Scattering of High-Velocity Neutral Particles. XIV. He–He Interaction below 1.1 Å

Abstract

Total elastic cross sections have been measured for helium atoms within energies between 300 and 2000 eV which have been scattered by room-temperature helium through an effective laboratory angle greater than about 1×10−2 rad. Special features and procedures used in the experiments include the use of a magnetic field to increase the positive-ion current in the source, acceleration of the positive ions extracted from the source in a region of relatively low pressure with a consequent reduction in their energy spread, production of neutrals by charge transfer in a chamber separated from the scattering chamber, modulation of the neutral beam to produce an ac signal on a compensated thermal detector, and amplification of the detector signal with synchronous rectification, at the modulation frequency, of the amplifier output. A detailed analysis of the scattering results treats effect on the measured cross sections of the intensity distribution in the beam and of the beam-detector geometry, interpretation of the results in terms of different empirical monotonic potential functions (the exponential, the screened Coulomb, and the point repelling inverse power), and the effects of quantum scattering. Numerical values for the He–He potential between 0.61 and 1.12 Å for the three assumed potential functions show good internal consistency and agree with the best theoretical values within the combined estimates of the maximum uncertainties in the experimental and theoretical values. The mean potential obtained from the three empirical functions may be represented within about 1.8% by V(r)mean=196 exp(−4.21r) eV; 0.61 Å<r<1.12Å.