APPARATUS FOR THE MEASUREMENT OF THE ELECTRONIC EXCITED-STATE STRUCTURE OF SINGLE-CRYSTALS USING X-RAY-DIFFRACTION

Instrumentation for measuring the X-ray diffraction pattern of optically excited crystals is described. The experiment uses a high-power (similar to 1 W) laser and a single-crystal diffractometer equipped with a helium cryostat (T < 70 K). The laser beam is modulated by a mechanical chopper and the diffraction signal gated in synchronization with the chopper phase. The modulation method is capable of observing small changes (down to about 0.01%) in the structure factors upon excitation of a fraction of the molecules in the crystal, given adequate counting statistics. The technique can be used for relatively long lived electronic excited states (tau similar or equal to 0.1-10 ms). The optical system is also suitable for time-resolved measurements using the time structure of synchrotron radiation.