S. R. Mielczarek

The GaAs spin polarized electron source

The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 μA/mW are obtained. The source is electron optically bright; the emittance phase space (energy‐area‐solid angle product) is 0.043 eV mm2 sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. T...

Energy Surface and Generalized Oscillator Strength of the 1A″ Rydberg State of H2O

The energies and wavefunctions of the lowest singlet and triplet Rybderg states in H2O were calculated in the expansion basis self‐consistent‐field procedure for a single configuration. A portion of the energy surface of the 1A′ Rydberg state for an HOH angle of 105° was determined that shows the asymmetric dissociation of this state into ground‐state H(2S) and OH(2Π) fragments. The energy along the reaction coordinate is almost separable into a repulsion depending only on the center‐of‐mass separation and an attractive potential that depends only on the OH internuclear separation. For the C2υ conformation a large basis set was used to approach the Hartree–Fock limit. Using these functions a minimum was calculated in the generalized oscillator strength for the 1A1 → 1B1 transition as a function of the momentum transfer function or the electron scattering angle. This behavior was then confirmed experimentally for an energy loss of 7.4 eV with an incident electron energy of 500 eV. Such a minimum is a gener...

Apparent Oscillator Strengths for Molecular Oxygen Derived from Electron Energy-Loss Measurements

Oscillator strengths for O2 from 6 to 14 eV are derived from the energy‐loss spectrum of 100 eV incident electrons. Integrated f values for the Schumann–Runge bands and continuum, which span four orders of magnitude in intensity, agree well with high‐resolution photoabsorption measurements. Vibrational structure superimposed on the Schumann–Runge continuum, previously assigned to the (3sσg)  3Πg Rydberg state, contributes less than 0.5% to the total oscillator strength determined for that region. These data also yield f values for discrete bands in the region between 9.5 and 14.0 eV, where line saturation problems complicate oscillator strength analysis of the optical data. An oscillator strength sum of 0.198 is obtained for all transitions below the ionization potential at 12.07 eV.

Electron energy loss spectroscopy of acetone vapor

High resolution, inelastic electron scattering data can provide new spectroscopic information on the electronic structure of polyatomic molecules. Features in the acetone energy loss spectrum from 0 to 15 eV obtained for 100 eV incident electrons correspond to vibrational, electronic discrete, and electronic continuum excitations. These data are compared with optical measurements in a wide spectral region extending from the infrared to the vacuum ultraviolet. A comprehensive interpretation of the energy loss spectra is attempted with the use of photochemical and photoelectron data, as well as quantum‐chemical calculations in the literature. Three Rydberg series with quantum defects of 1.03, 0.81, and 0.315 join onto bands previously discussed in terms of transitions to valence orbitals. These series converge to an ionization limit of 9.705 eV in good agreement with previous optical determinations. Dissociative continua underlie the Rydberg region and give rise to a variety of neutral products observed in ...

Inelastic Electron Scattering from Formaldehyde

The electron scattering spectrum of H2CO has been determined in the energy loss range between 0 and 16.0 eV. Three Rydberg series in the region 7–11 eV can be identified with the s, p, and d series reported in the literature by ultraviolet absorption studies. Oscillator strengths are determined for some of the Rydberg states and comparison is made with recent ultraviolet absorption work employing photoelectric methods of detection. A serious disagreement exists between the f values obtained by the two methods for the 3sa1 Rydberg. Another Rydberg series, probably converging to the third ionization potential of H2CO, is observed in the electron scattering spectrum in the region 12.4–14.0‐eV energy loss. Erratic behavior of the quantum defects of the first Rydberg series as well as failure to detect the π → π* transition in H2CO is discussed in the light of recent theories involving valence states which can seriously perturb members of a Rydberg series.

Absorption Spectrum of SF6 in the Far Ultraviolet by Electron Impact

Measurements of forward inelastic scattering of 400‐eV electrons from SF6 were made and used to derive relative ultraviolet‐absorption cross sections. The relative values were normalized to an ultraviolet‐absorption measurement at 23.00 eV. Agreement with additional uv measurements at 21.2 and 17.6 eV was excellent. Oscillator strengths for three absorption bands between 10 and 15 eV, as derived from the electron‐scattering measurements, are in fair agreement with the corresponding values measured by uv absorption. The total oscillator strength for excitations up to 32 eV is found to be 15.3.

Minima in Generalized Oscillator Strengths: C2H4

A characteristic of electron impact excitation of low‐lying Rydberg states is a minimum in the generalized oscillator strength as a function of K, the momentum transfer. This characteristic is used as a probe of the Rydberg character of four transitions in C2H4. Three, the 1Ag → 1B3u(π → 3s + 3s), 1Ag → 1B3u(π → 3pz − 3pz), and 1Ag → 1B3u(π → 4s) transitions, have long been identified as Rydberg and are found to exhibit the characteristic minimum. A fourth transition 1Ag → 1B1u(π → π*) is normally termed a valence excitation, and a theoretical calculation using Hartree–Fock molecular orbitals had predicted no minimum. For an energy loss of 8.0 eV which is identified with the valence transition a definite minimum is observed. Speculation on the source of this anomaly centers on a type of valence–Rydberg configurational mixing which can occur in many molecules. It is suggested that the presence or absence of minima in generalized oscillator strength curves be used to probe this aspect of the character of th...

Energy absorption by N2O in the 4 to 14 eV region

Apparent oscillator‐strength values for transitions in the 4 to 14 eV region in nitrous oxide have been derived from electron energy‐loss measurements. Detailed comparison with photoabsorption measurements in the ultraviolet region indicates a weak transition below the 1Δ←? 1Σ+ transition at 6.8 eV not observed optically. This analysis also provides oscillator‐strength values in the region between 11.5 and 12.4 eV, where no quantitative photoabsorption data are available.

Electron Energy Loss Spectrum of Ozone

Abstract Electron energy loss spectra for ozone are presented over the energy loss range 1–30 eV with an incident electron energy of 300 eV. The data are obtained using an electron monochromator—analyzer combination and a static gas cell, and have a resolution of 0.035 eV fwhm.

Resonances in Electron Scattering from H2, HD, and D2

Resonances in the transmission of electrons through H2, HD, and D2 have been studied for electrons of energy 11 to 13 eV. In H2 and HD pairs of resonances were observed, indicating that there exist two electronic states of H2− and HD−, each with well‐developed vibrational structure. In D2 only one series of resonances is observed. The single series is attributed to the overlap of two states of D2− because of a smaller vibrational separation. The absolute energy scale for the resonances has been recalibrated to an estimated accuracy of 0.1 eV.