Spatial Distribution Of Excited Atoms Research Articles

The influence of collective effects on the propagation of electromagnetic radiation in dense ultracold atomic ensembles

Based on the already-developed general theory (I.M. Sokolov, D.V. Kupriyanov, and M.D. Havey, JETP 112 (2), 246 (2011)), we have studied the spatial distribution of excited atoms and of the atomic polarization that a weak monochromatic field creates in a dense ultracold atomic medium. We show that, in the case of a homogeneous random spatial distribution of atoms, the amplitude of atomic polarization averaged over spatial configurations decreases outside boundary regions according to an exponential law, while its phase linearly increases. Based on this, we have numerically determined the extinction coefficient and the light wavelength in the medium, as well as its dielectric permittivity. The dispersion of the permittivity at different concentrations has been studied. We show that, for dense clouds, the real part of the dielectric permittivity acquires negative values in a certain frequency range. Based on the calculation of the spatial distribution of excited atoms, we have analyzed the character of the transfer and trapping of quasi-resonant radiation in atomic clouds of differing density.

Spatial distribution of metastable and resonance atoms in a low-pressure He–Xe discharge in spot mode

A dc low-pressure discharge in a helium–xenon mixture with a hot spot on a flat oxide cathode is investigated. The zone around this cathode spot is an interesting source of excitation of metastable and resonance atoms and can be effectively used for the study of fundamental aspects, e.g. transport phenomena of these excited atoms. The method of laser atomic absorption spectroscopy is used to measure the spatial distribution of these atoms. Since excitation sources are significantly distinct from the diffusion and radiation fundamental modes there is a need for correct interpretation of the experimental results based on simultaneous solution of the diffusion equation and the equation of radiation transport. It is shown that the conventional method for the description of resonance radiation transport, which uses the effective lifetime approximation according to Holstein–Biberman, cannot reproduce the spatial distribution of excited atoms in this type of discharge. The influence of various transport mechanisms and of the collisional coupling between the two lowest excited states of xenon on the spatial distribution of these densities is analysed. It is found that the excited atoms appear on the discharge periphery mainly due to resonance radiation transport.

One-dimensional coherent four-wave mixing as a way to image the spatial distribution of atoms in a laser-produced plasma

An experimental technique based on coherent one-dimensional hyper-Raman-resonant four-wave mixing in broad cylindrically focused light beams has been developed for line-by-line imaging of spatial distribution of excited atoms in a low-temperature plasma of optical breakdown. The technique was applied to map excited lead atoms in a low-temperature laser-produced plasma.

Imaging of the spatial distribution of atoms in an optical-breakdown plasma with one-dimensional coherent hyper-Raman scattering

A technique based on coherent four-wave mixing with a hyper-Raman resonance in wide beams was developed for line-by-line reconstruction of the spatial distribution of excited atoms in a laser-produced plasma. This method was used to study the spatial distribution of excited lead atoms in an optical-breakdown plasma formed on a metal target. A comparison was made of the technique of constructing plasma images line-by-line with a four-photon technique of point-by-point measurements. Estimates were obtained of the spatial resolution of the proposed technique. Potential applications of this approach in plasma diagnostics were analysed.

Application of coherent four-wave mixing for two-dimensional mapping of the spatial distribution of excited atoms in a laser-produced plasma

An experimental technique for the two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature plasma of optical breakdown is developed on the basis of coherent four-wave mixing (FWM) with hyper-Raman resonances. The possibility to extract information concerning the spatial distribution of the populations of excited atomic and ionic states from two-dimensional maps of the FWM intensity allowing for phase mismatch and one-photon absorption is considered. Conditions when FWM in a plasma occurs in the phase-matched regime and is not subject to a considerable influence of one-photon absorption are experimentally determined.

ASSOCIATIVE IONIZATION OCCURRING IN COLLISIONS BETWEEN TWO Na(3P) ATOMS

We have measured the cross section [σAl=4.2×10-17cm2±36％] for associative ioniza-tion occurring in collisions between two Na(3P) atoms, by measuring the current reselting from excitation of Na vapor with CW-laser radiation. The major experimental difficulty in measurements of this type is in the determination of the number density and spatial distribution of excited atoms. Here we have measured the excited-atom density by three independent methods, and we have studied the spatial distribution of excited atoms in detail. We have compared the cross section obtained by us at =523k with other experimental results.

Associative ionization in collisions between twoNa(3P)atoms

We have measured the rate coefficient [$k=5.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{3}$/s (\ifmmode\pm\else\textpm\fi{}37%)] for associative ionization occurring in collisions between two $\mathrm{Na}(3P)$ atoms, by measuring the current resulting from excitation of Na vapor by cw-laser radiation. The major source of uncertainty in measurements of this type is in the determination of the number density and spatial distribution of excited atoms. Here we have measured the excited-atom density by three methods, and we have studied the spatial distribution of excited atoms in detail. Our rate coefficient at $T\ensuremath{\sim}650$ K is compared to other experimental results.