Electron Excess Energy Research Articles

A model for prestabilized electron relaxation

A model for electron trap relaxation in glassy polar matrices is proposed. The relaxation process is considered as a result of two simultaneously occurring processes: molecular rotation under the influence of the field of the electron, and electron tunnelling from an occupied shallow unrelaxed trap to a slighly deeper pre-existing trap - this last process is followed by fast rotation of the trap constituting molecules on account of local “heating” of the matrix. This heating is the effect of the conversion of the electronic excess energy after transfer to the deeper trap into the local molecular motions of the trap neighbourhood. Numerical calculations are performed for ethanol and 1-propanol and the results obtained are in good accordance with experimental data. The ability of the model to describe the observed relaxation phenomena is discussed.

On the Scattering Behaviour of Low Energy Electrons Produced by Photoemission‐into‐Electrolytes from Gold and Silver Single Crystal Surfaces

AbstractThe dependence of the photocurrents Yp and Ys for p‐ and s‐polarized light on the electron scavenger concentration has been measured for fixed photon energy. In addition, the spectral dependence of the vector ratio Yp/Ys, for different electron acceptor concentrations has been investigated. The results, obtained on Ag(111) and Au(111) electrodes, indicate that a correlation between electron excess energy and thermalization length in the electrolyte is missing. This behaviour of the emitted electrons can be explained assuming a very rapid thermalization of the excess electrons with subsequent thermal diffusion before solvation takes place.

Ground state energy of excess electrons in liquid rare gas-propane systems

Ground state energy of excess electrons in liquid rare gas-propane systems

Thermalization of the Electron—Hole Plasma in GaAs.

AbstractThe band‐to‐band luminescence lineshape of the electron—hole plasma emission in GaAs is analyzed as a function of the excess energy of the photocreated electrons and holes. This yields experimental data for the dependence of the effective carrier temperature of the electron—hole plasma on the excess excitation energy for high electron and hole densities. Carrier temperatures as low as 5 K can be obtained in the case of resonant excitation. A discussion of these data in terms of the possible energy dissipation processes shows that hole—phonon interaction gives an important contribution to the cooling of the carrier system.

Filamentation of a laser beam in a strongly ionized magnetoplasma

On a time-scale of the order of the energy relaxation time, a high power laser beam, propagating in a strongly ionized magnetoplasma is shown to be unstable for small scale fluctuations. In the domain r0 < [mi/m]½ λm. v2/[ω2c + v2] (r0, λm, v, ωc, and m being respectively the spatial scale of the perturbation, electron mean free path, collision frequency, cyclotron frequency and mass and mi being the ion mass) the main loss of excess electron energy is due to thermal conduction; in the other limit collisional loss dominates. It is shown that for small scale fluctuations the growth rate increases with (i) increasing magnetic field and (ii) increasing r0. For large scale fluctuations the magnetic field does not show any effect; the growth rate, however, diminishes with increasing spatial scale. A maximum growth rate is obtained both for some optimum value of scale length and for intensity of the main beam.

Conduction state energy of excess electrons in condensed media. Liquid methane, ethane, and argon and glassy matrices

The temperature dependence of the conduction electron energy V/sub 0/ in liquid methane and ethane has been measured, and the theoretical model of Springett, Jortner, and Cohen (SJC) for the temperature dependence of V/sub 0/ is shown to apply to liquid alkanes as well as rare gases. Criteria for electron localization in liquid rare gases, molecular gases, and alkanes are critically discussed. The SJC criterion for electron localization in liquid rare gases is shown not be extendable to liquid alkanes or to molecular gases. In alkanes it is assumed that transient localization arises from rotational and translational fluctuations leading to cavities of molecular size. Then consideration of the kinetic and electronic polarization energies with a cavity radius given by Wigner-Seitz radius leads to a new localization criterion that seems generally applicable to alkane systems. Electrons in molecular gases probably move as molecular anions so neither of the above criteria apply to them. An empirical relation between the electron mobility and V/sub 0/ is found that seems valid for all alkanes at room temperature for both localized and quasi-free electron states. Values of V/sub 0/ for polar and nonpolar glassy matrices at 77/sup 0/K are derived from experimental data by twomore » independent methods. It is found that V/sub 0/ becomes more positive as the glassy matrix polarity decreases.« less

Mobility of excess electrons andO2−formation in dense fluid helium

Mobility measurements of excess electrons in dense fluid helium have been made at temperatures of 20.3, 52.8, 77.3, and 160 K, as a function of the helium density. An intrinsic mobility was observed, at all temperatures, which indicates increased electron self-trapping with increasing fluid density (constant $T$), and with decreasing temperature (constant $\ensuremath{\rho}$). These trends are continuations of those previously observed at lower temperatures. For 52.8 K and above, an additional low mobility signal was also observed. The intensity of this other signal is temperature dependent and nonmonotonic with helium density, and is interpreted as ${\mathrm{O}}_{2}^{\ensuremath{-}}$ formation. The probability of electron attachment to the minute concentration of ${\mathrm{O}}_{2}$ impurities is enhanced for helium densities at which the excess electron energies are in resonance with vibrational states of ${\mathrm{O}}_{2}^{\ensuremath{-}}$. The temperature effects indicate that electrons which are self-trapped are the ones predominantly involved in the ${\mathrm{O}}_{2}^{\ensuremath{-}}$ formation, as would be expected from their large geometrical cross section.

Microdipole model for trapped electrons in nonpolar liquids and glassy solids

A model is developed to treat the binding energy of excess electrons in nonpolar liquids and solids such as alkanes. Interaction of the excess electron with C–H bond dipoles is considered for several geometrical arrangements. On this basis it is found that a binding energy of about 0.5 eV can be explained for trapped electrons in condensed alkanes. Although long range polarization interactions are unimportant, the electron interaction with the bond microdipoles of a net nonpolar molecule appears to be the key to understanding electron binding in alkane matrices.

Electron and hole band structures and mobility in γ-polymorph platinum phthalocyanine

Excess electron and excess hole energy bands for γ-PtPc have been calculated using the tight-binding approximation, extended Hückel technique LCAO MO's as basis functions for constructing the Bloch functions and a modified molecular potential. The energy bands obtained are highly anisotropic. It is argued that the holes play a dominant role in the charge carrier transport in PtPc. In the absence of experimental data on PtPc, the calculated results are discussed in the light of the available results for H 2Pc and CuPc.

Determination of ionization potentials. Part 1.—The use of theoretical expressions which incorporate the effects of Maxwellian inhomogeneity

A study has been made of the feasibility of determining electron-impact ionization potentials by fitting various theoretical expressions to experimental ionization efficiency behaviour. The theoretical expressions which have been examined incorporate the effects of the Maxwellian energy distribution of a thermionic energy source, and are based on threshold behaviour proportional to the first or second power of the excess electron energy, and the occurrence of one or two ionization processes in the experimental potential range examined. The results indicate that first ionization potentials may be determined within an accuracy of ±0.3 eV, although the procedure in its present stage of refinement cannot be reliably extended to the determination of higher ionization potentials.

Localized Excess Electrons in Water and Ice. An Ice Cavity Model for the Localized Electron and the Microscopic Relaxation Time

In this paper we have identified the structure of the trapping center for localized excess electrons in water and ice with the structure of the large cavities existing in ordinary hexagonal ice (ice I). The short-range attractive interactions with the 12 water molecules on the cavity boundary were treated as charge point dipole interactions, and the long-range interactions were represented by a Landau-type polaron potential. The cavities were permitted to expand isotropically with increasing temperature, and the cavity size was calculated from the OO distance which was taken from x-ray data for ice and identified with the position of the first peak of the radial distribution function for water. Without the use of adjustable parameters this ``ice cavity model'' gives a reasonable agreement between calculated and experimental values for the optical transition energy of excess electrons in both water and ice, the general behavior of the temperature dependence of the transition energy is well reproduced between -180 and +83°C, and the pressure shift of the absorption peak is accounted for. The relaxation of the nearest-neighbor water molecules in ice has also been considered and their relaxation time was shown to be much less than the dielectric relaxation time.

Tight binding energy band structure calculations for phthalocyanines. I: β-hydrogen phthalocyanine

Excess electron and excess hole energy bands for beta -hydrogen phthalocyanine have been calculated using the tight binding approximation, omega technique LCAO-MO's as basis functions for constructing the Bloch functions and modified molecular potential. The energy bands are highly anisotropic. The mobilities of electrons and holes have been found to be independent of temperature in the range 200 K to 500 K. It has been suggested that the carrier scattering may be predominantly due to neutral molecules. An attempt has been made to explain some of the results of earlier workers.

Mass spectrometric study of ion-pair processes in diatomic molecules: H 2, CO, NO and O 2

The appearance of ion-pair processes has been studied in H 2, CO, NO and O 2. The different threshold successively observed for each process have been interpreted. Using the results from the convolution of theoretical models of the ionization efficiency curves and the deconvolution of their first differential, it was possible:(i) to propose a method for the determination of the threshold energy of an ion-pair process, (ii) to show that, except for H 2, the shape of the cross-section of an ion-pair process as a function of the electron excess energy is a step-function and (iii) to determine the kinetic energy distribution of the negative ions and consequently to form an idea about the relative position of the potential energy curves of the excited states of the molecule, decaying by ion-pair process, with respect to the ground state of the molecule. Our analysis suggests that, for CO and NO, some of the observed absorption bands, labelled by Tanaka as P i, ( i=1–5), might be related to the existence of some excited states of these molecules, correlated with an ion-pair as dissociation products.

Energy of Excess Electrons in Nonpolar Liquids by Photoelectric Work Function Measurements

The energy of the excess electron state has been determined for six nonpolar liquids by a photoelectric injection method. The functional dependence of the photocurrents on wavelength is the same in the liquid as in the vacuum and work functions can be evaluated from Fowler plots. For several liquids the work functions are less than the vacuum value and these lowerings are characteristic for each liquid. For tetramethylsilane, neopentane, cyclopentane, and 2,2,4-trimethylpentane the work functions are lowered by 0.62, 0.43, 0.28, and 0.18 eV, respectively. For n-pentane and n-hexane the work functions are close to the vacuum value. The magnitude of the lowering is correlated with the energy of the first electronic absorption band. A variation of electron mobility with the work function changes is noted and discussed relative to the trap theory. The photocurrents measured in the liquids increase with voltage, and at the voltage employed the currents in neopentane and tetramethylsilane are comparable to the vacuum photocurrent. The magnitude of the observed photocurrent depends on the distance the electrons penetrate.

Application of the fluctuation model for electron scattering in liquid alloys to hall coefficients for Hg−Tl and Hg−In systems

The Hall coefficients of liquid Hg−Tl alloys were measured at 20°C using a double ac method. Samples were subjected to an ac current of 77Hz and an ac magnetic field of 60 Hz, and the resulting Hall voltages were measured at the difference frequency of 17 Hz using a phasesensitive detection system. The results for alloys of compositions up to 38.9 at. pct Tl are accurate to ±4 pct. The measured Hall coefficients are significantly lower than the free electron tron values and agree with earlier measurements of Andreev and Regel. Using data for Hg-Tl and Hg-In alloys, the validity of a fluctuation scattering model was investigated. Calculated Hall coefficients agree well with measured values obtained here for the Hg-Tl system, but are lower than reported experimental values for the Hg-In system. Both systems are anomalous with respect to a previously observed correlation between electron concentration and excess free energy of formation of the alloys.

Single and Double Ionization of Na, K and Mg by Electron Impact

The ionization efficiency curves for Na + , Na ++ , K + , K ++ , Mg + and Mg ++ are studied with essentially monoenergetic electrons. It is proved that Na + , K + and Mg + follow the linear law up to 1.5 eV, 0.8 eV and 2 eV above the thresholds, respectively, and that Na ++ and K ++ follow the square law up to about 20 eV and 15 eV above the thresholds, respectively. These results agree with the theoretical prediction that n-fold multiple ionization would be proportional to a n-th power of excess electron energy. Two remarkable humps are found in the I. E. curve for Mg + , which are attributed to autoionization processes, These processes occur at 2.3±0.1 eV and 5.0±0.2 eV above the ionizotion potential, the first one may correspond to the excited states of Mg atom, 1 F° and 3 D°, but any excited states to be correlated to the second one is not known. Contrary to expectation, the I. E. curve for Mg ++ is not a square curve of excess energy but has a complex shape. In order to explain the shape, possibility of autoionization due to excitation of inner shell electron is suggested.

Double and Triple Ionization in Molecules Induced by Electron Impact

Double and triple ionization by electron impact in molecules is examined and, as was found for the monatomic gases, the data support the view that the threshold law for the probability of double ionization is a square-law function of the excess electron energy. Some excited states have been detected, and autoionization does not seem to be important. The vertical potentials for all the processes of multiple ionization observed have been measured. The relative electronic-transition probabilities for single, double, and triple ionization are compared. The factors affecting the stability of multiply charged molecular ions are discussed, and an attempt is made to correlate the electron impact data with the molecular structures. It is shown that Coulomb repulsion between the separated charges causes the potential energy functions describing these ions to be of unusual form.

Ionization of Cu, Ag, and Au by Electron Impact

The relative ionization probabilities of copper, silver, and gold by electron impact have been studied near threshold. Nearly monoenergetic electrons, obtained by means of the retarding potential difference method, were employed. Narrow peaks were found superimposed on the ion yield that arises from transition to the ion ground state. It was possible to correlate these peaks with groups of energy levels, many of which are known to autoionize. The ionization probability curve for formation of doubly ionized gold was also obtained and found to be a sum of linear functions of the electron excess energy. The onset of each linear section indicated the presence of an excited state of Au III. Three energy levels were located at 2.26, 3.92, and 7.81 ev above the ground state.

Probability of Double Ionization by Electron Impact for Neon, Argon, and Xenon

The ionization efficiency curves for the double ionization by electron impact of Ne, Ar, and Xe have been re-examined. It is shown that, near the threshold, the probability of double ionization varies as the square of the excess electron energy. Upper states of the ions are detected and the determination of appearance potentials for such processes is discussed.

Probability of Multiple Ionization by Electron Impact

The ionization efficiency curves for the 3-fold ionization of Ar, and the 3, 4, 5, and 6-fold ionization of Xe have been examined. It is shown that, near the threshold, the probability of n-fold ionization varies as the nth power of the excess electron energy for n = 3 and 4, and probably also for n = 5 and 6. The determination of appearance potentials for such processes is discussed.