Dissociative Ionization Cross Sections Research Articles

Cross-sections for electron impact ionization of O 2

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.

Cross sections for direct and dissociative ionization of NH3 and CS2 by electron impact

Cross sections for the production of positive ions by electron impact on NH3 and CS2 have been measured using a crossed electron beam‐molecular beam collision geometry. The present results on NH3 are considerably different from previous measurements which disagree among themselves. Except for the formation of CS2+, cross sections for other fragment ions resulting from CS2 have not been measured in the past. Threshold energies for the appearance of various positive ions are also reported. It is estimated that the present results are accurate to within 15%.

A pulsed electron beam time of flight apparatus for measuring absolute electron impact ionization and dissociative ionization cross sections

A new electron impact spectrometer has been constructed that utilizes a variable energy (4–500 eV) pulsed electron source with time-of-flight detection of electrons and ions. The apparatus can be used in a beam-beam scattering mode or in a constant pressure mode suitable for absolute measurements. A newly designed data processing system is described that uses standard CAMAC modules (LeCroy model 4208 TDCs) and allows up to 32 separate detectors to be used simultaneously in a single hit mode or up to four separate detectors in a multihit mode with each detector capable of recording up to eight hits in the same experiment. The dead time between experiments is 9.2 μs which allows up to 100 000 experiments/s for experimental flight times not exceeding 1 μs at 1 ns timing resolution. Longer flight times, up to 8.3 ms in duration, can be accommodated but with reduced timing resolution. The determination of the partial ionization cross sections for Ar+, Ar2+, and Ar3+ from threshold up to 500 eV is used as an illustration of some of the capabilities of the new instrument. The results obtained are in excellent agreement with other recent work for Ar+. For the multicharged ions the cross sections were found to lie between earlier reported results. In addition the utility of the variable ion extraction field capability of the instrument is demonstrated by the separation of N+ from N2+2 for the case of electron impact dissociative ionization of N2. A new method for placing the data on an absolute scale is presented.

Reaction cross section and product ion time-of-flight measurements for collisions of N+ and N+2 with CO2 at suprathermal energies

Charge exchange and dissociative ionization cross sections are reported for N+–CO2 and N+2–CO2 collisions at center-of-mass energies between 2 and 40 eV. Product ion kinetic energies are measured using time-of-flight techniques. Energy resonance considerations based on CO2 ionization Franck–Condon factors predict charge exchange to be slow for both of these reaction pairs. The N+–CO2 charge exchange, however, exhibits a large cross section and proceeds about one order of magnitude faster than the N+2–CO2 charge exchange. The formation of CO+ in N+–CO2 collisions is reported, while no dissociative ionization products are observed in the N+2–CO2 system.

Dissociative ionization of silane

The dissociative ionization cross-sections of silane (SiH4), have been measured by Fourier transform mass spectrometry. Neither the molecular ion SiH4+ nor the hydrogen ions H2+, H+ are formed by electron impact. Cross-sections for the other silicon hydride ions at 50 eV are: σ(SiH3+) = (0.5 ± 0.08) × 10−16 cm2, σ(SiH2+) = (0.74 ± 0.11) × 10−16 cm2, σ(SiH+) = (0.21 ± 0.03) × 10−16 cm2, and σ(Si+) = (0.19 ± 0.03) × 10−16 cm2.

Direct electron-impact collision cross sections involving vibrationally excited D 2( v) molecules relevant to D − sources

Complete sets of direct electron-impact cross sections of vibrationally excited D 2( v) molecules have been calculated by using a semiclassical approach. Calculated cross sections for dissociation, dissociative ionization, ionization and vibrational excitation according to the so-called E-V process show a decrease of threshold energy with increasing vibrational quantum number v of the target and a corresponding increase of the maximum cross sections. This last result is true for transitions to completely repulsive states (i.e. dissociation and dissociative ionization) while an intricate trend of maximum cross sections as a function of v is observed in the other cases.

Measurements of swarm parameters and derived electron collision cross sections in methane

A pulsed drift tube has been used to measure the electron drift velocity in methane over the range of E/N from 10 to 1000 Td. In addition, measurements of the positive ion mobility and ionization coefficient have been made over the range of E/N from 80 to 1000 Td. Within the experimental sensitivity, no evidence of attachment has been observed in this range. A set of electron collision cross sections has been assembled and used in Monte Carlo simulations to predict values of swarm parameters. The cross-section set includes a momentum transfer cross section which is based primarily on the present and previous drift velocity measurements, cross sections for vibrational excitation and ionization based on published experimental cross-section measurements, and a cross section for dissociation into neutral products obtained by subtracting a measured dissociative ionization cross section from a measured total dissociation cross section. Isotropic scattering is assumed for all types of collisions in the Monte Carlo simulations. Good agreement between the predicted and measured values of swarm parameters is obtained without making any adjustments to these cross sections. A two-term Boltzmann equation method has also been used to predict swarm parameters using the same cross sections as input. The two-term results are in poor agreement with experiment and confirm the well-known inadequacy of two-term methods in the case of methane.

Dissociative ionization of NH 3 and H 2O molecules by electron impact

A method is suggested to obtain the collisional parameter C i for different types of ions produced in the dissociative ionization of a molecule by electron impact from C T, the collisional parameter appropriate to the sum of direct and dissociative ionization cross-sections. These C i values are then used with the semi-empirical approach of Jain and Khare to obtain ionization cross-sections for the production of NH 2 +, NH +, OH +, H + and O + ions. The theoretical cross-sections, with the exception of those for H + ions, are found to be in satisfactory agreement with the experimental data. The method proposed here is an improvement over earlier work where C i was chosen to be C T for all values of i.

Mass spectrometric investigation of dissociative ionization of toxic gases by electrons at 20-1000 eV.

Measurements of the dissociative ionization cross-sections of CO, CO2, NO and N2O molecules for electrons from 20 up to 1000 eV were performed. Total collection of ions was achieved by using a specially constructed cycloidal mass spectrometer, an ion source without slit and a collector with large area. In that open ion source the effusion molecular beam formed by a capillary was crossed by the electron beam. Owing to such parameters of the mass spectrometer the mass discrimination effects were reduced. At the ionization of CO molecules, the ions CO+, C+, O+ and CO2+ were registered; at CO2, the ions CO2+, CO+, O+, C+ and CO2(2)+; at NO, the ions NO+, N+, O+ and NO2+; at N2O, the ions N2O+, NO+, N2+, O+, (N+,N2(2)+) and N2O2+. The relative values of the partial cross-sections were normalized to the absolute total ionization cross-sections obtained by Rapp and Englander-Golden.

Studies of the electron-impact ionization cross section of vibrationally excited oxygen employing a shock-heated molecular beam.

The shock-heated molecular-beam technique was used to obtain vibrationally excited oxygen molecules. The vibrational distributions of these diatomic molecules, in argon gas as the ``carrier'' gas of a shock-heated molecular beam, have been calculated by considering atomic recombination, collisional excitation, and deexcitation during the expansion. The vibrationally excited molecules, which correspond to vibrational temperatures ${T}_{v}$, in the range 1800--7000 K, are used to examine the role played by vibrational excitation in both direct and dissociative electron-impact ionization cross sections over a range of electron energies from 50 to 500 eV. The results show that the dissociative electron-impact ionization cross section increases only slightly with increasing vibrational temperatures, and results agree with the theory based on the modified reflection method.

Measurements of dissociative ionization cross section of SF 6 by using double collector cycloidal mass spectrometer

Electron impact diassociative ionization of SF 6 has been studied as a function of electron energy up to 600 eV witb a specially constructed double collector cycloidal mass spectrometer. High transmission of ions was obtained by using ion source without slit and the collectors with a large area.

Total and partial photoionization cross-sections of O 2 from 100 to 800 Å

The branching ratios and absolute values of the total and partial photoionization cross-sections for producing O 2 + in its various electronic states are given as a function of wavelength in the range 100–800 Å. Cross-sections for dissociative ionization are given based on the premise that some of the bound ionic states predissociate. Photoelectron spectroscopy and absorption techniques utilizing a double ionization chamber were used to obtain the data.

Calculation of dissociative ionization cross sections of diatomic molecules

A model for calculating the dissociative ionization cross sections of diatomic molecules is developed. The activated state for dissociative ionizations is modeled to occur via the ejection of a bound electron and an excitation of another bound electron. This process is described by successive two-electron scatterings in the first Born and the binary-encounter approximations. Comparison of calculated cross sections with available experimental data for five diatomic molecules (${\mathrm{H}}_{2}$, CO, ${\mathrm{N}}_{2}$, NO, and ${\mathrm{O}}_{2}$) shows good agreement with incident-electron energies above 200 eV.

Collisions between electrons and H2+ions. IV. Measurements of cross sections for dissociative ionization

For pt.III, see abstr. A64132 of 1972. Cross sections for the dissociative ionization of H2+ by electron impact have been measured for interaction energies below 940 eV. A source was used which produced H2+ ions under clearly defined conditions. The cross section attained a maximum value of 0.2 pi a02 at 103 eV which is in fairly good agreement with classical predictions by Alsmiller (1962) and Mathur et al. (1970). A Bethe plot of the results rolls over in a way which suggests that the Bethe approximation will only be valid for this reaction for energies of at least several hundred eV.

Photoionization of N2, O2, NO, CO, and CO2 by Soft X Rays

The total and relative abundances of thermal energy ions produced by absorption of 0.28, 1.25, and 1.5 keV x rays in N2, O2, NO, CO, and CO2 and the dissociative ionization cross sections at these energies were measured using a mass-spectrometric technique. The results demonstrate that there is a significant change in the character of the ionization at the K-absorption edges of the atomic constituents. At these energies dissociative ionization predominates for all the molecules accounting for over 95% of the ions formed at energies above the K edge. Quasistable doubly charged molecular ions make a significant contribution to the thermal energy ions at energies above the K edge. Our results for the total abundances of thermal singly and doubly charged molecular ions are compared with recent measurements by Carlson and Krause.

Experiments with Shock-Heated Molecular Beams

Shock-heated molecular beams have been produced in an apparatus similar in principle to that described by Skinner and Moyzis. Probes were used to measure particle velocities in shock-heated beams of argon, oxygen, and mixtures of these gases with helium. Good agreement was found between experimental and theoretical values. The velocity in a beam derived from a 5% mixture of argon with helium corresponded to an argon energy of 3.6 eV. Mass spectrometric measurements were made of the dissociation in shock-heated beams of argon mixed with either hydrogen or oxygen. Complete dissociation of hydrogen or oxygen was obtained by shock heating mixtures containing about 10% of these gases with argon. Excellent agreement between theory and experiment was found for the degree of dissociation of hydrogen, but there was substantial disagreement in the case of oxygen. It is suggested that this may be due to the variation with temperature of the dissociative ionization cross section of oxygen.

Laboratory measurements of photoionization, photoexcitation, and photodetachment

This review is devoted to a discussion of laboratory measurements of ionization, fluorescence, and photodetachment. The review is limited to molecules of aeronomic interest but excludes atoms. The section on photodetachment includes only work which has appeared since 1964. Current data are presented on photoionization cross sections, dissociative ionization cross sections, cross sections for the excitation of particular states of ions, relative vibrational excitation probabilities, photodetachment cross sections, angular distributions of electrons from photoionization, and dissociative excitation of neutral molecules. Some suggestions are made for further work.

A measurement of the cross section for proton production in collisions between electrons and H2+ions

A crossed beam method has been used to measure the cross section for proton production in single collisions between H2+ ions and electrons at incident electron energies from 5.0 eV to 1000 eV. The measured cross section is the sum of the dissociative excitation cross section plus twice the dissociative ionization cross section. Its magnitude is in good agreement with other experimental data and with theory, but there is some disagreement over the rate of fall-off of the cross section at high energies. The present data at electron energies above 40 eV can be represented approximately by Q = (255/E)log(E/13.8) where Q is in units of πa02 and E in eV. The measured cross section was found to be independent of ion source pressure over a wide pressure range, and the implications of this with regard to the vibrational-state population distribution of the H2+ ions are discussed.

Electron collisions in nitrogen

A mass spectrometer has been used to study electron collisions in nitrogen. The cross section for the ionization of N2 to form N22+ by electron impact has been measured up to 240 ev. By a comparison with the results of Peterson the dissociative ionization cross section of N2 has also been measured. N3+ formed from N2 has been observed and its appearance potential determined. The method developed by Baker and Hasted to study electron-ion collisions has been used to look at the following processes: N+ +e -> N2+ +2e (1) N2+ +e -> N22+ +2e (2) N22+ +e -> N2+ +N +e. (3) Reaction (3) appears to have a very large cross section.

Cross sections for dissociative ionization of molecules by electron impact: D Rapp et al, J Chem Phys, 42(12), June 1965, 4081–4085

Cross sections for dissociative ionization of molecules by electron impact: D Rapp et al, J Chem Phys, 42(12), June 1965, 4081–4085