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Time resolved superexcited state dynamics

Final Report Summary - EXTREME DYNAMICS (Time resolved superexcited state dynamics)

Understanding ultrafast molecular dynamics that govern the evolution of excited systems allows modeling, predicting and in some cases controlling the outcome of chemical reactions. Using femtosecond (fs) laser pulses to initiate and probe dynamics, we provide insight into the important quantum mechanical mechanisms that control molecular dynamics. However, range of excited dynamics that can be addressed is limited by the wavelengths of readily available laser systems.
Specifically, superexcited states (SES), which are states that lie above the threshold for electron emission, cannot be initiated directly from neutral molecular ground states with conventional laser systems. Such superexcited states exhibit extremely non Born-Oppenheimer dynamics, as electron and nuclear motion dynamics compete on ultrafast timescales, often leading to fragmentation rather than electron emission. The objective of the “Extreme Dynamics” project is to extend the range of
investigated ultrafast phenomena with conventional fs lasers by initiating dynamics not from neutral ground state molecules but from fast beams of negative ions or metastable neutral species that are energetically closer to superexcited states. Fast beam fragment imaging techniques are implemented to record time resolved kinetic energies and correlations of fragmentation products.
A custom made state of the art ultrafast amplified laser system with pulse duration of as low as 25fs and up to 4mJ pulse energy was purchased with funding obtained by D.S. from a separate grant. The laser system was successfully installed and equipped with a dedicated pulse shaper that allows controlling and optimizing the spectral phase of the ultrafast pulses. During the delays in the final laser system installation, we developed in collaboration with the Zajfman group, a new non-destructive method for characterization of trapped fast ion beams that can be valuable for future extensions of the "Extreme dynamics" project. In parallel, a fast ion beam setup was constructed and tested. The new setup allows efficient production of cold molecular and cluster anions of interest on a regular basis and interrogation of their interactions with ultrafast intense laser pulses. A fragment imaging setup that uses a Micro-Channel Plate (MCP) detector was integrated it into our experimental setup with a combination of fast CCD that can read out the 2D positions of fragment hits on the detector and a high resolution time to digital converter (TDC) that records the time of fragment impact on the detector. Thus a 3D data set is formed that will allow to measure separately the different fragmentation channels. A dedicated interaction region spectrometer was designed and constructed in our machine shop to allow reliable laser – ion beam alignment and separation of products with different charge over mass ratios based on their time of flight to the MCP detector as measured by the TDC.
While exploring the products of the non linear interaction of ultrafast intense laser pulses with molecular anions we observed an extremely efficient process of multiple detachment, in which two or more electrons are ejected from the parent anion molecule and produce cationic products. We performed extensive characterization measurements of the multiple detachment mechanism in a model SF6- system, using the full range of our now available capabilities. Based on pulse shaping experiments we found that although there is a large aspect ratio between the 1st and 2nd binding energies of molecular anions, a surprisingly efficient non-sequential mechanism is responsible for the multiple detachment by the intense laser pulse. Furthermore, by exploring the effect of laser polarization ellipticity we were able to show that the non-sequential mechanism for multiple detachment is different from the well established double ionization mechanism of neutral systems that relies on rescattering of the 1st electron from the parent system to liberate a 2nd electron. Based on the successful experiments with the SF6- model system we plan to continue this work and explore the intense field multiple detachment process in other loosely bound systems.

Summary of activities towards achieving project goals:
• A dedicated lab was constructed, combining ultrafast intense laser technologies and fast ion beam methods for photofragment spectroscopy.
M.Sc. students and a postdoc were recruited trained in cutting edge experimental techniques.
• Knowledge transfer activities include a new advanced course given at the host institution entitled “Introduction to ultrafast phenomena”, in addition to workshops and seminars given at other institutions in Israel (Tel Aviv, Bar-Ilan and Ben-Gurion Universities).
• In collaboration with the Zajfman group, a non-destructive and mass selective measurement of trapped fast ion beams was developed and described in a recent publication “Lifetime measurements in an electrostatic ion beam trap using image charge monitoring”, I. Rahinov, Y. Toker, O. Heber, D. Strasser, M. Rappaport, D. Schwalm, and D. Zajfman. Rev. Sci. Instrum. 83, 033302 (2012).
• Detailed investigations of the interaction of intense laser pulses with molecular anions revealed a non-sequential mechanism for multiple detachment, which is not based on recollision dynamics that dominate double ionization of neutral species as described in Albeck, Y , Kandhasamy, DM , Strasser, D. J. Phys.Chem. A. 118, 388 (2014)