Time Resolved Microcrystallography

X-ray structure determinations normally determine the structure of the electronic ground state of molecules. However, all processes involved in the colorful and living world, e.g. organic light emitting diodes or photosynthesis, proceed via excited states. The course of excitation and subsequent relaxation to the ground state, often by passing different intermediate excited states, can be well investigated using spectroscopic techniques. But the excited state structures have remained inaccessible until a few years ago. The lack of knowledge of the structure of the excited states is also the reason for many computational approaches to fail in calculating spectroscopic properties, especially of metal organic compounds. The pulsed structure of synchrotron radiation allows so called 'pump and probe' experiments to be conducted and hence the exploration of the excited state structure.


Figure 1: Time resolved microdiffraction: A 1 µm x 5 µm x-ray beam illuminates a portion of the sample directly beneath the surface. Thus only the optically excited molecules of the crystal surface are probed.

Figure 2a: Calculated LUMO of a perylene dye exhibiting a very high fluorescence quantum yield in the solid state.

Figure 2b: Calculated HOMO of a perylene dye exhibiting a very high fluorescence quantum yield in the solid state.

The new Petra-3 storage ring is well suited for time resolved experiments. Half of its operation time it will be operated in a 40 bunch mode, with a bunch separation time of about 360 ns. A chopper system for isolating single pulses and reducing the X-ray pulse frequency to 1 - 40 kHz has already been built and is currently in the phase of commissioning.

In contrast to existing time resolved beam lines, we will try to benefit from the very small and high flux density X-ray beams which will become possible at the Petra-3 storage ring. Using a very small X-ray beam of only 1 µm x 5 µm (h x v) should have several advantages in the field of time resolved X-ray crystallography. First, the penetration depth of the optical pump laser pulses on the maximum of the absorption band is in most cases only a few microns. Thus using an X-ray beam only one micrometer wide and irradiating the sample very close to the surface should cause it to mainly interact with molecules in their excited state. Using larger X-ray beams, the diffraction pattern is dominated by molecules located deeper inside the crystal and hence, due to insufficient pumping, in their electronic ground state. Second, by probing only a 5 µm broad stripe on the crystal surface, the optical pump laser energy required to pump all molecules in the probed volume can be reduced to about 1%. This drastically reduces the thermal heat load on the crystal and molecular changes are then dominated by the optical, rather than by thermal, excitation. In addition, more convenient and less expensive laser systems can be used. Due to the reduced thermal load on the sample and the new chopper system higher repetition rates become possible and should allow for much faster data acquisition.

One drawback of micro beam techniques in general is the high mechanical accuracy and positional stability required for all devices. To fulfill these stability requirements a new high precision diffractometer is currently under development at DESY. In addition, we are working on an active positional feedback system for the pump laser as well as for the probing X-rays.

The first experiments with the new experimental setup aim to investigate the excited state structures of organic light emitting diodes (OLED's) and biomimetic systems. The time resolution for these experiments is currently limited by the duration of the X-ray pulses - about 50 - 100 ps at current synchrotron sources. Future X-ray lasers will provide much faster X-ray pulses with a duration of a few femto-seconds only. The experiments at Petra-3 should therefore further assist in the development of experimental techniques and the aquisition of knowledge needed for future experiments at free electron X-ray lasers.