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Journal of Physics B Atomic and Molecular Physics

Cascade effects in neon life time measurements

C. Gerald Carrington

Published Aug 1, 1972• 14 citations
Publisher

Abstract

Alignment cascade phenomena have been studied in the neon 2p levels (Paschen notation) Hanle effect and double resonance techniques in a mild discharge. The levels are aligned directly by an optical excitation mechanism. Most 2p levels show evidence of alignment cascade from the 2s 4 , 3s 1 or 3d levels below 0.1 Torr; addition of argon impurities greatly enhances the cascade signal relative to the direct alignment signal. Data used previously in pressure broadening studies are shown to be free of cascade errors and are used to establish upper limits for the 2p level life times. A correction for coherence narrowing is estimated; this gives natural life times systematically less than those obtained by methods in which cascade effects are not carefully eliminated. In common with other Hanle effect measurements for these levels the values are marginally smaller than the threshold life times of Bennett and Kindlmann (1966).

DOI
10.1088/0022-3700/5/8/022

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Zero-field level crossing of the 633 nm Ne line in resonance absorption

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The authors have investigated the zero-field level crossing lineshape in resonance absorption. A single-mode HeNe laser beam at 633 nm is applied to an external neon-gas absorption cell in a longitudinal magnetic field. They compare the measured lineshapes with theoretical predictions. Two superposed nonlinear Lorentzian-shaped resonances, known as the population effect of the 2p 4 -3s 2 transition and the Zeeman coherence effect of the lower 2p 4 level, appear on the background caused by the inhomogeneous broadening. Taking into account the background and the power broadening the authors obtain the linewidths at zero pressure gamma (2p 4 ,3s 2 )=(7.7+or-2.1) MHz and gamma (2p 4 )=(8.4+or-1.2) MHz. For the neon pressure broadening the authors get for pressures below 200 Pa (1.5 Torr) d gamma (2p 4 ,3s 2 )/dp=0.20+or-0.02 MHz Pa -1 and d(2p 4 )/dp=0.040+or-0.007 MHz Pa -1 . Their values are in acceptable agreement with the values obtained with other spectroscopic methods.

Journal of Physics B Atomic and Molecular Physics•Feb 28, 1985•18 citations
Alignment cascade phenomena have been studied in the neon 2p levels (Paschen notation) Hanle effect and double resonance techniques in a mild discharge. The levels are aligned directly by an optical excitation mechanism. Most 2p levels show evidence of alignment cascade from the 2s 4 , 3s 1 or 3d levels below 0.1 Torr; addition of argon impurities greatly enhances the cascade signal relative to the direct alignment signal. Data used previously in pressure broadening studies are shown to be free of cascade errors and are used to establish upper limits for the 2p level life times. A correction for coherence narrowing is estimated; this gives natural life times systematically less than those obtained by methods in which cascade effects are not carefully eliminated. In common with other Hanle effect measurements for these levels the values are marginally smaller than the threshold life times of Bennett and Kindlmann (1966).
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.
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 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.
The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the 2s2s2s levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the 2s22{s}_{2}2s2​ and 2s32{s}_{3}2s3​ levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.
The theory of the cascade level-crossing effect as formulated by Happer is evaluated for zero-nuclear-spin atomic systems and extended to include the case of a polarized initial state. We have used this theory to interpret our cascade-Hanle-effect results (zero-field level crossings) to obtain the lifetime of the two resonance levels, 1s2(1P1)1{s}_{2}(^{1}P_{1})1s2​(1P1​) and 1s4(3P1)1{s}_{4}(^{3}P_{1})1s4​(3P1​), of the 2p53s2{p}^{5}3s2p53s configuration of neon. The resonance lines 744 \AA{} ($1{s}_{4}\ensuremath{-}^{1}S_{0}$) and 736 \AA{} ($1{s}_{2}\ensuremath{-}^{1}S_{0}$) are produced in the cascade to the ground 1S0^{1}S_{0}1S0​ state when a beam of metastable 1s3(3P0)1{s}_{3}(^{3}P_{0})1s3​(3P0​) and 1s5(3P2)1{s}_{5}(^{3}P_{2})1s5​(3P2​) neon atoms is excited by selected lines of the $2{p}^{5}3s\ensuremath{-}2{p}^{5}3p$ transition array. Our results are $\ensuremath{\tau}(1{s}_{2})=1.65\ifmmode\pm\else\textpm\fi{}0.16$ nsec and $\ensuremath{\tau}(1{s}_{4})=20.5\ifmmode\pm\else\textpm\fi{}1.5$ nsec.
The authors have investigated the zero-field level crossing lineshape in resonance absorption. A single-mode HeNe laser beam at 633 nm is applied to an external neon-gas absorption cell in a longitudinal magnetic field. They compare the measured lineshapes with theoretical predictions. Two superposed nonlinear Lorentzian-shaped resonances, known as the population effect of the 2p 4 -3s 2 transition and the Zeeman coherence effect of the lower 2p 4 level, appear on the background caused by the inhomogeneous broadening. Taking into account the background and the power broadening the authors obtain the linewidths at zero pressure gamma (2p 4 ,3s 2 )=(7.7+or-2.1) MHz and gamma (2p 4 )=(8.4+or-1.2) MHz. For the neon pressure broadening the authors get for pressures below 200 Pa (1.5 Torr) d gamma (2p 4 ,3s 2 )/dp=0.20+or-0.02 MHz Pa -1 and d(2p 4 )/dp=0.040+or-0.007 MHz Pa -1 . Their values are in acceptable agreement with the values obtained with other spectroscopic methods.