Rotational wave packets probed by high harmonic generation*

We prepare an aligned distribution of cold N$_{2}$ molecules by the interaction with an intense nonresonant fs laser pulse (pump pulse). The distribution is probed by high harmonic generation (HHG) using a time delayed probe pulse. The high harmonics show an enhancement if the molecules are aligned with the probe pulse polarization and a suppression if the molecules are aligned orthogonal to the polarization. We observe a first alignment 300 fs after the pump pulse. For longer time delays, we observe fractional and full revivals (the later at about 8 ps) of the rotational wave packet. We measure the relative phase of high harmonics from N$_{2}$ and Ar at the half revival for N$_{2}$, with a phase jump at the 23$^{rd}$ harmonic. The alignment contrast and phase relation among the high harmonics is discussed in the context of the two center model.