Acomplete quantum-mechanical description of matter and its interaction with the environment requires detailed knowledge of a number of complex parameters. In particular, information about the phase of wavefunctions is important for predicting the behaviour of atoms, molecules or larger systems. In optics, information about the evolution of the phase of light in time and space is obtained by interferometry. To obtain similar information for atoms and molecules, it is vital to develop analogous techniques. Here we present an interferometric method for determining the phase variation of electronic wave packets in momentum space, and demonstrate its applicability to the fundamental process of single-photon ionization. We use a sequence of extreme-ultraviolet attosecond pulses to ionize argon atoms and an infrared laser field, which induces a momentum shear between consecutive electron wave packets. The interferograms that result from the interaction of these wave packets provide useful information about their phase. This technique opens a promising new avenue for reconstructing the wavefunctions of atoms and molecules and for following the ultrafast dynamics of electronic wave packets.
Nature Phys.

Remetter, T., Johnsson, P., Mauritsson, J., Varju, K., Ni, Y., Lépine, F., … L'Huillier, A. (2006). Attosecond electron wave packet interferometry. Nature Phys., 2, 323–326. doi:10.1038/nphys290