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Journal of Physics B Atomic Molecular and Optical Physics

Zeeman effect on disalignment of excited atoms by radiation re-absorption: neon 2p2atoms in a discharge plasma

K. Deguchi, T Imagawa, T. Shikama, Masahiro Hasuo

Published Feb 23, 2009• 5 citations
Publisher

Abstract

We have measured the relaxation rate of alignment of neon atoms in a 2p2 (in Paschen notation) level, which were excited by a linearly polarized laser pulse in a glow discharge plasma at 77 K, in a magnetic field of up to 3 T in the Voigt configuration. The relaxation rate decreased with an increase in the magnetic field strength of up to 0.5 T and showed no magnetic field dependence above 0.5 T. We developed a Monte Carlo simulation method to calculate alignment relaxation, or disalignment, by radiation re-absorption of atomic resonance lines in a magnetic field. The simulated result was found to be consistent with the observed magnetic field dependence. We analysed the results of the simulation from a point of competition between the Zeeman splitting and the Doppler broadening of the transition lines from the 2p2 level.

DOI
10.1088/0953-4075/42/5/055403

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Disorientation and disalignment of 42P3/2 potassium atoms, induced in resonant collisions

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The total cross sections for disorientation (σ1) and disalignment (σ2) of 42P3/2 potassium atoms, induced in collisions with the ground-state atoms, have been determined using a modified Zeeman scanning method. Potassium vapor at densities of the order of 1011 cm−3, contained in a fluorescence cell located in a kilogauss variable magnetic field, was irradiated with circularly polarized 7665 Å resonance radiation emitted from a discharge lamp located in a constant field of 5.4 kG. Scans of the variable field permitted selective excitation of single Zeeman components in the absorbing vapor. The σ+ and σ− components of the resulting resonance fluorescence emitted parallel to the scanning field were monitored in relation to the vapor density, as were the π and σ components emitted in the perpendicular direction. As the vapor density increased so did the frequency of the collisions which caused transfers among the Zeeman states in the vapor and thus disorientation and disalignment. The observed dependence of circular and linear depolarization of the fluorescence on the potassium density yielded the cross sections σ1 = 17 × 10−12 cm2 and σ2 = 21 × 10−2 cm2, corrected for imprisonment of radiation.

Canadian Journal of Physics•Oct 1, 1980•2 citations
We have measured the relaxation rate of alignment of neon atoms in a 2p2 (in Paschen notation) level, which were excited by a linearly polarized laser pulse in a glow discharge plasma at 77 K, in a magnetic field of up to 3 T in the Voigt configuration. The relaxation rate decreased with an increase in the magnetic field strength of up to 0.5 T and showed no magnetic field dependence above 0.5 T. We developed a Monte Carlo simulation method to calculate alignment relaxation, or disalignment, by radiation re-absorption of atomic resonance lines in a magnetic field. The simulated result was found to be consistent with the observed magnetic field dependence. We analysed the results of the simulation from a point of competition between the Zeeman splitting and the Doppler broadening of the transition lines from the 2p2 level.
In a positive column of a glow discharge, a linearly polarized laser pulse produces aligned neon atoms in the 2p 2 (Paschen notation) level, and the subsequent fluorescence is observed with its polarized components resolved. The relaxation rate of the alignment, or the disalignment rate, is thus determined. The populations of the 1s 2 – 1s 5 levels are found to be strongly dependent on the discharge current, and so are the degrees of radiation re-absorption of the transition lines from 2p 2 to these levels. A Monte Carlo simulation is performed by which the disalignment rate by radiation re-absorption for these transitions is determined, where the isotope splitting of the transition lines is taken into account. This rate is subtracted from the observed disalignment rate, resulting in the rate being independent of the degree of radiation re-absorption. Thus, disalignment due to atom collisions and that due to radiation re-absorption are separated. For the latter disalignment the disalignment rate deduced by the simulation, treating the four transitions simultaneously, is found to be virtually equal to the sum of the disalignment rates for each of the transitions. The approximate expression for the disalignment rate given by D’Yakonov and Perel’ (1965 Sov. Phys.–JETP 20 997) is quantified, so that it can be applied to estimating the disalignment rate of an upper level having several transition lines with finite optical thickness.
In a temperature-controlled neon discharge cell under a magnetic field up to 7 T, a linearly polarized laser pulse has produced polarized neon atoms (atomic alignment) on the 2p 2 (in the Paschen notation) level from the 1s 3 level. The subsequent fluorescence to the 1s 2 level was observed with its polarized components being resolved. The disalignment rate coefficients of the 2p 2 atom due to neon atom collisions were determined at the temperature of 77 and 300 K. At each temperature, the rate coefficient shows no dependence on the magnetic field within the experimental accuracy.
The dichroism of a neon gas discharge plasma in a weak magnetic field is investigated by scanning the 3 s 2 -2 p 4 transition by monochromatic laser radiation. Magneto-optical resonances of the intrinsic macroscopic alignment and hidden alignment of the 2 p 4 level are separated. Against the background of these resonances, resonances attributed to coherence transfer from the 1 s 2, 4, 5 levels and the birefringence of the wings of the nearest absorption lines from the 1 s 4, 5 levels are observed. A new type of alignment is revealed—integral hidden alignment, whose lifetime, in contrast to the case with macroscopic alignment, increases without bound with increasing radiation trapping.
Summary form only given, as follows. We investigated the hidden alignment of Ne plasma in a magnetic field observing the optical characteristics of the medium-absorption and refraction. A tunable single frequency laser beam was used as a probe beam. Depending on the direction of the applied magnetic field and the geometry of observation, the dependence of the plasma birefringence or dichroism on the magnetic field was registered. The dye laser was tuned to /spl lambda/=607.4 nm (2p/sub 3/-1s/sub 4/ Ne transition). The dependence on the external magnetic field of the laser beam intensity passed through the Ne discharge cell was registered, for various laser frequencies in the limits of the absorption line. At the edge of the absorption line, the signal of the dichroism consists of two Lorentzian components while at the center of the absorption line the wide component is absent. The wide component width depends on neon pressure as: H(P)=(1.64/spl plusmn/0.03) gauss + P(torr) (0.33/spl plusmn/0.08) gauss/torr. This value is in a good agreement with other results concerning the lifetime of the 1s/sub 4/ state. The mechanism of alignment formation in this state is not clear. This state could not be aligned by light absorption following a quantitative calculation. The irradiation of atoms is isotropic within the limits of all the absorption line because of the high magnitude of absorption for the 1s/sub 4/-1s/sub 0/ transition. We consider the optical pumping as the most probable mechanism for alignment of this state. The magnitude of absorption on the s-p neon transitions are not high. The intensity of the spontaneous emission beam along the tube is greater than in the perpendicular direction. If the quantization axis coincides with the tube axis, this beam empties the magnetic sublevels with M=/spl plusmn/1, thus creating longitudinal alignment. The absence of a signal at the center of the absorption line can be explained by the high selfabsorption on the s-p transitions. The width of the narrow component of the signal is approximately 0.7 gauss. Its pressure dependence is small. We have established experimentally the absence of signals of such width in the spontaneous emission from 2p levels. Hence, all optical mechanisms of alignment transmission from higher or from lower states to the 1s/sub 4/ state are excluded. Apparently this signal arises from alignment of metastable state 1s/sub 5/. In our opinion the collisions are the most probable mechanism of alignment transmission.
Coherent population trapping in a gas discharge was studied for transitions between excited levels of neon atoms. Resonances corresponding to the arising of coherent population trapping in the Λ and V schemes for Zeeman sublevels of the lower and upper working states were observed in the presence of a longitudinal magnetic field. The effect of nonlinear polarization plane rotation under coherent population trapping conditions was studied. The possibility of using the results obtained in this work for diagnostics of local magnetic fields and other plasma parameters in gas discharges was considered.
The total cross sections for disorientation (σ1) and disalignment (σ2) of 42P3/2 potassium atoms, induced in collisions with the ground-state atoms, have been determined using a modified Zeeman scanning method. Potassium vapor at densities of the order of 1011 cm−3, contained in a fluorescence cell located in a kilogauss variable magnetic field, was irradiated with circularly polarized 7665 Å resonance radiation emitted from a discharge lamp located in a constant field of 5.4 kG. Scans of the variable field permitted selective excitation of single Zeeman components in the absorbing vapor. The σ+ and σ− components of the resulting resonance fluorescence emitted parallel to the scanning field were monitored in relation to the vapor density, as were the π and σ components emitted in the perpendicular direction. As the vapor density increased so did the frequency of the collisions which caused transfers among the Zeeman states in the vapor and thus disorientation and disalignment. The observed dependence of circular and linear depolarization of the fluorescence on the potassium density yielded the cross sections σ1 = 17 × 10−12 cm2 and σ2 = 21 × 10−2 cm2, corrected for imprisonment of radiation.