RESUMO
We investigate the out-of-equilibrium switching dynamics of a molecular Fe(III) spin-crossover solid triggered by a femtosecond laser flash. The time-resolved x-ray diffraction and optical results show that the dynamics span from subpicosecond local photoswitching followed by volume expansion (nanosecond) and thermal switching (microsecond). We present a physical picture of the consecutive steps in the photoswitching of molecular materials.
RESUMO
Recent progress in high intensity ultrafast laser systems provides the opportunity to produce laser plasma x-ray sources exhibiting broad spectrum and high average x-ray flux that are well adapted to x-ray absorption measurements. In this paper, the development of a laser based x-ray absorption near edge structure (XANES) beamline exhibiting high repetition rate by using the Advanced Laser Light Source (ALLS) facility 100 Hz laser system (100 mJ, 35 fs at 800 nm) is presented. This system is based on a broadband tantalum solid target soft x-ray source and a grazing incidence grating spectrometer in the 1-5 nm wavelength range. To demonstrate the high potential of this laser based XANES technique in condensed matter physics, material science, or biology, measurements realized with several samples are presented: VO2 vanadium L edge, Si3N4 nitrogen K edge, and BPDA/PPD polyimide carbon K edge. The characteristics of this laser based beamline are discussed in terms of brightness, signal to noise ratio, and compared to conventional synchrotron broadband x-ray sources which allow achieving similar measurements. Apart from the very compact size and the relative low cost, the main advantages of such a laser based soft x-ray source are the picosecond pulse duration and the perfect synchronization between this x-ray probe and a laser pulse excitation which open the way to the realization of time resolved x-ray absorption measurements with picosecond range time resolution to study the dynamics of ultrafast processes and phase transition.
