A. V. Phelps

Cross Sections and Swarm Coefficients for Nitrogen Ions and Neutrals in N2 and Argon Ions and Neutrals in Ar for Energies from 0.1 eV to 10 keV

Graphical and tabulated data and the associated bibliography are presented for cross sections for elastic, excitation, and ionization collisions of N+, N+2, N, and N2 with N2 and for Ar+ and Ar with Ar for laboratory energies from 0.1 eV to 10 keV. Where appropriate, drift velocities and reaction or excitation coefficients are calculated from the cross sections and recommended for use in analyses of swarm experiments and electrical discharges. In the case of N+ in N2, cross sections for momentum transfer, charge transfer, electronic excitation, and electron production are recommended. Drift velocity calculations predict runaway for N+ in N2 for electric field to gas density ratios E/n greater than 4.3×103 Td, where 1 Td (townsend)=10−21 V m2. For N+2 in N2, the cross sections include those for N+ and N+3 formation, electronic excitation, and electron production. Drift velocities and average cross sections are calculated for E/n≥500 Td. In the case of N in N2, only cross sections for momentum transfer are ...

Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons

We review the data and models describing the production of the electrons, termed secondary electrons, that initiate the secondary and subsequent feedback avalanches required for the growth of current during breakdown and for the maintenance of low-current, cold-cathode discharges in argon. First we correlate measurements of the production of secondary electrons at metallic cathodes, i.e. the yields of electrons induced by Ar+ ions, fast Ar atoms, metastable atoms and vuv photons. The yields of electrons per ion, fast atom and photon vary greatly with particle energy and surface condition. Then models of electron, ion, fast atom, excited atom and photon transport and kinetics are fitted to electrical-breakdown and low-current, discharge-maintenance data to determine the contributions of various cathode-directed species to the secondary electron production. Our model explains measured breakdown and low-current discharge voltages for Ar over a very wide range of electric field to gas density ratios E/n, i.e. 15 Td to 100 kTd. We review corrections for nonequilibrium electron motion near the cathode that apply to our local-field model of these discharges. Analytic expressions for the cross sections and reaction coefficients used by this and related models are summarized.

Predicted electron transport coefficients and operating characteristics of CO2–N2–He laser mixtures

Calculations have been made of transport coefficients of electrons in gas mixtures for ratios CO2:N2:He of 1:1:8, 1:2:3, 1:7:30, and 1:0.25:3. New cross sections for CO2 derived from swarm experiments are used together with previously published cross sections for N2 and He. Curves are presented of the predicted electron drift velocity, transverse and longitudinal diffusion coefficients, and ionization and attachment coefficients for E/N values ranging from 10−18 to 1 × 10−15 V cm2; E is the electric field strength and N the gas number density. Examples are given of derived distribution functions and comparisons are made with a Maxwellian distribution function. The percentage of the input electrical power which excites vibrational processes coupled to the 001 upper laser level of CO2 is given as a function of E/N. The maximum efficiency from these calculations increases for increasing ratios of N2:CO2, because the proportion of energy used to excite the bending and stretching modes of CO2 is then reduced. ...

Cross Sections and Swarm Coefficients for H+, H2+, H3+, H, H2, and H− in H2 for Energies from 0.1 eV to 10 keV

Graphical and tabulated data and the associated bibliography are presented for cross sections for elastic, excitation and ionization collisions of H+, H2+, H3+, H, H2, and H− with H2 at laboratory energies from 0.1 to 10 keV. Where appropriate, drift velocities and reaction or excitation coefficients are calculated from the cross sections and recommended for use in analyses of swarm experiments and electrical discharges. In the case of H+ in H2, cross sections for momentum transfer, rotational excitation, vibrational excitation, charge transfer, electronic excitation, and ionization are recommended. Energy‐loss or stopping‐power coefficients calculated from these cross sections are much smaller than obtained from stopping‐power theory. There are no relevant energy‐loss experiments for H+ in H2. Drift velocity calculations predict runaway for H+ in H2 for electric field to gas density ratios E/n greater than 700 Td, where 1 Td (townsend)=10−21 V m2. For H2+ in H2, the cross sections include H3+ formation, ...

The application of scattering cross sections to ion flux models in discharge sheaths

We suggest consistent sets of Ar++Ar and Ar+++Ar differential and integral cross sections for modeling ion scattering that take into account differential scattering data and the fact that symmetric charge transfer collisions are one aspect of elastic scattering collisions. These suggestions make possible a considerable improvement in the accuracy of future Monte Carlo calculations of the angular, energy, and temporal distributions of Ar+ and Ar++ ions passing through the electrode sheaths of low‐pressure, rf, and dc discharges in Ar. The cross sections necessary for a proper modeling of the energy dissipation in the gas and at the electrodes by fast neutral Ar atoms formed in symmetric‐charge‐transfer collisions of Ar+ and Ar++ with Ar are also reviewed.

Electron Attachment and Detachment. I. Pure O2 at Low Energy

Electron attachment and detachment coefficients are reported for pure oxygen from analyses of the current waveforms observed in drift‐tube experiments. The results are consistent with the identification of the negative ion as O2− with an electron affinity of 0.43±0.02 eV. The two‐body collisional detachment coefficient for O2− in thermal equilibrium with the gas increases from 9×10−17 cm3/sec at 375°K to 1.4×10−14 cm3/sec at 575°K. The three‐body attachment coefficient for thermal electrons increases from 2.0±0.2×10−30 cm6/sec at 300°K to 2.8±0.5×10−30 cm6/sec at 530°K. The O2− ions are found to survive at least 3×108 elastic collisions without de‐excitation and so are believed to be in their lowest vibrational state. At low oxygen densities the current of detached electrons is separated from the negative‐ion current by applying a high‐frequency voltage to the control grid. At high oxygen densities the electrons and negative ions cross the tube in a narrow pulse at a drift velocity determined by the equil...

Excitation of the b 1Σ+g state of O2 by low energy electrons

Rate coefficients for excitation of the b 1Σ+g state of O2 by low energy electrons have been measured using a drift tube technique. The time dependence of the absolute intensity of the 762 nm band emission was measured for O2 densities between 1016 and 2×1018 molecules/cm3. When corrected for electron attachment, ionization, and metastable diffusion, the number of b 1Σ+g molecules produced per centimeter of electron drift and per O2 molecule calculated from the 762 nm emission varied from 1.3×10−18 cm2 at E/N=5×10−17V cm2 to 2.1×10−16 cm2 at E/N=2×10−15V cm2. These values of electric field to oxygen density ratio E/N correspond to mean electron energies of 0.75 and 6 eV, respectively. Measured decay constants for the 762 nm radiation yield a value for the product of the diffusion coefficient and the O2 density of (5.0±0.3) ×108 cm−1 sec−1 and a quenching coefficient for the b 1Σ+g state of (3.9±0.2) ×10−17 cm3 sec−1. Comparison of measured excitation coefficients with values calculated using a recommended...

Survey of Negative‐Ion—Molecule Reactions in O2, CO2, H2O, CO, and Mixtures of These Gases at High Pressures

The negative‐ion species produced in O2 and some gases containing oxygen have been surveyed for various pressure and E/p conditions using an rf mass spectrometer coupled to an electron drift tube operating at pressures up to 5 torr. The gases studied include O2, CO2, H2O, and CO as well as the mixtures CO2–O2, H2O–O2, CO–O2 at moderate and high E/p. When CO2 is present, O2− and O− are converted to CO4− and CO3−, respectively. The rate coefficients for these reactions at 300°K are approximately 1.3×10−29 and 8×10−29 cm6/sec, respectively. When H2O is present, complex ions such as (H2O)n·O2−, (H2O)n·O− and (H2O)n·OH− are formed with n≤5. In CO–O2 and H2–O2 mixtures, ion destruction, consistent with associative detachment of O−, has been observed to proceed with rate coefficients of about 10−9 cm3 sec−1.

Electron-transport, ionization, attachment, and dissociation coefficients in SF6 and its mixtures

An improved set of electron‐collision cross sections is derived for SF6 and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF6 and mixtures of SF6 with N2, O2, and Ne. The SF6 cross sections differ from previously published sets primarily at very low and high electron energies. At energies below 0.03 eV the attachment cross section is adjusted to fit recent electron swarm experiments, while the elastic momentum transfer cross section is increased to the theoretical limit. At high energies an allowance is made for the excitation of highly excited levels as observed in electron beam experiments. The cross‐section sets used for the admixed gases have previously been published. Electron kinetic energy distributions computed from numerical solutions of the electron‐transport (Boltzmann) equation using the two‐term, spherical harmonic expansion approximation were used to obtain electron‐transport and reaction coefficients as functions of E/N and the fractional concen...

Electron Attachment and Detachment. II. Mixtures of O2 and CO2 and of O2 and H2O

Electron‐attachment coefficients in O2–CO2 and O2–H2O mixtures and electron‐attachment—detachment equilibrium constants for O2–CO2 mixtures are obtained from drift‐tube measurements similar to those used previously for O2. The three‐body attachment coefficients involving O2 and CO2 are 3.1×10−30, 4.0×10−30, and 2.7×10−30 cm6/sec at 300°, 477°, and 525°K. The electron—negative‐ion equilibrium constant data correspond to the formation of a CO4− with a large degree of freedom of internal motion and with an energy of dissociation into O2− and CO2 of 0.8 eV. The three‐body attachment coefficient involving O2 and H2O is 1.4±0.2×10−29 cm6/sec at 300° and 395°K. In O2–H2O mixtures at temperatures of 500° and 555°K we observe the conversion of O2− to a more stable negative‐ion form which shows no evidence of dissociation or detachment.