News | November 16, 2016

Chemical Reaction To The Fingers

Helmholtz cooperation lays the foundations for detailed observation of chemical processes with table-top lasers and special detectors

A research team from the Helmholtz Institute Jena and DESY has paved the way to make use of a conventional laser for the observation of the breaking of chemical bonds. For their experiments, the researchers combined high-performance light-generating and detector components from the two Helmholtz institutions. This arrangement forms the basis for observations of rapid processes with a time resolution of 30 billionths of a second (30 femtoseconds).The design is so compact and robust that it can be a prototype for measuring instruments, which can also be built and operated at smaller institutions and universities.

What happens when a chemical bond is dissolved? How do individual atoms couple to each other and to each other? The understanding of the dynamics of chemical processes is often referred to as the "holy grail" of physical chemistry; One understands the processes, one has the basis for influencing such connections and perhaps designing completely new substances.

For the precise observation of such chemical processes, high-speed cameras with the highest time and spatial resolution are needed, such as the X-ray laser European XFEL, which is currently being constructed in the metropolitan region of Hamburg and will make individual molecules and atoms visible. However, a laser for short-wave ultraviolet light, in combination with a coincidence detector developed for synchrotron and x-ray laser experiments, is also sufficient for the scientific observation of the breaking of chemical bonds of small molecules.

In their studies shelled the Helmholtz scientists iodomethane molecules (CH 3 I), consisting of an iodine atom and a methyl group (CH 3 ) are made, with short high-intensity XUV pulses. The bond between iodine and methylene group was broken by light; The fragments of the molecules were collected and measured in a spectrometer. From this, the rearrangement of electrons in the excited molecule and the following induced chemical processes can be deduced.

Basis of the experiments was a table-top laser system for light in the so-called extreme ultraviolet (XUV). The laser developed at the Helmholtz Institute Jena produces very short and high-intensity XUV pulses by first amplifying an infaram laser light pulse in an optical fiber and subsequently generating odd-numbered multiples of the original laser frequency. For these experiments, the wavelength of approximately 18 nanometers of these so-called higher harmonic frequencies was decoupled by means of special optics and used in the experiment.

"The XUV laser system produces light flashes with 1 million photons, which are only 30 femtoseconds long, with a pulse rate of up to 100 kilohertz," explains Prof. Limpert. Dr. Jan Rothhardt, who has developed the laser, says "The combination of high photon flux and very high repetition rate with very good stability, in principle, qualifies the system to perform useful experiments for chemical dynamics."

The nature of the pulse generation by higher harmonics offers a quasi-built-in advantage: a chemical reaction can be initiated with a light flash generated in the laser in order to examine it after a fixed time with an XUV flash generated in the same laser. "The delay of the second flash is adjustable with high precision", says Rothhardt. In this first series of experiments, this "pump & probe" technique has not yet been used; But it has already been tested and is scheduled for follow-up experiments.

Second important component of the experiments was a complex sample and detector chamber, which was developed for use on free electron lasers (FEL) and was already used on DESY's FLASH and PETRA III accelerators. In this CAMP experiment chamber, run by Daniel Rolles' working group, the sample was shot into the light beam in a thin beam with a supersonic velocity. In interaction with the XUV light, the molecules were destroyed and the properties of the flying fragments were measured with high precision in a built-in spectrometer. By means of coincidence measurements, the collected fragments could be assigned to their source molecule, and the precise characterization of the building blocks can be used to decipher the course of the break of the bond in a time-resolved manner.

"With the combination of experimental and scientific possibilities from Jena and Hamburg, we are opening up new possibilities in the observation of chemical dynamics," says the initiator of the experiments, DESY scientist Prof. Jochen Küpper, who also joined the Center for Free Electron Laser Science and the Hamburg Center for Ultrafast Imaging of the University of Hamburg. DESY researcher Tim Laarmann continues "In the next step, we will be carrying out pump-probe experiments. In principle, this structure should allow even higher time resolutions of femtosecond time to be possible and allow us to visualize the fastest electron motions in complex molecules."