|Atoms or molecules exposed to intense femtosecond laser fields are quickly ionized. The electron is then accelerated in the laser field and can be driven back to the parent ion. We have shown that the recolliding electron can form a diffraction image of the molecular structure [Meckel et al., Science 320, 1478 (2008)]. At the same time, the free electron can recombine with the parent ion and emit an XUV photon in the form of high harmonic emission. The emitted spectrum contains information about the electronic structure of the molecule [Itatani et al., Nature 432, 867 (2004)].
I will describe a recent experiment in which we used high harmonic emission to observe the dissociation of a bromine molecule. We used three femtosecond beams in a pump-pump-probe arrangement to align, dissociate and ionize the molecular sample. Emission from unexcited molecules acts as a local oscillator which makes it possible to measure both the amplitude and phase of the emission from the excited molecules.