Garrett A. Piech

University of Wisconsin, Madison, Madison, MS, United States

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Publications (13)23.13 Total impact

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    ABSTRACT: One requirement for plasma spectroscopy analysis and modeling of neon discharges is a set of electron-impact excitation cross sections covering excitation from both the 2p<sup>6</sup> ground state and from the four 2p<sup>5</sup>3s levels. We present experimental measurements for excitation cross sections into four J = 1 levels of the 2p<sup>5</sup>3p configuration from the J = 0 and J = 2 2p<sup>5</sup>3s metastable levels. A complete set of cross sections into all ten levels of the 2p<sup>5</sup>3p configuration ( 2p<sub>x</sub> in Paschen’s notation) from the ground state, the two metastable levels and the two resonance levels of the 2p<sup>5</sup>3s configuration ( 1s<sub>y</sub> in Paschen’s notation) are compiled in convenient form. The resonance cross sections are obtained from an empirical scaling relationship between the measured metastable excitation cross sections and the corresponding optical oscillator strengths.
    Journal of Applied Physics 07/2011; · 2.21 Impact Factor
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    ABSTRACT: This paper reports the results of measurements of cross sections for electron excitation out of the 1s3 and 1s5 metastable levels of argon (the J=0 and 2 levels, respectively, of the 3p54s configuration) into eight of the ten levels of the 3p54p manifold. The metastable atoms were generated by two methods: (a) an atomic beam emerging from a hollow-cathode discharge, and (b) charge-exchange collisions between a fast argon-ion beam and cesium atoms. The metastable argon atoms are excited by a crossed electron beam into the 3p54p levels and the emissions from these levels are utilized to determine the cross sections. Removal of the 1s5 atoms in the hollow-cathode discharge experiment by means of laser pumping allows us to determine the separate contributions from each metastable level to the observed fluorescence signal. The magnitudes of the cross sections for excitation out of the metastable levels into the different levels of the 3p54p manifold vary vastly. The patterns of the observed variations are interpreted by means of a multipole analysis. This multipole model is also used to discuss the comparison of excitation cross sections out of the metastable levels with those out of the ground level.
    Physical Review A 04/1999; 59(4):2749-2763. · 3.04 Impact Factor
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    ABSTRACT: We have measured electron excitation cross sections out of the 1s3 and 1s5 metastable levels of Ar into eight levels of the 3p5 4p manifold. We use the optical method to determine the cross sections by measuring the radiation of the excited atoms. By optically pumping the 1s5 metastable atoms with a laser, we separate the contributions that each metastable level makes to our observed signal, and thus we can determine the excitation cross sections out of the individual metastable levels. We also interpret the trends in both magnitude and energy dependence observed for the cross sections.
    Physical Review Letters 01/1998; 81:309-312. · 7.73 Impact Factor
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    ABSTRACT: We report cross section results for electron excitation out of the 0953-4075/31/4/029/img2 metastable level of He into the 0953-4075/31/4/029/img3, 0953-4075/31/4/029/img4, 0953-4075/31/4/029/img5, 0953-4075/31/4/029/img6, 0953-4075/31/4/029/img7, 0953-4075/31/4/029/img8, 0953-4075/31/4/029/img9, 0953-4075/31/4/029/img10, 0953-4075/31/4/029/img11 and 0953-4075/31/4/029/img12 levels for energies from onset to 18 eV. For each level, we use the optical method to measure the apparent cross section, which equals the sum of the direct excitation cross section and the cascade cross section. The direct cross section is then determined for each level by subtracting the measured cascade cross section from the apparent cross section. The shape of the excitation function is similar for levels of the same L and different n, but different for the S, P, and D families. The characteristic shapes for these three families closely resemble the corresponding ones for excitation out of the ground state of the Na atom. Trends of cross section versus principal quantum number, n, are also analysed and compared with theoretical predictions.
    Journal of Physics B Atomic Molecular and Optical Physics 01/1998; 31(4):859-872. · 2.03 Impact Factor
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    ABSTRACT: We have used a hollow cathode discharge to measure the cross sections (apparent and direct) for electron-impact excitation out of the 2^3S metastable level of He into more than ten higher triplet levels for electron energies up to 20 eV. For n>= 3 our results show a pattern of sharply peaked excitation functions into the ^3S and ^3P levels, and comparatively broad excitation functions into the ^3D levels. The 2^3P level, however, has a broad excitation function with a peak direct cross section of 112× 10-16; cm ^2. Cross sections for excitation into three of these higher triplet levels (3^3S, 3^3D, 4^3D) have been measured to larger electron energies using a He target formed via resonant charge exchange between a 1.6 keV He^+ beam and cesium vapor. There is excellent agreement between the absolute results generated by both experiments. Comparisons with theoretical calculations based on several different methods will be made.
    04/1997;
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    ABSTRACT: Cross sections for electron excitation out of the 2 3S metastable level of He into the 2 3P, 3 3S, 3 3P, 3 3D, 4 3S, 4 3P, 4 3D, 5 3S, and 5 3D levels have been obtained for energies up to 18 eV. We have observed a broad excitation function for the 2 3P level with a peak apparent cross section of 1.2×10-14ncm2. For the n=3, 4, and 5 levels, the excitation functions show a pattern of sharp peaks for excitation into the n 3S levels, slightly less sharp peaks for excitation into the n 3P levels, and relatively broad peaks for excitation into the n 3D levels. Absolute cross sections have been obtained for all the above mentioned levels using a laser-induced fluorescence technique and the results agree well with experimental values reported by Lagus et al. [Phys. Rev. A 53, 1505 (1996)] The cross sections for the 2 3S→n 3P excitations which correspond to dipole-allowed optical transitions are smaller than the corresponding 2 3S→n 3S and 2 3S→n 3D excitation cross sections, in contrast to the trends observed for excitations out of the ground level. This reversal behavior is discussed in terms of the dipole matrix element sum rule. Our cross-section data are compared with those of the alkali-metal atoms.
    Physical Review A 01/1997; 55(4). · 3.04 Impact Factor
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    ABSTRACT: We have measured the direct cross section for electron impact excitation out of the metastable 0953-4075/29/22/003/img2 level (0953-4075/29/22/003/img3 in Paschen's notation) into the 0953-4075/29/22/003/img4 level 0953-4075/29/22/003/img5 of argon from threshold to 800 eV. The direct cross section is 0953-4075/29/22/003/img6 at 10 eV.
    Journal of Physics B Atomic Molecular and Optical Physics 10/1996; 29:L795-L800. · 2.03 Impact Factor
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    ABSTRACT: Cross sections for electron impact excitation out of the 3p^54s J=0 and J=2 metastable levels of argon (1s3 and 1s5 in Pachen's notation) into levels of the 3p^54p configuration have been measured using the optical method and two separate metastable atom sources. For measurements at electron energies below the onset for excitation out of the ground state a hollow cathode discharge is used to generate the metastable target atoms. Higher energy results for a few selected excitations was obtained using a primarily Ar beam formed via charge exchange between 2.1 keV Ar^+ and cæsium vapour. In general, the peak values of the cross sections are 50 to 200 times the size of cross sections for excitation out of the ground state. The largest cross section measured is for excitation into the 3p^54p J=3 level (2p_9) with a value of 44× 10-16; cm^2 at 5 eV.(J. B. Boffard, et al.), J. Phys. B 29, 795 (1996) The cross sections have a broad shape as a function of electron energy, with the exception of the two J=0 levels (2p1 and 2p_5) which have sharply peaked excitation functions.
    Physical Review A 10/1996; · 3.04 Impact Factor
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    ABSTRACT: We have used two independent experiments to measure cross sections for electron excitation out of the 2^3S level of He into the n = 2, 3, 4, and 5 triplet levels. The first experiment uses a hollow cathode as its source of metastable He, and produces a metastable target density of 10^8cm-3. However, due to ground state He contamination of the beam, it is limited in energies to below 20 eV. The second experiment uses charge exchange of a fast He^+ beam with cesium to produce a metastable target density of 10^6cm-3. This target is relatively free of ground state contamination, and thus with this experiment we can extend our measurements to up to 1 keV. The two experiments are indpendently calibrated, and they agree quite well in both the absolute magnitude of the cross sections and in the shapes of the excitation functions that they observe. The trends and patterns of the results will be discussed and comparison with theoretical calculations will be made.
    10/1996;
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    ABSTRACT: Electron excitation out of the Ar metastable levels into levels of the 3p{sup 5}4p configuration has been studied by measuring the intensities of several lines of the 3p{sup 5}4p to 3p{sup 5}4s transition. The authors use two metastable atom sources. A hollow cathode discharge produces a relatively dense metastable target with, however, an extremely large ground state Ar concentration. The second experiment uses charge exchange of Ar{sup +} with cesium to produce a primarily Ar beam with a relatively low metastable density of 10{sup 6} atoms/cm{sup 3}. The absolute calibration is performed with the charge exchange source. In general, the excitation process is characterized by extremely large cross sections (e.g. much larger than or comparable to cross sections for excitation from the 2{sup 3}S level of He into the 3{sup 3}L levels of He) having broad shapes as a function of electron energy. Exceptions to this behavior are the cross sections for excitation into the 2p{sub 1} and 2p{sub 5} levels (Paschen`s notation).
    Bulletin of the American Physical Society. 04/1996; 41(3).
  • M.E. Lagus, J.B. Boffard, G.A. Piech
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    ABSTRACT: The comprehensive study of electron excitation out of metastable levels requires measuring cross sections into numerous levels over a broad energy range. The authors describe two different apparatus used to measure cross sections for electron excitation out of the metastable levels of He and Ar. The first apparatus uses a hollow cathode discharge as its source of metastable atoms (n {approximately} 10{sup 9} cm{sup {minus}3}), and it is calibrated using a laser induced fluorescence (LIF) technique. This apparatus is most suitable for measuring relative cross sections at energies below the threshold for ground state excitation. To extend the measurements to high energies the second apparatus uses charge exchange of a He{sup +} (or Ar{sup +}) ion beam with cesium to produce primarily metastable beams of He (or Ar) with a n {approximately} 10{sup 6} cm{sup {minus}3}. A thermal detector, calibrated with ions, is used to measure the metastable beam flux. This along with direct measurements of the spatial distributions of the metastable and electron beam allows the authors to measure absolute cross sections. The principles and methods of measurements are described in detail.
    Bulletin of the American Physical Society. 04/1996; 41(3).
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    ABSTRACT: We have developed two different metastable atom sources used to study electron excitation out of the metastable states of the rare-gas atoms (He, Ar,...) using the optical method. A hollow cathode discharge producing a thermal atomic beam is used for high signal to noise measurements. This apparatus is limited to incident electron energies below ground state threshold due to a high ground state concentration. In the case of argon, this apparatus was also used to measure the relative contributions from the two different metastable levels (3p^54s J=0 and J=2) by using a laser to quench one of the two levels. A second source uses a near-resonant charge exchange reaction (Rg^+ + Cs arrow Rg^* + Cs^+) of a fast ion beam with a cesium vapor to produce a primarily fast-metastable beam. This fast beam apparatus is used for electron energies up to 1 keV. Both experiments produce absolute results using two separate calibration procedures. Representative results will be presented.
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    ABSTRACT: Using a hollow cathode discharge as a source of He, the authors have measured absolute cross sections for electron excitation out of the 2S level of He into the n=2, 3, 4, and 5 triplet levels for energies up to 20 eV. Their results show a pattern of sharply peaked excitation functions into the n = 3, 4, and 5 ³S and ³P levels, and comparatively broad excitation functions into the ³D levels. The 2³P level has a remarkably broad excitation function, and its peak direct cross section is over 10 times the size of the next largest peak cross section observed. For energies above 20 eV, the authors use charge exchange of a fast He{sup +} beam with cesium to produce a primarily He target. This allows them to extend their measurements for selected triplet levels out to 1 keV. Their data indicate that even at this energy the excitations into the 3³S, 3³D, 4³D have not yet entered the Born regime. Comparisons with other theoretical calculations at low energies will be made.