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Gas Phase Structural Dynamics Imaging

Final Report Summary - GPSDI (Gas Phase Structural Dynamics Imaging)

Monitoring atomic motion as a function of time through Ultrafast (or Time-Resolved) Electron Diffraction (UED/TRED) is a novel approach to study the forces that govern natural phenomena. This atomic “motion picture” of the dynamics allows probing states of matter that traditional spectroscopy techniques cannot probe. The Gas Phase Structural Dynamics Imaging (in short hereafter GPSDI) project aimed at developing a new Time-Resolved Electron Diffractometer at IESL-FORTH featuring high electron brightness and adequate spatiotemporal resolution. The instrument would be used along with traditional dynamics techniques to investigate important phenomena such as DNA base relaxation and weak bonding effects in gas phase photodissociation.
As the main goal was the development of the TRED instrument, most of the project duration was spent on that. To choose a suitable photocathode material we considered the electron emission, damage threshold and vacuum characteristics of 27 candidate materials, including metals, alloys and semiconductors. Metals are rigid enough for high power laser irradiation and don’t need ultra high vacuum conditions but feature low electron quantum yields. Semiconductors exhibit high yields but need ultra high vacuum conditions and are rather fragile. We concluded that LaB6 appears to be the best compromise. A report on the results was published on the project’s website: .
To design the TRED instrument we started from simulations. Electron beam trajectories for different photocathode shapes in different electron gun configurations exploring several ideas were simulated, aiming to combine good spatiotemporal characteristics with increased number of electrons per pulse. Based on the results of this work, we prepared detailed mechanical drawings to be followed in the construction of the new diffractometer, taking into account previous experience in these instruments.
As the funds available from GPSDI were not enough to start construction, we secured additional funding before construction could begin. Procuring the parts and building the instrument took longer than expected, extending into most of the second part of the project. The new apparatus started with the vacuum system and was gradually equipped with a time-resolved electron gun featuring a LaB6 and a metal photocathode, a position-sensitive imaging detector and a nozzle source. A high repetition rate ultrafast laser necessary for the production of the short electron pulses was ordered, delivered and installed. Looking into the future, we invested in an inverted microscope with fluorescence imaging capabilities to combine TRED with imaging of biological samples. The team of the project expanded to 7 people. The result of all those efforts was a state of the art TRED instrument, however we run out of time before completing the diffraction experimental work initially envisioned.
Despite the delay, the project produced a significant amount of scientific knowledge, summarized in eight (8) peer-reviewed publications and presented in four conferences. Using traditional dynamics techniques such as Velocity Map Imaging (VMI) and Slice Imaging (SLIM), we explored the photolysis dynamics of CH3Br in monomer and inside Xe clusters and expanded into a number of benchmark molecules: CH3I, CH3NO2, ClN3, HBr. We found that weak bonding can indeed affect photochemistry significantly, through either stereochemistry or alterations to the Potential Energy Surface. These effects can provide a way of “controlling” chemical reactivity that deserves to be explored further in additional experiments.
In collaboration with a French company, pioneer in the field of ellipsometry, we explored the possibility to combine ellipsometry with electron diffraction, one providing spectroscopic and the other structural information on the same system. This effort led to the development of Multi-Pass Spectroscopic Ellipsometry (MPSE), a new ellipsometric method that improved the resolution afforded by the technique. In addition, we collaborated with an UK company on developing ultrafast electron pulse sources. These two examples demonstrate once again that curiosity-based basic scientific research can also produce technological advances.
Several collaborations started as a result of this project. Besides companies in France and the UK, the researcher worked and co-produced publications with research groups in France (Toulouse/Bordeaux), Spain (Madrid), Holland (Nijmegen), Iceland (Reykjavik), USA (Berkeley) as well as FORTH.
GPSDI, as a reintegration grant, aimed at re-integrating the researcher into the Greek and European scientific environment at the same time taking advantage of his scientific expertise, obtained during a several year stay in USA. This project was instrumental in initiating a new TRED activity at FORTH which -to a large extent- achieved the reintegration goal. This activity in a high-tech, cutting edge field is one of the few in Europe. Combined with additional funding secured by the researcher, the project trained 6 students and postdocs in developing and using high-tech, state of the art equipment, thus promoting the "economy of knowledge". It also promoted scientific excellence and technological innovation by investigating significant scientific problems with innovative approaches and developing new technological methods and tools. For all the above reasons we consider it to be a success.