Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics1. They arise in the visible and infrared range from femtosecond lasers2, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources3 and free-electron lasers4. Recent progress in laser wakefield accelerators has resulted in electron beams with energies from tens of mega-electron volts (refs 5,6,7) to more than 1 GeV within a few centimetres8, with pulse durations predicted to be several femtoseconds9. The enormous progress in improving beam quality and stability5, 6, 7, 8, 10 makes them serious candidates for driving the next generation of ultracompact light sources11. Here, we demonstrate the first successful combination of a laser-plasma wakefield accelerator, producing 55-75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. By demonstrating the wavelength scaling with energy, and narrow-bandwidth spectra, we show the potential for ultracompact and versatile laser-based radiation sources from the infrared to X-ray energies. (Abstract from: http://www.nature.com/nphys/journal/v4/n2/abs/nphys811.html

We report the growth of GdN thin films and a study of their structure and magnetic and conducting properties. It is demonstrated that they are semiconducting at ambient temperature with nitrogen vacancies the dominant dopant. The films are ferromagnetic below 68 K, and a significant narrowing of the band gap is signaled by more than a doubling of its conductivity. The conductivity in the low-temperature ferromagnetic state remains typical of a doped semiconductor, supporting the view that this material is semiconducting in its ground state and that no metal-insulator transition occurs at the Curie temperature.

Desorption of NO molecules from a Pd(l 11) surface induced by femtosecond visible laser pulses has been investigated in real time. The measurements, accomplished by means of a two-pulse correlation scheme, reveal a subpicosecond response time. The findings indicate that the desorption arises from coupling to the nonequilibrium electronic excitation in the substrate.PACS numbers: 68.45. Da, 42.65.Re, 78.90.+t, The nature of coupling and the rate of energy flow between an adsorbate and a solid surface constitute key issues in surface dynamics. l " 7 Despite considerable experimental and theoretical effort, a complete understanding of these fundamental issues concerning the interaction between the localized excitation of the adsorbate and delocalized electronic and lattice excitation in the substrate remains elusive. For metal surfaces, strong coupling is generally present and typical relaxation rates lie in the picosecond to femtosecond range. 6,7 While these processes cannot be examined in real time using conventional surface science probes, laser-based techniques provide the possibility of time-domain studies. 8 Recently, real-time studies of relaxation of electronic 9 excitation at surfaces and of vibrational 7,10,11 excitation in adsorbates have been performed using pulsed laser excitation. In this Letter we report the results of the first direct timeresolved measurements of desorption from a surface under femtosecond excitation. 12 The system of nitric oxide (NO) on Pd(lll) was chosen as a model for desorption of a nondissociating chemisorbed molecule from a surface. In our laboratory we have previously characterized the yield and final-state energy distributions for NO molecules desorbed from Pd (111) by femtosecond laser pulses. 13 In contrast to the results obtained for desorption by nanosecond pulses, 14 the findings for desorption by femtosecond pulses could not be understood on the basis of equilibrium behavior, nor could the results be explained by a simple photochemical mechanism. 4,5 In the present work, we investigate the dynamics of desorption in the time domain. The time-resolved experiments are performed using a two-pulse correlation scheme in which the total desorption yield is measured as a function of temporal separation between a pair of excitation pulses. In this manner, we obtain femtosecond time resolution despite the time delay required before the desorbed molecules can be detected. 15 Examining correlation signals for both equal and unequal excitation pulses, we demonstrate that the excitation responsible for desorption has a finite lifetime not exceeding 1 ps. This result implies that nonequilibrium electronic excitation in the substrate is responsible for the desorption process, since mechanisms associated with coupling to substrate phonons would reflect the longer time scale required for cooling of the phonons to occur. We further show that the principal features of the time-resolved data can be understood through consideration of the substrate electronic temperature, which in the su...

The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel.

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