Observation of femtosecond X-ray interactions with matter using an X-ray–X-ray pump–probe scheme

Resolution in the X-ray structure determination of noncrystalline samples has been limited to several tens of nanometers, because deep X-ray irradiation required for enhanced resolution causes radiation damage to samples. However, theoretical studies predict that the femtosecond (fs) durations of X-ray free-electron laser (XFEL) pulses make it possible to record scattering signals before the initiation of X-ray damage processes; thus, an ultraintense X-ray beam can be used beyond the conventional limit of radiation dose. Here, we verify this scenario by directly observing femtosecond X-ray damage processes in diamond irradiated with extraordinarily intense (∼10¹⁹ W/cm²) XFEL pulses. An X-ray pump–probe diffraction scheme was developed in this study; tightly focused double–5-fs XFEL pulses with time separations ranging from sub-fs to 80 fs were used to excite (i.e., pump) the diamond and characterize (i.e., probe) the temporal changes of the crystalline structures through Bragg reflection. It was found that the pump and probe diffraction intensities remain almost constant for shorter time separations of the double pulse, whereas the probe diffraction intensities decreased after 20 fs following pump pulse irradiation due to the X-ray–induced atomic displacement. This result indicates that sub-10-fs XFEL pulses enable conductions of damageless structural determinations and supports the validity of the theoretical predictions of ultraintense X-ray–matter interactions. The X-ray pump–probe scheme demonstrated here would be effective for understanding ultraintense X-ray–matter interactions, which will greatly stimulate advanced XFEL applications, such as atomic structure determination of a single molecule and generation of exotic matters with high energy densities.Since W. C. Röntgen discovered X-rays emitted from vacuum tube equipment in 1895, scientists have continuously endeavored to develop brighter X-ray sources throughout the 20th century. One of the most remarkable breakthroughs was the emergence of synchrotron light sources, which were much more brilliant than the early lab-based X-ray sources. Such dramatic increase in X-ray brilliance provided a pathway to obtain high-quality X-ray scattering data. This, in turn, enabled one to solve the structures of complex systems such as proteins, functional units of living organisms, and viruses. However, the increase in the brilliance is also accompanied by a severe problem of X-ray radiation damage to the samples being examined (1). X-rays ionize atoms and generate highly activated radicals that break chemical bonds and cause changes in the structures of the samples. To achieve structure determination precisely, a sufficient scattering signal should be recorded before the samples are severely damaged. Radiation damage was considered to be an intrinsic problem associated with X-ray scattering experiments, which imposed a fundamental limit on the resolution in X-ray structure determination (2).The recent advent of X-ray free-electron lasers (XFELs) (3–5), which emit ultraintense X-ray pulses with durations of several femtoseconds, may totally avoid the problem of radiation damage. The irradiation of intense XFEL pulses generates highly ionized atoms, and the strong Coulomb repulsive force leads to evaporation of the samples. Meanwhile, it has been predicted theoretically (6) that atoms do not change their positions before the termination of the femtosecond X-ray pulse owing to inertia, thus enabling the use of X-ray radiations beyond the conventional X-ray dose limit. This innovative concept, called a “diffraction-before-destruction” scheme (6, 7), has paved a clear way to high-resolution structure determinations of weak scattering objects, including nanometer-sized protein crystals (8), noncrystalline biological particles (9), and damage-sensitive protein crystals (10).Despite the potential impact of XFELs, detailed understanding of the ultrafast XFEL damage processes has been missing. As a pioneering work, Barty et al. (11) measured the diffraction intensities of protein nanocrystals by changing the XFEL pulse durations from 70 to 300 fs at intensities of ∼10¹⁷ W/cm². They found that the diffraction intensities greatly decrease for longer durations, clearly indicating sign of structural damage, i.e., X-ray–induced atomic displacements within the XFEL pulse durations. For further understanding of ultraintense X-ray interactions with matter, we need to directly measure the temporal changes of the structural damage. In particular, measuring the ignition time of the atomic displacements is crucial for realizing advanced applications with greatly intense XFELs. Although improving our knowledge of the X-ray damage processes is essential for all aspects of XFEL science, the experimental verifications have been missing because of the extreme difficulty in observation with ultrahigh resolutions in space (ångstrom) and time (femtosecond).As a new approach to investigate the femtosecond X-ray damage processes, we here propose an X-ray–X-ray pump–probe experiment using double X-ray pulses; a pump X-ray pulse excites a sample and a probe X-ray pulse with a well-controlled time delay characterizes the change in the sample. In this approach, it is highly useful to exploit two-color double pulses with tunable temporal separations (12–15), which have been developed at SPring-8 Angstrom Compact free-electron LAser (SACLA) (4) and Linac Coherent Light Source (3). In this article, we measured the X-ray damage processes of diamond by using an X-ray–X-ray pump–probe diffraction experiment at SACLA. As the carbon–carbon bond is one of the most fundamental bonds in biomolecules, our results should provide a benchmark for XFEL-induced damage to practical samples.     

Analysis of Passive Motion of Para- and Retropharyngeal Structures During Swallowing Using Dynamic Magnetic Resonance Imaging

The purpose of this study was to analyze passive motion of the para- and retropharyngeal space (PRS) during swallowing using dynamic magnetic resonance imaging (MRI). We conducted a preliminary study involving 30 healthy volunteers who underwent dynamic MRI. Consecutive MRI axial images were obtained by examining the plane parallel to the hard palate at the level of the anterior inferior corner of C2. Anterior displacement of the posterior pharyngeal wall (PPW) was measured as a motion index of pharyngeal contraction. The displacement and internal angle of the bilateral external and internal carotid arteries (ECA and ICA) and the bilateral centroids of the PRS area, as well as the increase in PRS area, were calculated at rest and at maximum pharyngeal contraction. In most participants, the bilateral ECA, ICA, and centroids were anterointernally displaced by pharyngeal contraction. The normalized ECA displacement (r = 0.64, r ² = 0.41), normalized ICA displacement (r = 0.60, r ² = 0.37), and normalized centroid displacement (r = 0.43, r ² = 0.19) were more than moderately positively correlated with the normalized PPW displacement. The normalized PRS area increase (r = 0.35, r ² = 0.12) was weakly positively correlated with the normalized PPW displacement. These results revealed that PRS area increased as the ECA and ICA were drawn anterointernally via its passive motion by pharyngeal contraction.