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Journal of Physics B Atomic Molecular and Optical Physics

Disalignment of excited atoms by radiation re-absorption: neon 2p2atoms in a discharge plasma

Manabu Seo, Masanori Nimura, Masahiro Hasuo, Takashi Fujimoto

Published Apr 24, 2003• 15 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/36/9/316

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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.
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.
Investigates the levels of the 2p 5 3p configuration of neon by studying the polarization of the light emitted by a dc neon discharge. The pressure broadening of the Hanle signals was studied and cross sections were obtained for the destruction of alignment for collisions with both helium and neon ground state atoms. The measurements have been made at 315 K and 85 K. The cross sections for the 2p 2 and 2p 6 levels (Paschen notation) change considerably with temperature, while the cross sections for other levels are almost independent of temperature.
Cross sections for the disalignment of excited caesium atoms, by collision with atoms in ground states, have been determined by studying the damping of quantum beats in fluorescence. Atoms in a sealed cell were excited selectively to 122D3/2 and 132S3/2 by a pulsed tunable laser and discharge lamp. Alignment was induced by linear polarisation of the exciting light and monitored correspondingly. The quantum beats originate in the coherent excitation of hyperfine states whose structures have been determined in previous work. Collisional damping is investigated studying the quantum-beat signal over a range of atomic densities. The fluorescence contains a component representing the decay of population as well as of alignment, and extraction of the alignment cross sections relies on previous measurements of the depopulation cross sections. The results for disalignment cross sections are: sigma (2)(122D3/2)=4.1+or-0.8*10-16 m2, sigma (2)(132D3/2)=5.3+or-0.9*10-16 m2, about three times larger than the depopulation cross sections. Radiative lifetimes obtained from the analysis agree with previous determinations.
The fluorescent light from two atomic levels is studied assuming that the atoms are subjected to a gas discharge, to a static magnetic field, and, simultaneously, to a laser beam having the transition frequency between the atomic levels. Calculations are developed in the formalism of irreducible tensor operators. One takes into account the existence of isotropic relaxations and cascade effects due to spontaneous emission between the upper and the lower levels. Following Lamb, the laser beam is described classically by its electric field, and its effects are calculated by perturbation theory. It is predicted that the σ component of fluorescent light exhibits the Hanle effect, which allows one to measure the relaxation rates\Gamma(0)and\Gamma(2)of the population and of the alignment of the emitting level, respectively. Experimental agreement with the theoretical analysis has been found for the3s_{2}, 2s_{2}, and2P_{4}levels of Ne. Among the experimental results obtained by this method are the natural widths and the collision cross sections for population (quenching collision) and for alignment. Several transition probabilities are also derived. Furthermore, the theory of coherent imprisonment of the fluorescent line has been found valid even if laser pumping light is used.
We have measured the atomic lifetime of the argon 2 p 4 level by the laser-induced-fluorescence spectroscopy to be 30.5 ns. This contradicts the recent measurement using the high-frequency plused electron-beam excitation. We conjecture that, in their experiment, the radiation trapping not only affects the decay of the upper-level population but also relaxes the alignment initially produced by the electron-collision excitation in the upper-level population. Thus, the directly observed decay of the emission-line intensity is different from the decay of the upper-level population. We have observed the radiative relaxation of alignment on the neon 2 p 2 level which is a complementary effect to coherence narrowing.