R. Shuker

Pulsed resonant optogalvanic effect in neon discharges

Pulsed resonant optogalvanic effect is investigated in a neon hollow cathode discharges utilizing a nitrogen pumped dye laser. We have studied by the pulsed technique transient effects in the discharge plasma such as fast relaxation of level population density and population inversion. The experimental results are correlated with a four states phenomenological model of the pulsed optogalvanic effect taking into account lumped relevant levels of the 3s and 3p manifolds of neon. The time integration of the pulsed optogalvanic siganls (OGS) yields the previously measured cw signals and explains their sign changes.

Low‐Frequency Vibrational Light Scattering in Viscous Liquids

The theory of Raman scattering from the vibrations of amorphous materials is applied to the spectra of B2O3 glass and its melt. Experimental evidence is obtained indicating that low frequency scattering in the melt liquid is also due to first order scattering from intermolecular vibrational states. The relationship among several scattering mechanisms in liquids is discussed.

Highly textured Bi2Te3-based materials for thermoelectric energy conversion

This work is concerned with Bi2Te3-based compounds known as being highly effective materials for thermoelectric applications near room temperature. These compounds are characterized by a remarkable anisotropy linked to their R3¯m crystal structure. Two textured p-type Bi0.4Sb1.6Te3 samples were prepared using a powder metallurgy approach, with the c axis parallel to the pressing direction. One sample was undoped while the second was doped with Pb which acts as an acceptor. The electrical conductivity, Hall coefficient, and magnetoresistivity were measured from room temperature down to 6K. The Seebeck coefficient α and electrical conductivity σ were measured along and perpendicular to the c axis from 300 up to 550K, and the thermal conductivity κ was measured at 300K. Different values of Seebeck coefficient were observed along and perpendicular to the c axis at temperatures above Ti, the beginning of intrinsic region in which the influence of the minority carriers becomes significant. Below Ti, the Seebeck...

Inverted population observation using pulsed optogalvanic effect

Abstract We report on the use of pulsed optogalvanic effect in the detection of inverted population. An early positive part of the optogalvanic signal is shown to be characteristic of a population inversion and is measured using a high temporal resolution from the start of the laser pulse. Such inversion is obtained on a few of the neon 3p → 3s transitions in a standard DC hollow-cathode discharge tube with neon buffer gas. We observed inverted optogalvanic signal with exciting laser wavelengths at 659.9 nm, 585.2 nm and others. In fact, transient laser transitions on some of these wavelengths, in fast neon discharges have been reported. The optogalvanic effects, corresponding to neon levels 1s 5 and 1s 2 , and their dependent signals have opposite features and signs and are essentially mirror images of each other. The necessity of using pulsed optogalvanic mode and detection in the case of inverted population is discussed.

Cherenkov emission due to laser-induced moving polarisation in sodium

Red shifted conical emission induced by a laser blue shifted with respect to the D2 line of sodium is observed and interpreted in terms of Cherenkov emission. Measured cone angles and wavelength shifts agree well with model calculations taking into account saturation and dispersion effects.

Penning ionization spectroscopy using the optogalvanic effect

The optogalvanic effect is proposed and demonstrated as a new technique for quasiresonant Penning ionization spectroscopy in a discharge plasma of mixtures of rare gases and metal vapors. A calcium and neon mixture is used as a prototype. Neon’s lowest metastable level, 3P2 at 134 034 cm−1, is within kT from the excited state of 2D3/2,5/2 of Ca+. Thus Penning ionization occurs to an excited state of the ion. This process strongly alters the optogalvanic signal and has its own signature. In fact, other energy transfer processes should also change the time dependence of the optogalvanic signal.

On the optical characteristics of the conical emission

Abstract An intense laser beam blue detuned with respect to an atomic transition produces a forward conical emission. We demonstrate new experimental evidence that in sodium vapor this emission is generated at the surface of self-trapped filaments. This includes comparison of the polarization of the incident and emitted light as well as comparing the output pattern obtained by cylindrical and spherical focusing modes. As a surface phenomenon, the conical emission shares its characteristics with Cherenkov-type processes.

Observation of Penning ionization in Sr/Ne discharge by the optogalvanic effect

A study of the pulsed optogalvanic effect in a Sr/Ne hollow cathode tube for Penning ionization spectroscopy in this system is described. In the case of strontium this ionization is a multichannel process due to the quasi-resonance between the four excited 3s states of neon with the high density of Rydberg states close to a Sr+ ionization continuum. A detailed investigation of the signal as a function of tube current and its spatial dependence as well as a comparison with Ca/Ne Penning rate is also reported.

Optogalvanic spectroscopy of quasi‐resonant Penning ionization

Quasi‐resonant Penning ionization in a Ne–Ca discharge is detected and studied by using the pulsed optogalvanic effect on the neon 1si→2pj transitions. The ensuing signals exhibit extra features which appear at longer times following the signals obtained in pure neon discharge. A phenomenological model is used to describe the signal and to extract an estimate of the Penning cross section, which is on the order of 10−14 cm2. At high discharge currents the Penning contribution becomes unimportant and the related features in the optogalvanic signal disappear. A correlation between the dependence of the emission intensity of certain transitions of Ca and Ca+ related to the Penning effect and the dependence of the optogalvanic signal due to the Penning ionization on the discharge current is demonstrated. A similar correlation between the I–V characteristics and the dependence of the optogalvanic signal on the discharge current is also shown.

Models of High-Power Discharges for Metal-Xe Excimer Lasers,

High‐power (∼108 W/l) discharges in metal‐doped Xe are modeled for typical metal atom densities of 1015–1017 cm−3 and Xe densities of ∼1020 cm−3, and electron densities of 1014–1017 cm−3 as appropriate for proposed excimer lasers. Na is used as a prototype species, while its properties are varied to indicate some of the changes that could result from the use of different metals. The model includes sixteen excited levels of Na, three ionic species, the excimer levels of NaXe, and Na2. The degree of ionization is determined by collisional multistep excitation and ionization of excited atoms versus dissociative recombination of electrons with Na+2. Steady‐state conditions in the positive column are calculated for typical gas temperatures of ∼0.06 eV and electron temperatures Te of 0.3–0.5 eV. The Na population distribution is largely Boltzmann at the electron temperature and the electron density is close to the Saha equilibrium value except at low electron temperatures and very high extracted laser powers. U...