Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at 4-7 10 13 W=cm 2 was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed. DOI: 10.1103/PhysRevLett.101.053001 PACS numbers: 32.80.Rm, 31.90.+s, 32.80.Fb, 32.80.Wr Within the past three decades extensive studies on the interaction of intense laser fields with atoms and molecules have resulted in a profound understanding of various strong-field phenomena. Prominent examples are abovethreshold ionization (ATI) [1] or high-order harmonic generation [2], both essentially treatable within the single active electron (SAE) approximation. Correlated fewelectron processes, on the other hand, most important, for example, in nonsequential double (multiple) ionization (NSDI) (for a recent review see, e.g., [3]) have, until the present day, resisted any comprehensive modeling.Recently, however, a breakthrough was achieved at high intensities ( PW=cm 2 ) [4]. The application of many-particle imaging techniques [reaction microscopes and cold target recoil ion momentum spectroscopy (COLTRIMS)] [5] has allowed recording (multi)differential data on strong-field few-electron reactions and sophisticated calculations (see, e.g., [6] and references therein) have advanced their theoretical interpretation. As a result, a commonly accepted though simple picture has emerged, characterizing NSDI. Here an electron first tunnels into the field, is then accelerated, and finally thrown back onto its parent ion by the oscillating laser field. During ''recollision'' n-fold ionization might occur either in a direct e; ne -like encounter or indirectly via recollision-induced excitation of the ion plus subsequent field ionization (RESI) [7]. Signatures of the former are that both electrons are exclusively emitted into the same hemisphere along the polarization direction leading to ''double-hump'' shaped parallel momentum distributions of the ions (compensating the electron momenta). For RESI instead, according to the present understanding, the electrons can be emitted either parallel or back to back, thus filling the valley in between the double hump for the ions. Beyond the well-accepted simple scenario, however, major questions about the correlated electron emission are still far from being understood and are extensively investigated [6,8] because recollision is at the very heart of attoscience, molecular tomography, or imaging [9].At low intensities one intriguing though still widely unexplored question did arise early on within the above picture [10]. What happens when the energy of the recolliding electron of up to 3:17U P is not sufficient t...

Using a reaction microscope, three-dimensional (3D) electron (and ion) momentum (P) spectra have been recorded for carrier-envelope-phase (CEP) stabilized few-cycle ( approximately 5 fs), intense ( approximately 4 x 10(14) W/cm2) laser pulses (740 nm) impinging on He. Preferential emission of low-energy electrons (E(e)<15 eV) to either hemisphere is observed as a function of the CEP. Clear interference patterns emerge in P space at CEPs with maximum asymmetry, interpreted as attosecond interferences of rescattered and directly emitted electron wave packets by means of a simple model.

In kinematically complete studies we explore double ionization (DI) of Ne and Ar in the threshold regime (I>3x10{13} W/cm{2}) for 800 nm, 45 fs pulses. The basic differences are found in the two-electron momentum distributions-"correlation" (CO) for Ne and "anticorrelation" (ACO) for Ar-that can be partially explained theoretically within a 3D classical model including tunneling. Transverse electron momentum spectra provide insight into "Coulomb focusing" and point to correlated nonclassical dynamics. Finally, DI threshold intensities, CO as well as ACO regimes are predicted for both targets.

We report on first proof-of-principles results on non-sequential double ionization of argon and neon achieved by using a newly developed long-cavity Ti:sapphire femtosecond oscillator with a pulse duration of 45 fs and a repetition of 6.2 MHz combined with a dedicated reaction microscope. Under optimized experimental conditions, peak intensities larger than 2.310(14) W/cm(2) have been achieved. Ion momentum distributions were recorded for both rare gases and show significantly different features for single as well as for double ionization. For single ionization of neon a spike of zero-momentum electrons is found when decreasing the laser intensity towards the lowest ionization rate we can measure which is attributed to a non-resonant ionization channel. As to double ionization, the longitudinal momentum distribution for Ne(2+) displays a clear double-hump structure whereas this feature is found to be smoothened out with a maximum at zero momentum for Ar(2+).

Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at 4-7 10 13 W=cm 2 was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.