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XUV/X-ray lasers for ultrafast electronic control in chemistry

Final Report Summary - XCHEM (XUV/X-ray lasers for ultrafast electronic control in chemistry)

Advances in generating controlled few-cycle laser pulses and novel ultrashort XUV/X-ray sources, from free electron laser (FEL)-based to attosecond high harmonic generation (HHG)-based, have opened completely new avenues for imaging electronic and nuclear dynamics in molecules, with exciting applications in physics, chemistry and biology. Processes such as ionization and dissociation of simple diatomic molecules can now be monitored in real time, but the access to (few-femtosecond) attosecond time scales in the XUV/X-ray domain may also allow one to uncover and control the dynamics of elementary chemical processes such as, e.g. ultrafast charge migration, proton transfer, isomerization or multiple ionization, and to address new key questions about the role of attosecond coherent electron dynamics in chemical reactivity. The success of current experimental efforts in explaining these phenomena, present in many biological processes, is seriously limited due to the difficulty in their interpretation. In this respect, theoretical calculations carried out in supercomputers are very valuable to guide experimental research. Such theoretical methods must necessarily lie outside the traditional quantum chemistry realm since, e.g. they must accurately reproduce the time evolution of the coupled electronic and nuclear motions in the electronic and dissociative continua, including electron correlation and non-adiabatic effects. The necessary extension to systems of chemical interest, the current bottleneck in this field, requires extensive and novel theoretical developments along a similar direction.
In this project, we have developed new methods that allow us to study electronic and coupled electronic-nuclear dynamics in simple and complex molecules generated by attosecond and few-femtosecond laser pulses or a combination of them. This has allowed us to develop new concepts and interpret a new generation of ultrafast time-resolved experiments. In particular, we have demonstrated the suitability of both UV-pump/IR-probe and UV-pump/UV-probe schemes to image and control the coupled electron and nuclear dynamics in diatomic molecules. In the first case, attosecond transient absorption spectroscopy has revealed as a promising simple tool to fully characterize this complex dynamics in molecules. We have also shown that, at variance with pump-probe schemes involving IR pulses that strongly perturb the molecule, UV-pump/UV-probe strategies allow one to image and control the intrinsic, unperturbed molecular dynamics. Such an approach should be soon accessible by using pulses generated in free electron lasers and/or high harmonic generation. We have also shown that the combination of RABITT measurements with theoretical calculations is a very powerful tool to reconstruct the electronic wave packet generated by attosecond pulses and eventually to control it.
We have also developed new theoretical methods to describe photoionization of more complex diatomic and polyatomic molecules, such as CO, F2, N2, BF3, CH4, CF4 and SF6. We have found that, at relatively high photon energies, vibrationally resolved photoelectron spectra carry the signature of intra-molecular scattering and Young’s double slit interferences, which encode structural information on the molecule. Extension of this methodology to the time domain, e.g. to study ionization induced by attosecond pulses, has allowed us to investigate charge migration induced by attosecond pulses in the aminoacids glycine and phenylalanine. Our results show that charge migration in such large molecules is extremely fast (less than 5 fs) and is entirely due to electron dynamics without a significant participation of the nuclear motion. Electron transfer is a fundamental process in biochemistry since it triggers the first crucial steps in a number of biochemical processes such as photosynthesis, cellular respiration and electron transport along DNA. Therefore, this result can open new perspectives in the understanding of the physical processes governing electron transport in biological phenomena.