Initial metal-metal bond breakage detected by fs X-ray scattering in the photolysis of Ru3(CO)12 in cyclohexane at 400 nm.

Using femtosecond resolution X-ray solution scattering at a free electron laser we were able to directly observe metal-metal bond cleavage upon photolysis at 400 nm of Ru3(CO)12, a prototype for the photochemistry of transition metal carbonyls. This leads to the known single intermediate Ru3(CO)11(µ-CO)*, with a bridging ligand (µCO) and where the asterisk indicates an open Ru3-ring. This loses a CO ligand on a picosecond time scale yielding a newly observed triple bridge intermediate, Ru3(CO)8(µ-CO)3*. This loses another CO ligand to form the previously observed Ru3(CO)10, which returns to Ru3(CO)12via the known single-bridge Ru3(CO)10(µ-CO). These results indicate that contrary to long standing hypotheses, metal-metal bond breakage is the only chemical reaction immediately following the photolysis of Ru3(CO)12 at 400 nm. Combined with previous picosecond resolution X-ray scattering data and time resolved infrared spectroscopy these results yield a new mechanism for the photolysis of Ru3(CO)12.

XFEL experiments: jitter of pump-probe time delays and pulse intensities.

Jitter of XFEL signals due to fluctuations in shot-to-shot time delays and intensities are explored in the frame of a statistical theory of X-ray diffraction from liquids. Deformed signals are calculated at different levels of pump-probe jitter. A new method is proposed to eliminate these distortions.

Liquid X-ray scattering with a pink-spectrum undulator.

X-ray scattering from a liquid using the spectrum from the undulator fundamental is examined as a function of the bandwidth of the spectrum. The synchrotron-generated X-ray spectrum from an undulator is 'pink', i.e. quasi-monochromatic but having a saw-tooth-shaped spectrum with a bandwidth from 1 to 15%. It is shown that features in S(q) are slightly shifted and dampened compared with strictly monochromatic data. In return, the gain in intensity is 250-500 which makes pink beams very important for time-resolved experiments. The undulator spectrum is described by a single exponential with a low-energy tail. The tail shifts features in the scattering function towards high angles and generates a small reduction in amplitude. The theoretical conclusions are compared with experiments. The r-resolved Fourier transformed signals are discussed next. Passing from q- to r-space requires a sin-Fourier transform. The Warren convergence factor is introduced in this calculation to suppress oscillatory artifacts from the finite qM in the data. It is shown that the deformation of r-resolved signals from the pink spectrum is small compared with that due to the Warren factor. The q-resolved and the r-resolved pink signals thus behave very differently.

Time-resolved observation of the Eigen cation in liquid water.

Experimental observation and time relaxation measurement of the hydrated proton Eigen form [H(3)O(+)(H(2)O)(3)] are presented here. Vibrational time-resolved spectroscopy is used with an original method of investigating the proton excess in water. The anharmonicity of the time-resolved spectra is characteristic of the Eigen-type proton geometry. Proton relaxation occurs in less than 200 fs. A calculation of the potential energy confirms the experimental result and the Eigen cation lifetime is in good agreement with previous molecular dynamics simulations.

Recombination of photodissociated iodine: a time-resolved x-ray-diffraction study.

A time-resolved x-ray-diffraction experiment is presented that aims to study the recombination of laser-dissociated iodine molecules dissolved in CCl4. This process is monitored over an extended time interval from pico- to microseconds. The variations of atom-atom distances are probed with a milliangstrom resolution. A recent theory of time-resolved x-ray diffraction is used to analyze the experimental data; it employs the correlation function approach of statistical mechanics. The most striking outcome of this study is the experimental determination of time-dependent I-I atom-atom distribution functions. The structure of the CCl4 solvent changes simultaneously; the solvent thus appears as a reaction partner rather than an inert medium hosting it. Thermal expansion of the system is nonuniform in time, an effect due to the presence of the acoustic horizon. One concludes that a time-resolved x-ray diffraction permits real-time visualization of solvent and solute motions during a chemical reaction.

Ultrafast x-ray diffraction of transient molecular structures in solution.

We report direct structural evidence of the bridged radical (CH2ICH2.) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with approximately 0.01 angstrom spatial resolution and approximately 100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2.. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.

Motion of hydrogen bonds in diluted HDO/D2O solutions: direct probing with 150 fs resolution

An experiment is described to study temporal variations of the hydrogen bond length in diluted HDO/D2O solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.5 microns spectral region at a 10 microJ power level is detailed next. The OH stretching band is reproduced for different excitation frequencies and different pump-probe delay times. A theory, based on statistical mechanics of nonlinear optical processes, is proposed to calculate the lowest two spectral moments. An effect is reported, the delay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the OH...O bond length directly, in real time. The corresponding time scales are of the order of 700 fs. No bond oscillations are observed.