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Surface Diffusion of Adsorbates

Fs-laser Induced Diffusion of O on Pt(111)

shg_scheme2.gif Diffusion of adsorbates is an important elementary step of many surface processes such as epitaxial growth or catalytic reactions. Usually, surface diffusion is a thermally activated process that is initiated by heating the substrate. In some cases it would be desirable to enable diffusion at a lower temperature where competing surface reactions have not yet set in. For this and other purposes one would like to induce diffusion by electronic instead of thermal excitation similar to the well studied phenomena of desorption induced by(multiple) electronic transitions (DI(M)ET). We have studied electronically induced diffusion of atomic oxygen on a vicinal Pt(111) surface using ultrashort laser pulses of near-infrared light for the generation of a hot electron distribution at the surface. Diffusion from the step edges onto the terraces have been monitored by exploiting the sensitivity of optical second-harmonic generation (SHG) on surface symmetry which is macroscopically broken by regular steps. More...

 

 

Thermally Activated Diffusion of H on Si(001)

thermal_hsi.gif Steps on a surface can have a strong impact on adsorbate diffusion since adsorbates have not only different binding energies at steps compared to terrace sites but steps also imply the existence of new microscopy diffusion pathways which can considerably affect macroscopic diffusion rates. This has been studied in detail for the thermal activated diffusion of hydrogen at D-step sites on a vicinal Si(001)-surface. We have determined microscopic diffusion rates and barriers by following the individual lateral motion of H atoms with an STM at variable temperature in real time. Surprisingly, the jump rate from the step site to the upper terrace is found to be much larger than the diffusion rate to the lower terrace. This preference, opposite to the trend for the respective binding energies, is explained by first-principles calculations that identify a meta-stable intermediate state responsible for lowering of the energy barrier for upward diffusion. More...


 

ns-Laser Induced Diffusion of H on flat Si(001)

laser_hsi.gif The rearrangement of silicon dangling bonds following pulsed laser heating of monohydride covered Si(001) surfaces has been studied with scanning tunneling microscopy (STM). Laser-induced thermal desorption (LITD) of small amounts of H2 via the interdimer pathway leads to the creation of isolated pairs of dangling bonds at two adjacent dimers. Hydrogen diffusion causes this arrangement of dangling bonds - which represents an excited state of the surface - to change quickly into the equilibrium configuration consisting of dangling bonds paired up at a single dimer. By using multiple nanosecond heating pulses we were able to freeze the surface at various stages of the equilibration process and take snapshots with the STM. In this way we were able to monitor hydrogen diffusion processes with atomic resolution which are associated with rates as high as 108 s-1. More...

Zuletzt aktualisiert: 14.04.2015 · armbrusn

 
 
 
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