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Journal of Physics B Atomic and Molecular Physics

Laser enhancement of the collisional broadening of the 4713 and 3188 AA lines in a helium plasma

P. Pignolet, Bernard Dubreuil

Published Mar 28, 1986• 1 citation
Publisher

Abstract

The enhancement of the electron collisional broadening of the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels in a helium glow discharge (n e =10 13 -10 14 cm -3 ), induced by the non-resonant interaction of the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels with a CO 2 laser field, is reported. This extra broadening depends on the product of the electron density with the laser field amplitude and manifests itself on the 4713 and 3188 AA linewidths (the transitions 4s 3 S 1 to 2p 3 P 2,1,0 degrees and 4p 3 P 2,1,0 degrees to 2s 3 S 1 respectively). This effect results from the destruction, by electronic collisions, of the coherence induced by the laser field between the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels, and can be viewed as an electron-atom collisional fluorescence redistribution process at relatively large laser detuning.

DOI
10.1088/0022-3700/19/6/011

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The enhancement of the electron collisional broadening of the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels in a helium glow discharge (n e =10 13 -10 14 cm -3 ), induced by the non-resonant interaction of the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels with a CO 2 laser field, is reported. This extra broadening depends on the product of the electron density with the laser field amplitude and manifests itself on the 4713 and 3188 AA linewidths (the transitions 4s 3 S 1 to 2p 3 P 2,1,0 degrees and 4p 3 P 2,1,0 degrees to 2s 3 S 1 respectively). This effect results from the destruction, by electronic collisions, of the coherence induced by the laser field between the 4s 3 S 1 and 4p 3 P 2,1,0 degrees levels, and can be viewed as an electron-atom collisional fluorescence redistribution process at relatively large laser detuning.
In a helium glow discharge, the A. C. Stark shift of the 4 3S 1 → 2 3P0,1,2 transition (λ ≅ 4 713 Å) induced by nonresonant interaction of the 4 3S and 4 3P levels with a multimode CO2 laser field is measured as a function of laser intensity in the range IL < 35 MW/cm 2. Departure from the linear intensity dependence given by second order perturbation theory is observed for IL > 2 MW/cm2. A three-level atom model taking into account the interaction with the laser field in a non perturbative way agrees with the experiment. Strong broadening is also detected for IL > 1 MW/cm2. This broadening grows with IL until a quasi-continuum spreading over 50 Å is observed for IL = 33 MW/cm2. This broadening results from amplitude and phase fluctuations of the laser field modes during interaction. For IL = 33 MW/cm2 the recorded 4 713 Å spectral line shape reflects the statistical distribution of the mode amplitudes.
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.
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.
The role of collision-induced transfer of electronic state coherence on the spectral response of an atomic vapor is investigated. It is shown that such collisional effects can lead to the appearance of narrow resonances in the nonlinear spectroscopy of three-level systems. Collisional transfer between different multipoles of electronic state coherence is shown to lead to a breakdown of several percent in Becquerel's relation between the Faraday rotation and dispersion of a vapor. All calculations are carried out in the impact limit of pressure broadening theory. Particular emphasis is given to the order in which the various averages over the velocity distributions of both the active atoms and perturbers must be performed. In particular, for the effects described herein, the various collisional transfer rates vanish when averaged over the active atom velocity distribution. It is essential, therefore, to obtain the final line-shape formulas, including velocity-dependent collisional transfer rates, before carrying out the average over the active atom velocity distribution. The experimental observation of these spectral features would provide direct evidence for the existence of these collisional transfer processes.