The velocity (v) dependence of the total cross section (Q) has been measured for the scattering of helium in the metastable 2 3S1 state by ground-state helium, argon, and krypton at relative velocities in the range 1000–3300 m/sec. The results are compared with those previously obtained for the scattering of lithium. For each scattering gas, the shape of Q(v) is similar to that of lithium, but the absolute values for Q are approximately 20% higher. The scattering is largely elastic. It is estimated that the inelastic contribution to Q is about 10% for the He*–He case, and that it is less than 3% for the He*–Ar and He*–Kr cases. The Q(v) for scattering by argon and krypton show undulatory behavior. From the extrema velocities, the product εσ is shown to be close to that for Li scattering, where ε is the depth of the interatomic potential well and σ is the interatomic separation at the zero of the potential. The separate parameters ε and σ are also deduced. A minimum to the number of diatom bound states is established for HeAr(3Σ+) and HeKr(3Σ+).
The total cross section for the scattering of electrons by atomic nitrogen has been measured as a function of electron energy from 1.6 to 10 eV. An electron gun was developed that produced a more intense beam of electrons than was used for similar experiments with atomic hydrogen and atomic oxygen. The number of electrons scattered from a region de6ned by the intersection of an electron beam and a modulated molecular nitrogen beam was compared with the number scattered when the nitrogen beam was partially dissociated. A pulsed dc discharge dissociated about 20~jo of the molecules. The degree of dissociation was measured with a mass spectrometer. From the data, the ratios of atomic to molecular scattering cross sections were obtained. The absolute atomic values were calculated by multiplying these ratios by the molecular nitrogen cross sections obtained by Normand. The results are compared with theoretical estimates of the cross section.
A different approach has been developed to measure two-body collision cross sections in the energy range from thermal to several hundred electron volts. Two molecular beams are merged and move in the same direction along a common axis. Ion-neutral reactions are discussed but the techniques could also be used for ion-ion and neutral-neutral collisions. Advantages of the merging beams technique over conventional beam-gas and crossed beam methods include accessibility (with good energy resolution) to the energy region from a few tenths of one to a few electron volts; the possibility of measuring cross sections for two general, labile species; and relative ease in collecting and detecting products for total reaction cross section measurements. The principles of the method and details of the instrument are discussed. A cross section for the resonant charge transfer of Ar was measured. Reasonable agreement with some previous measurements, and other tests, indicate that the basic principles of a merging beams technique are valid and that the apparatus performs satisfactorily.
The cross section for elastic scattering of electrons of less than 10-ev energy by free hydrogen atoms has been measured. The experimental approach was similar to that used by Bederson, Malamud, and Hammer in that a dc electron beam crossed an atomic hydrogen beam, which was chopped at a low frequency, and the signal was derived from the electrons scattered by the particles in the neutral beam. The electrons scattered by the beam were identified by their signal's appearing at the modulation frequency and at a specified phase. Direct measurements of the ratio of cross sections of the hydrogen atom and the molecule were made, and absolute values for the atomic cross section were obtained from knowledge of the absolute molecular cross section. The experimental results agree with theory.
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