Most chemical reactions, natural or man-made, are driven by thermal effects. This makes their monitoring in “real-time” difficult, and only equilibrium experiments are possible. It so happens that in the case of water (the most important solvent in earth) when using short laser pulses with the appropriate infrared wavelength and intensity, the irradiated volume can be impulsively heated by tens of degrees centigrade in 2 to 3 picoseconds.

This impulsive temperature jump (T-jump) has been exploited to impulsively trigger chemical reactions in aqueous media, which were monitored by spectroscopic methods ranging from the infrared to the ultraviolet. These methods deliver valuable information about the chemical reaction but are not sensitive neither to the elements involved, nor to the structure of the reactants.

These limitations have been overcome by the group of Majed Chergui within the Lausanne Centre for Ultrafast Science (LACUS) using X-ray spectroscopy. In a new paper, Olivero Cannelli and colleagues demonstrate for the first time the use of an X-ray probe in a T-jump experiment to observe structural changes over the course of a chemical reaction.

To demonstrate this, the researchers investigated a model thermally-driven reaction where cobalt ions in an aqueous solution containing chlorine ions, change their coordination from water molecules to chlorine ions, upon a temperature increase. This work is a first step towards the time-resolved investigations of thermally-driven chemical and biochemical reactions using X-ray methods.


Fig. 1. Schematic layout of the investigated chemical reaction. The equilibrium can be tuned with changes in temperature or chloride concentration, causing free chloride ions to gradually replace water molecules. In parallel, structural interconversion occurs from the hexa-octahedral to chloro-tetrahedral configuration. (b) Experimental layout of the laser T-jump/X-ray absorption spectroscopy probe. The sample jet schematically represents the closed loop wire-guided liquid jet employed in the experiment.

“So far, probing the structural evolution of reactions by X-rays has only been possible for light-driven reactions because photochemical triggers are impulsive,” says Chergui. “The T-jump method in aqueous media is another trigger that can now be used. This broadens the range of chemical processes that can be dynamically investigated at sources of X-ray pulses, such as synchrotrons and X-ray free electrons lasers."