E. Krishnakumar

Ionisation cross sections of rare-gas atoms by electron impact

Normalised values of partial ionisation cross sections for the rare gases (He+, Ne+-Ne3+, Ar+-Ar3+, Kr+-Kr4+ and Xe+-Xe5+) have been measured from threshold to 1000 eV using a pulsed electron beam and ion extraction technique. The cross sections have been obtained for the singly ionised species by normalising the relative data with the results of Rapp and Englander-Golden (1965) below the onset of production of the doubly ionised species. These cross sections for the singly ionised species have then been used to calibrate the mass transmission efficiency of the ion extraction+analyser+detection system by the relative flow technique. The mass transmission curve thus obtained has been employed to determine the absolute cross sections of the multiply ionised species. Summation of the individual partial cross sections with proper weighting for charge is employed to obtain the total ion cross sections. The measured partial cross sections are fitted to an empirical formula for the ease of future use.

Cross-sections for electron impact ionization of O2

Abstract Using a crossed electron beam—molecular beam collision geometry and the relative flow technique measurements have been made to generate a uniform set of electron impact ionization cross-section data for O 2 for electron impact energies from threshold to 1000 eV. Present values of total, partial and dissociative ionization cross-sections are compared with previously published data. Cross-sections for the formation of O 2+ from O 2 have been measured for the first time.

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium is a worldwide consortium which federates atomic and molecular databases through an e-science infrastructure and an organisation to support this activity. About 90% of the inter-connected databases handle data that are used for the interpretation of astronomical spectra and for modelling in many fields of astrophysics. Recently the VAMDC Consortium has connected databases from the radiation damage and the plasma communities, as well as promoting the publication of data from Indian institutes. This paper describes how the VAMDC Consortium is organised for the optimal distribution of atomic and molecular data for scientific research. It is noted that the VAMDC Consortium strongly advocates that authors of research papers using data cite the original experimental and theoretical papers as well as the relevant databases.

Cross sections for the production of N+2, N++N2+2 and N2+ by electron impact on N2

Cross sections for the production of N+2, N++N2+2 and N2+ by electron impact on N2 have been measured for an energy range from threshold to 1000 eV. These cross sections have been fitted to an empirical formula for future use in modelling various plasmas. Considerable differences from some previous data have been found.

Velocity slice imaging for dissociative electron attachment

A velocity slice imaging method is developed for measuring the angular distribution of fragment negative ions arising from dissociative electron attachment (DEA) to molecules. A low energy pulsed electron gun, a pulsed field ion extraction, and a two-dimensional position sensitive detector consisting of microchannel plates and a wedge-and-strip anode are used for this purpose. Detection and storage of each ion separately for its position and flight time allows analysis of the data offline for any given time slice, without resorting to pulsing the detector bias. The performance of the system is evaluated by measuring the angular distribution of O− from O2 and comparing it with existing data obtained using conventional technique. The capability of this technique in obtaining forward and backward angular distribution data is shown to have helped in resolving one of the existing problems in the electron scattering on O2.

Dissociative attachment of electrons to CS2

The formation of various negative ions from CS 2 by electron impact up to 50 eV is studied using a crossed beam geometry. A pulsed electron beam-pulsed ion extraction technique has been coupled with a specially designed time-of-flight mass spectrometer to make complete detection of all the ions irrespective of their kinetic energies and mass-to-charge ratios. The cross sections for the formation of different ions are put on an absolute scale using the relative flow technique. The formation of S - 2 through dissociative attachment is confirmed and is explained as arising from the presence of small number of CS 2 molecules with bending mode excitations in the neutral molecular beam. From the various thresholds the dissociation energy and electron affinity of CS are determined to be 6.5 and 0.8 eV respectively.

Absolute cross sections for dissociative electron attachment to H2O and D2O

The dissociative electron attachment (DEA) process to water (H2O) and heavy water (D2O) has been studied in the gas phase in a cross beam experiment for electron energies up to 20 eV. The apparatus used eliminates discrimination due to the kinetic energy and angular distribution of the ions. The cross sections are normalized to absolute values using the cross section for production of O− from O2 (Rapp and Briglia 1965 J. Chem. Phys. 43 1480). These are the first exhaustive measurements of absolute cross sections for both the H− and O− from H2O and D− and O− from D2O at all the three resonances. The results are compared with the scarce data available in the literature. Isotope effect is observed at the 12 eV resonance in the H− channel and at all the three resonances in the O− channel.

Cross sections for the dissociative electron attachment to ozone

The cross sections for the dissociative electron attachment (DEA) to ozone have been measured. Care has been taken to collect all the high kinetic energy fragments such that the present absolute DEA cross section measurements are believed to be the most accurate measurements made across the range from 0 to 10 eV for ozone. Our results have been interpreted with the help of both existing data in the literature and quantum chemical calculations performed on the neutral and negative-ion states of ozone.

Dissociative electron attachment to H2O2: a very effective source for OH and OH− generation

Abstract Dissociative electron attachment (DEA) to hydrogen peroxide (H 2 O 2 ) is studied by two independent electron beam experiments using mass spectrometric detection of the product ions. The fragments OH − and O − are observed from a prominent low energy resonance peaking near 0.4 eV. The partial absolute DEA cross-sections are 6.8×10 −17 and 1.7×10 −17 cm 2 for the OH − and the O − channels, respectively. These numbers are several orders of magnitude higher than for the corresponding DEA processes in simple OH containing organic compounds. These numbers are also important from the point of view of radiation damage in biological tissues.

Absolute cross sections for dissociative electron attachment to NF3

Abstract Absolute dissociative electron attachment cross sections for the formation of F−, F2−, and NF2− from NF3 have been measured in a crossed beams geometry by using the relative flow technique. Discrimination against ions with high kinetic energy are eliminated by employing a pulsed electron beam, pulsed ion extraction and a segmented time of flight mass spectrometer. The cross sections for the F− formation, which is the dominant channel, is found to be almost a factor of 2 larger than what is reported earlier. This descrepancy is explained in terms of the discrimination against ions of appreciable kinetic energy in earlier measurements.