Collision-induced Dipole Moment Research Articles

Molecular dynamics simulation of collision-induced absorption spectra of neon-krypton mixture thin layer confined between graphite walls

Molecular dynamics (MD) simulations have been used to calculate interaction-induced absorption spectra of neon-krypton mixture confined between graphite plates. The continuous phenomenological model with parameters fitted to couple clusters (CC) quantum mechanical method has been applied to calculate collision-induced dipole moments. The collision-induced dipole moment correlation functions and its 2, 3, 4-body contributions of Ne-Kr binary mixture at three temperatures have been calculated and analyzed. The dynamics of neon and krypton atoms between graphite slabs have been investigated by the mean square displacement functions and z-profile functions. The phase separation between neon and krypton has been observed at T=100K in the direction perpendicular to the graphite surface. We have found that collision-induced absorption spectra reflect the mobility of noble gas atoms in binary mixture between graphite slabs.

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.

Collision-Induced Dipole Moment and Millimeter and Submillimeter Continuum Absorption in Water Vapor

This work is devoted to estimation of the additional absorption of millimeter and submillimeter wavelengths in water vapor arising from collisional interaction of molecules due to the induced dipole moment. Absorption is modeled on the basis of ab initio data on the magnitude of the water molecule dipole moment at high densities, and common knowledge of the water vapor absorption spectrum. Using the model developed, we obtained a simple analytical expression for the absorption coefficient as a function of temperature, pressure, and frequency. Comparison of the results with known experimental data leads to the conclusion that in the range of pressures and temperatures typical of water vapor in the Earth’s atmosphere this type of absorption is negligible compared with the absorption arising due to association or dimerization of the water vapor molecules.

Asymptotic multipolar expansion of collision-induced properties

The collision-induced (CI) dipole moment, CI polarizability, and CI hyperpolarizability are considered for three H(2)-rare gas (Rg) pairs (Rg = He, Ne, Ar). In this study, the symmetry-adapted (SA) components, the projection of the CI dipole, polarizability, and hyperpolarizability on an appropriately tailored set of spherical harmonics are calculated. A set of equations for the respective SA components is derived. The Cartesian components of the CI properties calculated by quantum chemistry methods for three intermolecular geometries are used in our calculations as input data. The analytical, multipolar long-range behavior of the CI properties studied is considered within a multipole-induced multipole model. Taking the SA components at large distances, the ab initio SA numerical results and the model semianalytical data were compared. In general, a good agreement has been found. The results of our study are expected to be of value in spectral line shape analysis and in modeling of processes in the Earth's and planetary atmospheres.

Collision-induced hyperpolarizability and hyper-Rayleigh spectra in the He–Ar heterodiatom

The ab initio interatomic distance dependent collision induced dipole moment, polarizability and the vector b 1( R) and an irreducible 3rd rank tensor b 3( R) components of the first hyperpolarizability tensor b of He–Ar pair are obtained. The hyperpolarizability data are applied in order to calculate the collision induced He–Ar hyper-Rayleigh spectra both quantum-mechanically and semiclassically for the frequency shifts up to 1200 cm −1. Computations were carried out for two temperature values – T = 295 K and T = 95 K. Spectral features of both vector and the irreducible 3rd rank tensor parts of CIHR spectra are analyzed. The frequency dependent behavior of the HR depolarization ratio is discussed.

High-temperature (1000–7000 K) collision-induced absorption of H 2 pairs computed from the first principles, with application to cool and dense stellar atmospheres

The collision-induced absorption (CIA) spectra of H 2–H 2 and H 2–He are known to play an important role for modelling of low-metallicity cool and dense stellar atmospheres. In this paper we present collision-induced absorption spectra of H 2–H 2 complexes in the rototranslational (Δ v=0), the fundamental (Δ v=1), the first (Δ v=2) and the second (Δ v=3) overtone bands in the temperature range from 1000 to 7000 K, and in the frequency region from 0 to 20 000 cm −1. The translational spectral density functions are computed quantum mechanically, based on: (1) the newly developed ab initio collision-induced H 2–H 2 dipole functions of Zheng (Computational study of collision induced dipole moments and absorption spectra of H 2–H 2. Ph.D. thesis, Michigan Technological University, 1997), which account for the short-range H 2–H 2 intermolecular distances (as small as 2.5 a.u.) and for larger H 2 internuclear distances (as large as 2.15 a.u.); (2) semiempirical isotropic H 2–H 2 potential (Ross et al, J Chem Phys 1983;79(3):1487) suitable for high temperatures. We include the collision-induced absorption coefficient of the vibrational transitions as v 1, v 2, v′ 1, v′ 2≤3 which we computed rigorously. We also give our estimate for the collision-induced absorption coefficients of single vibrational transitions such as v i<3, v′ i>3 in the first and second overtone bands. The dependence of CIA spectra on rotational states of H 2 molecules is accounted for in our computations. We have previously (Borysow et al, Astronom Astrophys. 1997;324:185–95) studied the effect of CIA for stars of a wide range of fundamental stellar parameters (effective temperature, gravity, and chemical composition), and determined for which combinations of these parameters it is necessary to include CIA in the model and spectrum computation. These calculations showed that CIA from H 2–H 2 plays an important, and often even a dominating role for stellar atmospheres of a wide range of stars. The approximate character of the estimates of the H 2–H 2 absorption coefficient we used in our previous work combined with the large effect CIA had on the stellar atmospheres, were the main inspirations to initiate the more accurate computations of the absorption coefficient we present here. The absorption coefficient we compute in the present analysis is in qualitative agreement with our preliminary estimates (Borysow et al, Astronom Astrophys 1997;324:185–95), but in some spectral regions of high importance for the stellar structure, our computed absorption coefficient is up to a factor of 3 larger than our preliminary estimates. We therefore fully confirm our previous suspicion that H 2–H 2 CIA will have a pronounced effect on the atmosphere for a wide range of stars. In this paper we therefore quantify the effect the new data have on a typical cool dense stellar atmosphere, and compare our new results with our previous estimates.

Translational Band of Gaseous Hydrogen at Low Temperature

A comparison is made between the observed submillimeter absorption spectrum of normal hydrogen gas at three different temperatures near 25 K and a fully quantum mechanical ab initio calculated spectrum. Measurements cover the wavenumber range 20-320 cm-1, where, under the experimental conditions of low temperature and low pressure, the pure translational band is observed virtually isolated from the S(0) rotational line. The theoretical analysis shows that the H2 spectrum is primarily due to transient collision-induced dipole moments in pairs of colliding molecules, i.e., due to free-free transitions. Bound-free transitions of the H2 dimer are responsible for an additional ~10% of the absorption intensity. The agreement between the theoretical spectra and the measurements is excellent, thereby verifying that the intermolecular potential, the wave functions, and the model of the induced dipole moment have all been accurately represented. As a consequence, it is possible to predict with confidence the absorption spectrum of H2 in physical conditions not readily accessible to laboratory measurements.

Collision-induced b1Σg+–a1 Δg, b1Σg+–X3 Σg- and a1Δg–X3Σg - transition probabilities in molecular oxygen

Multireference configuration interaction (MRCI) calculations have been performed for the dipole transition moments of the Noxon band, b 1 Σ g + –a 1 Δ g , and for the red and IR atmospheric emission bands, b 1 Σ g + –X 3 Σ g - and a 1 Δ g –X 3 Σ g - , in O 2 in collision complexes with different solvent molecules. The spin–orbit coupling between the b 1 Σ g + and X 3 Σ g - (M S =0) states does not change after collision so the a–X transition borrows intensity from the collision-induced Noxon band b–a, and the b–X band borrows electric dipole transition probability from the collision-induced difference dipole moments of the b and X states. The calculations show that the b–a, a–X and b–X transition probabilities are enhanced by ca. 10 5 , 10 3 and 1.2 times by O 2 +H 2 collisions. An additional order of magnitude enhancement for all three transitions is possible for solvent molecules with larger polarizability than that of molecular hydrogen. The collision-induced b–a transition dipole moment depends not only on the intermolecular distance, but also on the internuclear O–O distance. Since a–X and b–a transition moments are directly connected (D a,X =0.013iD b,a ), the strong dependence on the O 2 bond length leads to an additional enhancement of the (0,1) vibronic band intensity of the a 1 Δ g –X 3 Σ g - transition. The dipole moments of the b and X states are also shown to depend on the O 2 bond length in the collision complex, which leads to additional collision-induced enhancement of the (0,1) band of the b–X system.

Symmetry-adapted perturbation theory applied to interaction-induced properties of collisional complexes

A symmetry-adapted perturbation theory (SAPT) is formulated for interaction-induced electrical properties of weakly bound complexes. Asymptotic (large R) expressions are reported for the contributions to the collision-induced dipole moments and polarizabilities up to and including second order in the intermolecular potential. These long-range expressions require knowledge only of the multipole moments and (hyper)polarizabilities of the isolated monomers. Numerical results are given for the dipole moment of He-H2 and the polarizability of He2 and the accuracy of the SAPT approach is examined by comparison with full configuration interaction results. The role of various physical contributions to the dipole moment of He-H2 and the polarizability of He2 is investigated. The validity of the long-range approximation and the importance of charge penetration (damping) effects are discussed.

Molecular dynamics study of liquid carbon disulfide and benzene: Effect of pressure on the far-infrared collision-induced absorption

We have performed molecular dynamics calculations of liquid carbon disulfide and liquid benzene, intending to examine the effect of pressure on the far-infrared collision-induced absorption as well as on the local structure and on the molecular reorientational dynamics. The calculations reproduced well the experimental results of the reorientational correlation times of liquid carbon disulfide at pressures, of the reorientational anisotropy of liquid benzene, and of the density dependence of the collision-induced dipole moments and their time derivatives of both liquids. Contributions of various interaction terms to the collision-induced dipole moments were examined as a function of density. The experimental results that the collision-induced dipole moments of liquid benzene decreases and those of liquid carbon disulfide changes little with increase in density are explained well by pressure-dependent change in degree of the cancellation between the positive two-molecule interaction term and the negative three-molecule interaction terms.

High-pressure study of the far-infrared collision-induced absorption of nondipolar liquids

Far-infrared collision-induced absorption of liquid benzene, hexafluorobenzene, and 1,3,5-trifluoro- benzene were measured at pressures and room temperature with a synchrotron radiation source. The zeroth and second spectral moments of the absorption bands were obtained as a function of pressure. The collision-induced dipole moments and their time derivatives were obtained from the spectral moments for these liquids and previously measured carbon disulfide and carbon tetrachloride. The induced-dipole moments decrease with increasing pressure for the liquids studied except carbon disulfide for which the moment is nearly independent of pressure. This decrease in the moments is probably caused by increase in the cancellation effect of three-molecule correlation with an increase in pressure. The time derivatives of the collision-induced moments, on the other hand, increase with increasing pressure. This is explained by increase in collision frequencies with pressure.

Coherent excitation of H(n=2) induced in H+-Ne and H+-Ar collisions

The collision-induced excitation of atomic hydrogen to the H(n=2) states in H+-Ne and H+-Ar collisions was measured in an energy range of 1 to 25 keV. From a polarization analysis of the emitted Lyman- alpha radiation as a function of an external electric field the partial cross sections sigma lm, for excitation to the H(2s) and the H(2pm) magnetic substates (m=0,+or-1) and the real part of the s0-p0 coherence were extracted. For both collision systems an energy dependent forward-backward asymmetry corresponding to a shift of the centre-of-charge relative to the centre-of-mass (electric dipole moment) was observed. The collision-induced electric dipole moment (Dz) is generally small or negative at incident energies below 10 keV and rises steeply towards higher energies. The present results are in reasonable agreement with close-coupling calculations employing a molecular representation.

The collision-induced electric dipole moment of H(n=2) in H-He, Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.

Long-range and overlap effects on collision-induced properties

Collision-induced dipole moments and dipole polarizabilities in ion-atom pairs are considered within the framework of the theory of intermolecular interactions. Contributions to collision-induced properties are classified according to their origin in the interaction energy formula. In the long-range electrostatic limit the expansions for dipole-induced-dipole and multipole-induced-multipole models are recovered. The role of different contributions to induced electric properties is investigated numerically for F- · · ·He and Cl- · · ·He pairs at the Hartree-Fock self-consistent field and fourth-order many-body perturbation theory correlated levels. Compression of the anion arising from exchange-overlap interaction terms is found to be important in the induced dipole moment, and indispensable for description of the parallel component of dipole polarizability. Overlap effects are, however, strongly anisotropic. As a result, even the simplest electrostatic model is capable of describing the perpendicular comp...

THz time-domain spectroscopy of nonpolar liquids

A newly developed terahertz time-domain spectrometer based on photoconductive dipole antennas driven by femtosecond laser pulses was used to measure the absorption and index of refraction for benzene, carbon tetrachloride, and cyclohexane in the far infrared. The spectra cover the region of 3-66 cm/sup -1/, where the dielectric response of the nonpolar liquids is dominated by collision induced dipole moments. With terahertz time-domain spectroscopy, information on both the real and imaginary part of the frequency response is obtained in a range difficult to access by standard techniques.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Erratum: Collision-induced rotovibrational spectra of H2-He pairs from first principles

A previous study of the collision-induced dipole moment has treated the ${\mathrm{H}}_{2}$-He complex as a molecule in self-consistent-field and size-consistent coupled-electron-pair approximation [Meyer and Frommhold, Phys. Rev. A 34, 2771 (1986)]. Based on that work, the vibrational dipole transition elements 〈v\ensuremath{\Vert}${A}_{\ensuremath{\lambda}L}$(R,r)\ensuremath{\Vert}v'〉 associated with the fundamental band (v=0\ensuremath{\rightarrow}v'=1) are obtained as functions of separation R of the collisional pair for the isotropic and anisotropic overlap induction components (\ensuremath{\lambda}L=01 and 21), and the quadrupole- and hexadecapole-induced parts (\ensuremath{\lambda}L=23 and 45). From these induced dipole components and Meyer, Hariharan, and Kutzelnigg's isotropic part of the ab initio potential surface, we compute in the (forbidden) fundamental band of hydrogen the collision-induced absorption spectra of the collisional complex of hydrogen (${\mathrm{H}}_{2}$) and helium from an exact quantum formalism. Both the shape of the computed spectral profiles and the theoretical absolute intensity agree closely with existing measurements at temperatures from 18 to 300 K. The fact that these spectra, and presumably the analogous overtone and ``hot'' (v&gt;0) bands of the ${\mathrm{H}}_{2}$-He complex which are not known from measurements, can be accurately obtained from basic principles is significant for research related to the atmospheres of the giant planets and late-type stars.

Collision-induced rotovibrational spectra of H2-He pairs from first principles.

A previous study of the collision-induced dipole moment has treated the ${\mathrm{H}}_{2}$-He complex as a molecule in self-consistent-field and size-consistent coupled-electron-pair approximation [Meyer and Frommhold, Phys. Rev. A 34, 2771 (1986)]. Based on that work, the vibrational dipole transition elements 〈v\ensuremath{\Vert}${A}_{\ensuremath{\lambda}L}$(R,r)\ensuremath{\Vert}v'〉 associated with the fundamental band (v=0\ensuremath{\rightarrow}v'=1) are obtained as functions of separation R of the collisional pair for the isotropic and anisotropic overlap induction components (\ensuremath{\lambda}L=01 and 21), and the quadrupole- and hexadecapole-induced parts (\ensuremath{\lambda}L=23 and 45). From these induced dipole components and Meyer, Hariharan, and Kutzelnigg's isotropic part of the ab initio potential surface, we compute in the (forbidden) fundamental band of hydrogen the collision-induced absorption spectra of the collisional complex of hydrogen (${\mathrm{H}}_{2}$) and helium from an exact quantum formalism. Both the shape of the computed spectral profiles and the theoretical absolute intensity agree closely with existing measurements at temperatures from 18 to 300 K. The fact that these spectra, and presumably the analogous overtone and ``hot'' (v&gt;0) bands of the ${\mathrm{H}}_{2}$-He complex which are not known from measurements, can be accurately obtained from basic principles is significant for research related to the atmospheres of the giant planets and late-type stars.

On the distribution of potential wells and its relation to collision-induced dipole moments in nonpolar solvents. - Part I.

Abstract The relaxation times of some rigid polar molecules in three non-polar solvents of different molecular geometries (and different collision induced dipole moments) are studied. There is some relationship to be observed between the arbritary distribution of surrounding potential wells and the dipole moments measured in the various solvents.

Collision-induced absorption of infrared radiation by N2, O2 and CO2

The collision-induced absorption of the symmetric vibration of CO 2 has been observed in the pure gas at densities from 20.0 to 40.0 amagat and at temperatures of 273, 298, and 323 K using infrared techniques. From the integrated intensities of the bands and using the (exp −4) model of van Kranendonk, it is possible to deduce a value for the first derivative of the quadrupole moment with respect to the vibrational coordinate. For CO 2 the contribution from quadrupole distortion to the binary absorption coefficient is reported for several temperatures. The (exp −4) model of van Kranendonk is used to calculate the binary absorption coefficients for the fundamental vibrational bands of N 2 and O 2 at temperatures from 70 to 340 K. The parameters λ and p /σ describing the magnitude and range of the short-range collision-induced dipole moments were determined using the known experimental absorption coefficients. The contributions from atomic distortion and quadrupole distortion to the binary absorption coefficient are calculated for N 2 and O 2 .

Collision-induced dipoles of rare gas mixtures

New ab initio calculations of the collision-induced dipole moment of the rare gas systems He–Ar, Ne–Ar, Ne–Kr, and Ar–Kr are obtained on the basis of a molecular Hartree–Fock treatment. With these and recent potential functions the spectral moments and line shapes of collision-induced absorption spectra are computed. Reasonable agreement with existing measurements is observed for the first time for the system Ne–Ar and Ne–Kr.