Atomic-motion-induced spectroscopic effects nonlinear in atomic density in a gas

The interatomic dipole-dipole interaction is commonly thought to be the main physical reason for spectroscopic effects nonlinear in atomic density. However, we have found that the free motion of atoms can lead to other effects nonlinear in atomic density $n$, using a previously unknown self-consistent solution of the Maxwell-Bloch equations in the mean-field approximation for a gas of two-level atoms with an optical transition at unperturbed frequency $\omega^{}_0$. These effects distort the Doppler lineshape (shift, asymmetry, broadening), but are not associated with an atom-atom interaction. In particular, in the case of $nk^{-3}_0<1$ (where $k^{}_0=\omega^{}_0/c$) and significant Doppler broadening (with respect to collisional broadening), atomic-motion-induced nonlinear effects significantly exceed the well-known influence of the dipole-dipole interatomic interaction (e.g., Lorentz-Lorenz shift) by more than one order of magnitude. Moreover, under some conditions a frequency interval appears in which a non-trivial self-consistent solution of the Maxwell-Bloch equations is absent due to atomic motion effects. Thus, the existing physical picture of spectroscopic effects nonlinear in atomic density in a gas medium should be substantially revised.