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

The effect of the velocity (v) dependence of the transport collision frequency nu (trv) 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 (M-b) particles, beta = M-b/M: it is at a minimum for beta < 1 and reaches a maximum for beta greater than or similar to 3. A power-law particle interaction potential, U(r) proportional to r (-n), is used as an example to show that, compared to nu (trv)(v) = const (n = 4), the line narrows if nu (trv)(v) decreases with increasing v (n < 4) and broadens if nu (trv)(v) increases with v (n > 4). At beta greater than or similar to 3, the line width can increase [compared to nu (trv)(v) = const] by 5 and 12% for the potentials with n = 6 and n greater than or similar to 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(Omega) has been found to be weakly sensitive to nu (trv)(v) at some detuning Omega (c) of the radiation frequency Omega . 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. (C) 2001 MAIK "Nauka/Interperiodica".