Subnanometer-Scale Measurements of the Interaction of Ultrafast Soft X-Ray Free-Electron-Laser Pulses with Matter

Abstract

Correspondence should be addressed to SPHR.Femtosecond pulses from soft-x-ray free-electron lasers (FELs) [1] are ideal for directly probing matter at atomic length scales and timescales of atomic motion. An important component of understanding ultrafast phenomena of light-matter interactions is concerned with the onset of atomic motion which is impeded by the atoms' inertia. This delay of structural changes will enable atomic-resolution flash-imaging [2][3] to be performed at upcoming x-ray FELs [4][5] with pulses intense enough to record the x-ray scattering from single molecules [6]. We explored this ultrafast high-intensity regime with the FLASH soft-x-ray FEL [7][8] by measuring the reflectance of nanostructured multilayer mirrors using pulses with fluences far in excess of the mirrors' damage threshold. Even though the nanostructures were ultimately completely destroyed, we found that they maintained their integrity and reflectance characteristics during the 25-fs-long pulse, with no evidence for any structural changes during that time over lengths greater than 3 Å.In the recently built FLASH FEL [7], x-rays are produced from short electron pulses oscillating in a periodic magnet array, called an undulator, by the principle of self-amplification of spontaneous emission [9][10]. The laser quality of the x-ray pulses can be quantified by the peak spectral brilliance of the source, which is 10 28 photons/(s mm 2 mrad 2 0.1% bandwidth) [8]; this is up to seven orders of magnitude higher than modern thirdgeneration synchrotron sources. For our studies, the machine operated with pulses of 25 fs duration at a wavelength of 32.5 nm and energies up to 21 μJ. We focused these pulses to 3x10 14 W/cm 2 onto our nanostructured samples, resulting in an the unprecedented heating rate of 5x10 18 K/s, while probing the irradiated structures at the nanometer length scale.The x-ray reflectivity of periodic nanometer-scale multilayers [11] is very sensitive to changes in the atomic positions and the refractive indices of the constituent materials, making them an ideal choice to study structural changes induced by ultrashort FEL pulses. The large multilayer reflectivity results from the cooperative interaction of the x-ray field with many layers of precisely fabricated thicknesses, each less than the x-ray wavelength. This Bragg or resonant reflection from the periodic structure is easily disrupted by any imperfection of the layers. The characteristics of the structure, such as periodicity or layer intermixing, can be precisely determined from the measurement of the Bragg reflectivity as a function of incidence angle. These parameters can be easily measured to a small fraction of the probe wavelength, as is well known in x-ray crystallography where average atomic positions of minerals or proteins are found to less than 0.01Å. Thus, we can explore ultrafast phenomena at length scales less than the wavelength, and investigate the conditions to overcome the effects of radiation damage by using ultrafast pulses.For our measurement...

Damage-resistant single-pulse optics for x-ray free electron lasers

“…The fluence dependence of the reflectivity of a Si/C multilayer at 32.5 nm wavelength at an angle of incidence of 45 degrees was measured at FLASH [11]. The experimental results and our model predictions are reproduced in Fig.…”

“…In recent experiments at FLASH it was demonstrated that the conventional damage threshold can be overcome by taking advantage of the extremely short pulse duration of the FELs [11]. It has been shown that during the pulse, only a limited amount of damage takes place, and the optic still functions.…”

Profiling structured beams using injected aerosols

“…These include ablation imprints, [7][8][9] scintillation crystals or x-ray sensitive Hartmann sensors. [10][11][12] However, high peak intensities of FEL pulses near their small foci have made this extremely challenging, either because of the potential damage to such instruments or the difficulty in interpreting their measurements or both.…”

“…The damage profile imprinted on silicon nitride membranes by polystyrene spheres acting as nano-lenses can help determine the near-field diffraction pattern that the latter creates when illuminated by intense soft x-ray FEL pulses. 9 wavefront of incident FEL pulse towards detector Figure 1. One dimensional phase tilts (wavefront shape) of an FEL pulse (dashed lines) cause the centers of diffraction patterns to diverge.…”

Macromolecular crystallography at synchrotron radiation sources: current status and future developments

“…In an important paper, Hajdu and colleagues (Neutze et al 2000) provided computer simulations of the predicted damage associated with photo-absorption and Auger electron emission and showed that coherently scattered X-rays from a single molecular complex could be collected before the sample explodes from Coulomb forces, provided that the pulse was short enough (in the range of femtoseconds). Later, it was calculated that some electronic damage (removal or rearrangement of electrons) may also take place on the femtosecond time scale (Hau-Riege et al 2007). In simple terms, scattering takes place on the femtosecond time scale, while most damage takes place on the picosecond time scale.…”