Center for Functional Nanomaterials Seminar

The multitude of possible processes that can occur at surfaces cover many orders of magnitude in the time domain. While large scale growth and structure formation, for instance, happens on a timescale of minutes and seconds, diffusion is already much faster, but can still be observed by PEEM or LEEM. Many other processes as chemical reactions, phonon dynamics, nanoscale heat transport, or phase transitions, however, take place on the femto- and picosecond timescale and are yet way to fast for imaging techniques.
In order to study such ultrafast processes at surfaces we have combined modern surface science techniques with fs laser pulses in a pump probe scheme. We use a reflection high energy electron diffraction (RHEED) setup with grazing incident electrons of 7 - 30 keV to ensure surface sensitivity [1,2]. Utilizing the Debye Waller effect the cooling of vibrational excitations in monolayer adsorbate systems or the nanoscale heat transport through a heterofilm interface is studied on the lower ps-time scale [3-5]: the cooling of ultrathin Bi(111) films on Si(001) is dominated by a pronounced non-equilibrium distribution in the phonon system resulting in a strongly reduced cooling rate.
In order to demonstrate the huge potential of this technique I will shortly present examples for the dynamics of strongly driven structural phase transitions at surfaces upon excitation with a fs-laser pulse: the famous order-disorder phase transition from c(4x2) to (2x1) on Si(001) at 200 K and the Indium induced Peierls-like transition from c(8x2) to (4x1) on Si(111) at 80 K which is additionally accompanied by the formation of a charge density wave [6].