L. Vuskovic

Critical minima in elastic electron scattering by argon

We determined experimentally two critical points in elastic electron scattering by argon where the differential cross section (DCS) attains its smallest values. The points were found to be at and at . Special attention was given to improve the angular resolution in order to determine the exact positions of the minima. These minima are important because they are a sensitive test of the validity of experimental procedures, and are used to verify theoretical predictions of DCS shapes and magnitudes, and of the polarization of scattered electrons. Normalized DCS were determined by measuring the angular distributions of elastically scattered electrons at incident energies of 10, 15, 20, 25, 30, 40, 50, 60, 75, 80, 90 and 100 eV in the angular range . Results are compared with the available experimental and theoretical data. In addition, integral, momentum-transfer, and viscosity cross sections were determined by numerical integration of the measured DCS extrapolated to and to .

Elastic and inelastic electron scattering by cadmium

Relative elastic scattering and excitation cross-sections of cadmium, up to the first ionization potential, are obtained from spectroscopic measurements on electrons with initial energies ranging from 3.4 to 85 eV. Relative differential cross sections are reported for excitation of the 55 0 (elastic), 53 P1 , 53 P)2 , 51 P1 , 63 S1 , 61 S0 , 51 D2 , 61 P1 , 71 S0 , (61 D2 -71 P1 ), 81 S0 and (71 D2 -81 P1 ) levels. Relative integrated scattering cross sections as well as momentum transfer and viscosity cross sections are also given.

Elastic electron scattering cross sections for Xe in the 1-100 eV impact energy region

Relative differential elastic scattering cross sections for Xe have been measured in the 1-100 eV impact energy and 10-146 degrees angular ranges. The data were subjected to phaseshift analysis, but the normalisation was achieved at each impact energy by utilising total electron scattering, ionisation and total excitation cross sections. Comparison is made with other recent experimental and theoretical results.

Elastic and inelastic scattering of electrons by Na

Utilising a crossed-electron-beam-metal-atom-beam scattering technique differential excitation cross sections at 10, 20, 40 and 54.4 eV incident energies and at scattering angles ranging from 10 to 120 degrees have been measured for the following transitions: (i) elastic scattering, and (ii) excitation from ground state (32S) to (a) 32P, (b) 42S, (c) 32D+42P and (d) 42D+42F+52P+52S states in Na. Comparisons with recent theoretical and experimental results have been made. Using previous theoretical and experimental results as guides the present differential cross sections have been extrapolated between angular regions of 0-10 degrees and 120-180 degrees . Integral and momentum cross sections have been computed from these cross sections.

Electronimpact cross sections for the 22P state excitation of lithium

Electron impact excitation of the 2p 2P state of Li was studied at 10, 20, 60, 100, 150 and 200 eV. Relative differential cross sections in the angular range 3-120 deg were measured and then normalized to the absolute scale by using the optical f value. Integral and momentum transfer cross sections were obtained by extrapolating the differential cross sections to 0 deg and to 180 deg. The question of normalizing electron-metal-atom collision cross sections in general was examined and the method of normalization to optical f values in particular was investigated in detail. It has been concluded that the extrapolation of the apparent generalized oscillator strength (obtained from the measured differential cross sections) to the zero momentum transfer limit with an expression using even powers of the momentum transfer and normalization of the limit to the optical f value yields reliable absolute cross sections.

The boundary effects of the shock wave dispersion in discharges

Interaction of shock waves with a weakly ionized gas generated by discharges has been studied. An additional thermal mechanism of the shock wave dispersion on the boundary between a neutral gas and discharge has been proposed [A. Markhotok, S. Popovic, and L. Vuskovic, Proceedings of the 15th International Conference on Atomic Processes in Plasmas, March 19–22, 2007 (NIST, Gaitersburg, MD, 2007)]. This mechanism can explain a whole set of thermal features of the shock wave-plasma interaction, including acceleration of the shock wave, broadening or splitting of the deflection signals and its consecutive restoration. Application has been made in the case of a shock wave interacting with a laser induced plasma. The experimental observations support well the results of calculation based on this model.

Scattering of intermediate-energy electrons by potassium

Utilising a crossed electron-beam-atom-beam collision technique electron impact differential cross sections at 7, 20, 40, 60 and 100 eV incident energies have been determined for elastic scattering and for the following transitions from the ground state (42S) to (a) 42P, (b) 52S+32D, (c) 52P, (d) 42D+62S+42F+62P and (e) 52D+72S+52F states in potassium. Measurements were done in the scattering angular range of 5 to 120 degrees . Values at angles smaller than 5 degrees and larger than 120 degrees have been obtained from extrapolation and the integral and momentum transfer cross sections have been calculated. Results are compared with available experimental and some selected theoretical data.

Subcritical microwave streamer discharge at the surface of a polymer foil

An innovative type of subcritical streamer discharge, generated by bursts of focused microwave (MW) radiation in transverse electromagnetic mode with wavelength of 10 cm, pulse duration of 40 μs, and intensity of 5.6×104 W/cm2, was initiated at the surface of a polymer foil at 420 Torr in air. The MW field of a quasioptical beam was three times lower than the critical breakdown field. Exposure of a polymer foil to the discharge caused a transition from hydrophobicity to hydrophilicity.

Electron-impact excitation of argon: I. The 4s´[½]1, 4p[½]1 and 4p´[½]0 states

We present absolute normalized differential cross sections (DCSs) for electron-impact excitation of the three individual electronic states of argon: 4s´[½]1 , 4p[½]1 and 4p´[½]0 . The measurements were performed with incident electron energies of 16, 20, 30, 40, 50 and 80 eV, scattering angles between 5° and 150° and an angular resolution of better than 2°. The absolute scale for the reference state (4s´[½]1 ) was established by experimental determination of inelastic-to-elastic intensity ratios for the 4s´[½]1 state at the above impact energies. Absolute inelastic DCSs for the other two states were determined using the intensity ratios of the peaks in the energy-loss spectra. The absolute DCSs were extrapolated to 0° and 180° and integrated numerically to yield integral, momentum transfer and viscosity cross sections.

Anomalous propagation of planar shock wave in weakly ionized gas

Shock propagation velocity along the centerline of a cylindrical glow discharge has to be much higher in the cathode region than in the positive column in order to account for the measured propagation time. This is an outcome of detailed analysis of planar shock waves propagating along a cylindrical glow discharge [B. N. Ganguly, P. Bletzinger, and A. Garscadden, Phys. Lett. A 230, 218 (1997)]. The analysis was restricted to shock front propagation time to the two well-defined positions at the centerline where the motion can be considered one dimensional. Although there is not yet a comprehensive interpretation of the anomalous propagation velocity, it seems to increase wherever number density of excited states increases. Intensified local ionization due to excessive presence of excited states combined with the excitation of ion acoustic waves could be the mechanism for anomalous shock propagation velocity.