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.     

Collisional — Radiative Model of Helium Plasma. II. Global Population Coefficients

We present results of our calculation of global population coefficients based on a collisional — radiative model of the helium plasma. The results are given for 2³P, 2¹P, 3³S, 3¹S, 3³P, 3¹D, 3³D, 3¹P, 4³ and 4¹ states of helium atoms. We consider electron densities from 10⁷ to 10¹⁸ cm^?3 and electron temperature from 1 to 10 eV. The electron distribution function is considered to be Maxwellian and the plasma to be radiatively thin.     

Helium emission spectra—II. Pressures to 0.91 torr

Electron impact excitation is employed to study the pressure variation of the steady state luminescence from 27 levels of helium. Apparent cross sections are determined for these levels over the pressure range 0.025–0.91 torr, a much broader pressure range for observing collisional excitation transfer processes than has been previously investigated. Model calculations are carried out in which the data is fit to a set of coupled steady state equations. Primary and secondary electron excitation, radiative transfer, and bimolecular and termolecular collisional processes are accounted for in the analysis. We find that n¹P→n¹D collisional excitation transfer is much more important as a populating mechanism for n¹D levels than has been previously believed. The cross section for 4¹P→4¹D collisional transfer is estimated to be (1.3±0.3)×10^?14 cm². A termolecular process, believed to involve formation of He₂⁺, becomes an important loss mechanism for the 3¹P and 4¹P levels at pressures above 0.5 torr. Rate constants for this process are estimated to be (6.6±1.0)×10^?27 cm⁶/sec and (1.9±0.4)×10^?26 cm⁶/sec for 3¹P and 4¹P, respectively. The magnitude of the 3³D and 4³D apparent cross sections require a large 4³F population, a result which leads us to conclude that the 4F level is singlet-triplet mixed. There is evidence that collision induced transitions between all sublevels do not occur with equal probability. The most internally consistent results are arrived at by assuming ΔJ=0 to be a favored collisional transfer process.     

Laser induced collisional energy transfer processes in a rubidium-potassium mixture

Two laser induced collisional energy transfer processes in alkali vapours are investigated. For process K(4²S_1/2)+ Rb(5²P_3/2)+?ω_T→K(6²S_1/2)+Rb(5²S_1/2), a population inversion between the final state K(6²S_1/2) and lower states, leading to a superradiant emission has been observed. Comparison between experiment and theory shows a qualitative agreement in the limit of low atomic densities and laser power. However the transfer profile width is about two times larger than the calculated one for both processes.     

The effect of the electric field on the spectral line shape

The effect of the external electric field on the spectral line shape under conditions of a uniform population of magnetic sublevels is studied in the density matrix formalism. The line shapes of spontaneous radiation for the atomic 4¹P₁-2¹S₀, 4³D₁-2³P₀, 4³S₁-2³P₀, 4³P₁-2³S₁, 4³D₁-2³P₁, and 4³S₁-2³P₁ transitions ofHe are calculated. The electric field is shown to be responsible for the spectral line shape asymmetry. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 23–28, November, 1999.     

M-dependence of collisional transfer of rotational energy in OCS mixtures

Collision-induced transitions between rotational levels of OCS in the ground vibrational state have been investigated by steady-state microwave double resonance, with the M sublevels separated by a Stark field. The (2 ← 1)_P-(1 ← 0)_S, (3 ← 2)_P-(1 ← 0)_S, and (4 ← 3)_P-(1 ← 0)_S systems have been studied for pure OCS and for mixtures with excess CH₃OH, He, and H₂. For four-level systems having dipolar connections (ΔJ = 1; ΔM = 0, ± 1; parity ± ? ?) between pump and signal levels, it is found for OCS and the OCS-CH₃OH mixture that the dipole-type ΔJ = 1 transitions always dominate the collisional transfer, but for the OCS-He and OCS-H₂ mixtures that ΔJ = 2 quadrupole-type transitions are dominant. For all four collision partners, significant ΔJ = 2 and ΔJ = 3 collisional transfer is observed in some systems, indicating the presence of high-order terms in the collisional interaction.     

Computational study of collisional energy transfer and quasi-continuous population inversions in laser-pumped Ca vapor

A computational study of population inversion between several pairs of excited states viz 3d4p ³ F-4s3d ³ D, 4s5s ³ S-4s4p ³ P and 4s3d ³ D-4s4p ³ P in Ca vapor pumped on the 4s ^{2 1} S ₀-4s4p ³ P ₁ transition is presented. The main aim is to investigate the influence of various atomic processes in creating and sustaining the population inversion for long times after the excitation pulse. The delicate interplay between superelastic energy transfer to free electrons, energy pooling collisions and cascaded recombination is particulary examined. It is noted that quasi-continuous population inversion can be readily excited on the 4s3d ³ D-4s4p ³ P transitions; and under some conditions, also on the 4s5s ³ S-4s4p ³ P transitions. Furthermore, inversion on the 3d4p ³ F-4s3d ³ D transitions can also be excited for a considerable length of time. The results may be useful in designing and developing quasi-cw metal vapor lasers.     

Population inversions in recombining hydrogen,nitrogen, and oxygen atomic plasmas

The possibility of population inversions in atomic hydrogen, oxygen and nitrogen plasmas has been investigated by solving a system of master equations describing the temporal evolution of the electronic levels exposed to the action of collisional and radiative processes. The results show that population inversions can be obtained for both quasistationary and fast, transient, recombining plasmas. In this case, the lasing durations of the transitions O(I) 3p³P-3s³S, 3p⁵P-3s⁵S and N(I) 3s²P-2p²D, 3s²P-2p²P⁰ are of the same order of magnitude as the collisional relaxation times of the original states.     

激发态钠原子间的碰撞能量合并

对钠蒸气中的碰撞能量合并过程Na(3P_J)+Na(3P_J′)→Na(nL_J″)+Na(3S_1/2)进行了实验研究．在低钠原子密度条件下，用激光将原子分别激发到3P_1/2或3P_3/2态，测量从nL_J″态发出的荧光强度比和Na(3P_3/2)与Na(3P_1/2)的密度比，给出了J′=J=1/2或3/2，nL_J″=4D_3/2，4D_5/2,4F_5/2，4F_7/2，5S_1/2的速率系数对打的比值、当销原子密度足够高时，3P态以统计比率布居，得到了k_(nLJ″)(1/2,3/2)对k_5S(3/2,3/2)的比值． 关键词：     

Etude spectroscopique de l'effet d'anode

Spectral lines of LiI emitted by an anodic plasma, produced during electrolysis of a KCl-LiCl mixture, have been observed. This paper deals with diagnostic studies of this plasma and is based on a comparison of observed line profiles for LiI(2P-3D), (2P-4S) and (2P-4D, 4F) with calculated values derived from the Griem-Baranger impact theory. We have found, from the ensemble of the results, that the electron density in the plasma center is N_e(0) ? 1·8×10¹⁷ cm^-3, the electron temperature is T_e(0) ? 9×10³ °K, and the plasma thickness is l ? 200 μ.     

Broadening and shift of neutral chromium absorption lines by various perturbing gases

The broadening and shift of the absorption lines (a⁷S-z⁷P°, a⁷S-y⁷P° and a⁵S-z⁵P°) of the neutral chromium atom were observed at various temperatures (1500–1800 K) and densities (0.24-3.2x 10¹⁹/cm³) of foreign perturbers (He, Ar, N₂, H₂). The dependence of width and shift on temperature and density was studied. The collisional broadening and shift rates were determined from the profile measurements. The results are interpreted in terms of the Lindholm-Foley interaction potential between radiating atom and perturbers. Semi-quantitative agreement of theory and experiment was found in some cases for the temperature dependence of width and shift and for the differences of attractive constants ΔC₆ between upper and lower states of chromium lines.     

Quenching of 6s6d and 6s7d Hg atoms by Hg*-Hg,Hg*-Ar and Hg*-N2 collisions

The radiative decays and collisional quenching of Hg 6¹D₂, 6³D₂, 7¹D₂ and 7³D₂ atoms were investigated by time-resolved methods. Hg vapour, pure or mixed with a buffer gas in a fluorescence cell, was irradiated with u.v. light pulses from a frequency-doubled N₂ laser-pumped dye laser, populating selectively each state in turn by 2-photon absorption from the 6¹S₀ ground state. Fluorescent components arising from the decays to the 6¹P₁ or 6³S₀ state were resolved with a spectrometer and their time evolutions recorded with a transient digitizer or time-to-amplitude converter and pulse-height analyzer, yielding the respective decay rates. The quenching cross sections were obtained from variations of measured decay rates with Hg or buffer-gas density.     

Impact broadening of Sr 5sns 1 S 0, 5snd 1D2 and 5snd 3 D 2 levels by He and Xe

The impact broadening of strontium 5sns ¹ S ₀, 5snd ¹ D ₂ and 5snd ³ D ₂ levels by He and Xe have been studied by measuring the collisional half-widths of Doppler-free two-photon absorption lines from the 5s ^{2 1} S ₀ ground state in a thermionic diode. Large inelastic contributions to the broadening rates of the levels have been found where the moduli of the quantum defects of different level series are close to each other. A similar but continuousn-mixing behaviour along the series has also been found in our recent investigation on the broadening of Rb levels by noble gases. The broadening rates of the very high levels in Sr were found to be independent on the principal quantum number and the angular momentum of the levels and in good agreement with our recent broadening data of the Rb-He and Rb-Xe systems and the data for Na-He and Na-Xe found by the Columbia group.     

Penning ionization electron spectroscopy of H2O,D2O,H2S and SO2

Electron energy peak shifts and peak shapes were determined in the ionization of H₂O, D₂O, H₂S and SO₂ by Ne(³P₂) and He(2¹S, 2³S) metastable atoms. The shifts are large, especially in ionization of H₂O and D₂O into the ionic ground state and are probably mostly due to chemical interaction during the collision.In a previous paper the electron energy distribution curves for ionization of CO, HCl, HBr, N₂O, NO₂, CO₂, COS and CS₂ by helium, neon and argon metastables and the characteristics of this ionization were described¹. In this paper the series of triatomic molecules was extended to the molecules H₂O, D₂O, H₂S and SO₂. Because all these molecules have considerable dipole moments it could be expected that the peak shifts might be enhanced as compared with other triatomic molecules.     

Collisional redistribution of energy in the infrared spectrum of potassium vapor under two-photon excitation

The cascade decay of the 6S(4D) levels of the potassium atom, 6S(4D)-5P-5S(3D)-4P, following two-photon excitation of potassium vapor (4S-4P-6S) is investigated experimentally. The dependences of 11 resonant IR line intensities on potassium and buffer gas densities are presented. It is shown that the presence of buffer gas leads to collisional redistribution of radiation, causing suppression of some lines and production of new ones.     

Measurement of total decay rates of excited states in a deuterium plasma by laser-resonance fluorescence

In a deuterium plasma with an electron density of 2.8 × 10¹³ cm^-3 and an electron temperature of 1850 K, the n = 2 level is coupled to the n = 6 level by absorption of short-pulsed laser light. Due to inelastic and superelastic electron collisions, the enhanced n = 6 population density is distributed to its neighbouring levels, i.e., n = 5 and n = 7. The laser-resonance fluorescences of the Balmer lines D₅, D₆, and D₇ were measured with time resolution. The transient behaviour of the fluorescence is compared with numerical calculations, which are based on the rate equations for population densities of a system with 5 states. It is observed that the rate coefficients of Johnson cannot explain our experimental data. The coefficients of Vriens and Smeets compare favourably with the measurements within experimental errors.     

Multi-state impact parameter approximation for many particle excitations in atomic collisions; Total cross sections for Na-Ne and N-Ne

A multi-state impact parameter treatment of many electron excitations in atomic collisions for the energy rangeE _lab=1 keV–1 MeV is applied to the 3s-3p excitation of Na and to the⁴ S ² D,² P excitations of N by collision with Ne. The Ne-target, for which simultaneous excitations are neglected, is described by a Hartree-Fock frozen-core potential. The calculated cross sections for Na-Ne turn out to be higher by an order of magnitude than the available experiment.     

Population of 2p 55s levels of neon in He-Ne plasma: I. The evolution of mechanisms in the discharge and the afterglow

The spectroscopic study of population processes of neon 2p ⁵5s states was carried out in helium afterglow with small admixture of neon at PHe = 38.1 torr, [He]/[Ne]=10^?5 with pulsed discharge afterglow in helium with small admixture of neon (pressure equal to 38.1 mm Hg; ). It is established that the main mechanism of population of 3s ₂ level (in Paschen’s notation) in the discharge and the initial after-glow is the excitation transfer from metastable atoms of He(2¹ S ₀). The other three levels—3s ₃, 3s ₄, and 3s ₅—corresponding to 2p ⁵5s configuration are populated in the afterglow as a result of the dissociative recombination HeNe⁺ of ions with electrons. The same process is also the main channel of population of 3s ₂ level in the late afterglow phase, when the concentration of He(2¹ S ₀) atoms is small. The hypothesis of recombination mechanism is confirmed by observation of the response of line intensities to pulsed electron heating. The partial coefficients of dissociative recombination into 2p ⁵5s states are estimated.     

Sodium resonances in fluorescence from wide-range laser scan in H2-O2-Ar flame

Several resonances and underlying continua were observed in the excitation profiles of sodium atoms in an H₂-O₂-Ar flame, irradiated by the focused beam of a tunable, pulsed dye layer. These profiles are obtained by tuning the laser through the spectral range of Rhodamine 6G and detecting the ensuing fluorescence signals from the 3D-3P and 4D-3P transitions. We observed resonances corresponding to the 3S_{$\text{1}\text{2}$}-3P_{$\text{1}\text{2}$}, 3S_{$\text{1}\text{2}$}-3P_{$\text{3}\text{2}$}, 3P_{$\text{1}\text{2}$}-5S_{$\text{1}\text{2}$}, 3P_{$\text{3}\text{2}$}-5S_{$\text{1}\text{2}$} one-photon transitions and resonances corresponding to the 3S_{$\text{1}\text{2}$}-5S_{$\text{1}\text{2}$}, 3S_{$\text{1}\text{2}$}-4D_{$\text{3}\text{2}$}, _{$\text{5}\text{2}$} two-photon transitions. The observed resonances result from different sequences of radiative and collisional excitation. The presence of the continua is explained by consecutive absorption of laser photons in the collisionally broadened wings of absorption lines. The spatial inhomogeneity of the laser beam and hence of the saturation causes submerging of some resonances into continua.