Physikalische Chemie - Direktor: Prof. Dr. Martin Wolf
Informal Seminar
Host: R. Ernstorfer

Tuesday, December 17, 2019, 4:00 pm
Room G 1.12
Hung-Tzu Chang
University of California
Electron Dynamics in Solids with Transient Absorption Spectroscopy in Extreme Ultraviolet
Understanding the dynamics of photoexcited carriers in solids is vital to the development of optoelectronics. Here I will present our studies of electron dynamics in solids using the optical pump – extreme ultraviolet (XUV) probe method, where snapshots of the dynamics of valence electrons are taken with the core-level absorption of sub-femtosecond long XUV pulses. In this talk, I will discuss our studies on the thermalization of electrons in nickel and dynamics of core-excitons in few-layer MoS2. In the study of electron thermalization dynamics in nickel, I will discuss the measurement of electron temperature using core-level absorption spectroscopy and the build-up of thermalized electrons after photoexcitation. In the study of MoS2, a two-dimensional material, I will show that the Mo N2,3 edge absorption spectrum exhibits an approximately 3 eV red-shift as the material thickness is reduced from bulk to few-layer, suggesting the presence of strong excitonic effect and that quantum confinement and dielectric screening affect not only the optical properties of MoS2 but also the core-level absorption. With XUV transient absorption spectroscopy, an order of magnitude increase of transition lifetime from sub-femtosecond to 3-5 fs during the reduction of material thickness is identified and the extent of the excitonic wavefunctions estimated to be approximately 2 Å, agreeing well with results of ab initio simulations on core-excitons of monolayer MoS2.