For many years chemists (and biologists) have dreamt about recording the atomic positions of reacting molecules in real time in order to understand the formation of new molecules. Indeed the 1999 Nobel Price in Chemistry was awarded to Ahmed Zewail from Caltech, who used femtosecond optical spectroscopy to study the formation (and breakage) of chemical bonds in gas molecules. These studies have shown that bond formation happens in 10 to 100 fs, a factor of 1000 faster than the duration of the X-ray pulse from a synchrotron. In these experiments, a molecular beam is excited by a femtosecond pulse and the reaction products are probed by a delayed probe pulse derived from the pump beam. Once the reaction products are formed, they usually proceed to the final state through a sequence of intermediates. The lifetimes of these intermediates can vary from 1 ps to 1 ms.

On beamline ID9 a pump & probe setup has been built which uses single pulses of X-rays combined with optical pulses from a femtosecond laser. It is a very flexible system which has enabled picosecond intermediates to be studied using Laue diffraction, oscillation diffractometry, powder diffraction and diffuse scattering techniques. Despite the synchrotron pulses being 1000 times longer than the laser pulses used by Zewail, X-rays are attractive since they probe interatomic distances directly. Moreover in those cases where the reacting species can be held in a crystal, diffraction techniques make it possible to record three-dimensional movies of atomic motion of all the atoms in the unit cell.

Recently the ID9 team has studied electron transfer reactions in chemical systems. Powder samples of organic compounds comprising linked donor- and acceptors units (PyDMA, DMABN) have been studied and the difference maps between the excited and non-excited states have shown the presence of large-amplitude motions on the picosecond timescale. Specifically, the angle between the planar donor and acceptor units in PyDMA and DMABN rotates 8 degrees in less than 200 ps. These systems relax back into the ground state after 3 - 4 ns, which makes it possible to repeat and accumulate the data stroboscopically.

The trans to cis isomerisation of the stilbene molecule has also been studied. This system is best characterised as a molecular hinge which rotates upon the absorption of a photon. The molecule consists of two benzene rings which are linked via an ethylene double bond, see Figure 27. In the planar ground state 1, the two benzene rings are positioned on the same side of the double bond. The system is excited by a 150 fs pulse, which brings the ethylene bond into an anti-bonding state 2. The molecule responds by rotating one of the benzene rings 90 degrees; the newly formed orthogonal state 3, then decays into either the original trans state 1 or to the cis state 4. The time it takes to decay is a function of the specific stilbene derivative and the friction from the surrounding solvent. Complete isomerisation takes place on a femtosecond to picosecond time scale. Note that the benzene rings are rotated 20 degrees in the cis state 4. In these experiment the stilbene molecule was dissolved in methanol and exposed to the laser/X-ray pump & probe beam. In practice a UV pulse initiates the reaction and a delayed X-ray pulse is used to record the diffuse scattering from the dilute sample. The pump and probe sequence is repeated at 900 Hz and the data accumulated on a CCD camera. The experiment is finally repeated at several time points between ­ 1 ns to + 1 ns. The measurements have shown that it is possible to isolate the weak stilbene signal (ca 1%) and that the difference patterns are in qualitative agreements with expectations from laser spectroscopy. Figure 28 shows the calculated change in the diffuse intensity between the trans, cis and orthogonal states.

Authors
S. Techert, F. Schotte, M. Wulff.
ESRF