Transit effects in the theory of a Lamb dip in a strong field at very low gas pressures

A theoretical analysis is made of the interaction of a spatially bounded plane standing optical wave with a molecular gas at a very low pressure. This theory applies to weak and strong (saturating) fields. Calculations are made of the profile, amplitude, and width of a Lamp dip as a function of the intensity of the saturating field and of the dimensions of the light beam. The role of slow molecules in the formation of the dip is identified. It is shown that in weak saturating fields the width of the resonance is governed by the average thermal velocity of molecules in a gas and by the transverse size of the light beam.