[en] Amphipod crustaceans are often used to measure the toxicity of bulk sediments. Acute lethal bioassays are commonly employed, but this study investigated the potential for using a chronic growth bioassay with Ampelisca abdita. A potential advantage of this method is that the growth rate could be a more sensitive measure of contamination than mortality. Growth rates for A. abdita in sediments spiked with cadmium and crude oil were compared to mortality rates in A. abdita, Eohaustorius estuaries, and Rhepoxynius abronius in sediments with the same concentrations of contaminants. A. abdita was more sensitive to cadmium than the other two species. For crude oil, there was a significant shift in size distribution from the control even at concentrations as low as 150 mg/kg of oil. The standard acute lethal tests for all species, on the other hand, did not show significant mortality until at least 1,600 mg/kg. The results confirm that growth rates are a more sensitive indicator of toxicity, and to at least the three contaminants tested, A. abdita is as sensitive as E. estuarius and R. abronius. This study also confirmed the reported high mortality rates of E. estuaries in San Francisco Bay sediments. The causes of this high mortality are unknown but give further reason for using A. abdita for toxicity tests in this region

No abstract available

No abstract available

[en] The abrasion-ablation model for reactions between relativistic heavy ions is derived from Glauber's multiple scattering theory. Simple expressions are found for the abrasion cross section and for the excitation energy of the projectile after abrasion. Both quantities depend on nuclear densities and on the nucleon-nucleon forward scattering amplitude. They do not contain any adjustable parameters. With a very simple model for the ablation process, fragmentation cross sections for the production of individual isotopes are calculated. The agreement with experiment is good for most cases. Systematic discrepancies are observed and are analyzed in terms of an extension of the abrasion-ablation model

[en] Strong-field phenomena driven by an intense infrared (IR) laser depend on during what part of the field cycle they are initiated. By changing the sub-cycle character of the laser electric field it is possible to control such phenomena. For long pulses, sub-cycle shaping of the field can be done by adding a relatively weak, second harmonic of the driving field to the pulse. Through constructive and destructive interference, the combination of strong and weak fields can be used to change the probability of a strong-field process being initiated at any given part of the cycle. In order to control sub-cycle phenomena with optimal accuracy, it is necessary to know the phase difference of the strong and the weak fields precisely. If the weaker field is an even harmonic of the driving field, electrons ionized by the field will be asymmetrically distributed between the positive and negative directions of the combined fields. Information about the asymmetry can yield information about the phase difference. A technique to measure asymmetry for few-cycle pulses, called stereo-ATI (above threshold ionization), has been developed by Paulus et al (2003 Phys. Rev. Lett. 91 253004). This paper outlines an extension of this method to measure the phase difference between a strong IR and its second harmonic. (paper)

[en] We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe and Ar using a tunable femtosecond laser in the wavelength range between 600 and 800 nm. Systematic analysis of the experimental data that are quantitatively reproduced by numerical solutions of the time-dependent Schroedinger equation with integration over the laser focal volume demonstrates the dominance of resonance-enhanced ionization. Continuous variation of the laser wavelength allows the identification of a number of consecutive channel-switching effects with a reliable assignment of the intermediate Rydberg states involved. The appearance of the resonant sub-structure in the electron energy spectra is influenced by the presence of a non-resonant contribution. At relatively low laser intensity, a coherent addition of resonant and non-resonant ionization processes is observed. Due to the absence of an intensity dependence in the resonance-enhanced ionization, we observe the persistence of Freeman resonances at the transition to the tunnelling regime.

[en] We model high-order harmonic generation (HHG) from an argon atom driven by an elliptically polarized laser field, using classical electron trajectories that are initialized from a discrete set of microcanonical distributions. Analysis of the returning trajectories reveals that when the atomic potential is accounted for, there are two possible ionization pathways leading to the same photon energy for both the short and the long trajectories found in the recollision model of HHG. We also demonstrate that the threshold ellipticity, a measure of the sensitivity to ellipticity, decreases as the harmonic order increases for both short and long trajectories, in qualitative agreement with recent experimental results (Larsen et al 2015 arXiv:1506.08660). We use a nonlinear dynamical analysis to show that this behavior is regulated by a set of recolliding periodic orbits (RPOs) that survive at ellipticities greater than 0.2 and can therefore drive the recollision dynamics even at relatively high ellipticity. The connection between the threshold ellipticity dependence we find and RPOs means that the effect should be general. (paper)

[en] We present angle- and energy-resolved measurements of photoelectrons produced in strong-field ionization of Xe using a tunable femtosecond laser. An occurrence of highly oscillatory patterns in the angular distribution at low photoelectron kinetic energy is observed that correlates with channel closing/opening over a wide range of laser parameters. The correlation is investigated both experimentally and by means of systematic analysis of numerical solutions of the time-dependent Schroedinger equation. Our experimental and numerical results are in quantitative agreement with the semi-classical model introduced by Arbo et al (2008 Phys. Rev. A 78 013406), which relates the oscillatory patterns to interference between photoelectrons produced during different cycles of the laser pulse in the course of non-resonant ionization of the atom. We observe that an increase of the laser intensity eventually leads to qualitative invariance of the pattern, defining a limit on the applicability of the semi-classical model.

[en] We show that we can probe the components of an attosecond bound electron wave packet by mapping the quantum beat signal produced by a synchronized delayed few-cycle infrared pulse into the continuum. In addition, spectrally overlapping peaks that result from one-, two- or three-photon processes from more or less deeply bound states can in principle be interferometrically resolved with high resolution. (paper)

[en] The exact delay-zero calibration in an attosecond pump-probe experiment is important for the correct interpretation of experimental data. In attosecond transient absorption spectroscopy the determination of the delay-zero exclusively from the experimental results is not straightforward and may introduce significant errors. Here, we report the observation of quarter-laser-cycle (4ω) oscillations in a transient absorption experiment in helium using an attosecond pulse train overlapped with a precisely synchronized, moderately strong infrared pulse. We demonstrate how to extract and calibrate the delay-zero with the help of the highly nonlinear 4ω signal. A comparison with the solution of the time-dependent Schrödinger equation is used to confirm the accuracy and validity of the approach. Moreover, we study the mechanisms behind the quarter-laser-cycle and the better-known half-laser-cycle oscillations as a function of experimental parameters. This investigation yields an indication of the robustness of our delay-zero calibration approach. (paper)