Trapping or imprisonment of atomic resonance radiation (i.e., radiation with frequencies corresponding to the transition between the ground state and first excited state) in vapour was first described by Milne [J. London Math. Soc. 1, 40 (1926)] Holstein [Phys. Rev. 72, 1212 (1947); 83, 1159 (1951)] and Biberman [Zh. Eksp. Teor. Fiz. 17, 416 (1947)] in the first half of the last century. If an excited atom is surrounded by other atoms of the same species in the ground state at high enough densities, the resonance radiation will be absorbed and reemitted many times before it escapes from the volume occupied by the atoms. The successive reabsorption and reemission of photons decrease the effective radiative decay rate Γeff of the ensemble of atoms in the sample compared to the spontaneous decay rate Γnat:
where g is a dimensionless parameter, the escape factor, which can be regarded as the reciprocal of the number of emission and absorption events before the escape. The escape factor g depends on a number of experimental conditions, like the number density and velocity distribution of absorbing atoms, the spectral line shapes, and the geometry of the region in which the atoms are confined. In optically thick media, resonant photons are 'trapped' and radiation escapes from the medium only after a chain of emission-reabsorption-emission events. At first glance it may seem that the density in atomic beams is too low to cause radiation trapping. However, in recent studies [JCP 119, 3174 (2003); 119, 7094 (2003)] it was shown that radiation imprisonment can be strongly expressed in collimated beams even at relatively small atom densities. It is therefore natural to suspect that it may also lead to peculiarities in the excitation spectrum in atomic beams. We observe such peculiarities in the excitation spectrum of Na atoms on 3s-3p transition, which are expressed as inversion of relative intensities of lines corresponding to transitions between various hyperfine components. Moreover, we observe narrowing of the excitation spectrum - a manifestation which at the first glance seems to be contradicting with the textbook examples, but it can be shown how the emission/absorption processes in quasi-independent sub-ensembles of atoms induce transparency in the absorbing medium, leading to anisotropically reduced effective opacity of the atomic beam. Moreover, we show, using calculations based on the Geometric Quantisation Technique [PRA 68, 063415 (2003)], that certain experimental conditions may lead to more unconventional manifestations of radiation trapping. For example, sub-natural decay times (i.e., shorter than natural lifetimes of excited atomic states) of fluorescence signals can be observed from cold atom samples in magneto-optical traps, and this effect depends on the atom number density. The above effects will be described in detail in forthcoming publications.
An interesting consequence of radiation trapping can be found in ultracold atom gases. In such gases, sub-natural decay times (i.e., shorter than natural lifetimes of excited atomic states) of fluorescence signals can be observed from cold atom samples in magneto-optical traps, and this effect depends on the atom number density. This effect is described in a paper to appear in Phys. Rev. A in 2008.