Scattering of thermal He beams by crossed atomic and molecular beams. IV. Spherically symmetric intermolecular potentials for He+CH4, NH3, H2O, SF6

Differential scattering cross sections are measured for He+CH4, NH3, H2O, and SF6, using the crossed molecular beams technique. These data, which are sensitive to the van der Waals attractive minima and adjacent regions of the intermolecular potential, are interpreted in terms of central‐field models. No evidence is found for quenching of the observed diffraction oscillations. The interactions of the isoelectronic hydrides CH4, NH3, H2O with He are found to have decreasing van der Waals radii in this sequence, and their attractive wells all have similar depths. However, the He+SF6 attractive well is found to be anomalously deep, and provides a counter example to the supposition that only the polarizability of the least polarizable of the interacting partners (atoms or molecules) correlates with the van der Waals well depth. Simple combination rules for predicting unlike‐pair potential parameters from the corresponding like‐pair ones are tested and found inadequate.

[1]  Hans Lishicka.  Ab initio calculations on intermolecular forces. II. An analysis of potential energy curves for HE… HF, He… H2O, H2… HF and H2 … H2O at large and intermediate distances , 1973 .

[2]  G. Scoles,et al.  Molecular beam scattering experiments with polar molecules: measurement of differential collision cross sections for H2O+H2, He, Ne, Ar, H2O and NH3+H2, He, NH3 , 1975 .

[3]  N. Draper,et al.  Applied Regression Analysis , 1966 .

[4]  Cheuk-Yiu Ng,et al.  Improved potentials for Ne + Ar, Ne + Kr, and Ne + Xe , 1974 .

[5]  G. A. Parker,et al.  Rotationally and vibrationally inelastic scattering in the rotational IOS approximation. Ultrasimple calculation of total (differential, integral, and transport) cross sections for nonspherical molecules , 1978 .

[6]  R. E. Leckenby,et al.  The observation of double molecules in gases , 1966, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[7]  James S. Cohen,et al.  Modified statistical method for intermolecular potentials. Combining rules for higher van der Waals coefficients , 1974 .

[8]  Edward A. Mason,et al.  Transport Properties of Polar Gases , 1961 .

[9]  R. Conn,et al.  Interaction potentials for He–HF and Ar–HF using the Gordon–Kim method , 1976 .

[10]  J. Parson,et al.  Intermolecular Potentials from Crossed‐Beam Differential Elastic Scattering Measurements. IV. Ar+Ar , 1972 .

[11]  C. F. Curtiss,et al.  Molecular Theory Of Gases And Liquids , 1954 .

[12]  G. Maitland,et al.  The forces between simple molecules , 1973 .

[13]  D. E. Freeman,et al.  Vacuum ultraviolet absorption spectra of binary rare gas mixtures and the properties of heteronuclear rare gas van der Waals molecules , 1977 .

[14]  H. Butz,et al.  Totale effektive Streuquerschnitte für die Streuung von He, HD und D2 an verschiedenen Molekülen , 1971 .

[15]  R. A. Aziz,et al.  Intermolecular forces in mixtures of helium with the heavier noble gases. [Differential cross sections] , 1977 .

[16]  R. Gordon.  The role of complex trajectories in elastic scattering , 1975 .

[17]  R. J. Cross.  EFFECTS OF ANISOTROPY IN THE POTENTIAL ON MOLECULAR SCATTERING. , 1970 .

[18]  H. Lekkerkerker,et al.  Dispersion forces in methane , 1977 .

[19]  J. Michael Finlan,et al.  New alternative to the Dunham potential for diatomic molecules , 1973 .

[20]  D. Herschbach,et al.  Combination Rules for van der Waals Force Constants , 1970 .

[21]  D. Margoliash,et al.  Pseudospectral dipole oscillator strength distributions and some related two body interaction coefficients for H, He, Li, N, O, H2, N2, O2, NO, N2O, H2O, NH3, and CH4 , 1978 .

[22]  A. Kuppermann,et al.  Differential Elastic Scattering of D2 by N2 in Crossed Molecular Beams , 1970 .

[23]  M. Keil,et al.  Scattering of thermal He beams by crossed atomic and molecular beams. I. Sensitivity of the elastic differential cross section to the interatomic potential , 1978 .

[24]  W. Jost,et al.  Physical Chemistry, An Advanced Treatise , 1974 .

[25]  James B. Anderson,et al.  Velocity Distributions in Molecular Beams from Nozzle Sources , 1965 .

[26]  R. Bernstein,et al.  The Simons-Parr-Finlan modified Dunham expansion: a generalized potential model for the analysis of differential elastic molecular beam scattering cross sections , 1974 .

[27]  G. A. Parker,et al.  An empirical anisotrophic intermolecular potential for He + CO2 , 1978 .

[28]  D. Marquardt.  An Algorithm for Least-Squares Estimation of Nonlinear Parameters , 1963 .

[29]  B. Alder,et al.  Pair Potential for Argon , 1969 .

[30]  R. Smalley,et al.  The structure of the HeI2 van der Waals molecule , 1978 .

[31]  A. Kuppermann,et al.  Scattering of thermal He beams by crossed atomic and molecular beams. II. The He–Ar van der Waals potential , 1979 .

[32]  T. Reed,et al.  Estimation of Properties of Dilute Polar Gases from a Potential Energy Function , 1967 .

[33]  G. Scoles,et al.  An intermolecular potential for (NH3)2 , 1978 .