Kuniya Fukuda

Relationship of the laser-induced population perturbation of excited levels with the dynamical optogalvanic signal in a discharge plasma

Abstract For a pulsed-laser excitation of various neon transitions (1s j → 2p k ) in a glow discharge the population perturbations in the upper and lower levels are measured by emission and absorption spectroscopy, and the dynamical optogalvanic signals are observed. We propose that the population perturbation in the lower levels (1s 2 –1s 5 ) as a whole is responsible for the optogalvanic signal, and that metastable-level populations determine its decay characteristics. The sign reversal of the optogalvanic signal that depends on the excitation condition is interpreted in this context.

Excitation Mechanism of 3250 and 4416 Å Laser Lines in the Cataphoretic He-Cd Laser Discharge

The hook method has been applied to the population measurement for low-lying excited levels of He and Cd including metastable levels as well as for the ground state of Cd in the cataphoretic He–Cd laser discharge (3 mm bore and 685 mm long) under the typical condition for lasing (He pressure of 4.0 Torr, evaporator temperature of 170 to 270°C and dc current of 30 to 160 mA). The population density of Cd ions in the 3250 A upper level was also obtained by the line intensity measurement combined with the hook method. The results have shown that Penning collisions of ground state Cd atoms with 23S metastable He atoms are dominant for the excitation of 4416 and 3250 A laser lines. The Penning-excitation cross section of 1.7×10-15 cm2 and the population density of the 4416 A upper level have been determined, which are consistent with the result so far obtained.

Population Density and LTE of Excited Atoms in a Positive-Column Plasma I. Calculation on Hydroegn

Collisional-radiative model is applied to hydrogen plasma in a positive column and it is shown that as electron density increases in plasma, population density of an excited level goes, in general, along coronal and quasi- and full-saturation phases and finally reaches LTE. Atomic processes responsible for these phases are examined so that a criterion is presented for each transition between the successive phases. Further, it is proved that the validity condition for LTE proposed by Griem corresponds to the transition from the coronal to the quasi-saturation phase in a positive-column plasma.

Potential Curves of 1u States of Rare Gas Dimers Derived from VUV Emission Continua

The oscillatory structures of vuv emission continua originating from the lowest excited states 1 u ( 3 P 2 ) have been observed for Ne 2 , Ar 2 and Kr 2 in the glow discharges with currents of the order of µA at pressures of a few Torr. From these structures the potential curves of the 1 u ( 3 P 2 ) states have been calculated for Ne 2 and Ar 2 in the region of internuclear distances up to near the well minimum by making use of the method by Smith based on the WKB stationary phase approximation. Although the present calculation depends on the assumed ground state potential curves, it has been found that Cohens calculation of the Ne 2 1 u ( 3 P 2 ) potential falls far from our results, but Saxons calculation of the Ar 2 1 u ( 3 P 2 ) potential is in good agreement with our results.

Population Density and LTE of Excited Atoms in a Positive-Column Plasma. II. Measurement on Argon

A calculation is made to relate external discharge parameters to internal plasma parameters for a positive column of Ar produced by a steady or pulsed DC-discharge. The plasma and energy balance equations and Ohms law based on the theory of Tonks and Langmuir, and on that of Schottky are numerically solved, and the calculated plasma parameters are compared with the measurements for various external parameters of the experiments. On the basis of the calculations, population densities of excited ArI levels are obtained as a function of the electron density at an electron temperature of 50,000 K in a steady DC-discharge of Ar, and it has been proved that, as shown in a previous paper, they change from coronal to full-saturation phases as the electron density increases.

Measurements of Oscillator Strengths for the Transitions from the Metastable ^3P Levels of Alkaline-Earth Atoms.II.Magnesium

Oscillator strengths ( f values) for the transitions 3 s 3 p 3 P –3 s n s 3 S ( n =4 and 5), -3 s n d 3 D ( n =4 and 6 to 9) and -3 p 2 3 P in Mg I have been determined by using the hook method. The metastable 3 s 3 p 3 P levels have been populated by a DC discharge of the Mg–He mixture. The absolute scale is based on the absolute f values for 3 s 3 p 3 P –3 s 3 d 3 D (∼0.62). The determined f values are in good agreement with recent theoretical predictions. An estimate of the photoionization cross section from the 3 P levels by extrapolating the f values results in 20.2 Mb at the threshold.

Measurement of population density of excited helium atoms by the hook method

The hook method has been applied to the positive-column discharge plasma of helium and the population density of the lowest-lying excited levels, 21,3S and 21,3P, was measured. The results are compared with data obtained by use of the line absorption method, and discrepancies between these data are discussed. The result is also compared with a calculation based on application of the collisional-radiative model.

New Oscillatory Structures of Ne2 and Ar2 Emission Continua in the Vacuum Ultraviolet

New oscillatory structures starting at the wavelengths of the atomic forbidden transitions from 3 P 2 to the ground states have been found in Ne 2 and Ar 2 emission continua in the vacuum ultraviolet for low current (I<1 mA) glow discharge in a liquid-nitrogen cooled discharge tube. These continua are associated with 1 u ( 3 P 2 ) states, and it is found that this state of Ne 2 has a potential hump.

Excitation cross section in He+-He collisions. I. Excitation function and potential curve crossing

The authors measured the excitation cross sections of He I 31,3S, 21P, 31,3P, 41,3P, 31,3D, 41,3D, 51,3D and He II 22P and 32P levels at ion energies from 50 to 2*104 (lab) eV. Two types of structures have been recognised on the energy dependence of the cross sections, the origins of which are ascribed to excitation from the incoming channel by radial and rotational couplings through potential degeneracies at small internuclear distances. Profiles of the excitation functions depend systematically on angular momentum and spin multiplicity; the outgoing channels of the Sigma state strongly populate the triplet levels of the He atom, following the statistical weights of the spin multiplicity and those of the Pi state mainly populate the singlet levels. Rotational coupling is important for P-level excitation, as is radial coupling for D-level excitation. The characteristics of the polarisation of emitted light are consistent with our assignments for the excitation processes. The two-level model calculations give excitation functions which agree qualitatively with the observed ones.

Determination of Atomic Lifetime by the Laser-Induced Fluorescence Method-Application to Neon 2p 53p Levels

A procedure to measure atomic lifetimes is proposed for the method of observing the decay of the atomic fluorescence following pulsed-laser excitation. This method is superior to others in that it is free from the cascading effect. Several effects taken into account are (1) radiation trapping, (2) polarization and anisotropy of the fluorescence and their decay, and (3) de-population and re-population collisions. Especially effect (2) is important since the observed fluorescence intensity is, in general, not directly proportional to the upper-level population. For each effect a method of correction to the experimental data is established. As an example lifetimes of the neon 2pj levels are determined. Excellent agreement is obtained between the present data and those by the delayed coincidence method by Bennett and Kindlmann.