Picosecond x-ray diffraction

X-rays are usually generated using thermionic electron sources. Even when driven by ultrashoit laser pulses [15] the emitted electrons have long pulse widths ( 10 ns) and are therefore not suitable for picosecond pulse x-ray generation. We have successfully utilized photoemission as a means of generating ultrashort electron bunches and subsequently picosecond duration x-ray pulses. Photoemission is known to have an extremely short response time in most materials, consequently the electron current practically follows the laser pulse intensity envelope under the appropriate conditions. 

The photocathode consists of a cylindrical, polished 15-mm diameter, nickel substrate. It is mounted on a high-voltage feedthrough, which is maintained at a pressure below 2 x 10 Torr. A schematic representation of the chamber that houses the electron and x-ray source is presented in Fig. 12. The photocathode substrate fits into Pierce focusing electrode with an additional field-shaping electrode added to increase the extraction field 

The electrons emitted by the photocathode are subsequently accelerated to 50 kV and focused on to a toroid-shaped anode. The anode is made of oxygen-free, high conductivity copper and is maintained at a high positive potential. The electron pulses interact with the copper anode forcing the emission of Cu-Ka x-ray photon pulses, which exit the vacuum chamber through a thin beryllium-foil window. A bend germanium crystal monochromator disperses and focuses the x-rays onto the sample. The duration of the x-ray pulses is measured by a Kentech x-ray streak camera fitted with a low density Csl photocathode. The pulse width of the x-rays at 50 kV anode-cathode potential difference is about 50 ps. This value is an upper limit for the width of the x-ray pulses because the transit time-spread of the streak camera has to be taken into consideration. A gold photocathode (100 A Au on 1000 A peiylene) is used to record the 266-nm excitation laser pulses. The intensity of the x-rays is 6.2 x 10 photons an r (per pulse), and is measured by means of a silicon diode array x-ray detector which has a known quantum efficiency of 0.79 for 8 kV photons. 

This experimental system has enabled us to perform picosecond timc-resolvcd x-ray diffraction experiments. One of the earliest experi-ments consist of a Ge (HI) crystal bent for maximum reflection of 8 keV x-ray photons, ilai to a sensitive double-crystal monochromator. The x-ray diffraction spectra are recorded before excitation. This diffraction exhibiu the normal diffraction pattern characteristic of this surface. In 

This somewhat brief description of the PXR system has made evident the general aspects of this unique experimental ultrafast x-ray system. The experimental procedure for time-resolved x-ray diffraction presented is based upon the pump-probe scheme first introduced several years ago in picosecond spectroscopy [17]. The laser in these experiments is used to create x-ray pulses and also functions as an excitation source for the sample. To detect the very weak signals of diffracted x-rays, a unique 

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P. Chen, I. V. Tomov, and P. M. Rentzepis, Time resolved heat propagation in a gold crystal by means of picosecond X-ray diffraction. J. Chem. Phys., 104(24), 10001-10007 (1996). 

J. Davidsson, J. Poulsen, M. Cammarata, P. Georgiou, R. Wouts, G. Katona, F. Jacobson, A. Plech, M. Wulff, G. Nyman, and R. Neutze, Structural determination of a transient isomer of CH2I2 by picosecond X-ray diffraction. Phys. Rev. Lett. 94(24), 245503 (2005). 

Guerin L, Collet E, Lemee-Cailleau M-H, Buron-Le Cointe M, Cailleau H, Plech A, Wulff M, Koshihara S, Luty T (2004) Probing photoinduced phase transition in a charge-transfer molecular crystal by 100 picosecond X-ray diffraction. Chem Phys 299 163-170... 

Techert S, Zachariasse KA (2004) Structure determination of the intramolecular charge transfer state in crystalline 4-(diisopropylamino)benzonitrile from picoseconds X-ray diffraction. J Am Chem Soc 126 5593-5600... 

Studies of Molecular Dissociation by Means of Ultrafast Absorption and Emission Spectroscopy and Picosecond X-Ray Diffraction P, M. Rentzepis and B. Van Wonterghem... 

Here, experimental results are presented that suggest that the decomposition of haloaromatics in the condensed phase, proceeds via the triplet manifold. We also present data that help to identify the intermediate states, their kinetics, and the radicals formed as a result of the photodb-sodation process. Additionally, a new method, picosecond x-ray diffraction (PXR) is described. This method is capable of time-resolved x-ray diffraction in the picosecond scale and has the potential of generating a set of diffraction histograms which depict, in real time, the evolution of the structure of excited states and intermediates during decomposition or in the course of a chemical or biological reaction. Processes su as dissociation, isomerization, melting, and nucleation are but a few examples that... 

However, time-resolved X-ray diffraction remains a young science. It is still impossible, or is at least very difficult, to attain time scales below to a picosecond. General characteristics of subpicosecond X-ray diffraction and absorption are hardly understood. To progress in this direction, free electron laser X-ray sources are actually under construction subject to heavy financial constraints. Nevertheless, this field is exceptionally promising. Working therein is a challenge for everybody ... 

Rose-Petruck C, Jimenez R, Guo T, Cavalleri A, Siders CW, Raksi E, Squier JA, Walker BC, Wilson KR, Barty CPJ (1999) Picosecond-milliangstrom lattice dynamics measured by ultrafast X-ray diffraction. Nature 398 310-312... 

Fig. 13. Schematic representation of the picosecond x-ray (PXR) system showing penmenlai setup for time-resolved x-ray diffraction.

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