Y. F. Hsieh

Bio: Y. F. Hsieh is an academic researcher from Wayne State University. The author has contributed to research in topics: Cross section (physics) & Inelastic scattering. The author has an hindex of 7, co-authored 8 publications receiving 592 citations.

Total-cross-section measurements for positrons and electrons colliding with H 2 , N 2 , and C O 2

Abstract: Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and C${\mathrm{O}}_{2}$ using a beam transmission technique. The projectile impact energies range from 1 - 500 eV for ${e}^{+}\ensuremath{-}{\mathrm{H}}_{2},2\ensuremath{-}500$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{H}}_{2},0.5\ensuremath{-}750$ eV for ${e}^{+}\ensuremath{-}{\mathrm{N}}_{2},2.2\ensuremath{-}700$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{N}}_{2},0.5\ensuremath{-}60$ eV for ${e}^{+}\ensuremath{-}{\mathrm{CO}}_{2}$, and $2\ensuremath{-}50$ eV for ${e}^{\ensuremath{-}}\ensuremath{-}{\mathrm{CO}}_{2}$. The onset of positronium formation is clearly seen by an abrupt rise in the total cross sections for positrons colliding with each of the molecules at the respective positronium-formation thresholds. The positron measurements are compared with the electron measurements at intermediate energies for ${\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}$. This comparison reveals a merging of the cross sections for ${\mathrm{H}}_{2}$ at energies above 200 eV, while for ${\mathrm{N}}_{2}$ the electron results remain higher than the positron results at all energies. Estimates are made of potential experimental errors, as well as the experimental resolution for discrimination against projectiles scattered at small forward angles.

Total-cross-section measurements for positron and electron scattering by O 2 , CH 4 , and SF 6

Abstract: Total cross sections (${Q}_{T}$'s) have been measured for 1--500-eV positrons and electrons scattered by ${\mathrm{O}}_{2}$, ${\mathrm{CH}}_{4}$, and ${\mathrm{SF}}_{6}$ using a beam-transmission technique. The positron ${Q}_{T}$'s are compared with the corresponding electron ${Q}_{T}$'s for each target gas. It is found that the positron ${Q}_{T}$'s are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as ${\mathrm{SF}}_{6}$. The positron ${Q}_{T}$ curve for ${\mathrm{CH}}_{4}$ reveals a significant increase in the vicinity of its positronium formation threshold, while the ${\mathrm{O}}_{2}$ and ${\mathrm{SF}}_{6}$ curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron ${Q}_{T}$'s for these gases. Interesting similarities are found in a comparison of the present positron and electron ${Q}_{T}$ curves for ${\mathrm{CH}}_{4}$ with prior comparison measurements for argon.

e(-CO and e(-CO/sub 2/ total cross-section measurements

Abstract: As part of a continuing series of investigations, total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with CO and C${\mathrm{O}}_{2}$ using a beam-transmission technique The projectile impact energies are in the range 1-500 eV for ${e}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{-}\mathrm{CO}$, 30-500 eV for ${e}^{+}\ensuremath{-}\mathrm{C}{\mathrm{O}}_{2}$, and 100-500 eV for ${e}^{\ensuremath{-}}\ensuremath{-}\mathrm{C}{\mathrm{O}}_{2}$ An important aspect of our work is to compare the corresponding positron and electron total cross section (${Q}_{T}$) curves for a given target gas For both CO and C${\mathrm{O}}_{2}$ the electron ${Q}_{T}$ values are generally larger than the positron results For both gases at low energies there are relatively narrow shape resonances for electron scattering and noticeable increases in ${Q}_{T}$ after the positronium-formation thresholds for positron scattering At the highest energies investigated there are indications of a tendency toward merging of the positron and electron curves for each gas A striking similarity is found between the present ${e}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{-}\mathrm{CO} {Q}_{T}$ curves and the ${e}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{-}{\mathrm{N}}_{2} {Q}_{T}$ curves obtained by Hoffman et al [Phys Rev A 25, 1393 (1982)], in that the corresponding shapes and magnitudes are very nearly the same Estimates of potential experimental errors, as well as the experimental discrimination against projectiles scattered at small forward angles, are made

Total-Scattering Measurements and Comparisons for Collisions of Electrons and Positrons with N 2 O

Abstract: Total cross sections (${Q}_{T})$ for 1-500-eV positron and electron scattering by ${\mathrm{N}}_{2}$O are measured by a beam-transmission method. Comparisons of these results with prior electron and positron measurements of ${Q}_{T}$ for C${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, and CO, made by our group, reveal several remarkable similarities in the shapes and magnitudes of the ${Q}_{T}$ curves for the triatomic and also for the diatomic molecules. Small differences in the absolute ${Q}_{T}$ values for ${\mathrm{N}}_{2}$O and C${\mathrm{O}}_{2}$, and for ${\mathrm{N}}_{2}$ and CO, may be due to the weak permanent dipole moments of ${\mathrm{N}}_{2}$O and CO.

Total-scattering cross-section measurements for intermediate-energy positrons and electrons colliding with Kr and Xe

Abstract: Total-scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with krypton and xenon in the energy range from 20 to 800 eV with the use of a beam-transmission technique. The unusual shapes of the peaks in the ${e}^{+}$-Kr and ${e}^{+}$-Xe total-cross-section curves suggest the possibility that inelastic processes may be contributing significantly to the respective total cross sections in those peaks. A distinct shoulder exists in the ${e}^{\ensuremath{-}}$-Xe total-cross-section curve in the vicinity of 20 eV, followed by a rapid decrease with the electron results dropping below the corresponding positron results in the energy range from 40 to 90 eV. This crossing of the curves at these energies is some-what anomalous compared to the situation for the other inert gases, where the electron results remain higher than the positron results. Estimates are made of potential experimental errors, including that associated with incomplete discrimination against small-angle forward scattering.

Anisotropic scattering of electrons by N2 and its effect on electron transport.

Abstract: As part of a systematic study of approximations commonly made in solutions of the Boltzmann equation for electrons in molecular gases, we have investigated the effects of anisotropic scattering on electron transport coefficients in ${\mathrm{N}}_{2}$ and have extended our study of the multiterm expansion technique to higher E/n. A critical survey of published data yields a set of differential and integral cross sections for electron energies from 0.003 to ${10}^{4}$ eV. For electric-field--to--gas-density ratios E/n between 10 and 500 Td (1 Td${=10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$), the changes in the commonly measured transport and reaction coefficients resulting from the introduction of anisotropic elastic and inelastic scattering, while keeping the elastic momentum-transfer cross section constant, are less than 1%.These calculations were made with use of the multiterm spherical-harmonic expansion solution of the Boltzmann equation. For 500lE/nl1500 Td the changes in scattering anisotropy cause changes in transport and reaction coefficients which increase with E/n to about 10%. The errors in drift velocity, mean energy, and the reaction coefficients resulting from the use of the two-term spherical-harmonic expansion rather than a six-term expansion are less than 3% at 1500 Td.However, the errors in the diffusion coefficients become large (g25%) at our highest E/n. The calculated transport coefficients are in generally good agreement with experiment for E/n less than 300 Td, but the differences increase at higher E/n. The importance of proper interpretation of ionization and excitation experiments at high E/n is illustrated by calculations which model either an exponential growth of density in time or an exponential growth with position. The calculated ionization coefficients are low compared to most experiments for E/n less than 200 Td. At E/ng600 Td the agreement is good for the spatial growth experiments, but the calculated values are below experiment from the temporal growth experiments. The calculated excitation coefficients are generally higher than experiment at low and high E/n but in agreement with experiment at E/n near 150 Td.

Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders

Abstract: [1] Streamers are thin filamentary plasmas that can initiate spark discharges in relatively short (several centimeters) gaps at near ground pressures and are also known to act as the building blocks of streamer zones of lightning leaders. These streamers at ground pressure, after 1/N scaling with atmospheric air density N, appear to be fully analogous to those documented using telescopic imagers in transient luminous events (TLEs) termed sprites, which occur in the altitude range 40–90 km in the Earth's atmosphere above thunderstorms. It is also believed that the filamentary plasma structures observed in some other types of TLEs, which emanate from the tops of thunderclouds and are termed blue jets and gigantic jets, are directly linked to the processes in streamer zones of lightning leaders. Acceleration, expansion, and branching of streamers are commonly observed for a wide range of applied electric fields. Recent analysis of photoionization effects on the propagation of streamers indicates that very high electric field magnitudes ∼10 Ek, where Ek is the conventional breakdown threshold field defined by the equality of the ionization and dissociative attachment coefficients in air, are generated around the tips of streamers at the stage immediately preceding their branching. This paper describes the formulation of a Monte Carlo model, which is capable of describing electron dynamics in air, including the thermal runaway phenomena, under the influence of an external electric field of an arbitrary strength. Monte Carlo modeling results indicate that the ∼10 Ek fields are able to accelerate a fraction of low-energy (several eV) streamer tip electrons to energies of ∼2–8 keV. With total potential differences on the order of tens of MV available in streamer zones of lightning leaders, it is proposed that during a highly transient negative corona flash stage of the development of negative stepped leader, electrons with energies 2–8 keV ejected from streamer tips near the leader head can be further accelerated to energies of hundreds of keV and possibly to several tens of MeV, depending on the particular magnitude of the leader head potential. It is proposed that these energetic electrons may be responsible (through the “bremsstrahlung” process) for the generation of hard X rays observed from ground and satellites preceding lightning discharges or with no association with lightning discharges in cases when the leader process does not culminate in a return stroke. For a lightning leader carrying a current of 100 A, an initial flux of ∼2–8 keV thermal runaway electrons integrated over the cross-sectional area of the leader is estimated to be ∼1018 s−1, with the number of electrons accelerated to relativistic energies depending on the particular field magnitude and configuration in the leader streamer zone during the negative corona flash stage of the leader development. These thermal runaway electrons could provide an alternate source of relativistic seed electrons which were previously thought to require galactic cosmic rays. The duration of the negative corona flash and associated energetic radiation is estimated to be in the range from ∼1 μs to ∼1 ms depending mostly on the pressure-dependent size of the leader streamer zone.

Cross Sections for Electron Collisions with Hydrogen Molecules

Abstract: Cross section data have been compiled for electron collisions with hydrogen molecules based on 71 references. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, emission of radiation, and dissociative attachment. For each process, the recommended values of the cross section are presented for use. The literature has been surveyed through the end of 2006.