Investigating Surface Femtochemistry with combined fs-laser LT-STM
The working principle of our combined fs-laser low temperature STM is illustrated above. First of all molecules adsorped on a metal surface are scanned by the STM, to indentify the molecules and their absorption sites (a). In the second step ultrashort laser pulses are given to generate hot electrons in the metal substrate. During the illumination the STM tip is retracted avoiding near and far field tip effects (b). The energy of the hot electrons is transferred to molecular vibrations due to non-adiabatic coupling. Because of the high density of hot electrons, the molecule gains enough energy to overcome energy barriers for surface processes such as diffusion or dissociation shown here. These processes are verified by scanning the same sample region again with the STM (c). The measured reaction yield depending on absorbed fluence and pulse-pulse-delay opens a fascinating look inside mechanisms and dynamics of elementary steps of surface reactions.
So far most of the fs-laser experiments investigating surface femtochemistry gives an integrated information about all molecules on the surface. In our set-up the STM is used to resolve processes on a single molecule scale in real space for example single molecules on terraces, at step edges, at close-by defects or in the proximity of other molecules.
For technical implementation see Mehlhorn et. al, Rev. Sci. Instr. 78 (2007) 033908.
For more information about surface femtochemistry please refer to the homepage of AG Wolf, FU Berlin and see also Frischkorn and Wolf, Chem. Rev. 106 (2006) 4207.
Scanning Tunneling Microscope
Our measurements are performed with a custom-build low-temperature scanning tunneling microscope (LT-STM) of the Besocke-type [Surf. Sci. 181 (1998), 148] providing submolecular and atomic resolution. It is based on a design of G. Meyer [Rec. Sci. Instrum. 67 (1996), 2960]. For our measurements with the combined laser-setup the STM is modified for optical access [Dissertation M.Mehlhorn, 2005]. We are using the femtosecond laser scientific xl (Femtolasers) to excite the "hot electrons" we need for our experiments. This image shows a schematic view of the modified Besocke STM-Scanner. The image to the left shows the complete STM-head. The ramp with the STM-tip is at the top. The golden part with the four electric contacs is the sample holder. The larger plate at the bottom is the baseplate. A eddycurrent damping serves for better stability (the other part of the damping unit is in the helium-cryostat to which the STM is pressed for cooling. On the image to the right you can see the STM-scanner while it is lighted with the HeNe-Laser used to adjust the laser to the position of the STM-tip.