Momentum distributions of Ne3-4+ ions created by 795 nm, 25 fs laser pulses have been measured at 10(15)-2 x 10(16) W cm(-2) using a 'reaction microscope' in order to characterize the transition regime between non-sequential and sequential strong-field ionizations. We can clearly separate pure and/or mixed recollision-induced (e, ne)-like reactions and sequential (field-induced) processes, and observe transition intensities which agree with recently reported ion-yield data. Our results provide consistent qualitative description of Ne strong-field multiple ionization for all non-relativistic intensities.

Recoil-ion momentum distributions for double and triple ionization of Ne and Ar, as well as for double ionization of N-2 molecule by intense (0(3-0(5PWcm(-2)), short (similar to 35-40 fs) laser pulses have been recorded in a so far unexplored long laser-wavelength regime at 1300 nm. Compared to earlier results at 800 nm, the direct (e, ne) ionization pathway during recollision is strongly enhanced manifesting itself in a pronounced double-hump structure in the longitudinal ion momentum spectra not only for Ne, but also surprisingly distinct for Ar and, found for the first time, for molecules. Observed wavelength dependence of the sub-laser-cycle correlated few-electron dynamics might be of paramount importance for possible future applications in attosecond science, in particular, for imaging of ultrafast molecular processes via recollision-induced fragmentation.

Momentum distributions of ${\mathrm{Ne}}^{2+}$ and ${\mathrm{Ar}}^{2+}$ ions created by linearly polarized $795\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $25\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ laser pulses have been traced at intensities from ${10}^{14}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ using a ``reaction microscope.'' Apart from the transition from nonsequential to sequential ionization, characterized by significant changes in longitudinal momentum distributions developing from a double hump, over a triple-peak structure to a narrow single Gaussian observed for both ions, for ${\mathrm{Ar}}^{2+}$ we find a similar behavior but reversed in its intensity dependence in the purely nonsequential regime, pointing to contributions of recollision excitation plus subsequent field ionization, or to the role of ``Z trajectories'' recently predicted within classical calculations.

Reaction Microscope-based, complete, and time-resolved Coulomb explosion imaging of vibrating and dissociating D(2)(+) molecules with femtosecond time-resolution allowed us to perform an internuclear distance (R-)dependent Fourier analysis of the corresponding wave packets. Calculations demonstrate that the obtained two-dimensional R-dependent frequency spectra enable the complete characterization of the wave packet dynamics and directly visualize the field-modified molecular potential curves in intense, ultrashort laser pulses.

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 the high-resolution multidimensional real-time mapping of H2+ and D2+ nuclear wave packets performed employing time-resolved three-dimensional Coulomb explosion imaging with intense laser pulses. Exploiting a combination of a "reaction microscope" spectrometer and a pump-probe setup with two intense 6-7 fs laser pulses, we simultaneously visualize both vibrational and rotational motion of the molecule, and obtain a sequence of snapshots of the squared ro-vibrational wave function with time-step resolution of ~ 0.3 fs, allowing us to reconstruct a real-time movie of the ultrafast molecular motion. We observe fast dephasing, or 'collapse' of the vibrational wave packet and its subsequent revival, as well as signatures of rotational excitation. For D2+ we resolve also the fractional revivals resulting from the interference between the counter-propagating parts of the wave packet.

An overview of results obtained with reaction microscopes is given. Due to the enormous diversity and amount of experimental results obtained with this technique, covering many different areas of collision physics, only a few and only the very recent results in the fields of charged particle-atom and intense laser-atom interactions will be presented here. For more comprehensive overviews the reader is referred to recent reviews [1, 2].

We rejort on the experimental realisation of time-resolved coincident Coulomb explosion imaging of H2-fragmentation in 1014 W/cm2 laser fields. Combining a high-resolution 'reaction microscope' and a fs pump-probe setup, we map the motion of wave packets dissociating via one- or two-photon channels, respectively, and observe two region of enhanced ionization in accordance with earlier theoretical predictions. The long-term interferometric stability of our system allows us to extend pump-probe experiments into the region of overlapping pulses, which offers new possibilities for the manipulation of ultrafast molecular fragmentation dynamics.

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.

The spatio-temporal evolution of H2 + (D2 +) nuclear wave packets is mapped using time-resolved Coulomb explosion imaging. We visualize the motion of both dissociating and bound parts of the wave packet, observe its dephasing and subsequent revivals. The reconstructed probability density of the wave packet is in good agreement with earlier theoretical predictions.

We present results of high resolution fully differential measurements on single ionization of He, Ne, and Ar by 7-25 fs linearly polarized 800nm laser pulses at intensities of up to 2.1015 W/cm2. Using a 'Reaction-Microscope' we were able to trace signatures of multiphoton ionization deep into the tunnelling regime. Surprisingly, in the low-energy electron spectra we observed several features (absence of the ponderomotive shifts, splitting of the peaks, their degeneration for few-cycle laser pulses) typical for resonantly-enhanced ionization. Other remarkable features, as the sharp cusp-like momentum distributions in the direction perpendicular to the laser field or the observed minima at zero longitudinal momentum for He and Ne, can be reproduced by semiclassical models, where the electron motion in the combined laser and Coulomb field is treated classically after the tunnelling.

Applying 7 fs pump-probe pulses (780 nm, 4 x 10(14) W/cm2) we observe electronic ground-state vibrational wave packets in neutral D2 with a period of T=11.101(70) fs by following the internuclear separation (R-)dependent ionization with a sensitivity of Delta

Momentum distributions of Ne2?5+ and Ar2?7+ ions created by 795 nm, 25 fs laser pulses at intensities of up to 20 PW cm?2 have been measured using a 'reaction microscope'. The experimental results indicate that below 2.0 PW cm?2 the production of Ne3?4+ ions is dominated by n-fold electron impact ionization (e, ne) collisions of the recolliding electron with its parent ion. Multiple ionization of Ar is found to be dominated by mechanisms involving a combination of recollision-induced and field-induced steps as well as intermediate excitations in the whole range of intensities where non-sequential ionization occurs. Whereas a clear double peak structure characteristic for electron recollision was observed in the momentum distributions of up to Ne5+, no signatures of non-sequential ionization were found in the spectra of Ar ions with the charge state higher than 4.

After preparing a coherent vibrational wave packet in the hydrogen molecular ion by ionizing neutral H2 molecules with a 6.5fs, 760nm laser pulse at 3×1014W/cm2, we map its spatio-temporal evolution by the fragmentation induced with a second 6.5fs laser pulse of doubled intensity. In this proof-of-principle experiment, we visualize the oscillations of this most fundamental molecular system, observe a dephasing of the vibrational wave packet and its subsequent revival. Whereas the experimental data exhibit an overall qualitative agreement with the results of a simple numerical simulation, noticeable discrepancy is found in the characteristic revival time. The most likely reasons for this disagreement originate from the simplifications used in the theoretical model, which assumes a Franck–Condon transition induced by the pump pulse with subsequent field-free propagation of the H2+ vibrational wave packet, and neglects the influence of the rotational motion.

Double, triple and fourfold ionization of rare gas atoms has been studied using a “reaction microscope”. We show that ionization dynamics drastically depends on atomic species and present the first data obtained with few-cycle pulses.

Double ionization of helium by charged particle impact belongs to the most simple and therefore fundamental, unsolved dynamical many-electron problems in atomic physics. At medium to small perturbations (Z/v < 1; Z, v projectile charge and velocity) mainly two mechanisms contribute to the double ionization cross section: First, one electron can be ionized directly by the interaction with the projectile, and the second one is ejected due to the rearrangement of the remaining target ion. Second, the target electrons can be emitted subsequently by two independent interactions with the projectile. These two contributions can essentially be assigned to first and second order amplitudes in a perturbative Born expansion. Since the First Born Approximation (FBA) is independent of the sign of the projectile charge, in numerous previous studies the interest was focussed on the projectile charge-sign dependence of the ionization process by exploring the total double ionization cross sections. By comparing the results for electron and positron respectively proton and antiproton impact, differences have been observed which were explained by an interference between first and second order contributions (for details see [1]). In the present contribution, we report on the first experimental fully differential cross sections (FDCS) for double ionization by positively charged particle impact [2]. The experiment was performed using a multi-electron recoilion momentum spectrometer (“reaction microscope”), which has been described in detail elsewhere [3]. A beam of protons was used as projectiles, which was provided by the Tandem accelerator of the MPI-K. An intermediate projectile velocity (15.5 a.u. i.e. 6 MeV) was chosen, since in this range a strong charge-sign dependence has been observed in total cross section measurements. The experimental cross sections were compared to previous results of electron impact double ionization [4] for a similar velocity regime (12 a.u., i.e. 2 keV). In figure 1, fully differential cross sections for proton (a) and for electron impact (b) are shown. A coplanar geometry is chosen, i.e. the momentum vectors of all particles are in the same plane, the so called scattering plane. The density plots represent the angular distribution of the two outgoing electrons with respect to the projectile beam direction for a momentum transfer to the target system of 0.8-1.5 a.u. and for an equal sharing of the excess energy (∆Ee1,e2 < 2.5 eV). For these conditions the angle of the momentum transfer vector ranges from 60 to 70 degrees, which is denoted by the thick bar in the centre of the plots. This representation elucidates some general properties of the FBA which are independent of the target description: The FBA and the experimental data both feature a four peakstructure, which is a consequence of selection rules in a dipole approximation, prohibiting emission along the broken lines (for details see [2, 4]). Moreover, since for an equal energy sharing the two electrons are indistinguishable, no emission along the diagonal line (lower left to upper right) occurs and the spectra are mirror-symmetrical along this line making the two peaks in the upper left equivalent to those in the lower right. In addition, FBA results have to be symmetric with respect to the momentum transfer direction. The corresponding symmetry axis is shown as a solid line in the graphs. Here a clear difference for electron and proton impact emerges. Whereas for proton impact the symmetry enforced by the FBA is closely fulfilled, for electron impact a strong asymmetry appears.

We report on the experimental realization of time-resolved coincident Coulomb explosion imaging of H2 fragmentation in 10(14) W/cm(2) laser fields. Combining a high-resolution "reaction microscope" and a fs pump-probe setup, we map the motion of wave packets dissociating via one- or two-photon channels, respectively, and observe a new region of enhanced ionization. The long-term interferometric stability of our system allows us to extend pump-probe experiments into the region of overlapping pulses, which offers new possibilities for the manipulation of ultrafast molecular fragmentation dynamics.