Contact: Tim Graber, +1 630.252.0462, graber@cars.uchicago.edu

Time resolved crystallography allows the study of transient molecular species which intermediate many chemical processes. This technique hopes to elucidate dynamic chemical processes, in particular, their reaction pathways and transition states.

Experimental Setup

The setup for a time-resolved crystallography experiment, such as a pump-probe experiment, is as follows:

• An x-ray beam is produced in APS Undulator A
• A wavelength is selected using a Double Crystal Diamond Monochromator
• Higher order harmonics are rejected using a double Mirror System
• The beam travels down to the 15IDB hutch where it is chopped by a Rotating Chopper

The interaction region has several components:

• Crystal (i.e. sample under consideration)
• Florescence Detector: measures the florescence lifetime and optimize the pointing of the laser
• Helium Cryostream: keeps the crystal at a temperature close to that of liquid helium
• CCD Detector: collects the diffraction pattern and the data is refined on a computer yielding a structure

Pump-Probe Procedure

In standard crystallography, a crystal mounted on a goniomenter is exposed to an x-ray beam and rotated simultaneously about a tenth of a degree. The diffraction spots produced during this time are recorded by a CCD detector. Typically this process is repeated until the crystal has rotated 360°.

The plot labeled frame duration above represents one of these rotations with a time duration of 2 seconds. In a time-resolved experiment, two frames are taken consecutively covering the same angular range. However, the first frame is taken with the pump laser operating and the second without the pump laser. Comparison of these two data sets can yield the difference in the ground and excited state molecular geometry.