General Colloquium
February 22 - 4:00pm Phys 223
(Coffee at 3:30p.m. in room 242)
February 22 - 4:00pm Phys 223
(Coffee at 3:30p.m. in room 242)
Qun Shen
Staff Scientist
Cornell High Energy Synchrotron Source (CHESS) and Department of Materials Science and Engineering
Title: "Solving the Phase Problem in X-ray Crystallography"
X-ray phase problem refers to the fact that only the intensities, not the phases, of Bragg diffracted waves from a crystal can be directly measured in an x-ray diffraction experiment. From the intensities only the magnitudes of structure factors can be determined, but both the phases and the magnitudes are needed to figure out the atomic positions in the crystal. In this talk we present a recent advance towards solving this classic phase problem, by using a three-beam data collection technique called reference-beam diffraction. The technique incorporates the principle of three-beam interference into the most popular oscillating-crystal setup in crystallography and allows simultaneous recording of many three-beam interference profiles on an area detector. From these measured interference profiles a large number of relative phases (triplet-phases) of the reflections involved can be obtained. These measured phases from a single reference-beam data set form a unique reference tripletpattern of individual-reflection phases that can reduce the three-dimensional problem into a two-dimensional one. The use of symmetry defined phases can further reduce the number of possible phase values, and thus has the potential to obtain all individual phases from a single reference-beam diffraction data set. Other strategies on how to utilize the measured triplet-phase information, such as the use of projection Fourier transforms, will also be discussed.Brief Bio:
Staff Scientist, Cornell High Energy Synchrotron Source (CHESS)
Adjunct Associate Professor, Materials Science & Engineering, Cornell University
Postdoctoral Research Associate, Applied & Engineering Physics, Cornell University (1987-89)
Ph.D. in Physics, Purdue University (1987)
B.S. in Physics Peking University (1981)
Research Interests:
Diffraction physics
Phase problem in crystallography
Strain in semiconductor & magnetic nanostructures
Polarization dependent scattering
X-ray optics
Research Activities:
High-heat-load designs
F-line optics upgrade
Reference-beam diffraction technique