Erhard W. Rothe

Total Collision Cross Sections for the Interaction of Atomic Beams of Alkali Metals with Gases

Total collision cross sections (Q) for the interaction of atomic beams of K and Cs with a number of molecules were measured with an apparatus of 30″ angular resolution. Although absolute determinations of Q are difficult, relative values are readily obtained (±3%). Results are reported as the ratio (Q*) of the cross section for a given molecule to that of argon for the same beam atom. Seventy‐seven molecules (of varied complexity and reactivity) were studied with K and 16 with Cs beams. Q* ranged from 0.29 to 2.8.The data were correlated using the Massey‐Mohr theory, assuming an attractive intermolecular potential V(r) = —C/r6. For this case Q=b(C/vr)2/5, where vr is the relative velocity and b a known constant. C was estimated from standard formulas for the London dispersion and dipole‐induced dipole forces, using known refraction and dipole moment data. The theoretical values of Q differ by a nearly constant factor from the experimental results; thus values of Q* are predicted with good accuracy. The de...

Total Collision Cross Sections for the Interaction of Molecular Beams of Cesium Chloride with Gases. Influence of the Dipole‐Dipole Force upon the Scattering

Total cross sections (Q) for the interaction of beams of CsCl with a number of molecules were measured using an apparatus of ca 4′ angular resolution in which the temperature of the scattering gas could be varied from 200°—735°K. The temperature dependence of Q was studied for Ar, CH4, CH2F2, CHF3, CF4, NO, H2S, NH3, and for cis‐ and trans‐CHCl=CHCl. Relative values of Q at 300°K were measured for eight additional gases.The data were correlated using the Massey‐Mohr theory, assuming an intermolecular potential V(r) = —C/r6, so that Q=b(C/vr)2/5, where vr is the relative velocity and b a known constant; the potential constant C was estimated from formulas for the dispersion, dipole‐induced dipole, and dipole‐dipole forces.For the nonpolar gases the observed small temperature dependence of Q agrees within experimental error (±3%) with that expected from the temperature dependence of vr. The theoretical values of Q differ by a nearly constant factor from the experimental results; thus relative cross sections...

Negative ions from phosphorus halides due to cesium charge exchange

An experiment has been conducted in which cesium atoms in the kinetic energy range 2–350 eV collide with phosphorus halides. Parent anions and fragments are formed. Molecular energies are obtained from threshold measurements. The electron affinites for PCl3, POCl3, PBr3, PCl2Br, PBr2Cl, and POCl2 are found to be 0.8, 1.4, 1.6, 1.5, 1.6, and 3.8 eV, respectively. The P–X bond energies for PCl3, POCl3, and PBr3 are 3.3, 3.5, and 2.6 eV, respectively.

Negative ion formation in halocarbons by charge exchange with cesium

A crossed molecular beam apparatus is used to study the formation of anions by charge exchange of fast cesium atoms with a variety of halocarbons. The branching ratios are determined from threshold to 350 eV. Comparison of the spectator stripping and information theoretical models to the data leads to the conclusion that this class of reactions is an example of an ultradirect impulsive mechanism. In the most favorable case, that of CF3I, we have determined the near threshold relative cross sections for the anions CF3I−, I−, CF−3, and IF−. From these we determine the electron affinity of CF3I− to be 1.4±0.2 eV and the bond dissociation energy of CF3‐I− as 0.38±0.1 eV, which disagrees with values obtained in a previous experiment.

Negative ion formation from energetic collisions of cesium with CO2, CS2, and COS

An experiment is described in which cesium atoms, with kinetic energies in the range 10–350 eV, collide with CO2, CS2, and COS. The measured products are negative ions from charge exchange (NICE). The parent anions, CO2−, CS2−, and COS− are observed, as are other fragment ions. Intensity ratios for the various anions are measured as a function of cesium atom energy. The electron affinity of CO2, determined from crude threshold measurements, lies in the range −2.1 to −1.1 eV, with a precision of about ±0.25 eV.

Ionizing collisions of cesium with Cl2, Br2, and I2

A crossed molecular beam apparatus is used to study ionizing collisions of energetic cesium atoms with Cl2, Br2, and I2. The cross sections for formation of Cs+ are reported in the near−threshold region. The experiment combines an energy resolution better than 0.1 eV (FWHM) with a deconvolution procedure. An electron affinity of 2.50 eV is deduced for all three halogens, in good agreement with previous work. In a separate experiment, with cesium energies from threshold to 350 eV, and with much poorer energy resolution, the intensity ratios X−/X−2 are obtained. The results can be reasonably explained with an electron jump model. At energies below 30 eV, the observed ratios are in agreement with two other investigators, but between 150−350 eV they are drastically different from work reported from a third. Complementary data recently reported by a fourth group are difficult to reconcile with the present results.

Measurement of the electron affinity of NO2

The translational energy dependence of the relative cross section for Cs+NO2→Cs++NO2− has been measured in the threshold region using crossed molecular beams. Good energy resolution in the center of mass is obtained with a time‐of‐flight technique for the Cs primary beam and by numerical analysis of the remaining c.m. energy spread. The data are well represented by convoluting a c.m. cross section with the experimental energy spreads. This relative cross section has a threshold at 1.39±0.05 eV and yields an adiabatic electron affinity for NO2: 2.50±0.05 eV.

Chemi‐ionization reactions of alkali dimers with halogen molecules

A crossed beam apparatus is used to characterize ionic products of thermal energy collisions of alkali dimers M2 with homonuclear halogens X2 and with IBr and ICl. All possible alkali–halogen combinations were studied, except Rb2 with ICl and IBr. When energetically possible the ionic products are mainly M+ and X−. When this path is closed two alternative paths are observed. These yield M2X+ and X−, or M+ and MX2 −. The results are discussed in the framework of a mechanism proposed by Lin, Whitehead, and Grice. The binding energy of the triatomic ions is computed with the use of an ionic model.

Two-step photoionization of Na2: dependence on alignment

Abstract Laser light is used for two-step photoionization of Na 2 . the first step depends on the moleculer orientation, the second step doesnot. This yields a method for the analysis of molecular alignment.

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.