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We have experimentally studied the nonradiative transport of excitation in a dense ͓Nϭ͑0.3Ϫ2.5͒ϫ 10 17 cm Ϫ3 ] 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.

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When the gas particles, which can be considered as a two level quantum system, are affected by frequent collisions with buffer gas particles (thermal reservoir), the spectral densities of the Einstein coefficients for absorption and induced emission are unequal to each other far from the absorption (emission) line. The relationship between the spectral densities at the specified condition reveals new physical phenomena. The mostly convincing example is the fact that population inversion is established in a two level system under nonresonance optical excitation. This effect is experimentally observed as a lasing on sodium resonance transitions.