Lowering of the Populations of Excited Hydrogen and Helium Atoms in Plasmas Irradiated by a Laser Beam

The populations of excited hydrogen and helium atoms in non-L.T.E. plasmas have been calculated for two different physical situations, namely (i) no external radiation field present, and (ii) strong superposed radiation originating from a CO₂, Ruby or Argon Ion Laser. The radiation field intervenes in the collisional-radiative model via two new terms: Photoionization and stimulated recombination induced by the Laser radiation field. The solutions of the rate equations yield a lowering of the populations when a plasma is irradiated by a Laser beam. The lowering is different for cold recombining and hot diffusion-dominated plasmas. At high radiation densities a saturation effect occurs, since the photoionization rate becomes equal to the rate of stimulated recombination. Measurements of the populations of excited H and He atoms in a glow discharge irradiated by a CO₂ Laser beam of power density 10⁵ W/cm² are in broad agreement with the theoretical predictions.     

Population of displaced levels of aluminum and copper atoms in plasma jets

Population studies have been made for the displaced levels of aluminum 4s⁴P_{1/2,3/2, 5/2} and copper⁴D_{1/2, 3/2, 5/2, 7/2} in a plasma jet entering an evacuated space. It is found that there is an increase in the relative population (which may exceed the equilibrium value) for unstable levels of high autoionization probability. This is considered to result from accentuated recombination to excited levels in a plasma jet having frozen-in ionization.We are indebted to Academician M. A. El'yashevich (AS Beloruss. SSR) for valuable discussions.     

The spectrum and decay of the recombination radiation from strongly excited silicon

The spectral distribution of the recombination radiation from silicon during and after excitation by a Q-switched ruby laser has been measured and analyzed. The interpretation assumes a third-order (Auger) recombination process and a simple parabolic band structure. It takes into account the heating of the sample at the surface and the reduction of the band gap due to the high carrier density. Measurements of the spectral distribution as a function of time gives a value of the Auger transition rate factor γ₃ = 2·10⁻³¹ cm⁶sec⁻¹.     

Spectroscopical Observation of a Relaxing High Speed Nitrogen Plasma Jet

A near sonic nitrogen plasma jet operating at pressures between 100 Torr and 1 atm has been investigated spectroscopically. From the absolute emission coefficient of a NI spectral line, local values for the electron temperature T_e have been derived. For pressures above 200 Torr, T_g was found to coincide within the limits of experimental error with the gas temperature T_g. The latter quantity has been determined via the relative emission coefficient of selected rotational line components of the N₂⁺ molecular band at 3914 Å. The results of these measurements together with control data for the electron density derived from the continuous emission coefficient indicate that at pressures above 200 Torr the existence of a thermal equilibrium between the degrees of freedom corresponding to particle translation, electron excitation, and ionization can be accepted, at least for the inner zone of the plasma jet. To the contrary, the data for the absolute emission coefficient of N₂⁺ molecular band lines show that the degree of dissociation in the plasma jet is much in excess of that corresponding to equilibrium. This phenomenon can be explained as a result of the rapid temperature decay in the plasma from initially 13000 K in the arc heating zone to T ≦ 9000 K in the plasma jet zone proceeding in a time interval of 10^?5 s which is much shorter than the time necessary for adjustment of dissociation equilibrium. In the outer cool zone of the plasma jet, an unusual high intensity of the N₂⁺ radiation was found thus indicating the existence of a nonequilibrium excitation mechanism typical for a decaying nitrogen plasma. From the supernormal high degree of dissociation in the high-speed subatmospheric nitrogen plasma jet, conclusions are drawn with respect to its applicability as source of reactive particles in plasma-chemical experiments.     

Systematic trends in the radiative emission rates for low- and medium-Z elements in high-temperature plasmas

The power radiated by an optically thin, low-density (N_e ≤ 10¹⁴ electrons/cm³) plasma has been calculated for the electron temperature range 1–10⁶ eV taking into account resonance line emission, direct recombination radiation, dielectronic recombination radiation, and bremsstrahlung from the ions of a given element. The ionization structure has been determined by using a corona equilibrium model in which collisional ionization and inner-shelled excitation followed by autoionization are balanced by direct radiative and dielectronic recombination. Based on the results for respresentative elements from carbon through nickel, graphs are presented of the maximum radiated power, the maximum emission temperature, and the mean charge at the maximum for each shell as functions of the atomic number Z. Assuming that the maximum emission temperature can be achieved, aluminum and iron are predicted to be the most efficient K-shell radiators for Z ≤28.     

Heterodyne detection of CO2 laser radiation scattered from a plasma arc jet

Observations have been made of CO₂ laser radiation scattered incoherently from an argon arc jet plasma. The heterodyne detection technique employed is discussed.     

X-ray study of microdroplet plasma formation under the action of superintense laser radiation

We have measured the X-ray emission spectra of a plasma generated by laser radiation with an intensity above 10¹⁹W/cm² and a pulse duration of 30 fs acting upon an argon jet target with a large abundance of micron-sized clusters. The time variation of the X-ray yield from ions of various multiplicities, calculated within the framework of a nonstationary kinetic model, shows a good qualitative agreement with the experimental time-integrated spectrum.     

Experiment on the electromagnetic radiation excited in an electron beam‐ion channel system

An experimental study on electromagnetic (EM) radiation in an electron beam‐ion channel system is reported, which indicates the same result predicted by our previous theory. The system is formed in an arc plasma jet with a plasma density of 10¹⁷m^?3 , and the electron beam is driven by a voltage pulse of 20 kV. The result shows that the system can excite EM radiation in the range of the plasma frequency. The scheme is also hopeful to be used for generating high‐frequency and wide‐band EM radiation up to terahertz by enhancing the plasma density.     

Some properties of luminescence lifetimes from quartz stimulated by blue light

Abstract

Measurements have been made of recombination lifetimes as a function of stimulation time in quartz, a natural dosimeter. Time-resolved luminescence techniques were used to study the influence on lifetimes of sustained 470nm optical stimulation of luminescence and modifications associated with beta irradiation of samples. Results are reported for samples differing in sensitivity to luminescence stimulation.     

Die Erzeugung von Seriengrenzkontinua mit Hilfe von Gleitfunken zur Absolutmessung im Vakuum-Ultraviolett

A lithium plasma is produced by discharging a 40 kV, 0,3 μF capacitor through a lithium-hydride-capillary (diameter 2 mm, length 20 mm) in vacuum (p～10^?4 Torr). During the first half-cycle (0,6 μs) Bremsstrahlung of Li III is observed in the visible and infrared, and the Lyman series of Li III together with the recombination continuum in the vacuum UV (100 Å). The high members of the Lyman series are broadened by Stark effect giving an electron density of about 6 · 10¹⁸ cm^?3. In the infrared the radiation is emitted from an optically thick plasma at a time when the free-free continuum in the visible is emitted from an optically thin plasma. Temporal development of electron temperatureT _e and electron density N_e has been measured from the absolute intensity in these spectral regions. Typical values ofT _e=230 000° K andN _e 5 · 10¹⁸ cm^?3 e.g. have been obtained. For these values the relaxation time for an ionisation equilibrium is short compared to the observation time. The complete ionisation of Li III has been checked by absorption measurements near 100 A. Thus the absolute intensity of the recombination continuum could be calculated. An experimental arrangement was built to measure simultaneously the time history of the intensity in the vacuum ultraviolet, in the visible, and infrared spectral regions.     

ARC study of the oxygen Schumann-Runge system

Other investigations have shown that the Schumann-Runge system of oxygen is excited by an inverse predissociation mechanism, O + O→ O^*₂. Under the condition where this excitation mechanism dominates, there is some question as to whether the radiation yielded a true rotational/vibrational electronic band spectrum, as opposed to a recombination continuum. Furthermore, there is controversy as to whether the spectral intensity is that given by equilibrium theory. The present work utilized a 1 atm arc jet facility to heat air and O₂-noble gas mixtures to temperatures between 3000 and 3500°K. Both photographic and photoelectric spectra were recorded. In the latter case, wavelength scans from 2000 to 6000 Å were carried out with 5 Å resolution to obtain absolute intensity data. The spectra were found to have true rotational/vibrational electronic band structure, and were identified as the O₂ Schumann-Runge ssytem. Comparison with an equilibrium radiation model showed excellent agreement. A slight adjustment was made in the electronic transition moment for the NO(β) system as demanded by the air runs.     

Forward-jet production in deep inelastic ep scattering at HERA

Forward jet cross sections have been measured in neutral current deep inelastic scattering at low Bjorken-x with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb^-1. Measurements are presented for inclusive forward jets as well as for forward jets accompanied by a dijet system. The explored phase space, with jet pseudorapidity up to 4.3 is expected to be particularly sensitive to the dynamics of QCD parton evolution at low x. The measurements are compared to fixed-order QCD calculations and to leading-order parton-shower Monte Carlo models.     

Charge exchange of laser-produced ions in a pulsed gas jet

Results of an experiment on the interaction of laser-produced plasma with a pulsed gas jet are reported. A resonant charge-exchange pumping of the n=3 level of the C³⁺ ion was observed. A spatial structure of the region of intensive interaction was obtained by a short time imaging of filtered plasma radiation. According to independent probe measurements, the interaction was realized at densities of ions and gas particles in excess of 10¹⁶ cm^-3. The obtained data provide a prospect for future experiments on laser gain in the EUV spectral range based on charge-exchange pumping of the C⁵⁺ ion. PACS 52.50.Jm; 52.25.Nr     

Phonon-assisted radiative electron-hole recombination in silicon quantum dots

The temperature dependence of the photoluminescence (PL) spectrum of silicon quantum dots (QDs) is studied both theoretically and experimentally, and the time of the corresponding electron-hole radiative recombination is calculated. The dependence of the recombination time on the QD size is discussed. The experiment shows that the PL intensity decreases by approximately 60% as the temperature increases from 77 to 293 K. The calculated characteristic recombination time has only a weak temperature dependence; therefore, the decrease in the PL intensity is associated primarily with nonradiative transitions. It is also shown that the phonon-assisted radiation is much more efficient than the zero-phonon emission. Moreover, the zero-phonon recombination time depends on the QD radius R as R⁸, whereas the phonon-assisted recombination time depends on this radius as R³.     

Transient optical absorption and luminescence of lithium triborate LiB<Subscript>3</Subscript>O<Subscript>5</Subscript>

The transient optical absorption and luminescence of LiB₃O₅ (LBO) nonlinear crystals in the visible and UV spectral ranges were studied. Measurements made using absorption optical spectroscopy with nsscale time resolution revealed that the transient optical absorption (TOA) in LBO originates from optical transitions in hole centers and that the kinetics of optical density relaxation are rate-limited by interdefect nonradiative tunneling recombination involving these hole centers and the Li⁰ electronic centers, which represent neutral lithium atoms. At 290 K, the Li⁰ centers can migrate in a thermally stimulated, one-dimensional manner, a process which is not accompanied by carrier delocalization into the conduction or valence band. It is shown that the pulsed LBO cathodoluminescence kinetics is rate-limited by a recombination process involving two competing valence-band-mediated hole centers and shallow B²⁺ electronic centers. The radiative recombination accounts for the characteristic σ-polarized LBO luminescence in the 4.0-eV region.     

Organic contaminants removal by oxygen ECR plasma

Recently electron cyclotron resonance (ECR) plasma have been explored for wafer cleaning applications, since it is known to do less damage to silicon surface than conventional plasma. Organic contaminants removal efficiency and plasma radiation damage of the ECR plasma cleaning have been investigated. In oxygen ECR plasma cleaning, the plasma exposure time needed to remove the organic contaminants on the silicon surface down to the detection limit is 40 s, but the one to reach the lowest surface roughness is 10 s. The leakage current level of the MOS capacitor made using the Si substrate exposed to oxygen ECR plasma for 40 s is 8 × 10⁻⁹ A. The optimum exposure time determined by considering the contaminants removal efficiency and the plasma radiation damage (or the leakage current level) is 40 s. Organic contaminants seem to be removed through both sputter-off mechanism by oxygen ion bombardment and evaporation mechanism by chemical reactions with excited oxygen atoms.     

Measurement of the argon continuum radiation in the near vacuum ultraviolet

Absolute argon continuum emission coefficients have been measured from 330 nm down to 110 nm in the near v.u.v. The radiation originates from a stationary 1 atm arc plasma with temperatures between 16900 and 21500 K. At these high temperatures, single ionization is almost complete and the plasma contains also a number of doubly ionized particles. These cause very intense recombination edges around 120 nm, which have been included in a recent theory. The agreement with these calculations is very good throughout the spectrum for lower plasma temperatures. Measurements at higher temperatures confirm only the results for the Ar(II) edges around 120 nm. At higher wavelengths the experimental values lie above the theoretical ones and do not show the predicted structure. This discrepancy may be partly due to omissions in the calculations and partly to line wing problems in the experiment.     

Laser plasma monitored by silicon carbide detectors

The excellent physical and chemical properties and the radiation hardness of silicon carbide (SiC) render this material particularly suitable for the realization of radiation detectors. In this paper we describe the main properties of SiC and the processes needed to realize good performance detectors. To this purpose, we made SiC Schottky diodes that were electrical characterized by using different techniques. In order to test the radiation hardness, the diodes were irradiated with different ion beams and the analysis of the electrical measurements allowed to identify the defects responsible of the device degradation. These detectors have been used to monitor the multi-MeV ions of the plasma emitted by irradiation of various targets with 300-ps laser at high intensity (10¹⁶?W/cm²). These measurements highlighted that the use of SiC detectors enhances the sensitivity to ions detection due to the cutting of the visible and soft ultraviolet radiation emitted from plasma. The small rise time and the proportionality to ion energy evidence that these detectors are a powerful tool for the characterization of ion generated by high-intensity pulsed laser.     

The electrical behaviour of abrupt ion implanted and diffused P+N junctions

Abstract

Results are reported of measurements of the properties of diodes formed by ion implantation, and for comparison boron diffused P⁺N diodes of similar area close by on the same chip. The four group III acceptor impurities were implanted separately to a dose of 5 × 10¹⁵ ions/cm² at room temperature into similar samples of suitably masked silicon. Boron ions were also implanted at liquid nitrogen temperature and 450°C. Annealing was limited to a maximum temperatare of 550 °C.

Measurements have been made of sheet resistance, forward and reverse I-V characteristics (from 10^?9 amps/cm²), reverse breakdown voltage, noise, minority carrier storage time and junction series resistance.

The bulk properties of boron implanted diodes were found to be reproducible. The introduction of recombination centres by implantation is the major factor influencing variation in these properties between one implantation condition and another. Changes in surface oxide conditions probably affect reverse leakage currents and breakdown voltages.

The properties of boron implanted diodes are considered suitable for applications such as the MOSFET, and are superior to those of the AI, Ga and In implanted diodes.