We report the observation of a new type of echo produced by applying two intersecting light pulses with different angles (θ and θ′) at t = 0 and τ. The laser induced population grating (LIPG) created by the first excitation and degraded due to the thermal motion of atoms is reformed by the second excitation at te= τ+mτ (αn-M), where m and n are integers and α sin ( θ 2)=sin ( θ′ 2). The experiment was performed in the ground state 3 2S 1 2 of atomic sodium vapor. This echo is not related to the atomic coherence but to the population distribution, and has novel feature for studying velocity-changing collisions.
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