Forbidden Components Research Articles

Suprathermal field fluctuations and high energy electrons in a low density plasma: A spectroscopic investigation

An oscillating magnetic field produces a hollow cylindrical helium plasma in the presence of a slowly varying bias field. The field penetrates to the center in the first quarter cycle, but is then shielded by a current layer without significant compression. The plasma is observed end-on in the helium I lines λ 4471.48 Å (23P←43D) and λ 4921.93 Å (21P←41D) and their nearby forbidden components at λ 4470.04 Å and λ 4920.59 Å, respectively. During the early phases of the discharge the forbidden components have intensities in the central, low density region much higher than those expected in a thermal plasma, indicating the presence of microinstabilities which cause suprathermal electric field fluctuations. The nonthermal or turbulent state of this low temperature (∼1 eV) plasma is consistent with the presence of an appreciable number of high energy (∼600 eV) electrons, indicated primarily by the spectroscopic investigations and verified by x-ray measurements.

Observation of ion dynamics in the He I 4026 AA Stark profile

A new experimental method allowing the heavy-particle temperature in an arc plasma to change considerably in otherwise fixed plasma conditions was applied to the study of the Stark broadening of the line He I 4026 AA and its associated forbidden components at an electron density of ne=1015 cm-3. Consistent with an earlier experiment, strong deviations from static ion calculations were found. The whole line shape was found to depend sensitively on variations in the heavy-particle temperature and, by including the earlier experiment as well as using the static ion theory, systematic trends could be observed. Though improving some features of the static profile, a recent calculation including ion dynamics still gives poor agreement with experiment.

Determination of the Plasmaparameters and the Suprathermal Microfields in a Critical Velocity Rotating Plasma

In a critical velocity rotating plasma the profiles of the Hel 4471 Å and Hel 4922 Å lines have been investigated with high time resolution using an optical multichannel system in order to determine suprathermal electric microfields. The plasma density has been measured simultaneously with a CO2 laserinterferometer. The existence of low- and highfrequency fields is proved by the occurrence of a strongly enhanced forbidden line component and plasma satellites in the front of the current spokes, which are typical for this plasma. Thus the occurrence of electrostatic instabilities and a turbulent heating process at the critical velocity is confirmed experimentally

Vibrational and low temperature optical spectra of the uranyl tricarbonato complex [UO2(CO3)3]4−

Abstract Diffuse reflectance (20,000–30,000 cm −1 ) and luminescence (18,000–21,000 cm −1 ) spectra of polycrystalline Na 4 [UO 2 (CO 3 ) 3 ] have been measured in the temperature range of 4.2–300 K; the i.r. (50–2000 cm −1 ) and Raman (50–2000 cm −1 ) spectra have been recorded at room temperature. The vibrational spectra exhibit all frequencies expected for a UO 2 L 3 complex of D 3 h symmetry with bidentate ligands L. The symmetric and asymmetric stretching vibrations of the uranyl ion occur at wave numbers as low as 808 and 843 cm −1 , respectively, indicating strong uranium ← carbonate bonding. The diffuse reflectance spectra consist of four overlapping electronic transitions, the lowest one with origin at 20,680 cm −1 is also seen in emission. The absorption bands are assigned to the spin and parity forbidden components of the π u →δ u and π u →φ u one-electron excitations. From the splitting of the excited configurations π 3 u δ 1 u and π 3 u φ 1 u the spin-orbit coupling constant is calculated to be ζ 5ƒ = 1750 cm −1 . Possible relationships between measurable spectroscopic properties of the uranyl ion and the strength of bonding in the equatorial plane are discussed.

Line Broadening Measurement in High Density Plasma. II. Neutral Helium Lines

The profiles of neutral helium lines at λ 4471, 4921, 5016, 6678, and 7065 A were observed in a plasma of electron density 1.0 to 3.5×10 17 cm -3 generated in an improved T-tube. Comparison of the measured profiles with the profiles predicted by line broadening theories shows that the λ 7065 A line is well described by the isolated line approximation but the λ 5016 and 6678 A lines should be treated by the overlapping line theory in a plasma of electron density of 3.0×10 17 cm -3 . On the profile of the λ 4471 A line which has intense forbidden components, disagreement between the observed and the theoretical result is seen on the wavelength separation between the allowed and the forbidden component. The observed ratio of the maximum intensity of the forbidden component to that of the allowed component is larger than the theoretical ratio.

Measurements of He I Line Profiles with Forbidden Components Induced by Fluctuating Fields in a Small Theta Pinch Plasma

Spectral line profiles of He I 4922 A line (2 1 P-4 1 D, 1 F) and 4471 A line (2 3 P-4 3 D, 3 F) with their forbidden components from a small linear theta pinch plasma have been measured and the plasma satellites have been observed on the wing of the line profiles. The observed profiles have broadened wings, and they are interpreted as the Doppler broadening due to fast neutral particles produced by the charge transfer with hot ions in the plasma of the neutral particle density of ∼10 14 cm -3 comparable with the plasma density. The low and high frequency fluctuating fields with the average strength of several kV/cm are deduced from the plasma satellites in the observed profiles.

Experimental study of stark broadening and shift of the HeI line [formula omitted] and its forbidden component [formula omitted] for electron densities 7 × 10 14 ≲ Ne [cm −3] ≲ 3 × 10 16

Measurements of the HeI line 4 1 D − 2 1 P, λ = 4921.93 A ̊ and its forbidden component 4 1 F − 2 1 P, λ = 4920.35 A ̊ have been carried out for the electron density range 7 × 10 14 ≲ N e (cm −3 ≲ 3 × 10 16. The lines were emitted from a continuous He afterglow plasma emerging from a plasmatron device with gas pressures ranging from 100 torr to 2 atm. Our results are in general agreement with the measurements of Birkeland et al., Nelson et al., Burgess et al. and Wulff, but not in agreement with the new calculations of Barnard et al.

Evidence for laser-induced effects in laser scattering diagnostics

Fluctuations of electric fields are excited in an arc plasma synchronous to the irradiation with an argon ion laser. Satellites of the forbidden component of the 4471 Å He line are observed.

Application of the model microfield method to Stark profiles of overlapping and isolated neutral lines

Describes a number of analytical and numerical calculations related to the model microfield method applied to Stark broadening (in particular, treatment of strong collisions and angular averages). This method is used for several types of profile: He I overlapping lines, Li I and Mg I resonance lines and Li I and O I non-resonance lines. For each of these, various approximations are discussed and the profiles are compared to those obtained from other theories and to experimental results. In particular, it is shown that the forbidden components cannot be obtained correctly using the usual approximation in which the electronic broadening is calculated independently of the ionic microfield.

Study of the 4471 Å and 4922 Å lines in the afterglow of an helium plasma produced by laser

An experimental investigation of the profiles of the 4471 A and 4922 A lines in the afterglow of an high density plasma of helium produced by laser is reported. The measured profiles of these two neutral helium lines and their forbidden components are given at electron densities between 5 · 1016 cm−3 and 2 · 1017 cm−3 and for electron temperature between 3 and 4 eV.

Measurement of Stark Broadening and Shift of the Helium I Line λ =4471.5 Ä and its Forbidden Component λ =4470 Ä for Electron Densities 7 · 1014 ≲ Ne [cm-3] ≲ 3 · 1016

Abstract The profiles of the HeI line λ=4471.5 Å (43D-23P) and its forbidden component λ=4470 Å (43F - 23P) have been measured for electron densities Ne ranging from 7 · 1014 cm -3 to 3 · 1O16 cm-3 . The line was emitted from a continuous afterglow plasma emerging from a plasmatron device. The working pressure ranged from 100 Torr to 2 atmospheres. Our measurements agree with those of Bekefi et al. (1971), Bretagne et al. (1971/1973) and Drawin and Ramette (1974). Relatively by large discrepancies are found with some theoretical calculations. Our measurements include the very interesting density region from Ne=2 · 1015 cm-3 to Ne = 1 · 1016 cm-3 for which only a very few experimental data are available.

Stark broadening tables for He I λ4922Å

The Stark broadening of He I λ4922Å and its forbidden components by both ions and electrons is calculated using a theory that includes ion dynamic effects. Tables are presented for temperatures from 5000°K to 40,000°K covering the density range 10 13 cm -3 to 10 16 cm -3 for both helium and hydrogen ionic perturbers.

Measurement of the HeI 4471 Å profile at an electron density of 10 15 cm −3

The profile of the HeI 4471 Å line and its forbidden component was measured using a pulsed arc plasma (electron density 10 15 cm −3, temperature 1·5 eV) as the light source. The profile is in good agreement with recent calculations in which the ion motion has been taken into account.

The Broadening of He I lines including ion dynamic corrections, with application to λ4471Å

A theory is developed for the broadening of He I lines with forbidden components which is valid at low densities (where the forbidden line is reasonably well isolated). This theory takes into account the effects of dynamic ion broadening. Application to λ4471Å gives good agreement with experiment and with a recent calculation of Lee; reasons for agreement with Lee are discussed in detail. Comprehensive tables for 4471Å are presented. These have been extended further into the line wings than previous tabulations. Static ion theory is found to be adequate for densities greater than ∼ 5 × 10 15 cm −3. A simple numerical profile for allowed lines, taking into account dynamic ion effects, is also presented.

The lowest singlet and triplet excited states of pyrazine

The phosphoresence, fluorescence and absorption spectra of pyrazine- h 4 and - d 4 have been obtained at 4°K in a benzene matrix. For comparison, those of the isotopically mixed crystal pyrazine- h 4 in d 4 were also taken. All these spectra show extremely sharp and well-resolved lines and reveal detailed vibronic structure. The coincidence of origins of absorption and emission shows that the lowest singlet state is an allowed transition, properly designated as 1B 3u ← 1A 1g . The forbidden component 1 B 2 g , predicted by both “exciton” and MO theories to be below the allowed component, must lie higher. Its exact location still remains uncertain. The phosphorescence spectrum, when compared with the singlet-triplet excitation spectrum, indicates that the lowest triplet state is also symmetry allowed, showing a strong 0-0 band and the coincidence of origins of absorption and emission. In accordance with previous work, the triplet state is designated as 3 B 3 u . The vibronic structure of the phosphorescence spectrum is very complicated. The first work on the analysis of this spectrum concluded that a simple progression of ν 6 a exists. Under the high resolution attainable in the present experiments, the supposed ν 6 a progression proves to have a composite structure, starting from the second member of the progression. Not only is the ν 9 a hydrogen-bending mode present as shown by the appearance of the CD bending mode in the d 4 spectrum, but a band assigned as 2 ν 6 b was also identified. The latter's anomalous intensity in the phosphorescence spectrum is interpreted as due to the Fermi resonance with 2 ν 6 a and ν 9 a bands. To help resolve the present controversy over the origin of the phosphoresence spectrum observed in crystalline pyrazine, detailed vibrational analyses of the emission spectra were made. The fluorescence spectrum has essentially the same vibronic structure as the phosphorescence spectrum.

Experimental Study of Stark Broadening of the Allowed and Forbidden Transitions 23P-43D, 4 3F (λ=4471.5 Å, λ=4470 Å) in Neutral Helium

Measurements were made of the profile of the Hel 4471.5 Å line and its forbidden component at λ = 4469.9 Å, emitted from an afterglow plasma of electron densities in the range 2.2 × 1014 to 1.5 × 1015 cm-3. The various quantities which define position and form of the profile have been compared with the theoretical calculations of Griem and of Barnard et al. It has been found that the wavelength shifts of the individual components and, thus, the peak separation are slightly smaller than predicted by theory, whereas the intensity in the valley between the two components is found to be slightly larger than the theoretical values. The peak intensity of the forbidden component is smaller than given by theory but not as small as found by D. D. Burgess and C. J. Cairns. At low electron densities the intensity of the forbidden component is proportional to the square of the normal field strength F0 of the electric microfield. At higher electron densities the intensity increases proportionally to F0 in agreement with the transition from quadratic to linear Stark effect at higher electric field strengths. The measurements show that the influence of the ion dynamics on the line profile is probably not as large as assumed by D. D. Burgess. The measurements show that the position of the intensity minimum between allowed and forbidden component is shifted towards the allowed component when the temperature decreases.

Corrections to Stark profiles of allowed and forbidden transitions in a moderately dense helium plasma

Comparison of experimental and theoretical Stark profiles of the allowed 21P-31D helium line at 6678AA and of the forbidden 21P-31P component at 6632AA are carried out. The calculated allowed and forbidden lines show good agreement with the experimental data. A study of some other theoretical profiles of this transition, which were previously determined, is performed to ascertain the reasons why poor agreement is found for these profiles when they are compared to the same experimental results.

High-density Stark profiles of two neutral helium lines and their forbidden components

Measured and calculated profiles are compared for two neutral helium lines, He I (4471 and 6678 A), and their forbidden components at high electron densities. Calculations were done at the measured plasma conditions and are compared directly with the experimental profiles. Good agreement is found between theory and experiment.

Experimental and theoretical studies on non-adiabatic ion dynamic contributions to the He I 4026 23P-53D, 53F profile at low electron densities

The He I 4026 23P-53D, 53F transition has been observed in a recombination plasma at low electron densities. The profile is in very poor agreement with all existing static ion theories and the discrepancies are different in form from those for the 2P-4D transitions studied previously. New theoretical predictions are also presented showing greatly improved agreement with the experiment, and emphasizing the importance of non-adiabatic ion dynamic broadening of the forbidden component. These results cast doubt on all existing criteria for the static approximation even when frequency separations are correctly measured from the centre of the forbidden component.

Stark profiles of forbidden and allowed transitions in a dense, laser produced helium plasma

Comparisons of experimental and theoretical Stark profiles of the allowed 21P-31D helium line at 6678AA and of the forbidden 21P-31P component at 6632AA in a dense plasma were carried out. The plasma was produced by optical breakdown of helium by means of a repetitive, high power CO2 laser. The allowed line shows good agreement with conventional theory, but discrepancies are found around the centre of the forbidden component. When normally neglected ion motions are taken into consideration, the observed discrepancies are partially removed. Tables of the Stark profiles for the pair of lines are given for electron densities in the range 3*1015-3*1017 cm-3 and temperatures in the range 10000-40000K.