Soft x-ray probing of ultrafast chemical dynamics |
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| Markus Guehr | |
| SLAC National Accelerator Laboratory, Menlo Park | |
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After presenting a short summary of the possibilities for understanding chemical processes given by x-ray lasers, I will concentrate on the specific example of nucleobase photoprotection.
Although nucleobases absorb strongly in the near ultraviolet region transmitted by the Earthís atmosphere, the UV excitation surprisingly does not lead to photoinduced chemistry or damage of the base molecules. Theory and previous experiments indicate that the photoprotection of the nucleobases proceeds via fast (femtoseconds to picoseconds) non-adiabatic transitions. Even for isolated nucleobases the understanding of the non-Born-Oppenheimer approximation (non-BOA) dynamics is currently controversial. Past experiments rely heavily on simulations to interpret the observed timescales and depending on the level of ab-initio approximations, different timescales are predicted. To shed new light on the photoprotection mechanism, we conducted an experiment at the Linac Coherent Light Source (LCLS) free electron laser to get a more direct insight into the transient valence occupation during photoprotection via element sensitive Auger spectroscopy. I will present the LCLS data together with dynamic Auger simulations and discuss the implications for the current understanding of nucleobase photoprotection. |