Biomedical subjects

H N Chapman

Publications and source records attributed to H N Chapman.

9 recordsLinked to original sources

PubMed2005

Damped and thermal motion of laser-aligned hydrated macromolecule beams for diffraction.

We consider a monodispersed Rayleigh droplet beam of water droplets doped with proteins. An intense infrared laser is used to align these droplets. The arrangement has been proposed for electron- and x-ray-diffraction studies of proteins which are difficult to crystallize. This paper considers the effect of thermal fluctuations on the angular spread of alignment in thermal equilibrium, and relaxation phenomena, particularly the damping of oscillations excited as the molecules enter the field. The possibility of adiabatic alignment is also considered. We find that damping times in a high-pressure gas cell as used in x-ray-diffraction experiments are short compared with the time taken for molecules to traverse the beam and that a suitably shaped field might be used for electron-diffraction experiments in vacuum to provide adiabatic alignment, thus obviating the need for a damping gas cell.

Chemistry, Physical↗

PubMed2005

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

Journal Article↗

PubMed2005

Diffraction with wavefront curvature: a path to unique phase recovery.

Modern X-ray optics is able to produce very tightly focused beams. The size of these focused spots is comparable to the scale of large molecules and therefore to the lattice spacing of crystals of these molecules. In this case, the phase of the illuminating beam may vary on the scale of the lattice and conventional diffraction theory needs to be modified. In this paper, coherent diffraction by non-planar beams is considered and it is shown that it is possible to uniquely recover the phase of the diffraction pattern.

Algorithms↗

PubMed2005

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

Journal Article↗

PubMed2005

Clocking femtosecond X rays.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

Journal Article↗

PubMed2003

Unique phase recovery for nonperiodic objects.

It is well known that the loss of phase information at detection means that a diffraction pattern may be consistent with a multitude of physically different structures. This Letter shows that it is possible to perform unique structural determination in the absence of a priori information using x-ray fields with phase curvature. We argue that significant phase curvature is already available using modern x-ray optics and we demonstrate an algorithm that allows the phase to be recovered uniquely and reliably.

Algorithms↗

PubMed2001

Dark field X-ray microscopy: the effects of condenser/detector aperture.

In order to visualize the functionality of a biological cell, it is often desirable to label specific proteins. In this work we concentrate on the optical theory of visualizing colloidal gold labels with soft X-ray microscopes, where scattering from small gold spheres used as labels dominates the image. Using numerical simulations of bright and dark field imaging, we compare different configurations of condenser and objective lenses in transmission X-ray microscopes, and configurations of detector and objective lens in scanning transmission X-ray microscopes. It is verified that the contrast of small, closely spaced features is strongly affected by changes in these configurations; the optimum situation is to have the condenser aperture (in TXM) or detector aperture (in STXM) equal to 3/2 that of the objective numerical aperture.

Journal Article↗

PubMed1996

A characterisation of dark-field imaging of colloidal gold labels in a scanning transmission X-ray microscope.

While X-ray microscopes provide images of biological specimens for which the contrast is mainly due to the difference in the absorption of carbon and oxygen when X-rays transmitted through the specimen are detected, signals other than absorption can also be used to form images. Using the Stony Brook scanning transmission X-ray microscope at the National Synchrotron Light Source, high-angle dark-field images have been formed of cells labelled with colloidal gold, with and without silver enhancement. The high density of the colloidal gold particles, or the silver particles seeded by the gold, leads to a large scattering signal, and the fact that the particle diameters are comparable to the width of the microscope point spread function results in good localisation of the label with high contrast. The dark-field images can have a greater signal to noise ratio than bright-field images acquired with the same incident X-ray dose. The theory of dark-and bright-field imaging is reviewed. Theoretical calculations of scattering from gold and silver particles are presented and good agreement is found between these and experimental dark-field images of 30 nm diameter gold particles. The signal to noise ratios of experimental bright-and dark-field images are measured and found to be in agreement with theory. Images are presented of cells labelled by immunolabelling and in situ hybridisation.

Fibroblasts↗

PubMed1995

X-ray microscopy.

The subject of X-ray microscopy (high-resolution X-ray imaging of general nonperiodic structures), an area in which much progress has been made in recent years, is reviewed. The main techniques are briefly described. Achievable performance levels, which for many years were highly speculative, can now be understood with fair accuracy in terms of basic X-ray and specimen properties, and techniques have progressed to the point where actual results are nearing those levels. In terms of specimen size and imaging resolution, X-ray microscopies lie between electron and light microscopy, and are thus suited to imaging extremely large and complex structures; in addition, they demand little or no specimen preparation, and can be used to observe local composition and chemical state as well as structure. Thus X-rays, which have played the leading role in imaging crystallizable materials, may also prove to be highly valuable in the imaging of very large non-crystalline structures. Throughout the treatment, attention is paid to the relationships connecting the subject with X-ray crystallography.

Crystallography, X-Ray↗