Photoemission-time-delay measurements and calculations close to the 3s-ionization-cross-section minimum in Ar

We present experimental measurements and theoretical calculations of photoionization time delays from the 3s and 3p shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random-phase approximation with exchange. The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results, especially in the region where strong intershell correlations in the 3s -> kp channel lead to an induced "Cooper" minimum in the 3s ionization cross section

Electron Wave Packet Dynamics on the Attosecond Time Scale

One objective of attosecond science is to study electron dynamics in atoms and molecular systems on their natural time scale. This can be done using attosecond light pulses. Attosecond pulses are produced in a process called high-order harmonic generation, in which a short, intense laser pulse interacts with atoms or molecules in a highly nonlinear process, leading to the generation of high-order frequencies of the fundamental laser with a large spectral bandwidth, supporting pulses with attosecond duration. In some condition the harmonics are locked in phase leading to a train of attosecond pulses or, in some cases, to a single attosecond pulse. This thesis presents experiments based on interferometry to study electron dynamics using attosecond pulses. The first part describes a series of experiments, in which the dynamics of electrons was studied after photoionization with an attosecond pulse train. The time resolution in these experiments was achieved by measuring the accumulated phase of the free electron wave packet after photoemission using an interferometric technique. The phase carries temporal information about the ionization process, from which the delay in photoemission can be determined with a much better time resolution than that given by the temporal structure of the pulse train. The same technique was applied to investigate the phase behavior of resonant two-photon ionization in helium atoms. The second part describes the application of an interferometric pump-probe technique to characterize bound electron wave packets. Single attosecond pulses are used to excite a broad electron wave packet containing bound and continuum states. The bound part of the wave packet is further ionized by an infrared laser with a variable delay. Analysis of the resulting interferogram allows for full reconstruction of the bound wave packet, since both the amplitude and the phase of all ingoing states in the wave packet are encoded in the interference pattern

Atomic and macroscopic measurements of attosecond pulse trains

We characterize attosecond pulses in a train using both the well established "reconstruction of attosecond beating by interference of two-photon transitions" (RABITT) technique and the recently demonstrated in situ method, which is based on a weak perturbation of the harmonic generation process by the second harmonic of the laser field. The latter technique determines the characteristics of the single atom emission, while RABITT allows one to measure attosecond pulses "on target." By comparing the results of the two methods, the influence of propagation and filtering on the attosecond pulses can be extracted

Exploring single-photon ionization on the attosecond time scale

One of the fundamental processes in nature is the photoelectric effect in which an electron is ripped away from its atom via the interaction with a photon. This process was long believed to be instantaneous but with the development of attosecond pulses (1 as 10(-18) s) we can finally get an insight into its dynamic. Here we measure a delay in ionization time between two differently bound electrons. The outgoing electrons are created via ionization with a train of attosecond pulses and we probe their relative delay with a synchronized infrared laser. We demonstrate how this probe field influences the measured delays and show that this contribution can be estimated with a universal formula, which allows us to extract field free atomic data.

Photoemission electron microscopy using extreme ultraviolet attosecond pulse trains

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed. (C) 2009 American Institute of Physics. [doi:10.1063/1.3263759

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 (N-2, 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