Effect of the Velocity Dependence of the Collision Frequency on the Dicke Line Narrowing

摘要

The effect of the velocity (v) dependence of the transport collision frequency v_(trv) of 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, β = M_b/M: it is at a minimum for β < 1 and reaches a maximum for β approx. > 3. A power-law particle interaction potential, U(r) ∝ r~(-n), is used as an example to show that, compared to v_(trv)(v) = const (n = 4), the line narrows if v_(trv)(v) decreases with increasing v (n < 4) and broadens if v_(trv)(v) increases with v(n > 4). At β approx. > 3, the line width can increase [compared to v_(trv)(v) = const] by 5 and 12% for the potentials with n = 6 and n approx. > 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 v_(trv)(v) at some detuning Ω_c 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.