Short pulse phenomena produced with long pulses*

We have found a way to produce to phenomena usually associated with ultrashort laser pulses using surprisingly long pulses. For example, the spatial asymmetry of a dissociating molecule has been observed to vary with the CEP for very short intense pulses [1-3]. By using chirped pulses with large bandwidths, however, our calculations show large, CEP-dependent asymmetry in dissociating H$_2^+$ even for 100 fs long pulses. This effect is also shown in the spatial asymmetry of an ionized atomic target. We also find varying the bandwidth and the direction of the chirp of the pulse can affect another multi-photon process, zero-photon dissociation of H$_2^+$ [4]. While it is not obvious how these unexpected phenomena can be explained in the usual time-dependent, field-based picture, their explanation in terms of a photon picture is rather straightforward, underscoring its utility even in the strong field regime. [1] M.~F.~Kling {\it et al.}, Science {\bf 312}, 246 (2006). [2] M.~Kremer {\it et al.}, Phys.~Rev.~Lett. {\bf 103}, 213003 (2009). [3] V.~Roudnev, B.~D.~Esry, and I.~Ben-Itzhak, Phys.~Rev.~Lett. {\bf 93}, 163601 (2004). [4] J.~H.~Posthumus {\it et al.} J.~Phys.~B: At.~Mol.~Opt.~Phys. {\bf 28}, 623 (2004).