K. A. Smith

Absolute and angular efficiencies of a microchannel‐plate position‐sensitive detector

This paper presents a characterization of a commercially available position‐sensitive detector of energetic ions and neutrals. The detector consists of two microchannel plates followed by a resistive position‐encoding anode. The work includes measurement of absolute efficiencies of H+, He+, and O+ ions in the energy range between 250 and 5000 eV, measurement of relative detection efficiencies as a function of particle impact angle, and a simple method for accurate measurement of the time at which a particle strikes the detector.

Absolute partial cross sections for electron-impact ionization of CH4 from threshold to 1000 eV

Absolute partial cross sections for the production of CH4+, CH3+, CH2+ , CH+, C+, H2+, and H+ from electron-impact ionization of CH4 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment ions. Although previous measurements are generally found to agree well with the present results for CH4+ and CH3+, almost all previous work for the remaining fragment ions lies lower than the present results and in the case of H+ is lower by approximately a factor of 4.

Absolute partial cross sections for electron-impact ionization of H2O and D2O from threshold to 1000 eV

Absolute partial cross sections for electron-impact ionization of H2O and D2O are reported for electron energies from threshold to 1000 eV. Data are presented for the production of H2O++OH++O+, O2+, H2+, and H+ from H2O and for the production of D2O+, OD+, O+, O2+, D2+, and D+ from D2O. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±4.5% for singly charged parent ions and is slightly greater for fragment ions. The cross sections for H2O and D2O are found to be the same to within experimental uncertainties except for the H2+ cross section which is approximately a factor of 2 greater than the D2+ cross section. Previous results are compared to the present measurements.

Absolute detection efficiency of a microchannel plate detector for kilo-electron volt energy ions

Measurements of the absolute detection efficiency of a commercial microchannel plate detector for kilo-electron volt energy ions are presented. The detector comprises two microchannel plates mounted in front of a resistive anode. It is found that when the detector is appropriately biased, and the ion impact energy is sufficiently high, ions with masses up to 131 amu are detected with equal efficiency and that this efficiency may remain constant over a period of years.

Ionization in K(nd)–SF6 and K(nd)–CCl4 collisions at intermediate n

Rate constants for ion production in collisions between potassium atoms in nd Rydberg states and SF6 and CCl4 have been investigated over a range of principal quantum numbers n between 9 and 45. The data suggest that, at the lower values of n, electrostatic interactions between the product ions become important and lead to a marked decrease in free ion production for both target species. The data also demonstrate that only a small fraction of the energy released by dissociative electron attachment to CCl4 appears as kinetic energy of the Cl− fragment.

Dissociative electron attachment to CCl4: Lifetime of the CCl4−* intermediate

Dissociative electron attachment to CCl4 is studied by measuring the angular and velocity distribution of Cl− ions produced in collisions with velocity selected K(np) Rydberg atoms. Analysis of the data using a Monte Carlo collision code that models the detailed kinematics of the reaction indicates that the lifetime of the CCl4−* intermediate initially formed by Rydberg electron capture is 7.5±2.5 ps and that, upon dissociation, only a small fraction of the excess energy of reaction appears as translational energy of the Cl− and CCl3 fragments.

Absolute partial cross sections for electron‐impact ionization of CO2 from threshold to 1000 eV

Absolute partial cross sections for the production of CO+2, CO+, CO2+2, O+, C+, O2+, and C2+ from electron‐impact ionization of CO2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time‐of‐flight mass spectrometer and detected with a position‐sensitive detector whose output provides clear evidence that all product ions are completely collected. The overall uncertainty in the absolute cross section values is ±3.5% for singly charged parent ions and is slightly greater for fragment and doubly charged ions. For the fragment ion cross sections, all but one of the previous measurements are observed to be significantly lower than the present results.

CONTROL OF LONG-TERM OUTPUT FREQUENCY DRIFT IN COMMERCIAL DYE LASERS

A simple technique using a scanning confocal Fabry‐Perot etalon and a stabilized helium‐neon laser is described that can be used to correct the long‐term drift in output frequency of actively stabilized commercial dye lasers. Using this technique the long‐term drift in the ultraviolet output frequency of an intracavity‐doubled Coherent Inc. CR699‐21 ring dye laser has been reduced to ≲±2 MHz.

Translational kinetic energy release in dissociative electron attachment to C2Cl4, CCl4 , CFCl3, and 1,1,1‐C2Cl3F3

Translational energy release in dissociative electron attachment to C2Cl4, CCl4 , CFCl3, and 1,1,1‐C2Cl3F3 has been investigated at subthermal electron energies through studies of collisions involving K(nd) Rydberg atoms. For CCl4 and CFCl3 , the observed mean translational energy release is in good agreement with the predictions of quasiequilibrium theories of unimolecular decay, suggesting that the excess energy of reaction is well‐distributed among the internal modes of the intermediate negative ion prior to dissociation. In contrast, the data for 1,1,1‐C2Cl3F3 indicate rapid dissociation before effective redistribution of the excess energy can occur. Implications of this as regards measurement of free‐electron attachment cross sections at very‐low energies are discussed.

Semiclassical model for analysis of dissociative electron transfer reactions involving Rydberg atoms

Collisions between atoms in high Rydberg states and molecules that dissociatively attach free low‐energy electrons can lead to ionization through capture of the excited electron by the target molecule. A Monte Carlo code is described that models the detailed kinematics of such dissociative electron transfer reactions. The model takes into account the velocity distributions of the reactants, the lifetime and decay energetics of the transient intermediate negative ion, and the electrostatic interaction between the product positive and negative ions. Data for CF3I are presented that illustrate how detailed comparisons between model predictions and experimental data can lead to a better understanding of the dynamics of dissociative electron attachment reactions. In particular, such comparisons can provide estimates of the lifetime of the intermediate negative ions and show how the excess energy of reaction is partitioned between translational and internal degrees of freedom in the dissociation process.