  Two-photon absorption is theoretically analyzed within the semiclassical
formalism of radiation-matter interaction. We consider an ensemble of
inhomogeneously broadened three-level atoms subjected to the action of two
counterpropagating fields of the same frequency. By concentrating in the limit
of large detuning in one-photon transitions, we solve perturbatively the Bloch
equations in a non-usual way. In this way we derive an analytical expression
for the width of the two-photon resonance that makes evident sub-Doppler
two--photon spectroscopy. We also derive an analytical expression for the Stark
shift of the two-photon resonance.
