the absorption coefficient. Using a simple empirical form to describe the variation of the collision-induced dipole moment with internuclear separation, and using plane-wave eigenstates (i.e., assuming straight-line collision paths), an analytic expression is obtained for the spectrum which agrees well with the experiments. Comparison with a previous classical calculation based upon the same model shows that quantum effects are small at room temperature except in the high-frequency wing of the spectrum. A number of analytic integrations allow us to derive simple expressions for the correlation function, relaxation time, static dielectric constant, and other spectral invariants.

Quantum Theory of Collision-Induced Absorption in Rare-Gas Mixtures

A theoretical calculation of the collision-induced far-infrared absorption, which has been observed in rare-gas mixtures, is carried out using the quantum-mechanical expression for the absorption coefficient. Using a simple empirical form to describe the variation of the collision-induced dipole moment with internuclear separation, and using plane-wave eigenstates (i.e., assuming straight-line collision paths), an analytic expression is obtained for the spectrum which agrees well with the experiments. Comparison with a previous classical calculation based upon the same model shows that quantum effects are small at room temperature except in the high-frequency wing of the spectrum. A number of analytic integrations allow us to derive simple expressions for the correlation function, relaxation time, static dielectric constant, and other spectral invariants.

Collision Induced Absorption in Carbon Dioxide in the Far Infra-red

The absorption spectrum of compressed carbon dioxide in the region 20-110 cm-1 has been measured for pressures up to 20 atmospheres in a 44 cm gas sample cell. The spectra show an absorption band attributed to collision induced dipole moments, centred at 50 cm-1.