Nonadherence to the conventional group velocity for nanosecond light pulses in Rb vapor

Propagation of nanosecond light pulses in a resonantly absorbing medium is studied. Deviation from the conventional group velocity and superluminal-to-subluminal transition were observed for the natural atomic transitions of Rb. The observed frequency and atomic density dependences of the propagation delay in the anomalous dispersion region, where the conventional concept of the group velocity breaks down, are reproduced well by the theory of the net group delay.