We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at 1.5 PW/cm;{2}. The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct (e,2e) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical (e,2e) kinematics.

We report on a real-time imaging of the ultrafast D(+)2 rovibrational nuclear wave-packet motion performed using a combination of a pump-probe setup with 7 fs laser pulses and a "reaction-microscope" spectrometer. We observe fast dephasing (collapse) of the vibrational wave packet and its subsequent revival and prove rotational excitation in ultrashort laser pulses. Channel-selective Fourier analysis of the wave packet's long-term (approximately 3000 fs) evolution allows us to resolve its individual constituents, revealing unique information on the mechanisms of strong-field ionization and dissociation.

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Recoil-ion momentum distributions for two-photon double ionization of He and Ne (@! 44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of 1 10 14 W=cm 2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models. DOI: 10.1103/PhysRevLett.101.073003 PACS numbers: 32.80.Rm, 41.60.Cr, 42.65.ÿk Since Einstein's revolutionary explanation of the photoelectric effect in 1905, the breakup of bound systems as a result of their interaction with single light quanta -the photons-has remained in the very focus of interest in experimental and theoretical physics as well as in chemistry and biology as one of the most fundamental reactions occurring in nature. Whenever there is more than one electron actively involved in the photoabsorption process, however, one faces serious problems in calculations as well as in measurements, even if only a single photon is absorbed at a time. Thus, the simplest situation where two electrons emerge from the He atom has numerically been solved only within the last decade when fully differential experimental cross sections have become available (see [1] for a review).Keeping the simple He target but increasing the number of photons, as, e.g., in strong-field double ionization at optical frequencies needing more than 50 quanta, still represents a serious challenge for computations (see, e.g., [2]). Likewise, kinematically complete experiments for this regime have been reported only within the last two months [3,4]. Also for the process of double ejection by Compton scattering, the comparison of experiment and theory does not go beyond the level of total cross sections [5].In this Letter we report the first differential measurement, recoil-ion momentum distributions, for the most basic nonlinear two-electron light-matter interaction, where two vacuum ultraviolet photons (44 eV each) ''simultaneously'' remove two electrons from He. The results are compared to the double ionization of Ne, where a sequential, stepwise absorption pathway with intermediate relaxation to a bound state of the Ne ion is energetically allowed. Vastly different momentum distributions are observed for both reactions and compared with theoretical predictions. Since the measured recoil-ion momentum spectra reflect the sum-momentum distributions of the emitted electrons and thus yield first information about the relative emission angles and the energy sharing between both electrons for different nonlinear processes, the data provide stringent test grounds for theoretical models. The experiments became feasible by exploiting a unique combination of modern multiparticle momentum imaging technique, ''reaction microscope' ' [6], and a novel light ...

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.