Showing papers on "Ionization published in 1971"

Radiative cooling of a low-density plasma

Abstract: The paper extends and improves the radiative cooling coefficient calculations of Cox and Tucker (1969) and Cox and Daltabuit (1971) for a low-density optically thin plasma with no molecules or dust, of cosmic abundances in the range from 10,000 to 100,000,000 K. Earlier rates applied to a plasma containing H, He, C, N, O, Ne, Mg, Si, and S; the present study added Ca, Fe, and Ni. For the elements included, many individual lines are calculated which previously has been averaged together. The cooling processes considered are permitted, forbidden, and semiforbidden line transitions, including contributions from dielectronic recombination and bremsstrahlung, radiative recombination, and two-photon continua. The ionization balance is calculated in collisional equilibrium using an approximate auto-ionization following inner-shell excitation and the low-density limit for the dielectronic recombination rate. Cooling in various observational bands in the soft X-ray region is determined.

Determination of the K-LL Auger spectra of N2, O2, CO, NO, H2O, and CO2

Abstract: A double‐focusing electrostatic electron spectrometer has been used to measure the K—LL Auger spectra resulting from electron impact for each of the elements in the gaseous molecules N2, O2, CO, NO, H2O, and CO2. An energy resolution of 0.09% full width half‐maximum was normally employed. A method for analyzing these complex spectra is described. It involves the identification of normal and satellite lines. The former are defined as arising from single electron ionization from the K shell without additional excitation followed by an Auger process in which one electron fills the vacancy while a second goes into the continuum, and where all the other electrons remain in their same orbitals. Satellite lines result when extra excitation occurs either in the initial formation of the K vacancy or in the subsequent Auger process. To aid in the identification of these satellite lines, auxiliary experiments have been performed such as the study of discrete energy losses in photoionization due to electron shake‐up,...

Measurements of Secondary‐Electron Spectra Produced by Electron Impact Ionization of a Number of Simple Gases

Abstract: The energy distribution and angular dependence of secondary electrons generated by the impact of 100–2000‐eV electrons on He, N2, and O2 and by the impact of 500‐eV electrons on Ne, Ar, Kr, Xe, H2, NO, CO, H2O, NH3, CH4, C2H2, and CO2 have been measured over the 4–200‐eV range. The measurements were made in a crossed‐beam apparatus with the use of a fixed hemispherical electrostatic analyzer and a rotatable electron gun. The observed spectra were integrated over angle to obtain relative cross sections for secondary‐electron production. It was found that the shapes of the spectra of all the gases (except Ar, Kr, and Xe, which contain intense electron emission features in this energy range) were smooth and qualitatively similar, approaching a constant cross section at low secondary energies, and falling off at high secondary energies slightly faster than Es−2, where E8 was the energy of the secondary. The shape of the spectrum was found to be nearly independent of primary energy in He, O2, and N2.

Two electrons in a coulomb potential. double-continuum wave functions and threshold law for electron--atom ionization.

Abstract: The Schr\"odinger equation for two electrons in a Coulomb field is studied in the critical region where both electrons have near-zero kinetic energies. The main feature of this problem is that the mutual screening between the two electrons determines and is determined by the partition of the available energy between them. This energy-dependent screening can be taken into account to yield a complex potential in the radial variable $R={({r}_{1}^{2}+{r}_{2}^{2})}^{\frac{1}{2}}$ of the six-dimensional configuration space of the two electrons. Solutions of this equation are obtained and are shown to correspond to the classical orbits given in an early paper by Wannier. A possible way is indicated of using these wave functions to establish the Wannier threshold law which, for ionization of neutral atoms, is $\ensuremath{\sigma}\ensuremath{\propto}{E}^{1.127}$. Finally, the interplay between the total energy and the Coulomb potential is discussed both for this problem and for the case of one electron in the field of a nucleus.

WKB approximation and threshold law for electron-atom ionization

Abstract: The solutions of the Hamilton-Jacobi equation and of the continuity equation for electron-atom ionization problem at zero total angular momentum are investigated in the neighbourhood of saddle points of the potential. Solutions are found which describe sets of orbits whose points of condensation coincide with the saddle points of the potential. The asymptotic behaviour of solutions corresponds to a spherically outgoing wave or to a plane wave. The energy dependence of WKB wave functions is determined and by means of a matching procedure the Wannier ionization threshold law has been confirmed. The connection between the WKB approximation and the asymptotic expansion of the zero- energy wave function has been considered.

Penetration of fast hydrogen atoms into a fusion reactor plasma

Abstract: The depth of penetration of fast hydrogen atoms into a fusion reactor plasma, before they become ionized and held by the magnetic field, is of importance in connection with the heating and refuelling of such a plasma. Here the experimental cross-sections for the important collision processes are reviewed and analytic expressions are formed which describe the data reasonably well.The ion and electron energy distributions to be expected in both toroidal and mirror reactors are used, with the analytic expressions for the cross-sections, to calculate rate coefficients. The plasma thickness T for an 1/e reduction in intensity of the atom beam is then obtained as a function of beam energy and plasma temperature over the range 0.1 to 2000 keV (D+, Do). As an example a 1 MeV deuterium atom beam entering a toroidal reactor plasma has T=1.7×1016 and 2.5×1016cm−2 for plasma temperatures of 1 and 100 keV, respectively.

Interpretation of Photoionization Threshold Behavior

Abstract: Analytical expressions are derived for treating the threshold regions of photoionization efficiency curves The effects of slitwidth, the initial thermal energy of the molecular system, and Franck–Condon factors are considered In the dissociative ionization of small molecules such as F2 and HCN, discrepancies can arise from failure to take these factors into account It is also shown that for large polyatomic molecules, the use of a Watanabe‐type plot to determine the adiabatic ionization potential can lead to large errors The paper concludes with detailed applications to HF, O2, S2, Se2, Te2, S6, and S8, and some general observations applicable to the diatomic halogens, hydrogen halides, and alkali halides

PLASMA DIAGNOSTICS OF AN rf-SPUTTERING GLOW DISCHARGE.

Abstract: The positive ions incident on the substrate plane in a planar diode discharge system are observed with an electrostatic deflection energy analyzer and a quadrupole mass spectrometer. Several targets have been rf sputtered in rare‐gas discharges and the mass spectra that are obtained indicate that the sputtered species, which leave the target predominantly as neutrals, are subsequently ionized in the discharge by Penning ionization rather than by electron‐impact ionization or other ion‐molecule reactions.

INELASTIC SCATTERING OF ELECTRONS AND PROTONS BY THE ELEMENTS He TO Na.

Abstract: We have calculated discrete and continuum generalized oscillator strengths for all the occupied shells of He-Na and the $3s$ and $3p$ continuum generalized oscillator strengths for Ar. The calculations are done with a one-electron common-central-potential unrelaxed-core approximation. The generalized oscillator strengths were used to compute proton-excitation and -ionization cross sections and stopping power, electron-ionization cross sections, and neutral-neutral-ionization and -stripping cross sections. For proton ionization above 200 keV and electron ionization above 200 eV, the calculated cross sections are in better than 20% agreement with experiment. The calculated proton stopping power is lower than experiment by 25% at 100 keV and within 10% at 1 MeV. The computed He-He ionization cross section agrees with the measurement by Wittkower, Levy, and Gilbody, while the computed He-Ar ionization cross section is a factor of 5 higher than the measurement by Puckett, Taylor, and Martin.

Dependence of fluorescence yield on atomic configuration

Abstract: A statistical weighting procedure is used to calculate the fluorescence yields omega K and omega L23 for various defect atomic configurations of the argon atom. The changes in fluorescence yield which accompany atoms with multiple vacancies in various shells should be included when determining inner shell ionization cross sections from X-ray data in heavy ion-atom collision studies. The general features discussed may be important for solid as well as gaseous targets.

Photoelectron spectra of the halogens and mixed halides ICl and IBr

Abstract: High-resolution photoelectron spectra of F2, Cl2, Br2, I2, ICl and IBr are reported and interpreted in terms of the electronic structures of these molecules. Vertical and adiabatic ionization potentials as well as vibrational data are given for the ionized states arising from the removal of an electron from each molecular orbital. Where possible, these values are related to the emission spectra of the molecular ion and to the vacuum u.-v. spectra of the neutral species.

Computer simulation of rapidly developing gaseous discharges

Abstract: A method is described for tracing theoretically the growth in space and time of a rapidly developing gaseous discharge between parallel-plate electrodes where the electric field is distorted by the space charge of the electrons and ions. The results obtained for a discharge in nitrogen initiated by a pulse of electrons leaving the cathode have been used to simulate streak photographs of the light output from the discharge. There is excellent agreement between the present results and the experimental streak photographs of Wagner. It is concluded that the occurrence of the anode- and cathode-directed streamers can be satisfactorily explanined in terms of the basic Townsend ionisation processes, provided that space-charge distortion of the field is allowed for correctly.

Free Radicals by Mass Spectrometry. XLIII. Ionization Potentials and Ionic Heats of Formation for Vinyl, Allyl, and Benzyl Radicals

Abstract: Using an energy-resolved electron beam, ionization potentials for the following free radicals have been measured: vinyl 8.95 V, allyl 8.07 V, benzyl 7.27 V. For vinyl and allyl ions new measurements of thresholds for dissociative ionization give ΔHf(C2H3+) = 266 kcal/mol and ΔHf(C3H5+) = 226 kcal/mol, leading to neutral radical heats of formation ΔHf(C2H3) = 59.6 kcal/mol and ΔHf(C3H5) = 40 kcal/mol. The data for benzyl radical and ion give ΔHf(benzyl cation) = 213 kcal/mol.

The Jahn-Teller Effect in the Electron Spectrum of Methane

Abstract: The high resolution He I photoelectron spectrum of the (a12t25)2T2 < - (a12t26)1A1 transition in methane has been recorded and analyzed and a value has been obtained for the adiabataic ionization potential. The electron spectrum resulting from ionization of a t2 electron is a composite of at least three overlapping electronic bands which are separated by several tenths of an eV. These bands are the split components of the 2T2 electronic state of the CH4+ ion which is distorted by strong Jahn-Teller forces. The symmetry of the molecule in these three electronic states is believed to be, in order of increasing energy, D2d

Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet

Abstract: The measurements have been conducted in the spectral range from 600 to 2000 A. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 A. From the photoionization curve the first adiabatic ionization potential was found to be 10.87 plus or minus 0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states.

Ionization disturbances caused by gravity waves in the presence of an electrostatic field and background wind

Abstract: Internal gravity waves in the neutral air propagating through the ionosphere cause a corresponding disturbance in the ionization. This note discusses additional effects that arise owing to the presence of an electrostatic field and background wind and in particular the possibility of a ‘spatial resonance’ when the natural drift of ionization irregularities equals the phase velocity of the gravity wave. The electrostatic field may be the one associated with the tidal motions that drive the Sq current system. It is shown that short-period (τ<30 min) disturbances in the F region may be appreciably affected by spatial resonance, if the component of E×B/B2 drift in the wave normal direction is equal to the phase velocity of the wave. In the E region a similar coincidence between the ion drift velocity and the wave phase velocity may cause intense patches of sporadic E to drift almost with the background wind at the sporadic E height.

A New Technique for the Elemental Analysis of Thin Surface Layers of Solids

Abstract: It is shown that the combination of rf glow‐discharge sputtering and mass spectrometry provides a very sensitive method for determining the composition of thin surface layers of solids. The sample is immersed in an rf sputtering glow discharge and a fraction of those sputtered atoms which are ionized in the discharge by Penning ionization are monitored with a quadrupole mass filter. 1011 sputtered atom/sec can readily be detected and considerable improvement in sensitivity can easily be anticipated. The sensitivity is comparable for all elements except He, Ne, and F and is independent of the matrix in which the species are contained; i.e., standards are not required. The matrix can be electrically insulating or conducting.

Threshold Photoelectron Detector for Use in the Vacuum Ultraviolet

Abstract: The device described responds selectively to those photoelectrons that are formed practically at rest in a gaseous target. The energy discrimination is achieved by accelerating the charged particles to reject those formed with appreciable transverse velocity. When this is used in connection with a light source of continuously variable wavelength over the range 10–20 eV, the resulting threshold photoelectron spectra allow the direct spectroscopic determination of ionization energies within a few millielectron volts. The potential of the method for detection of direct ionization and suppression of autoionization is illustrated by measurements of argon, krypton, and xenon. Simultaneously, ions can also be detected by use of a mass spectrometer. Thus the threshold photoelectron spectrum can be correlated with the photoion spectrum; and it is suggested that the method is applicable to coincidence measurements between electrons and ions.

Effect of Thermal Energy on Ionization Efficiency Curves of Fragment Ions

Abstract: The effect of thermal energy on the shape of photoionization‐efficiency curves is analyzed. Several methods for determining thresholds for dethermalized ionization‐efficiency curves are presented. The assumptions and approximations used are explicitly stated for each method. The effect of thermal energy on the photoionization‐efficiency curves of some alkanes is analyzed and the results are in fair agreement with the hypothesis that thermal energy is fully effective in the dissociation of the parent ions.

Thermal emission and capture of electrons at sulfur centers in silicon

Abstract: The electron thermal emission and capture rates and the electron impact ionization rate of trapped electrons at the sulfur donor centers in the depletion region of reverse biased silicon p-n junctions have been measured by the dark capacitance and current transient methods. Least square fits of the low field data give the following electron thermal emission rates: en0t = 1.64 × 1010(T/300°K)2exp [−276/kT] sec for the neutral center and en−1t = 1.03 × 1012 (T/300°K)2 exp [−528/kT] sec for the singly ionized center where kT is in meV. The thermal activation energies are then 276 and 528 meV respectively. The hole emission rates are much smaller and not determined. The electric field dependences of the thermal emission rates of electrons are considerably smaller than that predicted by the Poole-Frenkel theory applied to the ground state: en0t increased by 1.5 at 130°K from 0.2 to 1.0 × 105 V/cm and en−1t by 3 at 200°K. Electron capture coefficients are obtained from capacitance transient during steady state electron injection into the junction depletion layer either by transistor emitter or by optical generation at the surface next to the junction. The electron capture rate at the doubly charged center, cn−2t, decreases from 5 × 10−7 cm3/sec at 3 × 104 V/cm to 10−7 cm3/sec at 1.0 × 105 V/cm with essentially no temperature dependence. The electron capture rate at the singly ionized centers, cn−1t, obtained at 82°K was about two orders of magnitude smaller than cn−2t but had essentially the same electric field dependence. The electron impact ionization rate of trapped electrons at the neutral centers and its electric field dependences were also determined at 82°K.

Dissociation by Electron Impact of Oxygen into Metastable Quintet and Long‐Lived High‐Rydberg Atoms

Abstract: Excitation of O2 by low energy electrons leads to its dissociation with the formation of metastable oxygen atoms. The metastable atoms which have been detected are in the 3s5S0 state at 9.14 eV and in long‐lived high‐Rydberg states. A molecular beam time‐of‐flight method is used to measure their translational energy. Electron impact excitation functions are given for the formation of metastable atoms. High‐Rydberg atoms result from dissociation of initially formed high‐Rydberg molecules. Because a high‐Rydberg molecular orbital is nonbonding, dissociation is determined by states of the core O2+ ion. This mechanism is supported by general agreement between the observed kinetic energy distribution of high‐Rydberg atoms and the kinetic energy distribution of O+ from dissociative ionization of O2. The fragment kinetic energy distributions are discussed in terms of known and predicted states of O2 and O2+.