Single ionization of atoms in intense laser pulses: Evolution from multiphoton to tunnel ionization

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

Determination of the Carrier-Envelope Phase of Few-Cycle Laser Pulses with Terahertz-Emission Spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond time scale. In order to take full advantage of such pulses, it is crucial to measure and stabilise their carrier-envelope (CE) phase, i.e., the phase difference between the carrier wave and the envelope function. We introduce a novel approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range via plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows for a determination of the phase by measurement of the amplitude and polarity of the THz pulse

Sequential versus nonsequential two-photon double ionization of the D2 molecule at 38 eV

ABSTRACT: A simple theoretical model is used to interpret recent experimental results for two-photon double ionization (DI) of D2 at 38 eV. We show that the measured kinetic energy distribution associated with emission of two protons can be interpreted as a sum of two processes: a sequential and an instantaneous absorption of the two incident photons. These processes lead to peaks in di erent regions of the spaectrum

Ultrakurzzeit-Physik: Bewegte Bilder vom Aufbruch eines Wasserstoffmoleküls

Physikern des Max-Planck-Instituts für Kernphysik in Heidelberg ist es erstmals gelungen, mit Lasern verschiedene Quantenwege, die zum Aufbrechen eines Wasserstoffmoleküls führen, auf einer bisher nicht zugänglichen ultrakurzen Zeitskala zu visualisieren. Durch geschickte Formung der Laserpulse gelang es außerdem, die Moleküle auf bestimmte Quantenpfade zu zwingen - erste Schritte auf dem Weg zur gezielten Manipulation chemischer Reaktionen von komplexen Molekülen

Zeitaufgelöste Untersuchungen zur Fragmentationsdynamik von H₂ (D₂) in ultra-kurzen Laserpulsen

In course of this work pump-probe experiments aimed to study ultrafast nuclear motion in H2 (D2) fragmentation by intense 6-25 fs laser pulses have been carried out. In order to perform time-resolved measurements, a Mach-Zehnder interferometer providing two identical synchronized laser pulses with the time-delay variable from 0 to 3000 fs with 300 as accuracy and long-term stability has been built. The laser pulses at the intensities of up to 1015 W/cm2 were focused onto a H2 (D2) molecular beam leading to the ionization or dissociation of the molecules, and the momenta of all charged reactions fragments were measured with a reaction microscope. With 6-7 fs pulses it was possible to probe the time evolution of the bound H+2 (D+2 ) nuclear wave packet created by the first (pump) laser pulse, fragmenting the molecule with the second (probe) pulse. A fast delocalization, or "collapse", and subsequent "revival" of the vibrational wave packet have been observed. In addition, the signatures of the ground state vibrational excitation in neutral D2 molecule have been found, and the dominance of a new, purely quantum mechanical wave packet preparation mechanism (the so-called "Lochfrass") has been proved. In the experiments with 25 fs pulses the theoretically predicted enhancement of the ionization probability for the dissociating H+2 molecular ion at large internuclear distances has been detected for the first time

Playing with poetry: Poetic representation of research in children’s geographies of nature and adventurous play

In this article, we undertake a methodological and creative exploration of poetic representation in children’s geographies. Drawing on qualitative approaches to poetry as a method, we consider how poetic techniques have the potential to bring us into children’s experiences in different ways and inspire more playful engagement with our research data more broadly. We present an illustrated series of research poems derived from our project on children’s perspectives on nature and adventurous nature play in New Zealand. We invite readers and viewers to engage with these works and consider what disrupting traditional prose representations of research can add to critical children’s geographies and beyond