Time-Resolved Holography with Photoelectrons

Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics. DOI: 10.1103/PhysRevLett.109.073004 PACS numbers: 32.80.Fb, 32.80.Rm, 34.80.Qb When an atom or molecule is illuminated with a moderately intense femtosecond laser field ($ 10 14 W=cm 2 ), an electron wave packet will tunnel ionize and accelerate in the field before being turned around by the field and returning to the parent ion. The returning electron can either recombine with the parent ion, releasing its kinetic energy as a high-energy photon [1-3], or can elastically scatter from the potential of the ion. The photons and electrons generated by these strong-field processes have the potential to probe the dynamic structure of molecules and materials on the subnanometer length scale and femtosecond-to-attosecond time scale. Several recent papers have suggested that structures seen in angledependent photoelectron spectra may be useful for determining time-resolved molecular structures [4], characterizing attosecond electron wave packets [5], and studying the dynamics of electron wave packet propagation [6]. However, despite extensive analyses [7][8][9][10][11], many features observed in angle-resolved photoelectron spectra still lack a simple physical explanation.The recent development of midinfrared (mid-IR) femtosecond lasers [12] and angle-resolved detection schemes [13] has enabled new advances in visualizing strong-field physics. Electrons that are ionized in a mid-IR laser field reach higher velocities because of the larger ponderomotive energy, given by U P / I 2 , where I is the intensity and is the wavelength. The possibility of harnessing the high-energy electrons that are first ionized and then driven back to a molecule by a strong laser field has inspired several theoretical and experimental efforts to use strongfield ionization to probe molecular structure [4,[14][15][16]. Recently, Huismans and co-workers [17] used 7 m mid-IR lasers, in combination with angle-resolved detection, to observe angular interference structures in the photoelectron spectra. They presented a theoretical model that explains these structures based on the difference in the phase between two different paths that electrons can take to...

Time-resolved photoelectron holography from atoms using midinfrared laser pulses is investigated by solving the corresponding time-dependent Schrödinger equation (TDSE) and a classical model, respectively. The numerical simulation of the photoelectron angular distribution of Xe irradiated with a low-frequency free-electron laser source agrees well with the experimental results. Different types of subcycle interferometric structures are predicted by the classical model. Furthermore with the TDSE model it is demonstrated that the holographic pattern is sensitive to the shape of the atomic orbitals. This is a step toward imaging by means of photoelectron holography.

River delta degradation has been caused by extraction of natural resources, sediment retention by reservoirs, and sea-level rise. Despite global concerns about these issues, human activity in the world's largest deltas intensifies. Harbour development, construction of flood defences, sand mining and land reclamation emerge as key contemporary factors that exert an impact on delta morphology.Tides interacting with river discharge can play a crucial role in the morphodynamic development of delta channels under pressure. Emerging insights into tidal controls on river delta morphology suggest that -despite the active morphodynamics in tidal channels and mouthbar regions -tidal motion acts to stabilize delta morphology at the landscape scale under the condition that sediment import during low flows largely balances sediment export during high flows. Distributary channels subject to tides show lower migration rates and are less easily flooded by the river because of opposing nonlinear interactions between river discharge and the tide that lead to flow changes within channels and a more uniform distribution of discharge across channels. Sediment depletion and rigorous human interventions in deltas, including storm surge defence works, disrupt the dynamic morphological equilibrium and can lead to erosion and severe scour at the channel bed, even decades after an intervention.3

Midinfrared strong-field laser ionization offers the promise of measuring holograms of atoms and molecules, which contain both spatial and temporal information of the ion and the photoelectron with subfemtosecond temporal and angstrom spatial resolution. We report on the scaling of photoelectron holographic interference patterns with the laser pulse duration, wavelength, and intensity. High-resolution holograms for the ionization of metastable xenon atoms by 7-16  μm light from the FELICE free electron laser are presented and compared to semiclassical calculations that provide analytical insight.

At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in‐depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

Photoelectron holography is studied experimentally and computationally using the ionization of ground-state xenon atoms by intense near-infrared radiation. A strong dependence of the occurrence of the holographic pattern on the laser wavelength and intensity is observed, and it is shown that the observation of the hologram requires that the ponderomotive energy U p is substantially larger than the photon energy. The holographic interference is therefore favored by longer wavelengths and higher laser intensities. Our results indicate that the tunneling regime is not a necessary condition for the observation of the holographic pattern, which can be observed under the conditions formally attributed to the multiphoton regime.

Ionization mechanisms of C 60 molecules irradiated by a short intense 800-nm laser pulse are studied. Angleresolved photoelectron spectra show above-threshold ionization (ATI) patterns with a low peak contrast and a remarkably smooth angular distribution. The results are interpreted by combining two theoretical models. A time-dependent Schrödinger equation (TDSE) calculation based on the B-spline method that explicitly takes into account the molecular potential mimics the single-active-electron response while a statistical model accounts for the many-electron effects. We show that the latter are responsible for the loss of contrast in the ATI peaks. The smooth angular distribution arises in the TDSE calculation as a result of the high angular momentum of the C 60 ground electronic state and therefore is a manifestation of the atomic behavior of the molecule.