Time-dependent first-order perturbation theory is used to simulate the preparation of wavepackets that are created in picosecond pump-probe experiments on the B state of Na-3. Due to the large temporal length of the pump laser pulses used in these experiments, the wavepackets created are superpositions of only a few angularly excited eigenfunctions. How the composition of the wavepackets varies over the duration of the pump laser pulse is shown by observing the time dependence of the nodal structure of the wavepacket, its energy, and its norm. The possibility of simulating the ion signal detected in these picosecond pump-probe experiments is also discussed.