Reinhard K. B. Helbing

Crossed‐Beam Study of Ionization of Cs by Br2

The velocity dependence of the relative cross section for the formation of Cs+ from Cs and Br2 has been measured in the c.m. energy range 1.5–16.5 eV. A crossed‐molecular‐beam technique was used. The threshold energy is 1.66 ± 0.1 eV (c.m.). This leads to a nominal value of 2.23 ± 0.1 eV for the electron affinity of Br2. A maximum cross section is found near 6 eV.

GLORY SCATTERING OF LITHIUM BY A SERIES OF MOLECULES. I. DIATOMICS.

The velocity dependence of the total cross section has been measured for the scattering of lithium‐7 by D2, N2, O2, CO, NO, HF, HCl, DCl, HBr, HI, ICl, Cl2, and Br2. Glory undulations are observed for all systems except Cl2 and Br2 although in some cases these undulations are considerably smaller than might be expected for comparable atom–atom scattering. Potential parameters are derived for these systems. Chemical reaction, time‐dependent anisotropic potentials, and inelastic processes are discussed as possible mechanisms for the observed quenching.

Transformations and Averaging Processes for Differential Cross‐Section Measurements at Thermal Energies

Theoretical predictions on differential cross sections apply to the center‐of‐mass system of the colliding particles but not to the laboratory system. In order to compare theoretical results with measurements it is necessary either (a) to approximate the center‐of‐mass system by a suitable choice of the colliding particles, and the region of energy, or (b) to transform the differential cross section from the center‐of‐mass system into the laboratory system. If the target particles are at rest before collision, this transformation is simple and well known. If the target particles are not stationary before collision, the problem of transformation becomes more difficult, especially if the target particles have a distribution of velocities. In this paper, formulas are derived which allow the transformation from the center‐of‐mass system to the laboratory system (and vice versa). Both elastic and inelastic collisions are treated. In addition, the averaging process over all initial relative velocity vectors is ...

Glory Scattering of Lithium by a Series of Molecules. II. Fluorocarbons and Hydrocarbons

The velocity dependence of the total cross section has been measured for the scattering of lithium‐7 by 26 fluorocarbon and hydrocarbon molecules, and by CH3F, CH2F2 and CHF3, in the velocity range from about 1–4 km/sec. This work is similar to that described in a previous paper in which the scattering was from 13 diatomics. Potential parameters were derived for these systems. The quenching of the normal glory pattern may be qualitatively correlated with the geometric structures of the scatterers. Glory scattering persists even for rather complex molecules (e.g., CF3–cyclo‐C6F11). The saturated fluorocarbons may be roughly thought of as rare gases of varying shape. The data are in reasonable agreement with previous work, although our interpretation differs.

Additional Quantum Effects in Atom–Atom Scattering: Higher‐Order Glory Scattering

In atomic scattering theory the phase shift η has often been treated as a continuous function of angular momentum l, in order to replace summation over l by integration. However, because l is quantized, additional (higher‐order) forward and backward glories may occur. The effect is most pronounced in like‐particle scattering, where “hard‐sphere” glories show up in total cross section. General formulas are derived and applied to like particles. For comparison, exact quantum calculations are presented. A criterion for the applicability of the continuous phase approximation is derived. Construction of phase‐shift curves from experimental data only is discussed.

Alkali–Alkali Glory Scattering in the 3Σ State

The velocity dependence of the total cross section for the scattering of 7Li by Na, K, Rb, and Cs has been measured in a velocity range from about 1–4.5 km/sec, and that for Na–Cs scattering from 1–3 km/sec. The observed glory undulations are due to the 3Σ state of the diatom. A Lennard‐Jones (8,6) potential is assumed. The product of the potential parameters erm were obtained, and for the lithium scattering, the separated parameters e and rm were estimated. Independent of potential model, the number of extrema indicates that the minimum number of bound states for the Li–Na, Li–K, Li–Rb, Li–Cs, and Na–Cs triplet states is 3, 4, 5, 6, and 7, respectively. The Na–Cs results are not consistent with previous results.