Effects of the velocity dependence of the collision frequency on the Dicke line narrowing

Abstract

The effect of the velocity (v) dependence of the transport collision frequency νtrv on the Dicke line narrowing is analyzed in terms of the strong-collision model generalized to velocity-dependent collision frequencies (the so-called kangaroo model). This effect has been found to depend on the mass ratio of the resonance (M) and buffer (Mb) particles, β = Mb/M: it is at a minimum for β ≪ 1 and reaches a maximum for β ≳ 3. A power-law particle interaction potential, U(r) ∝ r−n, is used as an example to show that, compared to νtrv(v) = const (n = 4), the line narrows if νtrv(v) decreases with increasing v (n 4). At β ≳ 3, the line width can increase [compared to νtrv(v) = const] by 5 and 12% for the potentials with n = 6 and n ≳ 10, respectively; for the potentials with n = 1 (Coulomb potential) and n = 3, it can decrease by more than half and 6%, respectively. The line profile I(Ω) has been found to be weakly sensitive to νtrv(v) at some detuning Ωc of the radiation frequency Ω. Dicke line narrowing is used as an example to analyze the collisional transport of nonequilibrium in the resonance-particle velocity distribution in a laser field. The transport effect is numerically shown to be weak. This allows simpler approximate one-dimensional quantum kinetic equations to be used instead of the three-dimensional ones to solve spectroscopic problems in which it is important to take into account the velocity dependence of the collision frequency when the phase memory is preserved during collisions.