S. Y. Tang

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.

A cesium impact source for negative-ion mass spectra

Abstract Negative ions are produced in a single-collision charge-transfer source. A beam of cesium atoms with kinetic energies which are variable between 10 and 350 eV intersects a beam of target molecules. Negative ions and Cs + are formed. The energy dependence of these anion spectra are measured for SO 2 , CH 3 NO 2 , SF 6 , and CF 2 Cl 2 . The observed cesium impact spectra are qualitatively different from those produced by electrons. At the higher cesium-atom energies, they are relatively insensitive to energy. Large parent ion intensities are seen for SO 2 and CH 3 NO 2 . For Cs + SF 6 , our ratios (SF 6 − /SF 5 − ) and (F − /SF 5 − ) are in agreement with those of Compton and Cooper for the limited range of Cs energy employed in both experiments.

Negative gaseous ions from nitric acid

A crossed‐beam system of Cs atoms and HNO3 molecules has been used to study the ion chemistry of HNO3. The existence of an HNO2− anion was observed and EA(HNO3) and D(H–ONO2−), the electron affinity of HNO3, and the dissociation energy of the parent anion were measured. Also, value for EA(NO3) was deduced which requires additional thermodynamic data and compare it with previous work. (AIP)

Anion production from Cs + CF3X: evidence for stripping

Abstract Crossed beams of energetic cesium atoms (25–350 eV) and thermal Cf3X (X = Cl, Br and I) produce X− and F− as the major anions. The intensity ratio (F−/X−) is measured as a function of energy. A hard-sphere stripping model is in reasonable accord with the data.