Hyuck Cho

Measurements of elastic electron scattering by water vapour extended to backward angles

Absolute differential cross sections for elastic electron scattering by water vapour have been measured at nine incident electron energies between 4 to 50 eV and over scattering angles between 10° and 180°, using a crossed-beam electron spectrometer. A magnetic angle-changing device based on the design of Read and co-workers has been used to extend the measurements to backward angles (125° to 180°). The elastic integral and momentum transfer cross sections are derived from these elastic differential cross sections. Previous experimental results at backward angles, available only up to 156°, are consistently higher than the present results. We also compare, where possible, with results from recent theoretical calculations.

Cross sections and oscillator strengths for electron-impact excitation of the à 1B1 electronic state of water

The authors report absolute differential and integral cross section measurements for electron-impact excitation of the A (1)B(1) electronic state of water. This is an important channel for the production of the OH (X (2)Pi) radical, as well as for understanding the origin of the atmospheric Meinel [Astrophys. J. 111, 555 (1950)] bands. The incident energy range of our measurements is 20-200 eV, while the angular range of the differential cross section data is 3.5 degrees -90 degrees . This is the first time such data are reported in the literature and, where possible, comparison to existing theoretical work, and new scaled Born cross sections calculated as a part of the current study, is made. The scaled Born cross sections are in good agreement with the integral cross sections deduced from the experimental differential cross sections. In addition they report (experimental) generalized oscillator strength data at the incident energies of 100 and 200 eV. These data are used to derive a value for the optical oscillator strength which is found to be in excellent agreement with that from an earlier dipole (e,e) experiment and an earlier photoabsorption experiment.

Dispersion-like signals in velocity-selective saturated-absorption spectroscopy

We observed a dispersion-like signal from a caesium-vapour cell configured for saturated-absorption spectroscopy. Two parallel laser beams used as probes intersected a pump laser beam at a finite crossing angle. The dispersion-like signal was obtained from the difference in intensities of the two probe lasers passing through the vapour cell. The shape of the dispersion-like signal depended on the crossing angle and the crossing position of the intersecting beams. Using the dispersion-like signal as a frequency discriminator, we were able to stabilize the laser frequency without any frequency modulation. The frequency fluctuation was reduced to 22 kHz using a proportional amplifier and an integrator as a feedback circuit.

Substitution effects in elastic electron collisions with CH3X (X=F, Cl, Br, I) molecules

We report absolute elastic differential, integral, and momentum transfer cross sections for electron interactions with the series of molecules CH(3)X (X=F, Cl, Br, I). The incident electron energy range is 50-200 eV, while the scattered electron angular range for the differential measurements is 15 degrees-150 degrees. In all cases the absolute scale of the differential cross sections was set using the relative flow method with helium as the reference species. Substitution effects on these cross sections, as we progress along the halomethane series CH(3)F, CH(3)Cl, CH(3)Br, and CH(3)I, are investigated as a part of this study. In addition, atomic-like behavior in these scattering systems is also considered by comparing these halomethane elastic cross sections to results from other workers for the corresponding noble gases Ne, Ar, Kr, and Xe, respectively. Finally we report results for calculations of elastic differential and integral cross sections for electrons scattering from each of the CH(3)X species, within an optical potential method and assuming a screened corrected independent atom representation. The level of agreement between these calculations and our measurements was found to be quite remarkable in each case.

Elastic cross sections for electron collisions with molecules relevant to plasma processing

Absolute electron-impact cross sections for molecular targets, including their radicals, are important in developing plasma reactors and testing various plasma processing gases. Low-energy electron collision data for these gases are sparse and only the limited cross section data are available. In this report, elastic cross sections for electron-polyatomic molecule collisions are compiled and reviewed for 17 molecules relevant to plasma processing. Elastic cross sections are essential for the absolute scale conversion of inelastic cross sections, as well as for testing computational methods. Data are collected and reviewed for elastic differential, integral, and momentum transfer cross sections and, for each molecule, the recommended values of the cross section are presented. The literature has been surveyed through early 2010.

Cross sections for electron trapping by DNA and its component subunits I: Condensed tetrahydrofuran deposited on Kr

We report cross sections for electron capture processes occurring in condensed tetrahydrofuran (THF) for incident electron energies in the range of 0-9 eV. The charge trapping cross section for 6-9 eV electrons is very small, and an upper limit of 4 x 10(-19) cm2 is estimated from our results. This latter is thus also an upper bound for the cross section for dissociative electron attachment process that is known to occur at these energies for condensed THF. At energies close to zero eV electron trapping proceeds via intermolecular stabilization. The cross section for this process is strongly dependent on the quantity of deposited THF. Since THF may model the furyl ring found in deoxyribose, these measurements indicate that this ring likely plays little role in either initiating or enhancing strand break damage via the attachment of the low energy secondary electrons produced when DNA is exposed to ionizing radiation.

Differential two-signal picosecond-pulse coherent anti-Stokes Raman scattering imaging microscopy by using a dual-mode optical parametric oscillator

We propose and demonstrate a novel differential two-signal technique of coherent anti-Stokes Raman scattering (CARS) imaging microscopy using a picosecond (ps) optical parametric oscillator (OPO). By adjusting a Lyot filter inside the cavity, we operated the OPO oscillating in two stable modes separated by a few nanometers. The CARS images generated by the two modes are separated by a spectrograph behind the microscope setup, and their differential image is directly obtained by balanced lock-in detection. The feasibility of the technique is experimentally verified by imaging micrometer-sized polystyrene beads immersed in water.

A comparative experimental-theoretical study on elastic electron scattering by methane

Absolute differential cross sections for elastic electron scattering by methane have been measured at six incident electron energies between 5 and 100 eV and over scattering angles between 10° and 180°, using a crossed-beam electron spectrometer combined with a magnetic angle-changing device to extend the measurements to backward angles (125°–180°). Differential, integral and momentum-transfer cross sections are also calculated and reported for these energies. A complex optical potential was used to represent the electron–molecule interaction dynamics. The iterative Schwinger variational method combined with the distorted-wave approximation was used to solve the scattering equations. The comparison between our calculated and measured results, as well as with other experimental and theoretical data available in the literature, is encouraging.

Elastic electron scattering from sulfur hexafluoride

Absolute differential cross sections for elastic scattering of electrons from sulfur hexafluoride (SF6) have been measured at 11 incident energies between 2.7 and 75 eV and for scattering angles between 10° and 130°. The magnetic angle-changing device of Read and co-workers has been used to extend these measurements to backward angles (130°-180°) for incident energies below 15 eV. The measurements reveal some substantial differences with several previous determinations and a reasonably good level of agreement with a recent close-coupling calculation.

Cross Sections for Electron Collisions with Methane

Cross section data are compiled from the literature for electron collisions with methane (CH4) molecules. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational and vibrational states, dissociation, ionization, and dissociative attachment. The data derived from swarm experiments are also considered. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed through early 2014.