B. G. Lindsay

Charge transfer cross sections for energetic neutral atom data analysis

Energetic neutral atom (ENA) detectors have been successfully flown on missions such as the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) and are now an important tool for probing the geospace environment. Interpretation of ENA data, however, requires knowledge of a number of key charge transfer cross sections. Here we present a critical review of the published experimental measurements and recommend a set of parameterized cross sections. The processes considered are charge transfer of H + and O + with H, O, N 2 , and O 2 for collision energies from 10 eV to 100 keV. The limitations of the measurement techniques and the probable reliability of the recommended cross sections are addressed.

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.

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.

Electron-impact ionization of the simple alcohols

Absolute partial and total cross sections for electron-impact ionization of methanol, ethanol, and 1-propanol 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 ion species are collected with equal efficiency irrespective of their initial kinetic energies. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in most of the absolute cross sections is ±6%–8%. Significant discrepancies are seen between the only prior methanol partial cross section determination and this study but the majority of published total cross section measurements and calculations are in good agreement with this work.

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.

Absolute partial cross sections for electron-impact ionization of NO and NO2 from threshold to 1000 eV

Absolute partial cross sections for electron-impact ionization of nitric oxide and nitrogen dioxide 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 collected irrespective of their initial kinetic energy. Data are presented for the production of NO+, N+, O+, and NO2+ ions from NO; and for the production of NO2+, NO+, N+, O+, and (N2++O2+) ions from NO2. The overall uncertainty in the absolute cross section values is ±5% for singly charged parent ions, while that for the fragment ions may be as large as ±20%. For NO, the previously published data are generally in reasonable agreement with those presented here, although there remain some significant discrepancies with even the most recent work. No previous comprehensive study of NO2 has apparently been reported but the experimental NO2+ cross section data that are availab...

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

Absolute partial cross sections for electron-impact ionization of CO are reported for electron energies from threshold to 1000 eV. The product ions are mass analysed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output unequivocally demonstrates that the various product ions are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of CO+, C+, O+ and CO2+, and for the total cross section which is obtained as the sum of these partial cross sections. The overall uncertainty in the absolute cross section values is ±5% for singly charged parent ions and ±6% for fragment ions. Comparison is made with prior experiments and calculations.

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.