Influence of Laser radiation on the collisional-radiative recombination and ionization coefficients

It is shown that irradiation of a plasma by a Laser beam lowers the collisional-radiative coefficient for recombination (α) and increases the one for ionization (S) compared to unirradiated plasmas. Numerical values ofα andS are given for a hydrogen plasma irradiated by CO₂, Ruby, and Ar-Ion Lasers. The calculations are based on a collisional-radiative model in which photoionization and stimulated recombination intervene. Stimulated recombination has an important effect and can lead to saturation of free-bound transitions at high power density of the Laser beam as long as multiphoton ionization effects and plasma heating by inverse bremsstrahlung remain negligibly small.     

Non-linear effects in the spectrum of 90° scattered laser light from gas breakdown plasmas

Laser radiation scattered at 90° from gas breakdown plasmas induced by focussed 10 nsec, 1.06 μm wavelength laser pulses was simultaneously spatially and spectrally analysed. The scattered spectra showed non-linear, side band generation, smearing out of spectral structure and Doppler shifts. The side bands are attributed to a non-linear refractive index n₂ = 0.6 × 10^-12 e.s.u. which causes phase modulation when a weak side band frequency beats with a strong centre frequency. The observations suggest a fast response non-linearity such as would arise from the near resonant non-linear polarizability of excited atoms.     

The violet emissions produced by laser excitation of Na vapor in the 570–595 nm region

We have observed atomic Na and molecular Na₂ emissions in the violet region when Na vapor in a heatpipe (～10³—10¹⁶ atoms/cm³) is irradiated with a pulsed dye laser with output wavelength in the 570–597 nm region. The Na atomic emissions probably result from recombination of Na⁺ + e^- and energy-pooling involving highly excited atoms and molecules, while the diffuse violet emission bands are probably produced through collisions among excited Na atoms and Na₂ molecules.     

Photoreduction processes and nanocluster formation induced by a CO2 laser on silicate surfaces

The photoreduction processes stimulated either the cw (power density 10⁵–10⁶ W/cm²) or pulsed (pulse energy 3–4 J, pulse duration 200 ns, effective laser spot diameter 1 mm) radiation of a CO₂ laser on the surfaces of fused and crystalline quartz, as well as of natural silicates (nepheline KNa₃[AlSiO₄]₄, rhodonite CaMn₄[Si₅O₁₅], and zircon ZrSiO₄), are studied. The X-ray emission analysis of irradiated surfaces showed that the laser irradiation of these materials leads to the sublimation of silicon oxides and the enrichment of surfaces with constituent metal elements. Laser radiation also stimulates the formation of silicon and metal nanoclusters on irradiated surfaces. The appearance of these nanoclusters is confirmed by both photoluminescence and X-ray emission studies of irradiated surfaces.     

The Influence of Admixtures of Molecular Gases on the Efficiency of Radiation Production by Ar?Hg Mixture Plasmas used in Fluorescent Lamps

Starting from former investigations of pure Ar? Hg mixture plasmas in parameter ranges typical of fluorescent lamps we studied the influence of additional admixtures of molecular gases (N₂, H₂) on the energy transfer from the electrons heated by an electric field to the lowest excited states of Hg atoms which are the energy source for the resonance radiation production. By calculation of the different power loss rates via solving the appropriate Boltzmann equation for three component mixture plasmas it was found that already a threshold level of molecular impurities of about 10^?4 Torr leads to a marked energy dissipation by the impurities and thus to a pronounced reduction of the efficiency of the resonance radiation production. This is caused by the great effectivity of vibrational excitation of molecules in electron collisions due to the great cross sections for such collisions and their low thresholds.     

Excitation transfer between excited electronic states of the H2 molecule due to interaction with a CO2 Laser beam

A spectroscopic study of the band systemsG ¹ Σ _g →B ¹ Σ _u andI ¹ Π _g →B ¹ Σ _u of H₂ emitted by a glow discharge in H₂ gas showed that the populations of theG ¹ Σ _g (v, J) states decrease for the benefit of theI ¹ Π _u (v′, J′) states when the discharge is irradiated by a powerful CO₂ Laser beam (power density 70 kW/cm²). The effect is interpreted as an absorption process in which two Laser photons intervene according to the transition scheme:G ¹ Σ _g →B″¹ Σ _u →I ¹ Π _g. A change of the populations of excited electronic states should lead to a change of the populations of excited atomic states as well.     

Ca和Sr原子n′dnl自电离态在低电场中的线性Stark效应

用激光多步激发技术观察了Ca和Sr原子n′dnl双电子激发自电离态的电场效应。Stark光谱显示,在F<1/3n⁵的低电场中,Stark能级分裂与外电场强度线性相关。也观察了近简并l亚能级的Stark混合使自电离共振谱线在外电场中的增宽。这一效应在高温等离子体中对双电子复合速率的增加起重要作用。 关键词：     

Reactions of excited I*(2P1/2) and unexcited I(2P3/2) iodine atoms in perfluoroalkyl radicals

A method of investigating reactions of excited and unexcited atoms is discussed. It is based on pulsed photolysis of molecules with simultaneous passage of laser radiation through the working medium. The method proposed is used to investigate the reactions that accompany the photolysis of the molecules RI(CF₃I, n-C₃F₇I, i-C₃F₇I). The rate constants of the recombination of iodine atoms into I₂ in the presence of RI molecules are calculated for the atoms I(²P_3/2) and I*(²P_1/2), as are the recombination constants of the radicals R into R₂ and with the atoms I*(²P_1/2) and I(²P_3/2) into the RI molecule. It is shown that the I(²P_3/2) atoms are much more active in the recombination into Ia and RI than the I*(²P_1/2) atoms. The role of the investigated reactions in the kinetics of a photodissociation iodine laser (PDIL) is discussed. The results are compared with the published data.     

Direct measurement of lifetimes of infrared multiple-photon dissociated molecules

A beam of mass selected SF ₅ ⁺ ions is crossed with a pulsed CO₂ laser beam. The distribution of lifetimes of infrared multiple-photon pumped SF ₅ ⁺ ion-molecules prior to dissociation into SF ₄ ⁺ +F has been measured directly using a time-of-flight ion-beam technique. The data provide new information on the distribution of the total internal energy of molecules excited by absorption of infrared radiation.     

Continuous wave field ionization laser spectroscopy

A new technique is described which allows Doppler-free, isotope-selective excitation of atoms by continuous wave laser radiation and continuous ionization of the atoms by an electric field. The atoms are excited to high Rydberg states in an electric-field-free region of a collimated atomic beam. Because the lifetimes of Rydberg atoms are long they can reach a spatially separated region of the atomic beam where they are ionized by a continuous electric field with a probability of unity. In the case of lithium we obtained a 10³ times larger ion signal by field ionization of Rydberg atoms than by direct photoionization from low excited states.     

Equilibrium and non-equilibrium radiation observed in shock-tube studies of methane oxidation

Equilibrium emission intensities between 3700 and 5152 A have been accounted for in terms of absolute emission intensities associated with the recombination reactions between CO and O to form CO₂ and O plus O yielding O₂, as well as emission from the Schumann-Runge bands of O₂. Absolute radiation intensities observed for CO-O recombination are consistent with the experimental results described in earlier publications.Observed equilibrium emission intensities in fuel-lean and stoichiometric mixtures at 3100 and 3180 A correspond to radiation from electronically excited OH. Our results indicate that simplified smeared-line band models do not agree with the observed data at 3100 and 3180 A. In view of the known spectral structure of the twelve contributing main and satellite vibration-rotation bands near the band origins, failure of the simplified band models is readily intelligible. In a fuel-rich mixture, the observed radiation is much greater than can be accounted for by radiation from known emitters.Previously published, non-LTE populations for the first electronically excited states of OH, CH and C₂ have been reevaluated for argon-diluted mixtures of CH₄ and O₂ behind reflected shock waves. In these studies, absolute emission intensities were measured from OH at 3100 and 3180 A, from CH at 3922 and 4297 A, and from C₂ at 4689 and 5152 A.Equilibrium absorption coefficients have been measured for H₂O at temperatures between 2200 and 3100°K.Induction times observed for the onsets of measurable emission from electronically excited species and the times required to reach maximum emission increase in the order OH¹, CH¹, C¹₂.In conclusion, we define an elaborate spectroscopic procedure for mapping the complete concentration-time profiles for all of the chemical species that occur in a representative reaction scheme. The consistency of the postulated reaction mechanisms and rates may be verified by detailed comparisons of calculated and observed concentration-time histories. This program has not yet been successfully completed.     

Investigation of stimulated Brillouin scattering under well-defined interaction conditions

Ruby-laser light scattering was used to gain spatially and temporally resolved information about plasma parameters for nanosecond CO₂ laser-plasma interaction studies. The results confirm a heating model, where heat conduction in dense model plasmas can be described classically. CO₂-laser excited stimulated Brillouin scattering could directly be shown to arise from near thermal level by simultaneous Thomson scattering with a ruby-laser, and turbulence enhanced density fluctuations could be excluded.     

Electron acceleration below quarter-critical density in CO2 laser-produced plasmas

We present detailed characteristics of heated electron distributions in long-scale-length underdense plasmas irradiated by intense nanosecond CO₂ laser radiation. Below n_c/4, the heated electron distributions are consistent with heating by Raman instability.     

Effect of combined radiation processing on parameters of Si-based MOS transistors

The variation of parameters of Si-based MOS-transistors affected by combined infrared and X- ray photons processing were presented. It was found that X-ray irradiated transistor under incoherent infrared radiation is characterized by lowering of radiation sensitivity. As positive charge and its radiation-induced rate of generation in undergate SiO₂ insulator decreases, we observe a reduction of both ΔU_No and voltage threshold U_th transistor components. Under pulsed laser processing (λ?=?1.06?µm, τ?=?10^?3s) of the X-rays irradiated transistor the energy density of laser beam corresponding to the minimum threshold voltage was established. It was shown that the combined treatment can be effectively used to control the transistor parameters. The model describing the radiation changes of parameters in Si-SiO₂ structure was proposed. The effect of Na⁺, K⁺ and H⁺ ions in undergate insulator on variation of charge state and interface reconstruction under laser irradiation were predicted by proposed model.     

On the mechanism of formation of helium molecules in a low-temperature plasma

The glow of He₂ molecules in a low-pressure (P ≤ 2 Torr) helium plasma excited by a monochromatic electron beam is detected and studied. The experimental data point to the existence of two basic mechanisms of formation of an excited He₂ molecule, namely, the collision of an excited helium atom with two unexcited atoms and the recombination of a molecular ion.     

Decomposition of methylene chloride by electron beam and pulsed corona processing

Experiments on the plasma-assisted decomposition of dilute concentrations of methylene chloride in atmospheric-pressure N₂ and dry air streams by electron beam and pulsed corona processing are presented. This paper presents the first experimental evidence showing that the decomposition of methylene chloride in a non-thermal plasma at ambient gas temperature proceeds via reaction with nitrogen atoms. The decompositions is more efficient with energy deposition in electron beam generated plasmas because of the higher rate for electron-impact dissociation of N₂. In dry air mixture, the decomposition of methylene chloride is degraded substantially because the nitrogen atoms are consumed in the production of nitrogen oxides. At higher gas temperatures (300°C), the decomposition of methylene chloride in dry air is shown to proceed via reaction with oxygen atoms. The main products of methylene chloride decomposition in dry air mixtures are CO, CO₂, HCl, and probably Cl₂.     

Multiphoton excitation of molecules by single mode CW lasers

An intracavity laser interaction zone with molecules expanded in a molecular beam set-up permits multiphoton excitation under collision-free conditions by narrow band radiation. Two- and three-laser experiments are performed to study rapid adiabatic passage processes, stimulated de-excitation and hole burning effects. The detection of excitation occurs by means of a sensitive bolometric molecular beam detector. The essential information obtained for SF₆ concerns the vibrational bottleneck, the conservation of excited eigenstate characteristics for at least 1 ms, the ease with which about 15 photons can be deposited in the molecules with the help of two cw CO₂ lasers and the influence of simultaneous two-frequency radiation, yielding a significant extra-excitation. Other molecules where multiphoton excitation is observed utilizing the same set-up are CF₃I and CF₃Br.     

Study of boron and molybdenum plasmas on multipulse interaction of femtosecond radiation with a target

Laser plasmas generated by femtosecond radiation on the surface of boron and molybdenum targets are investigated by the shadowgraph method. The modes of single-pulse and multipulse interaction of laser radiation with a target are compared. The occurrence of plasma bullets is discussed, which were observed on both single-pulse and multipulse interaction with the same area of a target. The wavefront velocities of expanding boron and molybdenum plasmas were measured to be 5 × 10⁴ and 6 × 10³ m s^?1, respectively. The electron density measured by interferometry using a time delay of 800 ps in a boron plasma excited by 795-nm radiation with an intensity of 10¹⁶ W cm^?2 amounted to 8 × 10¹⁹ cm^?3. The correlation between some specific features of the plasma and the generation of the 3/2 harmonic, observed on multipulse interaction of femtosecond radiation with a boron target, is discussed.     

Electron impact excitation of higher energy states of molecular oxygen in the atmosphere of Europa

Recent measurements of integral cross sections for electron impact excitation of the Schumann-Runge continuum, longest band and second band of molecular oxygen are applied to calculations of emissions from the atmosphere of Europa. Molecules excited to these bands predissociate, producing O(¹D) (excited oxygen) atoms which subsequently decay to produce 630.0-nm radiation. Radiation of this wavelength is also produced by direct excitation of O atoms and by the recombination of O \hbox{₂⁺_2^+} + 2 with electrons, but these two processes also produce O(¹S) atoms which then emit at 557.7 nm. It is shown by modeling that the ratio of 630.0-nm to 557.7-nm is sensitive to the relative importance of the three processes, suggesting that the ratio would be a useful remote sensing probe in the atmosphere of Europa. In particular, the excitation of the Schumann-Runge continuum, longest band and second band is produced by magnetospheric electrons while the recombination is produced by secondary electrons produced in the atmosphere. This difference raises the possibility of determination of the secondary electron spectrum by measurement of light emissions.     

Intensity enhancement of the C5+ Balmer radiation excited by capillary discharge pumping

The intensity of the 18.2 nm Balmer α-transition in C⁵⁺ excited in a capillary discharge using alumina and polyacetal tubes was studied. For discharge currents of up to 80 kA in tubes filled with C₂H₂, intense radiation from the excitation of C⁵⁺ ions and from the recombination of C⁶⁺ ions was observed. With increasing length of the discharge, the intensity in the falling edge of the recombination pulse rises faster than proportional. In contrast to previous investigations, gain by stimulated emission is excluded. The enhancement is ascribed to an optical guiding of the XUV radiation in the dense plasma created by ablation from the tube walls. Received: 1 April 1999 / Revised version: 22 July 1999 / Published online: 30 November 1999