September 2012 Archives

In time-resolved X-ray analysis based on the pump-probe scheme, an increase in the repetition rate is crucial for improving efficiency. At the same time, it is crucial to maintain or improve pulse to pulse stability. Recently a Swiss research team developed a fast multichannel detection system for pump-probe spectroscopy, capable of detecting single shot super-continuum spectra at the repetition rate (10-50 kHz) of an amplified femtosecond laser system. The setup is not for synchrotron X-rays, but many points discussed in the report will be useful. For more information, see the papers, "Femtosecond pump/supercontinuum-probe setup with 20 kHz repetition rate", G. Aubock et al., Rev. Sci. Instrum., 83, 093105 (2012).

When spatially coherent X-ray beams pass through a sample, edge-enhancement is observed in the transmission X-ray image because of the refraction effect. At Canadian Light Source (Saskatoon, Saskatchewan, Canada), Professor P. C. Johns (Carleton University) and his colleague have recently reported an interesting extension of this type of X-ray refractive imaging. They used multiple pencil beams (up to five) to create both transmission and refractive projection images, simultaneously, during the sample scan crossing the beams. The radial data were extracted from the overlapped image by a Maximum Likelihood-Expectation Maximization (MLEM) algorithm. For more information, see the papers, "Synchrotron-based coherent scatter x-ray projection imaging using an array of monoenergetic pencil beams", K. Landheer et al., Rev. Sci. Instrum., 83, 095114 (2012).

Readers might recall several previous news articles on X-ray spectra of neon excited by ultra-short, high-intensity pulses from an X-ray free electron laser source at LCLS, Stanford ("Observation of non-linear resonances of inner-shell electrons by X-ray free electron laser", No.1, Vol. 41 (2012), "Calculation of X-ray emission from doubly ionized neon", No.1, Vol. 40 (2011), ""Hollow" neon atom created by X-ray laser excitation", No.5, Vol. 39 (2010) and "Removing all electrons from neon by X-ray laser", No.6, Vol. 38 (2009) ). Recently, a research team led by Professor C. H. Keitel (Max-Planck-Institut für Kernphysik, Germany) has published its calculation of the resonance X-ray fluorescence spectra of neon, based on a so-called two-level model, which is used to study the transition of 1s2p_z^-1→1s^-12p_z in Ne⁺ at an energy of 848 eV. As X-rays induce Rabi oscillations so fast, they compete with Ne 1s-hole decay. The research group discusses resonance X-ray fluorescence spectra for two different cases; the first is chaotic pulses, which are most likely based on the SASE principle employed in the present XFEL facilities, and the second is Gaussian pulses available from the more ideal types of X-ray lasers expected in the future. For more information, see the paper, "Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses", S. M. Cavaletto et al., Phys. Rev. A86, 033402 (2012).