Time-resolved x-ray diffraction from small molecules in solution

We describe beamline ID09B at the European Synchrotron Radiation Facility (ESRF), a laboratory for optical pump and x-ray probe experiments to 100-picosecond resolution. The x-ray source is a narrow-band undulator, which can produce up to I x 1010 photons in one pulse. The 3% bandwidth of the undulator is sufficiently monochromatic for most diffraction experiments in liquids. A Ti: sapphire femtosecond laser is used for reaction initiation. The laser runs at 896 Hz and the wavelength is tunable between 290-1160 ran. The doubled (400 nm) and tripled wavelength (267 nm) are also available. The x-ray repetition frequency from the synchrotron is reduced to 896 Hz by a chopper. The time delay can be varied from 0 ps to 1 ins, which makes it possible to follow structural processes occurring in a wide range of time scales in one experiment. We present a preliminary study on the structural dynamics of photo-excited iodine in methanol. At early time delays after dissociation, 1 - 10 ns, the change in the diffracted intensity DeltaS(q, tau) is oscillatory and the high-q part 4-8 Angstrom(-1) is assigned to free iodine atoms. At later times, 10-100 ns, expansive motion is seen in the bulk liquid. The expansion is driven by energy released from the recombination of iodine atoms. The AS(q, T) curves between 0.1 and 5 mus coincide with the temperature differential dS/dT for static methanol with a temperature rise of 2.5 K. However, this temperature is five times greater than the temperature deduced from the energy of dissociated atoms at I ns. The discrepancy is ascribed to a short-lived state that recombines on the sub-nanosecond time scale.