This work by H. Xie and co-authors opens a new dimension for probing both the structure of atoms and molecules and the dynamics of electron and ions on the attosecond time scale using photoelectron holography in ionization by strong elliptically polarized laser pulses. Discovered in 2011, photoelectron holography was explained by interference of wave packets of directly ionized electrons (reference wave) and forward scattered electrons (signal wave) generated by a linearly polarized field. Replacing linear with elliptical pulses, the dynamics of electron between the ionization and forward scattering events is no longer one dimensional. H. Xie and co-authors discovered in this work that the second vector component of the elliptically polarized pulse qualitatively changes the photoelectron spectra. The spacing of the holographic fringe gradually decreases with the increase of the ellipticity. With an increase of the ellipticity above 0.3, a further qualitative change of the results is observed: a clear ridge structure for low electron momenta due to forward scattered electrons and a two-lobes structure for larger momenta, which comes from the direct electrons. The experimental results are explained by simple analytical formulas. The method presented in this work can be easily generalized to more complex systems; in particular, the separation of the forward scattered electrons this work demonstrates (shown by the ridge structure in the holography data) will allow time-resolved imaging of molecular structures.
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