Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·10⁴ to 5·10⁹ W/cm². It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C⁺ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·10⁹ W/cm².     

ϑ-pinch in extrinsic InSb

The compression of an electron-hole plasma, caused by a ?-pinch in extrinsic InSb of 140 K, was investigated by measuring the absorption of 10·6 μm radiation and the change of the magnetic flux in the sample. A plasma density of 5 × 10¹⁵ cm^?3 was hereby found. The temporal development of the plasma density indicates that the electrons and holes resulted from impact ionization in the electric field when the magnetic field was low. Since the electric field is highest at the sample surface, the ionization was limited to the outer region of the sample and the plasma was transmitted to the inner sample volume by the ?-pinch.     

Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

The Mott transition in cold semiconductors

We discuss the Mott transition when a semiconductor is externally excited at low temperatures. We demonstrate that hysteresis is inevitable unless a phase separation occurs at higher temperatures or lower average excitation densities than necessary for the predicted ionization catastrophe. The critical temperature and pair density for hysteresis in Ge is of the order 6K and 3 · 10¹⁶ cm^-3, respectively, for a model involving dynamical screening.     

Spectroscopic Studies of Indium Plasma Produced by Fundamental (1,064 nm) and Second (532 nm) Harmonics of an Nd:YAG Laser

In this paper, we present spectroscopic studies of a laser-induced indium (In) plasma produced by fundamental (1,064 nm) and second (532 nm) harmonics of an Nd:YAG laser along with the characteristics determined by plasma parameters. The electron temperature is determined using four lines of neutral indium at 260.17, 271.02, 275.38, and 325.85 nm, in view of the Boltzmann plot method. The temperature varies from 6,470 K at 0.05 nm to 4,990 K at about 2 mm from the target surface for the fundamental wavelength and from 6,250 to 4,880 K for the second harmonic. The electron density is ±300 calculated using the Stark broadened profiles recorded at laser pulse energy 130 mJ (for fundamental) and 72 mJ (for second harmonic) as 5:8·10¹⁶ and 6:9·10¹⁶ cm^?3, respectively. These values decrease to 3:5·10¹⁵ and 4:9·10¹⁵ over a distance of 2 mm from the target surface, respectively. Moreover, we study the variation of N _e as a function of laser irradiance as well as its spatial variation from the target surface.     

Influence of electron-atom cross section uncertainties on shock wave structure

The exact value of the electron-atom collisional ionization cross section for argon is not accurately known. The purpose of the present research is to determine numerically the effect of varying the magnitude of the electron-atom cross section on nonequilibrium shock-wave structure. Mach 18 shock waves propagating into an argon-like gas at 1 cm-Hg and 300°K have been analyzed. Thermal, ionizational, and excitational non-equilibrium are considered in the relaxation region behind the shock wave. Electrons in the relaxation region are formed by a two-step collisional process, wherein the atom is first excited and then it is ionized. The precursor is formed by ground and excited state continuum radiation and line radiation which is emitted, but not reabsorbed, in the region behind the shock wave. When the electron-atom ionization cross section is varied from 1·86 × 10^?4to 1·86 × 10^?2cm²/erg, the results show that (1) it influences the coupling between the precursor and relaxation region through the radiative source functions, (2) it does not influence the distance necessary to attain equilibrium behind the shock wave, (3) it inversely influences the magnitude of the precursor ionization, and excitation, and (4) it inversely influences both the free electron and excited state population in the relaxation layer.     

Evaluation of the parameters of electron transport in liquid hydrocarbons using the photoconductivity technique

The ballistic model of electron transport proposed by Mozumder (Chem. Phys. Lett. 1993. V. 207. P. 245) is applied to interpret experimental data on the photoconductivity of solutions in three hydrocarbon liquids: methylcyclohexane, isooctane, and squalane. For methylcyclohexane, the values of the quasi-free electron mobility 100 cm² V^?1 s^?1 and electron trap density 3 · 10¹⁹ cm^?3 agree with experimental data. For isooctane, the motion of a quasi-free electron is nearly diffusive, whereas the trap density is ～0.8 · 10¹⁸ cm^?3. As to squalane, the two-state model turned out to be inapplicable to describing electron transport. The results obtained are used to discuss initial stages of ionization in liquids.     

Bestimmung von Temperatur und Dichte eines Gleitfunkens aus der Bremsstrahlung im Infrarot

The absolute intensity of a vacuum sliding spark (length 2–10 cm, half cycle 0,8 μsec) has been measured in the spectral region between 0,4 and 3μ. From the long wavelength radiation emitted from an optically thick layer, one gets the temperature, from the short wavelength radiation emitted from an optically thin layer, one obtains the density. Using polyethylene as an insulator we reached a temperature of 4·10⁵ °K at electron densities of 8·10¹⁸ cm^?3 and current densities of 1.2·10⁶ A/cm². The temporal development of temperature and density has been determined. The maximum intensity at λ=0.43 μ was found to be 5·10⁴ (7·10¹⁰ W/cm³ ster) as large as that of the positive crater of a properly driven carbon arc. At λ=3 μ this same parameter turned out to be 300 times as large (5·10⁷ W/cm³ ster).     

Copper and native defects in zinc telluride

Zinc telluride crystals were grown from tellurium-rich solutions containing 10¹⁷–10²⁰ cm^?3 atoms of copper. The copper concentrations in these crystals were measured by activation analysis. Hall effect and resistivity measurements were performed. Photoluminescence spectra were also determined. Our interpretation of the different results is that copper brings about both acceptor defects Cu_Zn with a 0·12–0·13 eV ionization energy, and donor defects. The second acceptor level of the zinc vacancy was found to be at 0·15 eV.     

Low-flux neutron irradiation of ceramic YBa2 Cu3O7−δ

Summary Pellets of sintered YBa₂Cu₃O_7−δ with three different oxygen contents have been irradiated with fast neutron beams of energies 6.5, 3.3 and 4.4 MeV at fluences of 7.7·10⁴, 1.3·10⁵ and 1.4·10⁹ n/cm², respectively. The radiation damage has been investigated by comparing the critical temperature (T _c ^mid ), the zero resistivity and the onset temperature before and after neutron irradiation. The critical current has been measured for a few samples in the same experimental conditions. In all transport measurements two different responses to the neutron radiation are observed and discussed.     

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τ_E, τ₁ and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency. The experimental conditions are: Helium-gas, p = 1 …? 5 Pa, T_e = 4 eV, n = 1 …? 5 · 10¹⁰ cm^?3, B = 6,5 · 10^?2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …?_E = 10 …? 15 μs is given by inelastic collision losses. The ionization time constant τ₁ is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm-like in the transition phase.     

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n_cr ≈ 6 × 10¹²cm⁻³ at onset of emitted microwaves.     

Radiative decay and autoionization in the4 D-states of the 3d 94s5s configuration in CuI

The CuI 3d ⁹4s5s ⁴ D-states, which lie above the ionization limit, were populated by stepwise collisional and laser excitation. With respect to their total angular momentum, the⁴ D-states have completely different decay modes. Whereas the lifetimesτ(⁴ D _7/2)=8.2(4)·10^?9s andτ(⁴ D _1/2)=7.9(6)·10^?9 s are determinrd by radiative transitions and were measured by time-resolved spectroscopy, the lifetimesτ(⁴ D _5/2)=1.59(6)·10^?12s andτ(⁴ D _3/2)=1.28(5)·10^?12s are due to autoionization and were deduced from the widths of the excitation signals observed by ion detection.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

Kinetics of Rb<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and Rb<Superscript>+</Superscript> formation in laser-excited rubidium vapor

We have studied the formation of the molecular ion Rb₂⁺ and the atomic ion Rb⁺. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb⁺ and Rb₂⁺. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×10¹³cm⁻³ and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb₂⁺ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb⁺ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb⁺ and Rb₂⁺ is obtained.      

An investigation of the energy exchange mechanisms involving the 23S metastable level in an RF helium plasma

The energy exchange mechanisms present in a pure helium and a helium-neon plasma were investigated using spectroscopic diagnostic techniques. The plasma was spatially resolved and only the volume element at the plasma centerline was considered in the energy- exchange analysis. The experiment was conducted with a constant total pressure of 0·7 torr, a fixed oscillator frequency of 4 MHz, and a constant input power of 1·8 kW. Emission line spectroscopy was used to determine the population densities of 16 levels in the n³S, n³P, and n³D series. Spatially resolved, self-absorption measurements of the 2³P-2³S transition were used to determine the 2³S metastable level number density. The electron number density of 3·3 × 10¹³ cm^-3 was determined from the spatially resolved H_β blue wing profile, and a lower bound excitation temperature of 8800 °K was determined from a Boltzmann plot of the spatially resolved lower bound levels of the excited helium. The addition of 10% and 20% by volume of neon gas caused a measurable decrease in the population densities of the lower bound levels of helium, while the electron number density and lower bound excitation temperature remained unchanged. Three energy exchange models (local thermal equilibrium, corona, and collisional- radiative) were examined, and the collisional-radiative model was found to best describe the excitation processes for the 2³S level. This model was also appropriate for describing the helium-neon plasma at this level.     

Nuclear orientation of156Tb in gadolinium matrix

The anisotropy of the angular distribution of gamma-rays from the decay of¹⁵⁶Tb, oriented in a gadolinium matrix at low temperatures, has been measured at the angles of 0 and π/2 with respect to the applied magnetic field direction in the range of temperatures from 14·6 to 68·4 mK. The temperature dependence of anisotropy was measured for the first time. The parameters of hyperfine magnetic dipole and electric quadrupole splittings have been determined and the values of the magnetic dipole moment ¦Μ¹⁵⁶¦=(9·6±1·3)×10^?27 J/T and the electric quadrupole momentQ ¹⁵⁶=(2·9±0·9)×10^?28 m² of the¹⁵⁶Tb ground state have been calculated. Multipole mixing ratios andB(E2) branching ratios of many gamma-ray transitions occurring in¹⁵⁶Gd have been found and the results have been discussed in terms of the rotational-vibration and pairing-plus-quadrupole models.     

Generation of multiply charged refractory metals in an electron-cyclotron resonant discharge in a direct magnetic trap

The possibility of additional ionization of refractory metal ions in the vacuum arc plasma injected to a magnetic trap due to additional heating of electrons by microwave radiation under the conditions of electron-cyclotron resonance is demonstrated. High-power short-wave radiation of gyrotrons used in experiment makes it possible to work with a higher (on the order of 10¹³ cm⁻³) density of the plasma and to ensure the confinement parameter at a level of 3 × 10⁸ cm⁻³ s at an electron temperature sufficient for multiple ionization.     

An analytical method for DC negative corona discharge in a wire-cylinder device at high temperatures

This paper proposes an analytical solution for DC negative corona discharge in a wire-cylinder device based on experimental results in which both the corona and drift regions are considered; this approach aims to provide a theoretical method for analyzing electrostatic precipitation at high temperatures. The inter-electrode space is divided into three zones, namely, the ionization layer, the attachment layer (corona region) and the drift region, to investigate the space charge concentration and the electric field distribution. The boundary of the ionization layer is assumed to be the radius at which the rate of ionization balances that of electron attachment. The radius where the value of E/N equals 110 Td is recommended as the boundary of the attachment layer. It was determined that an increasing temperature leads to a decrease in the largest space charge number density and the largest electric field in the drift region that can be provided by a discharging device. With respect to the device in the present work, when the temperature increases from 350 °C to 850 °C, the largest electric field decreases from ∼9 × 10⁶ V/m to ∼3 × 10⁶ V/m, and the largest charge number density decreases from ∼1.3 × 10¹⁵ m⁻³ to 6.4 × 10¹⁴ m⁻³. The radius of the corona region, the space charge number density and the electric field increase as the applied voltage increases at a given temperature. For example, at a temperature of 550 °C, when the applied voltage increases from 10,500 V to 18,879 V, the radius of the corona region increases from ∼2.9 mm to ∼4.9 mm. It appears to be unreasonable to use a constant value that is calculated from Peek's formula as the electric field at the surface of the cathode under all of the conditions.     

Messung der quasi-elastischen Neutronenstreuung in Glyzerin mit einem hochauflösenden Kristallspektrometer

A new neutron crystal spectrometer with an energy resolution of 6 · 10^?7 eV was installed at the Munich reactor. The high energy resolution is based on the backscattering of neutrons on silicon single crystals. The energy of the neutrons striking the glycerol sample is varied by Doppler shifting. With this spectrometer the quasielastic line broadening in glycerol was investigated at constant momentum transfer (κ=1.4 ± 0.25) Å^?1 as a function of temperature. The diffusion constants deduced from the experiment are in the order of 10^?7 cm²/s to 5 · 10^?9 cm²/s.