Decreasing Electron Energy Research Articles

Dissociation of fluorine by electron impact

Using a previously developed F-atom diagnostic technique, the efficiency of F2 dissociation by an electron beam or an electron-beam-sustained discharge was systematically studied. By monitoring the transient concentration of ions generated during the electron-beam-sustained discharge, we have determined the F−+F+2 ion-ion recombination and F2 dissociative attachment rate constants at various E/P conditions. The dissociative attachment rate constant of F2 at an average electron energy of 1 eV was found to be about (2.3±0.3) ×10−9 cm3/molecule sec−1 and increased with decreasing electron energy. The F−+F+2→3F recombination rate constant at an E/P of 18 kV/cm/atm was found to be about (1.5±0.2) ×10−8 cm3/molecule sec−1 and also increases with decreasing E/P values. The sustained-discharge enhancement of F-atom production from these experiments is F2 is small, ∼1.7.

Dissociative attachment of electrons to F2

By measuring the equilibrated sustainer current density in F2/N2 mixtures under controlled discharge conditions, we have determined the F2 dissociative attachment rate constant at various E/P conditions. The F2+e→F∓F rate constant at an average electron energy of 1.0 eV was found to be 2.3±0.3×10−9 cm3/molecule/sec and increased with decreasing electron energy.

Pure rotational excitation ofH2at electron impact energies of 3 to 100 eV

Cross sections for pure rotational excitation in H2 are obtained using two different crossed-beam type electron-impact spectrometers with three sets of conditions: at electron energies of 15-100 eV and scattering angles of 115 deg; 3-40 eV at 20 deg; 40 eV at 10-135 deg. For intermediate-energy electrons the pure rotational excitation cross section at large scattering angles exceeds the elastic scattering cross section at electron energies greater than about 30 eV. Rotational cross sections are found to decrease slowly with decreasing electron energy, with a magnitude at their peak (at 4 eV) about 20 times that at 100 eV. It is suggested that these results may account in part for the large population of excited rotational states observed in interstellar H2.

Low-energy (< 1 eV) electron attachment to molecules in very-high pressure gases: C6H6

Benzene has been found to capture slow (≲ 0.3 eV) electrons in high densities of N2 (and Ar) with a rate that increases with increasing N2 density and decreasing electron energy at near-thermal energies. The dependence of the attachment rate on the N2 density and the electron energy has been studied in detail, and a model is presented that accounts for the experimental results. On the basis of this model the autodetachment lifetime of C6H6-* has been estimated and found to vary from ∼1 psec at 0.04 eV to ∼0.2 psec at 0.18 eV. From the high-pressure experiments the ``liquid state behavior'' has been predicted also. The finding that benzene negative ions are formed in the gas phase forces the conclusion that the benzene molecule has a positive (>0 eV) electron affinity, (EA)B, contrary to the accepted view that (EA)B <0 eV. The short autodetachment lifetime of C6H6-*, however, indicates that (EA)B is small. Benzene was also studied in mixtures with C2H4 and C2H4–N2. The results of these latter experiments strongly indicated that the probability of electron detachment in C6H6-*-C2H4 collisions is large.

Magnetic lens analysis of fast electrons in the negative glow

The short magnetic lens technique used in beta-ray spectroscopy is applied to the fast electrons entering the negative glow from the cathode dark space in a helium glow discharge. Information can be obtained about the decrease of electron energy and number density through the negative glow. The method also gives information about the range of the fast electrons.

Modulus effects in metals after low temperature electron irradiation—I. Cu

A study of electron irradiated Cu has been made by means of modulus measurements. Damage production data obtained at 20°K have been analyzed in terms of a two dislocation model and compared with damage data taken at 77°K. Analysis of the data yields information on the types of dislocations which are being pinned by irradiation-produced point defects and the relative effectiveness of these dislocations as sinks for defects which arrive both dynamically during irradiation (20°K) as well as thermally (77°K). Information is also obtained concerning the range over which defects can travel dynamically. Detailed isochronal annealing studies have been made following 20°K irradiations at various electron energies. After irradiation at sufficiently high electron energies, isochronal measurements show three dislocation pinning stages; these are centered at approximately 55°K (Stage I), 225°K (Stage III), and 300°K (Stage IV). The magnitude of Stage I relative to that of Stage III decreases sharply with decreasing electron energy. This result suggests that the defect responsible for Stage I is a separate entity from that responsible for Stage III. A two interstitial model is used in explaining Stages I and III; Stage IV is attributed to vacancies. Results of quenching studies substantiate the latter assignment. Migration of defects to dislocations in Stages I and III is found to occur with a 2 3 -dependence on annealing time. The activation energy for migration in Stage III is about 0.6 eV while that in Stage IV is approximately 1.0 eV. A model for crowdion motion to dislocations in Stage I is outlined.

Absolute Total Electron-Helium-Atom Scattering Cross Sections for Low Electron Energies

Abstract : The Ramsauer technique has been used to measure the absolute total electron-helium-atom scattering cross section as a function of electron energy from 0.30 to 28 eV with an estimated probable error of =3%. No 'fine structure' has been observed at the lower electron energies studied. The variation of the cross section with energy for energies less than 3 eV is in reasonable agreement with the modified effective-range formula given by O'Malley, using a scattering length of 1.15a sub o. The cross section first increases with decreasing electron energy from 2.2 A sq at 28.0 eV to a maximum of 5.6 A sq at about 1.2 eV and then decreases to 5.4 A sq at 0.300 eV. The cross section has been found to decrease sharply with increasing energy at about 0.5 eV below the first excitation energy. This resonance, predicted by Baranger and Gerjuoy and originally observed by Schulz, first decreases with increasing energy to a minimum of about 10% below the background at 19.285=0.025 eV and then increases to a gentle maximum of about 3% above the background at 19.65=0.05 eV. The resolution of this resonance as well as the 10% decrease in the cross section at the minimum is determined by the half-width of the electron beam at this energy which is about 0.1 eV. (Author)

Ionization Coefficients in Helium at High Pressures

IN recent years there has been considerable effort devoted to the study of the ionization processes which give rise to ionization growth in different gases. Of particular interest from the fundamental point of view is the growth of ionization in helium, because in this case quantum mechanical methods may be more easily applied than in the more complex case of diatomic gases. Consequently, direct comparison may be made between experimental data on ionization coefficients, for example, and the quantum mechanical computations. Despite its simple atomic structure, however, the existence of high-energy metastable states of the helium atom, as well as of the helium molecule, results in the possibility of highly complex ionization processes occurring in this gas1–3. Moreover, experimentally, helium is a difficult gas on which to make observations because it has the highest ionization potential (24.6 eV) of any known gas. Most common impurities have ionization potentials ( 20 eV) to the ground-state. The effects of impurities, even when present in quantities as low as 1 part in 106, become increasingly important as the parameter E/p (E, the electric field, p, the gas pressure) decreases; because, as E/p decreases, the mean electron energy decreases and the ratio of ionized impurity atoms to ionized helium atoms then increases. These considerable experimental difficulties probably explain why there are so far no published data on the measurement of ionization growth in helium at high gas pressures and low values of E/p.

Cascade Capture of Electrons in Solids

Enormous capture cross sections in the range ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ to ${10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{2}$ have been observed for a wide variety of Coulomb attractive centers in Si and Ge, some involving binding energies many times the Debye energy. Whereas multiphonon transitions to the ground state yield cross sections five to ten orders of magnitude too small, capture into excited states of large radius followed by a cascade of one-phonon transitions leads to cross sections of the right order of magnitude. The initial capturing event is likely to involve an optical phonon or an intervalley collision in the room temperature range, but the acoustic phonon contribution will predominate at low temperatures.Subsequent collisions may eject the electron or cause it to increase its binding energy. The "sticking probability," or probability of eventual capture into the ground state, becomes significant for binding energies of order $\mathrm{kT}$. As the temperature is reduced capture into orbits of larger radius becomes effective, and, at least for the acoustic phonon case the cross section increases rapidly with decreasing temperature, and with decreasing electron energy. The large cross sections ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ to ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ found for neutral centers can be explained on a similar basis, the attractive potential in this case being provided by the large polarizability of the neutral center.

The polarization of atomic line radiation excited by electron impact

The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable

The Second Born Approximation in Inelastic Collisions of Electrons with Atoms

The second approximation in Born's theory of collisions is worked out for the excitation of the 2p state of hydrogen and of the 21P state of helium from the corresponding ground states by electron impact. The method is not applicable to electron energies too close to the threshold. The correction introduced by the second approximation, which reduces the intensity of the small angle inelastic scattering and the total inelastic cross section by an amount increasing as the electron energy decreases, is of the correct sign and about the right magnitude, in the energy range to which it is applicable, to provide an explanation of the discrepancies between calculated and observed cross sections for excitation of optically allowed transitions.

Formation of Negative Ions in Gases by Electron Attachment Part I. NH3, CO, NO, HCl and Cl2

With the same method and apparatus employed in the study of negative ion formation in O2 by electron attachment, the results have been extended to all the common diatomic gases and NH3. No negative ions are formed in NH3 below an X/p of 7.5; beyond that point they are formed with increasing probability as the energy of the electrons increases. The phenomenon is interpreted as dissociation of the molecule with the formation of NH— at an electronic energy of approximately 3 volts, the NH3 molecule having itself no electron affinity. No negative ions could be formed in CO at the electronic energies available, and the molecule is presumed to have no electron affinity. Negative ions are formed by electron attachment in NO, the probability increasing with decrease in electronic energy. A linear variation in probability of attachment with pressure is also observed in NO which is due to a collision with a pair of NO molecules held together by weak attractive forces. Negative ions are formed in HCl with a probability which increases with increasing electronic energy suggesting that dissociation of the molecule occurs here as well. Similar results are obtained in Cl2 where it has been known that the energy of the attachment process must be more than sufficient to dissociate the molecule. The experiments indicate several types of attachment processes which can occur in gases and the possibilities of energy dissipation in ion formation. In general the most favored process is the carrying off of the energy by a third body involved in the process.