Electronic Excitation Threshold Research Articles

Time-dependent aspects of electron degradation. V. Ar-H2 mixtures.

Time-dependent electron degradation spectra and yields of initial products are studied for the Ar-H{sub 2} mixture by using the time-dependent Spencer-Fano (TDSF) theory. An integral form of the TDSF equation is derived and applied in numerical solutions. The present study shows, in the time-dependent context, that addition of a small amount of H{sub 2} in Ar greatly enhances the yield of the triplet states of H{sub 2} because of the presence of subexcitation electrons whose energies are below the electronic excitation threshold of Ar. The production of the triplet states is slow; it takes a period ten times longer than fast processes such as ion production. Secondary processes such as ion-molecule reactions following the electron degradation process are also discussed.

Energy spectra of subexcitation electrons

Subexcitation electrons are those electrons that are produced in irradiated matter and have kinetic energies ( T) lower than the first electronic excitation threshold of the major component. The abundant subexcitation electrons play an important role in many contexts such as the excitation of minor-component molecules. For quantitative assessment of effects of the subexcitation electrons, their energy spectra must be known. Often they are assumed to be of the Platzman form: A+B/( T+I) 3, where I is the ionization threshold. We point out that this form is justified for He and H 2, but not in general. For Ar we have carried out an accurate calculation and have obtained a spectrum not representable by the Platzman form.

Experimental Determination of Energy Dependent Inelastic and Elastic Scattering Rates of Hot Electrons in Large Bandgap Insulators

Internal photoemission for transport analysis (IPTA) and low energy electron transmission (LEET) experiments allow the determination of energy dependent quasi-elastic (acoustic phonon) and inelastic (LO-phonon) scattering rates of hot electrons in wide bandgap insulators for energies smaller than the electron excitation threshold. A model for the analysis of experimental data is presented. The model is used to analyse IPTA and LEET experiments in a variety of insulators such as rare gas solids, amorphous SiO2 and organic dielectrics. The results are compared to scattering rates obtained from electron-phonon interaction theory. The implications of the energy dependence of scattering rates on high field electron transport are discussed.

Near-threshold electronic excitation by electron impact of multilayer physisorbed N2 and CO.

Electronic excitation of ${\mathrm{N}}_{2}$ and CO condensed (i.e., physisorbed) on a metallic substrate has been investigated by high-resolution electron-energy-loss (HREEL) and low-energy electron-transmission (LEET) spectroscopies in the incident energy range 0\char21{}19 eV. The HREEL results yield a more complete picture of the spin-forbidden vibronic bands of the solids than previously available from photon spectroscopies. All levels are found to be shifted down by a few tens of meV and broadened with respect to the gas-phase values. As expected, Rydberg states are absent from such spectra. The LEET experiments provide a measurement of inelastic transitions near the electronic excitation threshold. By recording the second energy derivative of LEET spectra, transitions having a strong cross section near threshold are identified by the sharp structure they produce. These are the a ${}^{3}$\ensuremath{\Pi} state in CO and, in ${\mathrm{N}}_{2}$, the A ${}^{3}$${\ensuremath{\Sigma}}_{u}^{+}$ and B ${\mathrm{}}^{3}$${\ensuremath{\Pi}}_{g}$ states and a new previously unobserved vibronic band near 12 eV. The magnitudes of thresholds cross sections in the solid phase are discussed in terms of single-electron\char21{}molecule scattering mechanisms.

A Theory of Resonant Photoemission Spectra in Black Phosphorus

Our previous theory of the resonant photoemission spectra in black phosphorus is improved by using a more realistic density of states of conduction and valence bands and by taking account of the effect of a core hole potential. From comparison of the calculated spectra with recent experimental data, it is shown that a core exciton plays an important role both in Fano-type resonance and the enhancement of the Auger electron intensity at the core electron excitation threshold.

The Schwinger variational method in electron-atom and electron-molecule scattering theory with polarisation

A method of implementing the Schwinger variational expression in electron-atom and electron-molecule scattering calculations with polarisation included via an ab initio optical potential is described. The Schwinger expression is also used to obtain approximate molecular Hartree-Fock continuum orbitals at arbitrary energies which are needed to calculate the optical potential above the first electron excitation threshold. Explicit closed-form expressions of integrals required for a Gaussian orbital basis are given. The method is applied to low-energy electron-helium scattering with a second-order optical potential.

Lifetimes of exciton-polariton luminescence of anthracene crystal at low temperature

The lifetimes of anthracene crystal luminescence have been measured at 1.6K The wavelength dependence of the lifetime has been determined near the threshold for electronic excitation. The results are discussed in terms of the exciton-polariton model of luminescence.

Unpolarised and spin-change collisions between He+and H at low energies

He+-H collisions are studied for energies lower than the electron excitation threshold. Unpolarised cross sections having resonances and values greater than 4000 a02 and spin-change cross sections showing high structures are obtained. Comparisons with the corresponding H-H collision cross sections are displayed. The sensitivity of the cross sections to the potential energy curves is discussed.

Elastic Scattering of Positrons by Hydrogen Molecules

Elastic positron–hydrogen molecule collisions are considered below the first electronic excitation threshold. The interaction of the incoming positron with the hydrogen molecule is represented by a superposition of single center static and nonspherical polarization potentials. The static potential is expanded to second degree in Legendre polynomials. The Chebyshev polynomials are used to improve the truncated Legendre series approximation and the effect of including the fourth degree contribution is shown to be slight. Positronium formation effects are neglected. Two semiphenomenological polarization potentials are used, and it is shown that the parameters can be suitably adjusted to reproduce the results obtained if the Henry and Lane polarization potential is used. It is shown that the partial Ramsauer–Townsend effect is useful in determining the values of polarization potential parameters. Total elastic scattering cross sections are given for incoming positron energies between about 0.4 and 14 eV while the positron annihilation coefficient is given to about 60 eV.

Chemiluminescence in Molecular Beams. Electronic Excitation of Alkali Atoms in Exchange Reactions of Vibrationally Excited Alkali Halides

Energy transfer by a chemical-exchange process, KBr†+Na→K*+NaBr (where † denotes vibrational excitation and * electronic excitation), has been observed in a ``triple-beam'' experiment. The KBr† was formed at the intersection of crossed beams of K+Br2 and entered a second scattering chamber containing a Na cross beam, where light emitted by K* was observed with a photomultiplier behind an interference filter. The cross section for the exchange process is estimated to be at least 10 Å2. The angular variation of the light intensity agreed precisely with the angular distribution of KBr measured (by a surface-ionization detector) in the same experiment. The light disappeared when propyl iodide was substituted for Br2 (as expected, since the K+PrI reaction exothermicity is well below the threshold for electronic excitation of K). An attempt was also made to observe the nonreactive exchange process, NaBr†+K→NaBr+K* (by interchanging the K and Na beams). However, this was obscured by background light arising from collisions of K with ambient Br2; at present it can only be concluded that the cross section appears to be <10% of that for the reactive exchange process. The light yield from chemical exchange was about as large when the experiments were repeated with Br2 replaced by ICl. Thus the primary K+ICl reaction must give an appreciable yield of KCl (since the exothermicity is above the threshold for K excitation if KCl is formed but below if KI is formed). In this case, the light intensity falls off more rapidly at wide angles than the angular distribution of the alkali halide. This suggests that the primary reaction also forms KI which scatters mainly at wide angles, whereas the KCl scatters mainly forward with respect to the incident K atoms.