Ultrafast soft x-ray spectroscopy of
molecular dynamics

Markus Guehr

Mo, 16 May 2016, at 10:00. Aula Querzoli

pdf of the presentation

Nearly all sources of energy, from wind over fossil to solar photovoltaic, rely on the sun as the primary energy source. This photon energy is highly abundant but it takes very sophisticated systems to funnel it into particular and useful forms of energy like chemical energy, electric current or heat.

Photo energy conversion starts with light absorption to the molecular valence electrons, which provide the binding forces between the positively charged atomic cores. In the ground state, all forces among nuclei and electrons are balanced. Photoabsorption leads to a coupled motion of nuclei and electrons, commonly presented in a potential energy scheme depicting different electronic states as a function of the multidimensional nuclear geometry. It is that motion which determines the exact conversion mechanism. The common approach to simplify molecular dynamics is the Born-Oppenheimer approximation (BOA) allowing for the separation of electronic and nuclear degrees of freedom. Over the recent two decades ultrafast spectroscopy has discovered the breakdown of the BOA in many important optically excited molecules and new experimental methods are needed to understand the interplay of electronic and nuclear coordinates in energy conversion.

Over the past couple of years, we have been witnessing novel experiments with soft x-ray pulses (SXR, above the C K-edge at 280 eV) to provide an alternative view on molecular dynamics. The exciting property of SXR light is the spectroscopic element and site sensitivity. Soft x-ray induced core-to-valence or core-to-continuum transitions address spatially narrow core electronic wavefunctions with large binding energy changes depending on the element. Even the distinction of equal elements in different valence bonds is possible using site specific chemical shifts. Ultrafast soft x-ray pulses can thus be used as a probe of the transient nuclear and electronic dynamics confined to well defined positions within the molecule. This lecture will give a overview of the molecular basics behind photoenergy conversion and discuss recent result from SXR probing using free electron lasers.

[1] M. Guehr "Probing Molecular Photoexcited Dynamics by Soft X-Rays" in: Ultrafast Dynamics Driven by Intense Light Pulses, eds.: M. Kitzler S. Graefe (Springer, Heidelberg, 2016)