Probe-field spectroscopy in two-level systems with low Doppler broadening

The spectrum of absorption (amplification) of a weak probe field by two-level atoms colliding with atoms of a buffer gas in the field of a strong electromagnetic wave is studied theoretically. A universal, free of any collision model, solution is obtained for systems with small (compared to collision rates) Doppler broadening with no restrictions on the strong-field intensity and with the possible absence of dephasing of the light-induced dipole moment in elastic collisions of the gas particles. The relation of the collisional parameters of the problem with the characteristics of an elementary scattering event and with the macroscopic characteristics connected with the transport phenomena is established. It is found that the nontrivial features of the probe-field spectrum discovered by us previously Zh. Éksp. Teor. Fiz. 127, 320 (2005)] in the framework of the strong-collision model are not connected with a particular model of collisions and have a universal nature.