Aust. J. Phys., 1982,35, 521-31 A general description of electron-photon angular correlations is given which relates the quantummechanical observables to the structure of the coherently excited state. The wavefunction is expressed in terms of magnetic sublevel amplitudes and state vectors. A semiclassical impact parameter model of orientation is introduced, the experimental considerations are discussed, and experimental results for the orientation and polarization fraction are reviewed.

Photoelectric work functions of flat thin metal oxides of interest as potential field emission sources for field emission displays have been measured using ultraviolet photoelectron spectroscopy. The samples which include molybdenum, lanthanum strontium cobalt oxide, lead zirconium titanate, lead niobium zirconium titanate have also been characterized with X-ray photoelectron spectroscopy and X-ray diffraction spectroscopy. In addition, field emission measurements have been made with molybdenum and molybdenum coated field emission tips using single tip-gated aperture diodes mounted on a micromechanical motion control system. In these measurements, an electron decelerating lens and a hemispherical electron spectrometer is positioned behind the gated aperture so field emitted electrons passing through the aperture may be decelerated and their energy distribution measured. The emission characteristics and electron energy distributions were measured at different electric field strengths to determine the range of validity of the Fowler-Nordheim equation for the various emitter surfaces.

We report a significant decrease in the photoelectric threshold of chemical vapor deposition grown diamond films as the fraction of sp 3 carbon to sp 2 plus sp 3 carbon in the films decreases. Raman spectroscopy and x-ray photoelectron spectroscopy are used to characterize the different forms of carbon in the films and the sp 3 /( sp 2 + sp 3 ) carbon fraction at the surface. We observe a decrease in the photoelectric threshold from 4.5 eV to 3.9 eV as the sp 3 /( sp 2 + sp 3 ) carbon fraction at the surface decreases from 71% to 55%. Ultraviolet photoelectron spectroscopy of the films shows that they have a negative electron affinity surface. Therefore, the work function of the films decreases from 4.5 eV to 3.9 eV. We propose that the decrease in photoelectric threshold is due to a decrease in the band gap of sp 2 - sp 3 carbon networks at the grain boundaries. The observed decrease in photoelectric threshold can be used to tailor the electronic properties of diamond films for specific applications.

Boron doped polycrystalline diamond films grown on p-type single-crystal Si substrates using chemical vapor deposition with a gas mixture of hydrogen, methane and diborane were characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy and photoelectric current measurements. The energy distributions are not sensitive to boron doping for diborane concentrations from 0 to 4.75 ppm, although the boron doping modifies the surface morphology and the photoemission intensity. The photoemission intensity is high where the microcrystalline content is highest (at diborane concentrations of 2.91 and 4.75 ppm. The photoelectric threshold is found to be at 4.38 eV, in agreement with earlier measurements. The present results are characteristic of valence band emission at 4.38, 4.63, 4.92, 5.12 and 5.30 eV for incident photons between 4.87 and 5.63 eV.

Collisional ionization by electrons and positrons is a major unsolved problem in atomic collisions. Because the projectile can transfer significant momentum to the loosely bound atomic electrons and/or the target core, this cannot be treated as a semi-classical three-body collision between the projectile, the target core and a loosely bound electron, even at 100 times the ionization energy. To elucidate this problem, we are studying single and double binary effects in correlation experiments between outgoing projectile and ejected electrons as a function of projectile energy, and projectile and ejected electron exit angles. We have made, what we believe to be, the first observation of anti-capture to the continuum, the electron equivalent of the Thomas peak in ion-atom scattering. We also have observed a very sharp drop in the double differential cross section at 45 degrees. This is probably due to the fact that two identical particles cannot have the same momentum and energy. In addition, our results show that the probability of scattering one electron to a particular angle or two is given only by the geometry in zero order. Thus, the double differential cross section is much larger than one might expect on the basis of a second Born calculation.

We have constructed an apparatus to measure multiparameter angular correlation between reaction products following electron impact ionization of ionic targets to obtain alignment and orientation information. With molecular targets, ground and excited state potential energy diagrams may also be obtained. For incident energies sufficiently large compared to the ionization energy, we will also study final‐state interactions between the charged projectile and the outgoing electrons. We will measure the momentum of all raction products and provide a stringent experimental test of near threshold three‐body theory which could lead to a general theoretical treatment for three particles interacting via a Coloumb field.

Several problems of fundamental importance to physics involving highly correlated and few body systems are being addressed in our laboratories. In particular, the breakup of three low energy charged particles is being studied using crossed beams of electrons and ions. The systems involved can consist of the excitation and dissociation of molecular ions or the resonant excitation-auto-double ionization (READI) of simple atomic ions. The use of particle accelerators in conjunction with electron beams provide a unique combination of tools for studies of this nature. We have constructed an apparatus to measure multiparameter angular correlations between reaction products following electron impact ionization of ionic targets to obtain alignment and orientation information. With molecular targets, ground and excited state potential energy diagrams may also be obtained. We will measure the momentum of all reaction products and provide a stringent experimental test of near threshold three-body theory which could lead to a general theoretical treatment for three particles interacting via a Coulomb field.

The electron-impact single-ionization cross section of Ar7+ has been measured from 18 to 1186 eV using a crossed-beams technique. The present results are higher than previous measurements at the higher energies and are in good agreement with recent distorted-wave calculations that include contributions from excitation autoionization. Below the threshold for direct ionization at 144 eV, the cross section contains contributions due to space-charge modulation and excitation of metastable ions in the target beam into autoionizing states. These transitions are predominantly spin allowed and dipole forbidden. Scaling laws for sodiumlike ions are discussed.

A photoelectron source that produces an electron current of as high as 200 x 10(-6) A with an effective electron brightness as high as between 10(7) A/cm2 sr has been developed. This electron source has been shown to have an energy width as low as 0.036 eV FWHM.

A crossed-electron-ion-beam experiment was used to measure the total ionization cross section for Ar8+ in the energy range from 50 to 600 eV. The present results are roughly in agreement with the previous measurements of Defrance et Defrance et al. [Nucl. Instrum. Methods Phys. Res. B 23, 256 (1987)] above 450 eV. However, the present measurements show a significant contribution below the threshold for direct ionization. The present measurements agree with the distorted-wave calculations of Pindzola et al. [Phys. Rev. A 41, 1375 (1990)] when 3% of the incident ion beam is assumed to be in metastable states and excitation autoionization from metastable states is included.

The University of North Texas Ion Beam Modification and Analysis Laboratory consists of three accelerators. These are: a 200 kV, 10 MA Cockcroft-Walton, a 2.5 MV single-ended Van de Graaff machine and a newly installed National Electrostatics Corporation Model 9-SDH-2, 3 MV tandem. The tandem accelerator was received in November of 1987 and acceptance tests were completed in December of 1987. The tandem — and the 2.5 MV Van de Graaff — are now both installed in a new 7000 ft2 laboratory in the physics building. The tandem which will be used for many of the activities in the laboratory was purchased with a rf charge exchange (Alphatross) source and a SNICS-type cesium ion sputter source. Each of these sources injects through a 30° magnet into a 90° analysis magnet and then into the tandem accelerator. This configuration gives the high mass resolution which is needed for the AMS system. A table will be shown that overviews the analyzed beams that have been obtained to date with the tandem accelerator. A summary of the beam lines that are being constructed for all the activities of the laboratory will also be given. These include: accelerator mass spectrometry (AMS), nuclear reaction analysis (NRA), Rutherford backscattering and channeling (RBS&C), particle-induced X-ray emission (PIXE), high-energy ion implantation (HEII) and atomic collision physics.

Electron-photon angular correlations between electrons which have excited the $2^{1}P$ state of He and photons from the $2^{1}P\ensuremath{\rightarrow}1^{1}S$ transition have been studied for 27-, 30-, 35-, and 40-eV incident electrons. Values of $\ensuremath{\lambda}$ and $|\ensuremath{\chi}|$ obtained from these measurements are compared to values obtained in distorted-wave and $R$-matrix calculations. The values of $\ensuremath{\lambda}$ and $|\ensuremath{\chi}|$ have been combined to examine the behavior of $|{O}_{1\ensuremath{-}}^{\mathrm{col}}|$ [$\ensuremath{\lambda}(1\ensuremath{-}\ensuremath{\lambda})sin|\ensuremath{\chi}|$], the nonvanishing component of orientation. At 27 eV, a substantial decrease was observed in the values of $\ensuremath{\lambda}$ and $|{O}_{1\ensuremath{-}}^{\mathrm{col}}|$, compared with their values for $E>~30$ eV.

The radiative lifetimes and optical excitation functions have been measured for the e 3Σ−(v = 2, 3, 4) levels of CO using pulsed low energy electron impact excitation and delayed coincidence detection. The v = 2, 3, and 4 lifetimes were found to be 4.12±0.14, 4.08±0.29, and 3.73±0.06 μs, respectively. Cascade components were observed to feed all three vibrational levels and these lifetimes were measured to be 13.8±6.5, 11.8±1.3, and 14.9±3.6 μs. The threshold energies of these states are discussed. The optical excitation functions indicated the presence of thresholds above those of the e states which appear to be due to nearby Asundi band vibrational levels ranging from v = 8 to v = 16.

The electron-photon angular correlation function was measured between 80-eV electrons which excited the $2^{1}P_{1}$ state of helium and 58.4-nm photons from the decay of that state for electron scattering angles ranging from 5\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{}. The data have been analyzed to yield values of the ratio $\ensuremath{\lambda}$ of the differential cross section for exciting the ${M}_{j}=0$ sublevel to the total differential cross section and the magnitude $|\ensuremath{\chi}|$ of the phase difference between the ${M}_{j}=0$ and ${M}_{j}=1$ excitation amplitudes. The data agree with all previous measurements within one standard deviation, with the exception of the large-angle values of $\ensuremath{\lambda}$ obtained by Hollywood, Crowe, and Williams. Possible causes of these discrepancies are discussed. The values of $\ensuremath{\lambda}$ and $|\ensuremath{\chi}|$ obtained in this work agree quite well with those given by the distorted-wave calculations of Madison over the entire angular range.

Electron-photon angular correlation measurements have been made for scattered electrons which have excited the $2^{1}P_{1}$ state of helium and photons from the $2^{1}P_{1}\ensuremath{\rightarrow}1^{1}S_{0}$ transition at an incident electron energy of 80 eV for a range of electron scattering angles ${\ensuremath{\theta}}_{e}$ from 5\ifmmode^\circ\else\textdegree\fi{} to 155\ifmmode^\circ\else\textdegree\fi{} and for a photon angle ${\ensuremath{\theta}}_{\ensuremath{\gamma}}$ of 90\ifmmode^\circ\else\textdegree\fi{} in the scattering plane. These measurements have been analyzed to obtain relative values of the ratio $\ensuremath{\lambda}$ of the differential cross section for exciting the ${m}_{j}=0$ sublevel ${\ensuremath{\sigma}}_{0}$ to the total differential excitation cross section $\ensuremath{\sigma}$, for $J=1$ ($\ensuremath{\lambda}=\frac{{\ensuremath{\sigma}}_{0}}{\ensuremath{\sigma}}$). The results extend the measurement range to both larger and smaller values of ${\ensuremath{\theta}}_{e}$ at this energy and are consistent with previous measurements and the most recent calculation for the process.

The excitation and subsequent decay of the E /sup 3/..sigma../sub g//sup +/ state of N/sub 2/ has been studied in a delayed coincidence experiment with a pulsed electron beam by monitoring the time and pressure dependence of the second positive C /sup 3/Pi/sub u/..-->..B /sup 3/Pi/sub g/(0,0) radiation at 3371 A. The rate equations for the populations of the E and C states have been analyzed for the conditions of the experiment and the relevant equations obtained. The analysis shows the 3371 A radiation detected, at times long compared to the C state lifetime, to be associated with the initial E state population. Analysis of the time and pressure dependence of this long lived radiation yields a value of 3.8 x 10/sup 3/ sec/sup -1/ mtorr/sup -1/ for the collisional deactivation rate coefficient for the E state and a value of 1.9 x 10/sup 3/ sec/sup -1/ mtorr/sup -1/ for the energy transfer rate coefficient between the E and C states. (AIP)