Coherent coupling effects in pump-probe measurements with collinear, copropagating beams.

We demonstrate the existence of coherent coupling effects in pump-probe measurements with collinear, copropagating beams, despite the absence of any induced spatial gratings in this geometry. The coherent interaction, which is found to be similar but not identical to that for crossed beams, must be taken into account in analyzing relaxation processes occurring on the time scale of the laser pulse. These coherent effects cannot generally be eliminated by detecting the total change in energy in both the pump and probe beams.