Ultrafast Soft X-ray Probing of Gas Phase Molecular Dynamics

The molecular ability to selectively and efficiently convert sunlight into other forms of energy like heat, bond change, or charge separation is truly remarkable. The decisive steps in these transformations often happen on a femtosecond timescale and require transitions among different electronic states that violate the Born-Oppenheimer approximation (BOA) 1 W. Domcke, D. Yarkony, and H. Köppel, Conical Intersections Electronic Structure, Dynamics &; Spectroscopy, World Scientific, Singapore (2004).Crossref] , Google Scholar]]. Non-BOA transitions pose challenges to both theory and experiment. From a theoretical point of view, excited state dynamics and nonadiabatic transitions both are difficult problems 2 F. Plasser, Theor. Chem. Acc. 131, 233902 (2012).Crossref], Web of Science ®] , Google Scholar], 3 J. C. Tully, J. Chem. Phys. 137, 22A301 (2012).Crossref], PubMed], Web of Science ®] , Google Scholar]] (see Figure 1(a)). However, the theory on non-BOA dynamics has advanced significantly over the last two decades. Full dynamical simulations for molecules of the size of nucleobases have been possible for a couple of years 4 H. R. Hudock, J. Phys. Chem. A 111, 8500–8508 (2007).Crossref], PubMed], Web of Science ®] , Google Scholar], 5 J. J. Szymczak, J. Phys. Chem. A 113, 12686–12693 (2009).Crossref], PubMed], Web of Science ®] , Google Scholar]] and allow predictions of experimental observables like photoelectron energy 6 S. Ullrich, Phys. Chem. Chem. Phys. 6, 2796 (2004).Crossref], Web of Science ®] , Google Scholar]] or ion yield 7 H. Tao, J. Chem. Phys. 134, 244306 (2011).Crossref], PubMed], Web of Science ®] , Google Scholar]–9 M. Assmann, T. Weinacht, and S. Matsika, J. Chem. Phys. 144, 34301 (2016).Crossref], Web of Science ®] , Google Scholar]]. The availability of these calculations for isolated molecules has spurred new experimental efforts to develop methods that are sufficiently different from all optical techniques. For determination of transient molecular structure, femtosecond X-ray diffraction 10 M. P. Minitti, Phys. Rev. Lett. 114, 255501 (2015).Crossref], PubMed], Web of Science ®] , Google Scholar], 11 J. Küpper, Phys. Rev. Lett. 112, 083002 (2014). Google Scholar]] and electron diffraction 12 J. Yang, Nat. Commun. 7, 11232 (2016).Crossref], PubMed], Web of Science ®] , Google Scholar]] have been implemented on optically excited molecules.