Ar HCl Research Articles

Experimental and calculated cross sections for pressure broadening of pure rotational Raman lines of HCl

The pure rotational Raman transitions J = 0 → 2 through J = 5 → 7 of HCl have been measured. Results are presented for HCl without admixture at 1 atm and 300 K, and for HCl with argon at pressures of about 10 and 20 atm. The HCl self-broadening is compared with earlier results. The linewidths of HCl broadened by argon are compared with the result of calculations. Cross sections are calculated based on different potentials which have been suggested for the ArHCl system.

Comparison of planar trajectory and classical centrifugal decoupling cross sections for rotationally inelastic Ar–HCl collisions

Comparisons of planar trajectory and classical mechanical calculations are presented for the rotationally inelastic Ar–HCl collisions.(AIP)

Anisotropic intermolecular force effects in the far infrared absorption spectra of the Ar-HCl van der waals molecule

Far infrared absorption spectra of the Ar—HCl van der Waals molecule are discussed. The frequencies of the spectral features and their intensities suggest a librational assignment. An analysis of the observed spectra in terms of a simple angular intermolecular potential is given and a comparison with several proposed potentials is made. The data strongly suggest a large anisotropy of P 1 symmetry.

Internal dynamics of van der Waals complexes. I. Born–Oppenheimer separation of radial and angular motion

An efficient method has been developed for calculating ground state properties of atom–diatomic van der Waals complexes. The wide amplitude bending motion is decoupled from the radial motion in a way analogous to the Born–Oppenheimer separation of electronic and nuclear motion. The treatment furnishes rigorous upper and lower bounds to the ground state energy that are typically ±3 cm−1 for the Ar–HCl surfaces considered. Extensive comparison of this technique with the close-coupling method of Dunker and Gordon [J. Chem. Phys. 64, 354 (1976)] using intermolecular potentials for the Ar HCl complex indicates excellent agreement in the prediction of angular expectation values and rotational constants for the ground state, while realizing an approximate 700-fold reduction in computation time.

A far infrared study of the Ar–HCl van der Waals molecule

Results of far infrared measurements on the system HCl–Ar at low density and temperature are presented. Distinct spectral features are observed that must be attributed to Ar–HCl van der Waals molecules. Possible explanations of the observed spectra in terms of a simple picture of the internal motion of the complex are discussed. An estimate of the enthalpy of formation is made.

Rotational excitation of HCl by ArIMPACT

Making use of an empirical potential energy surface, the coupled states approximation has been employed to study elastic and rotationally inelastic ArHCl collisions. Converged integral and differential cross sections are presented for transitions j → j′ out of the j = 0 and 1 initial rotational states of HCl. The behaviour of the experimental total differential cross section of Farrar and Lee cannot be explained with the assumed potential surface. Modifications of the anisotropy in the surface are considered and a correlation is found between the deepest potential well and the rainbow angle in the total differential cross section.

On the relation between collinear and three dimensional collision rates with applications to vibrational energy transfer

A simplistic procedure for the generation of three dimensional rate constants is introduced and applied to vibrational energy transfer. The method is based on correcting for the different volumes in phase space available to the products of a collinear and a 3-D collision. Explicit expressions are derived for the 3-D distribution of final vibrational states and the effect of reagent vibrational excitation on the rate for collisions where the initial translational (and rotational) energy is thermally distributed. The procedure is applied to an exactly soluble collinear model of vibrational excitation. The resulting 3-D detailed rate constants are shown to correspond to an exponential decay of the bulk mean vibrational energy. The temperature dependence of the rates is compared to experimental results for the He–CO, Ar–HCl, He–HCl, and He–H2 systems. A surprisal analysis of the rates conforms to an ’’exponential gap’’ representation.

Calculations on the HCl–Ar van der Waals complex

Previously determined anisotropic intermolecular potentials for HCl–Ar are tested for comparison with the recent molecular beam resonance experiments on the van der Waals complexes. The Schrödinger equation for the coupled rotational and vibrational motion of the complex is solved accurately by a close coupling method. Energy levels, and expectations of the dipole moment, cosϑ, and cos2ϑ, where ϑ is the Ar–HCl center of mass-H angle, have been computed for the lower states of 40Ar complexes with H35Cl, H37Cl, D35Cl, and D37Cl. Rotational and centrifugal distortion constants were also computed for the lowest level. The assignment of the angle ϑ is discussed, and our results support the acute choice (i.e., H on the average between the Cl and Ar nuclei). The shift in angular expectations with HCl isotope distinguishes between the potentials tested here, and comparison with the experiments suggests that the true potential has an absolute minimum near ϑ=0°. However, none of the available potentials fits both the scattering data and the bound state spectroscopic data quantitatively. This demonstrates that the simultaneous analysis of scattering and bound state data constrains the potential much more than either alone.

Semiclassical three-dimensional model for vibrational relaxation. II.

A three−dimensional semiclassical model for vibrational/rotational transitions has been extended to include an infinite order treatment of the rotational transitions. An analytical expression for the transition probabilities is obtained by using an effective potential and a simple collision trajectory. Numerical results for the Ar−N2 and the I2(B3Π0u+) + He4 systems have been obtained.

ArHCl interaction potential from differential elastic scattering cross section measurements

The crossed molecular beam technique has been used to determine the ArHCl spherically symmetric potential. Analysis of the data obtained at a relative collision energy of 12.4 × 10 −14 erg (1.78 kcal mole −1) yields a potential with ϵ/ k = 191°K, r m = 4.0 Å. These results are shown to be in agreement with other experimental conclusions on the ArHCl van der Waals complex as well as with transport phenomena.

Comment on determination of the interaction potential between Ar and HCl

To test the accuracy of the Gordon-Kim theory of intermolecular forces, predicted and experimental values are compared for Ar–HCl. The method appears to accurately predict the short-range repulsive forces and also the position (but possibly not the depth) of the potential well.

Determination of the structure of ArHCl

The radio frequency and microwave spectra of K = 0 states of ArHCl and ArDCl have been measured by molecular beam electric resonance spectroscopy. The rotational and hyperfine structure constants for the lowest vibrational state of the four isotopic species are ArH35 ClArD35 ClB̄ = (B+C)/2 = 1.678511 (5)GHzB̄ − 2DJ = 1.657596 (10)GHzDJ=20.33 (20)kHz(eqQ)aC1 = −36.250 (20)MHz(eqQ)a=−23.027(10)MHz(eqQ)aD = +102.3 (50)kHzμa=0.81144(10)Dμa=1.00355(70)DArH37 ClArD37 ClB̄ − 2DJ = 1.631566 (5)GHzB̄ − 2DJ = 1.611876 (10)GHzOnly the lowest vibrational state has been observed. The molecule is extremely nonrigid. The average Ar–Cl distance is 4.006 Å in ArHCl and 4.025 Å in ArDCl. The Ar–Cl–H angle is acute and the amplitude of the zero point angle bending is extremely large. The average Ar–Cl–H angle is near 45°, while the average Ar–Cl–D angle is near 34°. The vibrational frequency of Ar–HCl bond stretching is computed to be 32.2 cm−1 from the observed centrifugal distortion constant.

Semiclassical Three-Dimensional Model for Vibrational Relaxation

In a semiclassical three-dimensional treatment of the atom-diatom scattering problem, we have included the intermultiplet transitions to infinite order, whereas simultaneous rotational and vibrational transitions are treated to first order. Thus we have found criteria for neglect of intermultiplet transitions. With this model we have investigated three systems, namely, the He–H2, the Ar–HCl, and the Ar–O2 systems. We found that transitions with change in both vibrational and rotational quantum number were dominant for the vibrational relaxation process in the Ar–HCl case, while they were important for the Ar–O2 system and negligible for the He–H2 system.

Thermal Diffusion in Polyatomic Gases: A Generalized Stefan—Maxwell Diffusion Equation

A study is made of the influence of inelastic processes on thermal diffusion in polyatomic gases, based on the Wang-Chang—Uhlenbeck—deBoer treatment of the Boltzmann equation, in which it is assumed that the distribution function for molecular spins is isotropic. A generalized Stefan—Maxwell diffusion equation is obtained for multicomponent mixtures of polyatomic gases, carried to the second Chapman—Enskog approximation. The external form of this equation is the same as the well-known result for monatomic gases, but contains higher-order corrections for the binary diffusion coefficients (i.e., the composition dependence of these coefficients), and correction terms for the effect of inelastic collisions on the diffusion and thermal-diffusion coefficients. Numerical calculations for several selected systems show that the effects of inelastic collisions on the thermal-diffusion factor are not negligible and must be considered in any attempt to derive information on intermolecular forces from thermal-diffusion measurements. However, inelastic effects are not capable by themselves of explaining the known anomalies in systems such as Ar–HCl and D2—HT.