Relaxation of a dip in the atomic velocity distribution due to elastic collisions

A calculation is made of the relaxation kinetics of a dip formed in a luminescence line of a gas by a short laser pulse. If the initial width of the dip Δ_h is much less than the quantity kδ (δ is the characteristic scale of the change in the velocity in a collision and k is the modulus of the wave vector), the relaxation kinetics of the dip depth is exponential. An analysis of the kinetics makes it possible to determine the total cross section of the elastic collisions which alter the velocity. In the opposite case of Δ_h >> kδ, the dip depth decreases in accordance with the power law and the center of the dip moves toward the line center; in principle, this can be used to determine by two independent methods the frequency of the velocity-changing collisions and the corresponding transport scattering cross section.