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Soviet Journal of Quantum Electronics

Optical gain in halogen atom recombination reactions

I. A. Izmaǐlov, V. A. Kochelap

Published Dec 31, 1980• 1 citation
Publisher

Abstract

The radiative channel of halogen atom recombination reactions is discussed using experimental data. It is found that various types of population of excited electronic states are formed during recombination. Calculations are made of the optical amplification cross sections for various transitions. It is found that for recombining atom concentrations of (1–5)×1018cm−3 at temperatures not exceeding 600–800°K, an optical gain of the order of 10−3cm−1 may be achieved in the near infrared.

DOI
10.1070/qe1980v010n12abeh010224

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The radiative channel of halogen atom recombination reactions is discussed using experimental data. It is found that various types of population of excited electronic states are formed during recombination. Calculations are made of the optical amplification cross sections for various transitions. It is found that for recombining atom concentrations of (1–5)×1018cm−3 at temperatures not exceeding 600–800°K, an optical gain of the order of 10−3cm−1 may be achieved in the near infrared.
Spontaneous radiative recombination between protons and electrons to form hydrogen atoms with 8\ensuremath{\le}n\ensuremath{\le}19 has been measured. CO2{\mathrm{CO}}_{2}CO2​ laser light has been shown to induce stimulated radiative recombination to the n=11 and 12 levels with an inferred gain in the cross section of 1720\ifmmode\pm\else\textpm\fi{}860 and 4790\ifmmode\pm\else\textpm\fi{}2830 for a laser power of 12.6 and 15.3 W, respectively. This is in line with that predicted theoretically.
Introduction Previous discussions (see Section 10.4) suggested that stimulated emission can be used to generate optical gain, that is, to amplify radiation. The reader certainly has experience in the amplification of electronic signal. For example, radio receivers capture faint radio waves and turn them into a signal that is powerful enough to drive large speakers. This electronic amplification increases the amplitude of the signal while faithfully preserving its acoustic frequencies and modulation characteristics. Similarly, optical amplification is expected to increase the amplitude of an optical signal while preserving its frequency, its modulation characteristics, and its coherence. The latter requirement is of particular significance for optical radiation, where the coherence of naturally occurring radiation rarely exceeds one micrometer. Lasers are the primary source for coherent radiation. They depend on stimulated emission for amplification and for the generation of coherent radiation (the word laser is the acronym of Light Amplification by Stimulated Emission of Radiation). For amplification, an atomic system that is part of the laser medium must be prepared with a sufficiently large number of particles in the excited state. (The term atomic system is used here to describe all microscopic systems including molecules and free electrons.) Radiation passing through that excited medium encounters multiple events of stimulated emission, each event contributing one photon that is added coherently to the propagating beam. When the number of events of stimulated emission exceed all losses by absorption or scattering, the incident radiation is amplified.
Using exact analytical expressions of the nonrelativistic dipole recombination cross sections for a pure Coulomb potential summed over the quantum numbers j and l, we numerically compute the recombination cross sections in the various n states for 1\ensuremath{\le}n\ensuremath{\le}500 and the total cross section for all possible values of the incoming electron energy. We present a parametrization of the recombination cross sections in the various n states, of the total cross section, and of the energy emission cross section. A few low- and high-energy approximations are also derived and discussed.
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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 $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.