1. Photography methods 2. Diffraction intensity

1. 1. Photography methods2. Diffraction intensity Lecture 5 1-30-2006

2. Data collection methods • Either the crystal has to rotate or the wavelength has to vary. According to Bragg’s law, only these lattice points that intercept with Ewald sphere will give rise to reflections. Fig 4.19

3. Data collection methods: oscillation photography Crystal Detector X-ray • Crystal is gently rocked (oscillated) over very small angle (1 - 3 degrees) while the film is held stationary perpendicular to the incident beam • Crystal needs to be rotated through enough reciprocal space to collect all unique data • The choice of the rotation angle depends on: • unit-cell size (the larger unit cell the smaller oscillation angle) • spot size or mosaic spread (the larger the spot size, the smaller oscillation angle) • resolution (the higher the resolution desired the smaller the oscillation angle) • Crystal sits at a random orientation • Very efficient, simple experimental setup Experimental setup

4. Data collection methods: oscillation photography • A typical oscillation photograph • Reciprocal lattice is distorted

5. Data collection methods • Either the crystal has to rotate or the wavelength has to vary. According to Bragg’s law, only these lattice points that intercept with Ewald sphere will give rise to reflections. Fig 4.19

6. Data collection methods: Laue photography • First Laue picture was taken by M. von Laue, W. Friedrich and P. Knipping in 1912. • Employs multiple wavelengths (white radiation). • Gives many more diffraction spots at once than the monochromatic radiation. • This method is mainly used for studying reaction intermediates, or for collecting entire data sets within few seconds or even milliseconds • Synchrotron radiation provides extremely intense 50 to 150ps long X-ray pulses • Experimental setup is rather complex The area between the two Ewald spheres will be covered by varying l

7. Data collection methods: Laue photography Experimental setup used to acquire picosecond time-resolved X-ray diffraction data. The crystal is photolysed by a laser pulse and then probed by a time delayed X-ray pulse A typical Laue picture A series of structures provide information about structural dynamics

8. Diffraction intensity • Temperature factor • Calculation of electron density from structure factors • Symmetry of diffraction pattern • Effect of unit cell sizes on diffraction intensities

9. The temperature factor • The temperature B is caused by both dynamic disorder (temperature-dependent vibration) and static disorder (molecules in different unit cells occupying slightly different positions) • B is usually treated as an isotropic term • B modify diffraction intensity by: • B is related to mean square displacement of the atomic vibration by Scattering of a carbon atom *** B factor affects high resolution limit of crystal diffraction