Electronic movement flashing into view Numerous chemical processes begin with ionization: the ejection of an electron from a molecule. What happens in the immediate aftermath of that event? Kraus et al. explored this question in iodoacetylene by detecting and analyzing the spectrum of emitted high harmonics (see the Perspective by Ueda). They traced the migration of the residual positively charged hole along the molecular axis on a time scale faster than a quadrillionth of a second. They thereby characterized the capacity of a laser field to steer the hole's motion in appropriately oriented molecules. Science , this issue p. 790 ; see also p. 740

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gammarays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

We propose and analyze a scheme for ultrafast spectroscopy with nanometer spatial and femtosecond temporal resolution. The interaction of polarization-shaped laser pulses with a nanostructure allows us to control the spatial and temporal evolution of the optical near field. Employing a learning algorithm, the field is tailored such that pump and probe excitation occur at different positions and at different times. Both excitations can be restricted to subdiffraction extensions and are separable on a nanometer length scale. This enables the direct spatial probing of nanoscale energy transfer or charge transfer processes.

In steep mountain streams, macro-roughness elements typically increase both flow energy dissipation and the threshold of motion compared to lower-gradient channels, reducing the part of the flow energy available for bed load transport. Bed load transport models typically take account of these effects either by reducing the acting bed shear stress or by increasing the critical parameters for particle entrainment. Here we evaluate bed load transport models for mixed-size sediments and models based on a median grain size using a large field data set of fractional bed load transport rates. We derive reference shear stresses and bed load transport relations based on both the total boundary shear stress and a reduced (or ''effective'') shear stress that accounts for flow resistance due to macro-roughness. When reference shear stresses are derived from the total boundary shear stress, they are closely related to channel slope, but when they are derived from the effective shear stress, they are almost invariant with channel slope. The performance of bed load transport models is generally comparable when using the total shear stress and a channel slope-related reference shear stress, or when using the effective shear stress and a constant reference shear stress. However, dimensionless bed load transport relations are significantly steeper for the total stress approach, whereas they are similar to the commonly used fractional Wilcock and Crowe (WC) transport model for the effective stress approach. This similarity in the relations allows the WC model, developed for lower-gradient streams, to be used in combination with an effective shear stress approach, in steep mountain streams.

Bed load transport during storm events is both an agent of geomorphic change and a significant natural hazard in mountain regions. Thus, predicting bed load transport is a central challenge in fluvial geomorphology and natural hazard risk assessment. Bed load transport during storm events depends on the width and depth of bed scour, as well as the transport distances of individual sediment grains. We traced individual gravels in two steep mountain streams, the Erlenbach (Switzerland) and Rio Cordon (Italy), using magnetic and radio frequency identification tags, and measured their bed load transport rates using calibrated geophone bed load sensors in the Erlenbach and a bed load trap in the Rio Cordon. Tracer transport distances and bed load volumes exhibited approximate power law scaling with both the peak stream power and the cumulative stream energy of individual hydrologic events. Bed load volumes scaled much more steeply with peak stream power and cumulative stream energy than tracer transport distances did, and bed load volumes scaled as roughly the third power of transport distances. These observations imply that large bed load transport events become large primarily by scouring the bed deeper and wider, and only secondarily by transporting the mobilized sediment farther. Using the sediment continuity equation, we can estimate the mean effective thickness of the actively transported layer, averaged over the entire channel width and the duration of individual flow events. This active layer thickness also followed approximate power law scaling with peak stream power and cumulative stream energy and ranged up to 0.57 m in the Erlenbach, broadly consistent with independent measurements.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms. DOI: 10.1103/PhysRevLett.94.114801 PACS numbers: 41.60.Cr, 41.75.Ht, 42.65.Re Ultrafast x-ray pulses are providing our first view of subpicosecond atomic motion. New sources based on high harmonic generation [1,2] and laser-produced plasmas [3] as well as femtosecond laser-sliced synchrotron emission [4] have been demonstrated. These sources produce x-ray pulses with durations of less than a few hundred femtoseconds, the time scale of vibrations in solids and molecules and the making and breaking of chemical bonds. While these sources provide the time resolution necessary to study these dynamics, their relatively low brightness limits their application and often hinders attempted experiments.A new generation of linear-accelerator-based x-ray free electron lasers (XFELs) will be more than 20 orders of magnitude brighter than laser-plasma-based sources and have the potential to produce x-ray pulses below one femtosecond in duration [5]. With x rays from an XFEL, researchers can expect to image chemistry in real time on the atomic scale. While these new XFELs will be far brighter than any other ultrafast x-ray source, their physical size and complexity introduce new challenges which, if left unaddressed, will restrict their application. A major obstacle will be the inability to precisely synchronize the time-dependent process being studied with the x-ray pulse generated by a large accelerator-based source.Subpicosecond time-dependent phenomena are typically studied with pump-probe techniques in which the dynamics are initiated by an ultrafast laser or laser-driven source and then probed after a time delay. If these experiments can be self-synchronized, with the pump and probe having a common laser source, then precise time delays can be produced using different optical path lengths. The time resolution is then limited by the overlap of the pump and probe pulses that can be as short as a fraction of a PRL 94, 114801 (2005) P H Y S I C A L

Critical Shields values ( τc*) suitable for specific applications are back‐calculated from representative bed load samples in mountain streams and a flow competence/critical flow approach. The general increase of τc50* (for the bed D50 size) as well as τc16* and τc84* (for the bed D16 and D84 sizes) with stream gradient Sx and also the stratification of τc50* by relative flow depth and relative roughness are confirmed. Critical Shields values τc16* are shown to exceed τc50* by about threefold, while those for τc84* are nearly half of τc50*. However, it remains unclear to what extent physical processes or numerical artifacts contribute to determining critical Shields values. Critical bankfull Shields values ( τcbf*) back‐computed from the average largest particles mobile at bankfull flow DBmax,bf approach τc16* at steep gradients and τc84* at low gradients and therefore increase very steeply with Sx. The relation τcbf*=f(Sx) is stratified by bed stability (D50/DBmax,bf) and predictable if bed stability can be field categorized. Noncritical Shields values ( τbf50*) computed from bankfull flow depth and the D50 size differ from τc50* and τcbf*. Only in bankfull mobile streams where D50/DBmax = 1 can τ*cbf, τc50*, and τbf50* be used interchangeably. In highly mobile streams, substituting τcbf* by τbf50* overpredicts the DBmax,bf size by up to fivefold and underpredicts DBmax,bf by the same amount in highly stable streams. A value of 0.03 is appropriate for τcbf* only on low stability beds with Sx ≅ 0.01, but overpredicts DBmax,bf by 30‐fold on highly stable beds with Sx ≅ 0.1. Differences in field and computational methods also affect critical Shields values.