Electron Impact Ionization Of Molecules Research Articles

Ionization of guanine, adenine and thymine molecules by electron impact

The method and results of the study of positive ions yield in the mass-spectrometric investigations by electron impact ionization of molecules of guanine, adenine and thymine are described. Mass spectrum obtained by electron impact has been measured in a crossed electron–molecule beam experiment ionization at the electron energy 70 eV. The ionization energy of the molecule and the appearance potential of some fragment ions were obtained. The mass spectra and fragmentation schemes of adenine and thymine molecules by electron impact were analyzed. The analysis of measured mass spectra is carried out, schemes of fragmentation of guanine, adenine and thymine molecules are proposed, which illustrate the most probable channels for the formation of ionic fragments in an electron impact, when the energy of the incident electrons is much higher than the ionization potential of the molecule. An explanation of the behaviours of the obtained temperature dependences of ions can be due to the difference in the character of the formation and fragmentation of the guanine, thymine and adenine molecules by electron impact.

Recent theoretical progress in treating electron impact ionization of molecules

(e,2e) ionization differential cross sections are presented for several molecules. We will compare experimental results with theoretical calculations using the molecular three body distorted wave (M3DW) approximations for H2, N2, H2O and Formic Acid (FA) using better wave-function for the molecules than we had in previous works. Generally, good agreement is found between the M3DW approach and experiments.

Direct measurement of supra-thermal energy distribution of fragments following electron-impact ionization of molecules

The energy distribution function of the molecular fragments produced by electron impact is measured using a combination of a delayed pulse extraction, a free-fall degradation of the ionic density along the beam path, and a time-of-flight spectroscopy. This new technique is able to measure energy distribution of ions from thermal energies up to few electron volts and allows measuring the energy distribution of singly charged ions which is difficult to obtain through the more usual method using the increase of the width of the peaks in the time-of-flight spectrum.

Effects of ion–neutral chemical reactions on dynamics of lightning-induced electric field

Secondary lightning phenomena in the upper atmosphere called sprites attract interest from the viewpoint of atomic–molecular and plasma physics. Lightning-induced electric field accelerates the ionospheric electrons up to tens of electron-volts, inducing electrical breakdown as well as strong optical emissions, through electron impact ionization of molecules. A large-scale structure of sprites is constructed by collective dynamics of filamentary streamer discharges in a rarified gas, which in turn is controlled by the distribution of the background electric field. In this paper, we firstly reanalyze the relationship between quasi-static field formation and local ion chemistry with first-order perturbation techniques. Secondly, we investigate with a full ion chemical model the effects of electron attachment to oxygen molecules on its density in moderate cases of undervoltage lightning electric fields rather than the cases of intense ionization in streamers. We estimate the minimum values that are provided by the chemical balance with electron detachment from negative ions. We also investigate the recovery timescale of the electron density and find that the scale (⩾1 s) is occasionally much larger than the interval of each lightning stroke (≈10 ms). We suggest that the subsequent sprite event as well as the field formation could be well affected by the ghost of the primary event. We discuss further the negative ion chemistry triggered by electron attachment in the nighttime mesosphere.

(e,2e) ionization of helium and the hydrogen molecule: signature of two-centre interference effects

Relative (e,2e) triply differential cross sections (TDCS) are measured for the ionization of the helium atom and the hydrogen molecule in coplanar asymmetric geometry at a scattered electron energy of 500 eV and ejected electron energies of 205, 74 and 37 eV. The He experimental results are found to be in very good agreement with convergent close-coupling calculations (CCC). The H2 experimental results are compared with two state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. Both models yield an overall good agreement with experiments, except for some intensity deviations in the recoil region. Similar (e,2e) works were recently published on H2 with contrasted conclusions to the hypothesis that the two H nuclei could give rise to an interference pattern in the TDCS structure. Murray (2005 J. Phys. B: At. Mol. Opt. Phys. 38 1999) found no evidence for such an effect, whereas Milne-Brownlie et al (2006 Phys. Rev. Lett. 96 233201) reported its indirect observation. In this work, based on a direct comparison between experimental results for He and H2, we observe an oscillatory pattern due to these interference effects, and for the first time the destructive or constructive character of the interference is observed, depending on the de Broglie wavelength of the ejected electron wave. The experimental finding is in good agreement with the theoretical prediction by Stia et al (2003 J. Phys. B: At. Mol. Opt. Phys. 36 L257).

Triply differential (e,2e) cross sections for ionization of the nitrogen molecule at large energy transfer

Measurements of the (e,2e) triply differential cross sections (TDCS) are presented for the ionization of the nitrogen molecule in coplanar asymmetric geometry at an incident energy of about 600 eV and a large energy transfer to the target. The experimental results are compared with state-of-the-art available theoretical models for treating differential electron impact ionization of molecules. The experimental TDCS are characterized by a shift towards larger angles of the angular distribution with respect to the momentum transfer direction, and by a large intensity in the recoil region, especially for ionization of the ‘inner’ 2σg molecular orbital. Such shifts and intensity enhancement are not predicted by the model calculations which rather yield a TDCS symmetrically distributed around the momentum transfer direction.

Electron impact ionization of molecules at low to intermediate energies—A search for Young's double slit type interferences

Experimental and theoretical work studying the (e,2e) collision process from diatomic molecules is reported. In particular, recent work carried out from near threshold to ∼100 eV incident electron energy ionizing H 2 and N 2 in coplanar symmetric and asymmetric geometries is compared to calculations using a molecular three body distorted wave model. Of interest is the possibility of observing the effects of Young's double slit-type interference terms in the measured cross-sections. There is considerable interest in this type of scattering, since simple models predict the effects should be equivalent to a modulation of the cross-section from individual atoms which constitute the molecule. Such effects have recently been inferred at 250 eV incident energy in an asymmetric geometry, however at lower energies the complexity of the scattering process and the requirement to integrate over all possible alignments of the targets means that these effects remain unclear. Experimental data and results from a molecular three-body distorted wave (M3DW) model are presented, and possible methods to experimentally align and detect the molecular direction prior to ionization are suggested.

Electron impact ionization of small molecules at intermediate energies: the molecular R-matrix with pseudostates method

A procedure for the ab initio study of electron–molecule collisions at intermediate energies is presented in detail. The molecular R-matrix with pseudostates method is based on the inclusion of discretized continuum states in the close-coupling expansion. This method allows, for the first time, the calculation of totally ab initio cross sections for electron impact ionization of molecules as well as for electronic excitation above the first ionization threshold. The method is general and can be applied to multielectron targets. Results for collisions with H+3 and H2 are presented. Numerical considerations necessary for performing a successful calculation are detailed.

Comment on the accuracy of absolute electron-impact ionization cross sections for molecules

Electron impact ionization of molecules has been studied extensively in many laboratories and it is perhaps surprising therefore that uncertainty in the values of many cross sections has persisted. In an effort to rectify this situation absolute partial and total electron-impact ionization cross sections have been measured in this laboratory for a range of molecular targets, including H2, N2, O2, CO, NO, CO2, CH4, and SF6. These total cross sections for these molecules are compared here to those measured by Rapp and Englander-Golden, which have long been widely adopted as the de facto standard. The excellent agreement observed between these two independent data sets would seem to warrant a high level of confidence in them, and in the corresponding partial cross sections determined in this laboratory.

The influence of pressure and conductivity on the pulsed breakdown of water

The influence of hydrostatic pressure and liquid conductivity on the dielectric breakdown of water solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. Well-defined pulses (80 kV, 3 ns risetime, 100 ns duration) have been applied to a gap (0.4 to 2.1 mm), between Rogowski profile electrodes (thus ensuring a uniform electric field), containing de-ionized water (non-distilled, and distilled and ultrasonically treated), sodium chloride solutions (0.001 to 1.0 molar), or magnesium sulfate solutions (0.01 to 0.1 molar). Breakdown in these liquids has been studied at pressures from atmospheric up to 40 MPa. The inter-electrode potential and the current response were measured indicating the time lag to breakdown, breakdown voltage, and temporal characteristics of the breakdown process. The breakdown time lag increases with increasing pressure, and is insensitive to the liquid conductivity. These findings have relevance to the ongoing discussion concerning 'thermal' vs. 'electronic' mechanisms for dielectric breakdown in liquids. In particular, the results suggest that breakdown evolves via 'bubble' formation by field emitted currents near asperities on the cathode, and that the time for the change in liquid conductivity as a result of breakdown is limited by processes other than ionization growth (due to electron impact ionization of molecules in the 'bubble') of prebreakdown electron currents.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Discrimination effects for ions with high initial kinetic energy in a Nier-type ion source and partial and total electron ionization cross-sections of CF 4

Fragment ions produced by dissociative electron impact ionization of molecules often have large amounts of kinetic energy (up to several electronvolts). Presently performed computer simulations of the ion trajectories in a Nier-type ion source show strong discrimination effects in the extraction characteristics of such ions, leading to huge errors in measured fragmentation patterns and absolute partial ionization cross-section functions. Using the simulation results a correction function (extraction efficiency vs. fragment kinetic energy) is claculated here for a given potential distribution of a Nier-type ion source extraction system. This correction procedure is applied to previously measured partial ionization cross-sections of CF 4. The corrected partial and total CF 4 cross-sections are in excellent agreement with other recent measurements using advanced methods.

Anomalous fragmentation in the electron-impact ionization of SF 6 and SiF 4 in Ar clusters

Anomalous fragmentation in the electron-impact ionization of molecules has been observed under conditions in which the molecules would be in van der Waals clusters with argon. This is interpreted as evidence for the Penning transfer of excitation energy within clusters from the inert gas atom to the molecular species. The effect may have important implications for the interpretation of mass spectra of molecular beams.