We report on measurements of ion energy distributions from hydrogen clusters irradiated by intense laser pulses of duration 40 and 250 fs. Contrary to the predictions of a simple Coulomb explosion model, we observe a pronounced spatial anisotropy of the ion energies from these explosions with the highest energy ions ejected along the laser polarization direction. The origin of the anisotropy is distinct from that previously seen in clusters of high $Z$ atoms such as Ar and Xe. Furthermore, a measured increase in ${\mathrm{H}}^{+}$ ion energy when longer, lower intensity pulses are employed suggests that multiple-pass, vacuum heating of the cluster electrons is important in the deposition of energy by the laser.

• Received 2 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.123401