Bernhard W. Adams

Picosecond X-ray absorption measurements of the ligand substitution dynamics of Fe(CO)5 in ethanol

Ultrafast X-ray absorption near edge spectroscopy has been carried out for photo excited iron pentacarbonyl in ethanol with 2 picosecond resolution. A temporal resolution limited dissociation process was observed, followed by the formation of the mono-substituted complex Fe(CO)(4)EtOH within a few tens of picoseconds. The measurements have been carried out with a newly developed X-ray absorption instrument at station 7 ID-C of the Advanced Photon Source. The results show that single picosecond temporal resolution can be achieved at a synchrotron beam line.

X-ray quantum optics

Quantum optics with X-rays has long been a somewhat exotic activity, but it is now rapidly becoming relevant as precision x-ray optics and novel X-ray light sources, and high-intensity lasers are becoming available. This article gives an overview of the current state of the field and an outlook to future prospects.

Subnanosecond piezoelectric x-ray switch

We report an ultrafast piezoelectric switch for synchrotron x rays. A thin epitaxial film of piezoelectric Pb(Zr,Ti)O3 works as a diffractive optical switch at frequencies from dc to >1GHz. The broad frequency range allows single bunches of synchrotron x rays to be selected in an arbitrary sequence. The piezoelectric effect introduces mechanical strains of a fraction of 1% in the Pb(Zr,Ti)O3 film, which can be used for blocking or passing diffracted x rays.

Invited Article: A test-facility for large-area microchannel plate detector assemblies using a pulsed sub-picosecond laser

The Large Area Picosecond Photodetector Collaboration is developing large-area fast photodetectors with time resolution </~10 ps and space resolution </~1 mm based on atomic layer deposition-coated glass Micro-Channel Plates (MCPs). We have assembled a facility at Argonne National Laboratory for characterizing the performance of a wide variety of microchannel plate configurations and anode structures in configurations approaching complete detector systems. The facility consists of a pulsed Ti:Sapphire laser with a pulse duration ≈100 fs, an optical system allowing the laser to be scanned in two dimensions, and a computer-controlled data-acquisition system capable of reading out 60 channels of anode signals with a sampling rate of over 10 GS/s. The laser can scan on the surface of a sealed large-area photodetector, or can be introduced into a large vacuum chamber for tests on bare 8 in.-square MCP plates or into a smaller chamber for tests on 33-mm circular substrates. We present the experimental setup, detector calibration, data acquisition, analysis tools, and typical results demonstrating the performance of the test facility.

Time-Resolved Research at the Advanced Photon Source Beamline 7-ID

The Sector 7 undulator beamline (7‐ID) of the Advanced Photon Source (APS) is dedicated to time‐resolved x‐ray research and is capable of ultrafast measurements on the order of 100 ps. Beamline 7‐ID has a laser laboratory featuring a Ti:Sapphire system (average power of 2.5 W, pulse duration <50 fs, repetition rate 1–5 kHz) that can be synchronized to the bunch pattern of the storage ring. The laser is deliverable to x‐ray enclosures, which contain diffractometers, as well as motorized optical tables for table‐top experiments. Beamline 7‐ID has a single APS Undulator A and uses a diamond (111) double‐crystal monochromator, providing good energy resolution over a range of 6–24 keV. Available optics include Kirkpatrick‐Baez (KB) mirrors to microfocus the x‐ray beam. A variety of time‐resolved diffraction and spectroscopy research is available at 7‐ID, with experiments being done in the atomic, molecular, optical, chemistry, and solid state (bulk and surface) fields.

Picosecond laser-pump, x-ray probe spectroscopy of GaAs

A laser-pump, x-ray probe spectroscopic experiment is described, and the results are shown. The Ga Kα x-ray fluorescence following x-ray absorption, at the Ga K absorption edge was measured, and its increase due to excitation with subpicosecond pulses of laser light at 4.6 eV photon energy was determined. The x-ray absorption, and thus the fluorescence, is increased for about 200 ps after the laser pulse because additional final states for the x-ray absorption are cleared in the valence band by the laser excitation. The technique could eventually lead to a femtosecond pump-probe spectroscopy with an absolute reference energy level and also to a femtosecond x-ray detector. This is of particular importance to future short-pulse x-ray sources, such as free-electron lasers.

An active-optic x-ray fluorescence analyzer with high energy resolution, large solid angle coverage, and a large tuning range

A crystal-optic x-ray fluorescence energy analyzer has been designed and tested, which combines the features of electron-volt energy resolution, large solid angle coverage, and tunability over several kilo-electron-volts. The design is based upon the principle of active optics, with ten actuators available to optimally adjust the shape of a silicon crystal used in the Bragg geometry. In most applications the shape is that of a logarithmic spiral for high energy resolution with a spatially nonresolving detector, but a wide range of other shapes is also possible for applications such as imaging or single-shot spectroscopy in a spectral range of the operators choosing.

A novel experimental technique for atomic X-ray holography

A new experimental technique for reciprocal X-ray holography has been developed. The experimental set-up makes it possible to measure a reciprocal hologram without unwanted mixture of the X-ray fluorescence holography signal. The data are recorded during continuous rotation of the sample, and are accumulated over many revolutions. Thus the sensitivity to fluctuations of the source characteristics is reduced. A very high resolution over a large angular range is also achieved, which allows recording of X-ray standing-wave shapes near Bragg angles. The reconstruction of the measured hologram of a Cu(3)Au crystal gives positions of the nearest and next-nearest neighbours of a fluorescing Cu atom.

X-ray Holography for Structural Imaging

X-ray atomic resolution holography is a new method for direct evaluation of three-dimensional electron density distribution in solids. The practical implementation of the multiple-energy technique on a synchrotron radiation source as well as image reconstruction from the experimental data are described. Holograms at several different energies were processed together to suppress twin images and artifacts from long-range-order effects in the experimental data sets. Reconstructed images of copper atoms in Cu(2)O crystals are presented.

2-ps Hard X-Ray Streak Camera Measurements at Sector 7 Beamline of the Advanced Photon Source

A hard X-ray streak camera capable of 2-ps time resolution is in operation at the Sector 7 beamline of the Advanced Photon Source. It is used for laser-pump, X-ray probe experiments using the Ti:Sapphire femtosecond laser system installed on the beamline. This streak camera, combined with standardized and prealigned experimental setups, can perform time-resolved liquid-phase absorption spectroscopy, reflectivity, and diffraction experiments.