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Optics Letters

Photon echoes in plasmas: collisional relaxation in helium on 2 ^3S_1–3 ^3P_012

Thomas M. Baer, Isaac D. Abella

Published Nov 1, 1978• 7 citations
Publisher

Abstract

Photon echoes are observed for the first time in plasmas on atomic metastable states populated by rf discharge in helium at pressures from 0.01 to 0.5 Torr. The echoes are generated by tunable dye-laser excitation. The collision cross section for echo dephasing on the He 2 (3)S-3 (3)P transition at 3888 A by collisions with ground-state atoms is 5 x 10(-14) cm(2) +/- 20%. Echoes are also observed on the 3P metastable manifold in neon.

DOI
10.1364/ol.3.000170

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Optical Pumping of Neon P23 Metastable Atoms

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The $^{3}P_{2}$ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-$S$ state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable $^{3}P_{2}$ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.

Physical Review•Apr 5, 1969•26 citations
Photon echoes are observed for the first time in plasmas on atomic metastable states populated by rf discharge in helium at pressures from 0.01 to 0.5 Torr. The echoes are generated by tunable dye-laser excitation. The collision cross section for echo dephasing on the He 2 (3)S-3 (3)P transition at 3888 A by collisions with ground-state atoms is 5 x 10(-14) cm(2) +/- 20%. Echoes are also observed on the 3P metastable manifold in neon.
Photon echoes generated from visible transitions in the noble gases helium, neon, and argon are observed and the collisional relaxation rates are measured. The photon echoes are excited on transitions initiating from metastable states which are populated by electron impact in an rf discharge. The photon-echo intensity is measured as a function of time after the discharge is turned off and as a function of gas pressure. The reduction of the echo intensity in these two experiments is attributed to relaxation of the metastable population in the afterglow and dephasing collisions, respectively. The dependence of the photon-echo damping time on transition wavelength and perturber gas is interpreted by means of a simple van der Waals model for the excited-atom and perturber-gas interaction.
We report a total collisional quenching rate of (6.4\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{sec}}^{\ensuremath{-}1}$ at 292\ifmmode^\circ\else\textdegree\fi{}K (or a thermally averaged cross section of 0.36\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2) for He 33S3^{3}S33S atoms colliding with ground-state He atoms. This rate is determined by using a pulsed dye laser to populate the 33S3^{3}S33S level in a He aftergow and then observing the decay of the $3^{3}S\ensuremath{\rightarrow}2^{3}P$ fluorescence. We see no evidence for three-body collisional quenching of 33S3^{3}S33S atoms at pressures up to 300 Torr.
The 3P2^{3}P_{2}3P2​ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-SSS state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable 3P2^{3}P_{2}3P2​ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.