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arXiv:2608.27250

Influence of Many-Body Dipole-Dipole Interactions on Excitation Transfer in a Dense Gas

A. A. Bobrov, S. A. Saakyan, B. B. Zelener, Vladimir A. Sautenkov

Published Aug 27, 2026• 0 citations
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Abstract

We study non-radiative dipole-dipole induced excitation transfer in dense Rb vapour. We show that density dependence of characteristic time of the excitation diffusion changes from linear to non-linear for high Rb density. It is attributed to the breakdown of binary collisions approach to the dipole-dipole excitation transfer. It is shown that the observed result can be qualitatively described if the mean free path is replaced by mean interatomic distance for the chracteristic diffusion length.

DOI
10.48550/arxiv.2608.27250

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We study non-radiative dipole-dipole induced excitation transfer in dense Rb vapour. We show that density dependence of characteristic time of the excitation diffusion changes from linear to non-linear for high Rb density. It is attributed to the breakdown of binary collisions approach to the dipole-dipole excitation transfer. It is shown that the observed result can be qualitatively described if the mean free path is replaced by mean interatomic distance for the chracteristic diffusion length.
We study dependence of dipole-dipole interaction in dense Rb vapour on optical excitation. We measure the dipole-dipole contribution to the selective reflection spectrum of D2 line. Optical excitation is created by intense laser beam, which depletes the 5S1/2 population and decreases the dipole-dipole interaction, narrowing the observed spectrum. The limit of the dipole-dipole interaction change is determined by the optical saturation of the D2 transition and corresponds to the 5S1/2 population approximately equal to 1/3.
We have experimentally studied the nonradiative transport of excitation in a dense [N[N[N=(0.3-2.5)\ifmmode\times\else\texttimes\fi{}${10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}]$ potassium vapor. We show that in such a high-density vapor the diffusive nonradiative transport of excitation via ``hopping'' of excitation in dipole-dipole collisions between excited- and ground-state atoms is dominant over the radiative transport through photons. The nonradiative transport mechanism becomes visible in the fluorescence-excitation spectrum signal as a sharp dip close to the resonance frequency. The appearance of the dip, its shape, width, and density dependence can all be explained in terms of a simple diffusion model for the nonradiative transport. This yields values for the diffusion coefficient and the resonance-exchange rate coefficient.
In this paper, our experimental studies of dipole-dipole interactions in excited resonance gases are discussed. In weakly excited gas the dipole broadening of optical transitions is proportional to its density N. A gas medium can be identified by its density N as rarefied gas, where dipole broadening of atomic transitions is much less than Doppler width, or as dense gas, where dipole broadening is much more than Doppler width. According to our experiments the dipole broadening in rarefied gas does not depend on the optical excitation. This behavior is in a good agreement with predictions of standard theory for impact atomic collisions. In excited dense gas we observed the strong reduction of the dipole broadening. These experimental results are not described by standard theory for impact collisions. We suggested a simple description of the optical properties for excited dense gas.
This thesis presents a comparative study of the measured and calculated absolute absorption and dispersion properties of the Rb D lines through a dense thermal vapour in the absence and presence of an applied magnetic field. A detailed theoretical model valid in the weak-probe regime is calculated. The model uses a matrix representation of the atomic Hamiltonian including the magnetic field interaction for Rb in the completely uncoupled basis. Numerical diagonalisation allows the frequency detunings and transition strengths to be calculated. The lineshape of each transition is modelled as a Voigt profile, a convolution of the inhomogeneous and homogeneous profiles. The medium’s susceptibility is found by summing over all the electric-dipole-allowed transitions. For dense thermal vapours a modification to the homogeneous linewidth of each transition, which grows linearly with the number density of atoms, arises due to resonant dipole-dipole interactions between identical atoms in superpositions of the ground and excited terms. In the presence of an applied magnetic field we investigate the Stokes parameters of light propagating through a dense thermal vapour. For fields larger than 0.33 T we enter the hyperfine Paschen-Back regime on the Rb D lines. We present a compact optical isolator based on an atomic vapour, exploiting the spectral region of high transmission and large dispersion where we would normally expect absorption on the Rb D lines. Frequency up-conversion is shown in the fluorescence measurements over the visible and near infra-red regions for strong excitation. Low density transfer arises due to the energy-pooling effect between two identical atoms in their first excited terms. At high densities resonant dipole-dipole interactions give rise to a threshold for the energy transfer. We characterise the threshold behaviour with increasing number density.
A theory is developed for the resonant dielectric susceptibility of a high density gas which consists of two level atoms interacting through the dipole–dipole potential. A many body collision model, which includes the binary as well as the shielding effects, is investigated. The linear response of the dielectric is given by a generalized transport equation. Self-consistent coupled equations are derived for the various collision operators, using the propagator techniques. In the two limiting cases, the results of the binary collision model and those of the shielded interaction model, respectively, are recovered. The equations are simplified by neglecting the nonresonant contributions. Finally, a solution is obtained in the resonance broadening approximation. The resulting expression for the dielectric susceptibility includes the two body collision as well as the many body shielding effects. In particular, it includes the important collective excitation transfer effects. The proposed many body collision model should be appropriate for the resonant, linear response of high density gases.