Determine the pump/probe delays that you want to measure. Typically you want the first delays to be from before the PUMP crosses the probe path, and the last ones to be after the last cycle of the PUMP cross the probe path.
For example, if the probe arrives at the centre after 30 us and the S-wave arrives after 40 us, and the PUMP signal is 20 us in length, then you could start with a 0 delay (P-wave arrives 10 us before S-wave) and then measure until a delay of 40 us. This will give you 10 us before the PUMP arrives, 20 us while the two waves interact and 10 us after the PUMP has passed the interaction region.