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We report on non-sequential double ionization of Ar by a laser pulse consisting of two counter rotating circularly polarized fields (390 nm and 780 nm). The double ionization probability depends strongly on the relative intensity of the two fields and shows a "knee"-like structure as function of intensity. We conclude that double ionization is driven by a beam of nearly monoenergetic recolliding electrons, which can be controlled in intensity and energy by the field parameters. The electron momentum distributions show the recolliding electron as well as a second electron which escapes from an intermediate excited state of Ar + .Strong laser fields efficiently lead to the ejection of electrons from atoms and molecules. In the continuum the electron wave packet is driven by the laser field and its trajectory can be controlled by tailoring the time evolution of the electric field vector of the laser pulse on a sub cycle basis. A laser pulse composed of two harmonic colors offers already a significant amount of control parameters, such as polarization, relative intensity and phase between the two fields. This allows shaping the light field and thus to steer the electron motion in the continuum or in a bond [1][2][3][4][5][6][7][8][9]. Particularly versatile and in addition well controllable waveforms are generated by counter rotating circular two-color fields (CRTC) shown in Fig. 1 panels (c), (f), (i). These waveforms have spawned recent activities because, unlike circularly polarized light consisting of only a single color, CRTC fields can initially drive electrons away from the atom they have escaped from but later drive them back -often on triangularly shaped trajectories to re-encounter their parent ion. The recapture of these electrons gives rise to the emission of circularly polarized higher harmonic light as predicted in pioneering work by Becker and coworkers [10] and confirmed by recent experimental studies [11]. The recollision in such fields has also been identified by high energetic electrons [12] as well as by characteristic structures in the electron momentum distribution at very low energies where the electrons are Coulomb focused [13].In the present work we experimentally show that CRTC fields also lead to efficient double ionization mediated by the re-colliding electron as predicted by recent classical ensemble calculations [14]. Studying the probability of double ionization and in particular the three dimensional momentum distribution of the emitted electrons gives unprecedented insight into the recollision dy- * Electronic address: doerner@atom.uni-frankfurt.de namics occurring in these two-color laser fields. In particular they support that CRTC fields can be used to create a nearly monoenergetic electron beam for attosecond time-resolved studies. Additionally, very recent theoretical work [15] building on [16] predicts that these recolliding electrons can be generated such that they are spin polarized [17].In order to generate two-color fields, we use a 200 µm BBO to double the frequency of a 7...

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Even though ion/atom-collision is a mature field of atomic physics great discrepancies between experiment and theoretical calculations are still common. Here we present experimental results with highest momentum resolution on single ionization of helium induced by 1 MeV protons and compare these to different theoretical calculations. The overall agreement is strikingly good and already the first Born approximation yields good agreement between theory and experiment. This has been expected since several decades, but so far has not been accomplished. The influence of projectile coherence effects on the measured data is shortly discussed in line with an ongoing dispute on the existence of nodal structures in the electron angular emission distributions.