Photoelectron distributions from femtosecond pump/probe excitation with chirped probe pulses

We calculate photoelectron distributions obtained from the two-photon pump/probe ionization of ${\mathrm{Na}}_{\mathrm{2}}.$ A first ultrashort pulse prepares a vibrational wave packet in the ${\text{(2)}}^1{\sum }_u^{+}$ electronic state. The dynamics is probed by time-delayed ionization caused by a chirped femtosecond laser pulse. Time-resolved photoelectron spectra are determined for different chirp parameters. It is shown that the spectra are very sensitive to the choice of the chirp. In particular the width can be selectively modified and the in- or outward motion of the wave packet can be distinguished by using an appropriately chirped electric field. An analytical expression for the photoelectron spectrum explains its dependence on the chirp parameter.