Professor Dr. Martin Wolf, Fritz Haber Institute of the MPG, Deartment. of Physical Chemistry, Berlin, Germany

Ultrafast Dynamics of Photoinduced Surface Reactions and Phase Transitions in Solids

Electronic excitations at surfaces and in solids can lead to rich variety of processes, which often include transfer of energy of the electronic system into nuclear motion. Femtosecond laser excitation of metals or more complex materials results in a highly non-equilibrium electron distribution, which may induce chemical reactions of adsorbed molecules (surface femtochemistry) or lead to displacive excita­tion of coherent phonons or ultrafast phase transitions in solids. Both types of processes occur on ultrafast (femto- to pico­second) timescales and are accompanied by pronounced changes of the electronic structure.

In this talk, I will discuss different experimental approaches to probe such tran­sient electronic structure changes on ultrafast timescales by employing (i) time-resolved resonant inelastic x-ray scattering (trRIXS) and (ii) time- and angle-resolved photoelectron spectroscopy (trARPES). Experiments performed with trRIXS at the X-ray free electron laser LCLS provide direct insight into the changes of the chemical bonds after photoexcitation (like  CO or O on Ru(001)), leading to a pronounced bond weakening and CO desorption or oxidation. These studies provide new insights into dynamics and details of the potential energy landscape.

Furthermore, we have investigated the mechanism of the photoinduced insulator-to-metal transition in a prototypical charge-density wave (CDW) system (RTe₃, R=Te, Dy). In CDW materials a periodic lattice distortion leads to the opening of an electronic gap at the Fermi surface at low temperatures. trARPES allows for probing directly the transient evolution of the electronic structure and collective phonon dynamics after photoexcitation through their influence on the electronic band structure. This enables a systematic study of the opening and closing of the CDW gap and the dynamics of the CDW amplitude mode during the photo-induced non-equilibrium phase transition.

Host: Professor Philip Hofmann, Department of Physics and Astronomy & iNANO, Aarhus University