Excitation and breakdown of Ar at very high ratios of electric field to gas density

Noble-gas resonant radiation effects on electron emission in plasma devices

Experimental investigation results for photoemission affected by vacuum ultraviolet radiation of xenon and krypton atoms from a solid in vacuum and the target surface in contact with plasma (gas) are presented. It is demonstrated that, for adsorption (or implantation) of gas atoms into the target, the photoemission response considerably (to an order of magnitude) increases. This is caused by a change in the mechanism of photoemission from a solid surface in contact with plasma (gas), as compared to vacuum. This phenomenon can be characterized by the term adsorption- (implantation-) induced resonant photoemission. The inclusion of this phenomenon has largely transformed our view of gas discharge ignition and glowing, in addition to operating a variety of plasmic and photoelectron devices. A different class of gas discharge instruments can be realized on this basis as well.

Energetic ion, atom, and molecule reactions and excitation in low-current H2 discharges: H(alpha) Doppler profiles

Absolute spectral emissivities for Doppler broadened H(alpha) profiles are measured and compared with predictions of energetic hydrogen ion, atom, and molecule behavior in low-current electrical discharges in H2 at very high electric field E to gas density N ratios E/N and low values of Nd , where d is the parallel-plate electrode separation. These observations reflect the energy and angular distributions for the excited atoms and quantitatively test features of multiple-scattering kinetic models in weakly ionized hydrogen in the presence of an electric field that are not tested by the spatial distributions of H(alpha) emission. Absolute spectral intensities agree well with predictions. Asymmetries in Doppler profiles observed parallel to the electric field at 4<or=E/N<or=20 kTd result primarily from excitation by fast H atoms directed toward the cathode and diffusely reflected from the cathode. (1 Td=10(-21) V m(2)) The effects of reflection of hydrogen particles and of changes with cathode material are modeled accurately without adjustable parameters. Maximum measured wavelength shifts result from acceleration of H+ ions and charge transfer to fast H atoms. The Doppler profiles are consistent with models of reactions among H+, H2+, H3 , H, and H2 leading to fast H atoms and then fast excited H(n=3) atoms.

Energetic ion, atom, and molecule reactions and excitation in low-current H2 discharges: spatial distributions of emissions

Spatial distributions of H alpha , H beta , and the near-uv continuum emission from the H2 a ;{3}Sigma g;+ state are measured and compared with a model for low-current electrical discharges in H2 at high E/N and low Nd , where E is the spatially uniform electric field, N is the gas density, and d is the electrode separation. Data are analyzed for 300 Td<E/N<45 kTd , d=0.04 m , and 2 x 10;{21}<N<2.6 x 10;{22} m;{-3} . (1 Td=10;{-21} V m;{2}) The excitation is produced by electrons and by hydrogen atoms and molecules with mean energies from 5 to 1500 eV. Electron-induced emission, dominant at low E/N and low pressures, is distinguished by its buildup toward the anode. Excitation of H alpha by fast H atoms dominates at high E/N and increases toward the cathode. The observed H alpha emission at low E/N is normalized to previous experiments to yield absolute experimental excitation coefficients for all E/N and Nd . Small adjustments of model parameters yield good agreement with H alpha data. Cross sections are derived for excitation of the H2 near-uv continuum by H atoms. Spatial and pressure dependencies of H alpha and H2 near-uv emissions agree well with a model in which reactions of H2+ , H3+ , and H+ ions with H2 lead to fast H atoms and H2 molecules, which then excite H atoms or H2 molecules.

Energetic ion, atom, and molecule reactions and excitation in low-current H2 discharges: model

Models of the elastic, inelastic, and reactive collisions of energetic hydrogen ions, atoms, and molecules are developed for predicting H_{alpha} and H2 near-uv emission, H_{alpha} Doppler profiles, and ion energy distributions for low-pressure, low-current discharges in H2 . The model is applied to spatially uniform electric field E to gas density N ratios of 350 Td< or =E/N< or =45 kTd and 8 x10;{19}< or =Nd< or =10 x10;{21} m;{-2} , where d is the electrode separation and 1 Td=10;{-21} V m;{2} . Mean ion energies at the cathode are 5-1500 eV. Cross sections for H+ , H2+ , H3+ , H, H2 , and excited H(n=3) collisions with H2 and reflection probabilities from electrodes are updated and summarized. Spatial and energy distributions of ions and fast neutrals are calculated using a "multibeam" technique. At the lower E/N and Nd , electron excitation of H_{alpha} dominates near the anode. Excitation of H_{alpha} by fast H atoms near the cathode increases rapidly with pressure through a multistep reaction sequence. At higher E/N , fast H atoms produced at the cathode surface excite much of the H_{alpha} . The model agrees with experimental spatial distributions of H_{alpha} emission and Doppler profiles. Ion energy distributions agree with experiments only for H2+ . Cross sections are derived for excitation of the near-uv continuum of H2 by H atoms.

Excitation in low-current discharges and breakdown in He at low pressures and very high electric field to gas density ratios E/N

We investigate optical emission from low-current discharges in He at very high electric field to gas density ratios E/N between parallel plate electrodes. We also determine the electrical breakdown and the voltage-current behavior at low currents. The E/N are 300 Td to 9 kTd (1 Td = 10(-21) V m2) at pressures times electrode separations p(0)d from 3 to 0.9 Torr cm. Absolute optical emission probabilities versus distance are determined for the 501.6 nm line (3 (1)P -->2 (1)S) and for the 587.6 nm line (3 (3)D -->2 (3)P) by reference to Boltzmann calculations at our lowest E/N and to published pressure dependent electron beam experiments. At E/N below 1 kTd, the emission follows the exponential growth of the electron density, while at above 7 kTd heavy particle excitation is evident near the cathode. Collisional transfer of excitation from the singlet to the triplet system dominates the 587.6 nm excitation. Comparisons of models with experiments show the importance of excitation and of electron production at the cathode by fast He atoms produced by charge transfer collisions of He+ with He. The breakdown voltage versus p(0)d is multivalued for p(0)d approximately 1.5 Torr cm. At currents below 100 microA and our lower E/N, the discharge voltage decreases linearly with current as expected for an increasing electron yield with ion energy and E/N at the cathode.