GPSDI

Gas Phase Structural Dynamics Imaging

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 Obiettivo del progetto (Objective)

'Chemical reaction mechanisms are at the core of fundamental science questions (like the functionality of biological molecules), as well as important technological advances (like the development of new materials). Understanding reaction mechanisms with atomic detail promotes both basic understanding of natural phenomena and knowledge-based technological applications. Despite the giant steps in the last decades in understanding chemical reactivity and achieving more control over chemical reactions at the atomic level, current knowledge comes mainly from spectroscopic and dynamics techniques which probe only a small fraction of the potential energy surface. We propose a different approach: to use Ultrafast Electron Diffraction (UED) to study chemical, physical and biological phenomena by imaging the structure of the intermediates involved as a function of time, all the way from reactants to products. In UED, a short laser pulse triggers the dynamics and diffraction of a short burst of electrons probes the transient structures at ultrashort intervals at the atomic/molecular level. The novelty of the technique has been demonstrated in gas phase as well as surface and materials science problems but there is a lot of room for improvement. If successful, we will construct a novel, state of the art UED instrument and use it to : (1) Probe the structural change during relaxation of photoexcited DNA, seeking to identify the contribution of spectroscopically dark channels, detectable only with UED, and (2) Image transient structures in reaction dynamics problems involving cluster chemistry and conical intersections'

Introduzione (Teaser)

One of the open challenges in science today is to directly observe atomic motions as they occur. Ultrafast electron diffraction (UED) uses electron beams to achieve this goal.

Descrizione progetto (Article)

For EU-funded researchers, the challenge was to combine high spatial and temporal resolution with sufficient electron beam intensity to bring atomic motions into focus. They started with simulations of kilo electron-volt electron beams generated by a photocathode. In such a scheme, a laser is used to illuminate the photocathode and to control the initial spatial and temporal distribution of the beam.

Based on the simulation results, researchers prepared technical drawings for the construction of an electron diffractometer for the Institute of Electronic Structure and Laser (IESL) at the Foundation for Research and Technology - Hellas (FORTH) on the island of Crete. The new apparatus consists of a high-brightness electron gun with a lanthanum hexaboride photocathode, an ultrafast laser and a position-sensitive imaging detector. The electron diffractometer integrated into the Institute's research infrastructure during the GPSDI (Gas phase structural dynamics imaging) project offers the opportunity to investigate structural changes in gas phase and solid materials.

Concurrently, researchers investigated possible combinations of UED with other techniques such as spectroscopic ellipsometry and slice imaging.

In the ellipsometry case, multipass spectroscopic ellipsometry (MPSE) was developed, where, the light beam is reflected multiple times off the sample in a single measurement. This way, MPSE allows more sensitive, simultaneous measurements of the refractive index, absorption coefficient and thickness of very thin films with possible applications in the semiconductor industry.

On the other hand, using traditional techniques such as velocity map imaging and slice imaging, researchers explored the photolysis mechanisms of methyl bromide and other molecules inside and outside clusters. The results have been described in a series of publications in renowned scientific journals.

GPSDI project research, conducted in collaboration with pioneers in France, Iceland, the Netherlands, Spain and the United States, has opened a new window onto the microscopic world. When scientists look with new eyes, they have a chance to see things in new ways.