By Lars Hjortshøj and Jakob Mikael Espersen
With a sample of platinum-based molecules and use of an ultra-short laser pulse with the right wavelength, it is possible to "freeze" the molecules in their otherwise random vibrations.
In addition to this, a series of ultra-short X-ray pulses from a kilometer-long X-ray laser can be used to study, not only the molecules that have been photo-excited and "stopped" in the excited state, but also the majority of the molecules that haven't been excited by the laser light.
These are the main points of an article selected as one of "Editors Suggestion" in the highly recognized physics journal Physical Review Letters (PRL). The research is a collaboration between researchers from DTU Chemistry, DTU Physics and Stanford University.
Read the article in Physical Review Letters and the mentioning at SLAC National Accelerator Laboratory.
A unique look into the behavior of molecules
At the LCLS x-ray laser at Stanford University, the research team investigates the molecule "PtPOP"; a model system for some types of photo-catalysis. It is especially the details of light-powered chemistry that are of interest to the researchers.
"We are interested in investigating how the molecular structure of some molecules ring like a bell, after they absorbe a light photon. It gives us direct information on how and how much of the absorbed light energy is distributed as vibrations in the molecule and how much is potentially available to drive, e.g., a catalytic reaction”, explains Kristoffer Haldrup, Senior Researcher at DTU Physics .
Most chemical reactions take place between molecules in their ground state, so the researchers use a trick called "off-resonance excitation" to lock some of the molecules into a particular structure.
"Off-resonance excitation makes it possible to study how the structure of the ground state develops on the ultra-short, ie femtoseconds and picoseconds, time scales, which are absolutely crucial in chemistry. When we combine that with detailed calculations carried out by Professor Klaus B. Møller's group at DTU Chemistry, the X-ray measurements give a completely unique look into the behavior of the molecules ”, Kristoffer Haldrup says. The group at DTU Chemistry consisted - in addition to Professor Klaus B. Møller - of former PhD students Gianluca Levi and Asmus O. Dohn as well as Associate Professor and co-supervisor Niels Engholm Henriksen.
The researchers hope the new-found insight will allow refinement of the very basic understanding of chemical reactions. This better understanding can eventually pave the way for "rational design" of molecules with very specific properties within, e.g., photo-catalysis.
"To understand how something works, we need to study it while it works, right down to the ultra-short time scales. When we understand how a given molecule "works", we can, together with our colleagues from chemistry, begin to consider how to design similar molecules that are better at, e.g., absorbing light energy - And save this light energy in a way so it is available either for chemical reactions or as part of a solar cell ”, Kristoffer Haldrup explains.
The results and the method, which has now been published, has the potential of wide usage in future chemical reaction studies.