Ion-photoelectron entanglement in photoionization with chirped laser pulses

The investigation of coherent dynamics induced by photoionization of atoms or molecules by extreme ultra-violet (XUV) attosecond laser pulses requires careful consideration of the degree of ion + photoelectron entanglement that results from the photoionization process. Here, we consider coherent H2+ vibrational dynamics induced by photoionization of neutral H2 by a chirped attosecond laser pulse. We show that chirping the attosecond laser pulse leads to ion + photoelectron entanglement and the transition from a pure to a mixed state. This transition is characterized by evaluating the purity, which is close to unity for a transform-limited attosecond laser pulse and which decreases to a value that is determined by the number of vibrational states populated in the photoionization process for increasing values of the chirp parameter. In the calculations, the vibrational dynamics is probed by calculating time-delayed dissociation of the H2+ cation by a short ultra-violet (UV) laser pulse. Independent of the magnitude of the chirp, the coherent vibrational dynamics can be recovered by recording the XUV-UV delay-dependent kinetic energy release in coincidence with the kinetic energy of the accompanying photoelectron

Attosecond streaking in a nano-plasmonic field

A theoretical study of the application of attosecond streaking spectroscopy to time-resolved studies of the plasmonic fields surrounding isolated, resonantly excited spherical nanoparticles is presented. A classification of the different regimes in attosecond streaking is proposed and identified in our results that are derived from Mie calculations of plasmon fields, coupled to classical electron trajectory simulations. It is shown that in an attosecond streaking experiment, the electrons are almost exclusively sensitive to the component of the field parallel to the direction in which they are detected. This allows one to probe the different components of the field individually by resolving the angle of emission of the electrons. Finally, simulations based on fields calculated by finite-difference time-domain (FDTD) are compared with the results obtained using Mie fields. The two are found to be in good agreement with each other, supporting the notion that FDTD methods can be used to reliably investigate non-spherical structures

Probing Time-Dependent Molecular Dipoles on the Attosecond Time Scale

Photoinduced molecular processes start with the interaction of the instantaneous electric field of the incident light with the electronic degrees of freedom. This early attosecond electronic motion impacts the fate of the photoinduced reactions. We report the first observation of attosecond time scale electron dynamics in a series of small- and medium-sized neutral molecules (N2, CO2, and C2H4), monitoring time-dependent variations of the parent molecular ion yield in the ionization by an attosecond pulse, and thereby probing the time-dependent dipole induced by a moderately strong near- infrared laser field. This approach can be generalized to other molecular species and may be regarded as a first example of molecular attosecond Stark spectroscopy

Phase-locking of time-delayed attosecond XUV pulse pairs

We present a setup for the generation of phase-locked attosecond extreme ultraviolet (XUV) pulse pairs. The attosecond pulse pairs are generated by high harmonic generation (HHG) driven by two phase-locked near-infrared (NIR) pulses that are produced using an actively stabilized Mach-Zehnder interferometer compatible with near-single cycle pulses. The attosecond XUV pulses can be delayed over a range of 400 fs with a sub-10-as delay jitter. We validate the precision and the accuracy of the setup by XUV optical interferometry and by retrieving the energies of Rydberg states of helium in an XUV pump–NIR probe photoelectron spectroscopy experiment

adiabatic versus nonadiabatic dressed-state dynamics

We discuss how a recent pump-probe study [Kelkensberg et al., Phys. Rev. Lett. 103, 123005 (2009)] of the dissociative ionization of H2, under the combined effect of a single extreme ultraviolet attosecond pulse and an intense near- infrared pulse, actually represents a transition-state spectroscopy of the strong-field dissociation step, i.e., of the (probe-pulse-)dressed H2+ molecular ion. The way the dissociation dynamics is influenced by the duration of the near-infrared probe pulse, and by the time delay between the two pulses, is discussed in terms of adiabatic versus nonadiabatic preparation and transport of time-parametrized Floquet resonances associated with the dissociating molecular ion. Under a long probe pulse, the field-free vibrational states of the initial wave packet are transported, in a one-to-one manner, onto the Floquet resonances defined by the field intensity of the probe pulse and propagated adiabatically under the pulse. As the probe pulse duration shortens, nonadiabatic transitions between the Floquet resonances become important and manifest themselves in two respects: first, as a vibrational shake-up effect occurring near the peak of the short pulse, and second, through strong interference patterns in the fragment's kinetic energy spectrum, viewed as a function of the time delay between the pump and the probe pulses

Observation of correlated electronic decay in expanding clusters triggered by near-infrared fields

When an excited atom is embedded into an environment, novel relaxation pathways can emerge that are absent for isolated atoms. A well-known example is interatomic Coulombic decay, where an excited atom relaxes by transferring its excess energy to another atom in the environment, leading to its ionization. Such processes have been observed in clusters ionized by extreme- ultraviolet and X-ray lasers. Here, we report on a correlated electronic decay process that occurs following nanoplasma formation and Rydberg atom generation in the ionization of clusters by intense, non-resonant infrared laser fields. Relaxation of the Rydberg states and transfer of the available electronic energy to adjacent electrons in Rydberg states or quasifree electrons in the expanding nanoplasma leaves a distinct signature in the electron kinetic energy spectrum. These so far unobserved electron-correlation-driven energy transfer processes may play a significant role in the response of any nano- scale system to intense laser light

Interference stabilization of autoionizing states in molecular N2 studied by time- and angular-resolved photoelectron spectroscopy

An autoionizing resonance in molecular N2 is excited by an ultrashort XUV pulse and probed by a subsequent weak IR pulse, which ionizes the contributing Rydberg states. Time- and angular-resolved photoelectron spectra recorded with a velocity map imaging spectrometer reveal two electronic contributions with different angular distributions. One of them has an exponential decay rate of 20 ± 5 fs, while the other one is shorter than 10 fs. This observation is interpreted as a manifestation of interference stabilization involving the two overlapping discrete Rydberg states. A formalism of interference stabilization for molecular ionization is developed and applied to describe the autoionizing resonance. The results of calculations suggest, that the effect of the interference stabilization is facilitated by rotationally-induced couplings of electronic states with different symmetry

Recombination dynamics of clusters in intense extreme-ultraviolet and near- infrared fields

We investigate electron-ion recombination processes in clusters exposed to intense extreme-ultraviolet (XUV) or near-infrared (NIR) pulses. Using the technique of reionization of excited atoms from recombination (REAR), recently introduced in Schütte et al (2014 Phys. Rev. Lett. 112 253401), a large population of excited atoms, which are formed in the nanoplasma during cluster expansion, is identified under both ionization conditions. For intense XUV ionization of clusters, we find that the significance of recombination increases for increasing cluster sizes. In addition, larger fragments are strongly affected by recombination as well, as shown for the case of dimers. We demonstrate that for mixed Ar–Xe clusters exposed to intense NIR pulses, excited atoms and ions are preferentially formed in the Xe core. As a result of electron-ion recombination, higher charge states of Xe are efficiently suppressed, leading to an overall reduced expansion speed of the cluster core in comparison to the shell

High power, high repetition rate laser-based sources for attosecond science

Within the last two decades attosecond science has been established as a novel research field providing insights into the ultrafast electron dynamics that follows a photoexcitation or photoionization process. Enabled by technological advances in ultrafast laser amplifiers, attosecond science has been in turn, a powerful engine driving the development of novel sources of intense ultrafast laser pulses. This article focuses on the development of high repetition rate laser-based sources delivering high energy pulses with a duration of only a few optical cycles, for applications in attosecond science. In particular, a high power, high repetition rate optical parametric chirped pulse amplification system is described, which was developed to drive an attosecond pump-probe beamline targeting photoionization experiments with electron-ion coincidence detection at high acquisition rates

Wave Function Microscopy of Quasibound Atomic States

In the 1980s Demkov, Kondratovich, and Ostrovsky and Kondratovich and Ostrovsky proposed an experiment based on the projection of slow electrons emitted by a photoionized atom onto a position-sensitive detector. In the case of resonant excitation, they predicted that the spatial electron distribution on the detector should represent nothing else but a magnified image of the projection of a quasibound electronic state. By exciting lithium atoms in the presence of a static electric field, we present in this Letter the first experimental photoionization wave function microscopy images where signatures of quasibound states are evident. Characteristic resonant features, such as (i) the abrupt change of the number of wave function nodes across a resonance and (ii) the broadening of the outer ring of the image (associated with tunneling ionization), are observed and interpreted via wave packet propagation simulations and recently proposed resonance tunneling mechanisms. The electron spatial distribution measured by our microscope is a direct macroscopic image of the projection of the microscopic squared modulus of the electron wave that is quasibound to the atom and constitutes the first experimental realization of the experiment proposed 30 years ago