Surface Diffusion of Adsorbates
contact: Prof. Dr. Jens Güdde, Prof. Dr. Ulrich Höfer
Thermally Activated Diffusion of H on Si(001)
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.
- M. Lawrenz, P. Kratzer, C. H. Schwalb, M. Dürr and U. Höfer
Diffusion pathways of hydrogen across the steps of a vicinal Si(001) surface
Phys. Rev. B 75, 125424 (2007).
ns-Laser Induced Diffusion of H on flat Si(001)
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.
- C. H. Schwalb, M. Dürr, and U. Höfer
Real-space investigation of high-barrier H-diffusion across the dimer rows of Si(001)
Phys. Rev. B 80, 085317 (2009).
- C. H. Schwalb, M. Lawrenz, M. Dürr, and U. Höfer
Real-space investigation of fast diffusion processes of hydrogen on Si(001) by combination of nanosecond laser heating and STM
Phys. Rev. B 75, 085439 (2007).
fs-laser Induced Diffusion of O on Pt(111)
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.
Idea of the Measurement
The principle of this method is sketched in the figure. First, a non-equilibrium distribution of adsorbate atoms (O) is produced by decorating step edges of a vicinal surface (Pt(111)) selectively by dissociative adsorption. Then, diffusion from the step edges onto the terraces is induced by femtosecond laser pulses. The depopulation of the step sites is monitored by optical second-harmonic generation (SHG). SHG is an optical probe of electronic, structural, and magnetic properties of surfaces and interfaces that owes part of its sensitivity to the occurrence of a symmetry break at surfaces. Since the presence of regular steps on a vicinal surface breaks the symmetry parallel to the surface, step sites can be a very efficient source of SHG. The capability of SHG to monitor step coverage in situ and in real time makes it possible to determine diffusion rates as a function of laser fluence and delay between two pump pulses.
The optical setup of the second-harmonic diffraction pump and probe experiment is shown in figure left. Laser pulses of 50-fs duration at 800 nm were produced by a Ti:sapphire amplifier system with 1kHz repetition rate. The original beam was separated into two parts, a weak probe beam and a strong pump beam with the help of a 5%-95% beam splitter. The spatial overlap of the combined pump and probe beams was adjusted under observation of the sample through an UHV-viewport with an near IR-sensitive CCDcamera equipped with a large-working distance macro objective. The temporal overlap of the two pump beams was set when the two pump beams were both p polarized. The temporal and spatial overlap of the two beams can be controlled by the observation of interference fringes.
The weak p-polarized probe beam was focused on the sample at an angle of incidence of 45° by a f=250 mm fused silica lens down to 90 μm in diameter resulting in an absorbed fluence of about 0.5 mJ/cm2. The polarization of the reflected second-harmonic (SH) light was analyzed by a Glan-Taylor prism. The p-polarized SH radiation was separated from the reflected fundamental beam by a combination of 400-nm dielectric bending-mirrors and a Schott BG39 color filter. A photomultiplier was combinated with a 300 MHz Preamplifier and with a boxcar averager for detection.
The p-polarized pump beam was incident on the surface at 40° off the normal and slightly focused to produce a Gaussian spot with 0.94 mm diameter (FWHM). The intensity was varied by a combination of a half-wave plate and a thin-film polarizer and was measured by a power meter. For two-pulse correlation measurements, the pump beam was split into two beams with the help of an interferometer. The two beams were orthogonally polarized using a zero order half-wave plate in one arm of the interferometer. The intensity of the two pump beams were chosen in a way that the adsorbed fluence of the s and p polarized beams on the platinum sample were nearly equal. Between the two splitted pump pulses a variable time delay could be set: the length of one arm of the interferometer can be changed with two mirrors fixed on a linear positioning stage. The stage was driven by a motion controller. Behind the interferometer the two pump beams were combined collinearly. A Schott RG715 filter was used to block possible contributions at short wavelengths in order to exclude any direct induced photoreactions, which were observed for molecular oxygen. The absorbed fluence was calculated using the optical constants of platinum and assuming a constant.
Chemisorption of atomic oxygen at the steps leads to a strong reduction of the SHG signal until the steps are saturated. For the study of laser-induced diffusion we cool the sample down to 80 K, where oxygen is immobile even on the terraces, and irradiate the sample with femtosecond laser pulses. We observe a continuous recovery of the SHG signal for pulses that exceed an absorbed fluence of 3,5 mJ/cm2. The recovery of the SHG signal is due to the depletion of the step sites by oxygen diffusion onto the terraces and not due to desorption.
The measured hopping probability pdiff depends in a strongly nonlinear way on the pump fluence Fabs. It increases by more than 2 orders of magnitude in the fluence range 3,8 mJ=cm2 to 6,0 mJ/cm2 and can be described by a power law of the form pdiff ∝ F15.
The nonlinear dependence of pdiff on laser fluence enables the application of a two-pulse correlation scheme. For these experiments the pump pulse is split and the step depletion rate is determined as a function of the time delay between the two pulses. The two-pulse correlation has a high contrast between pdiff at zero and large delays, which is related to the high nonlinearity of the fluence dependence. The width of 1.45 ps (FWHM) is much larger than the cross correlation of the two laser pulses and has the value of a typical electron-phonon coupling time. This unambiguously shows that diffusion is driven by the laser-excited electrons of the metallic substrate. The fluence dependence and the two-pulse correlation cannot be described simultaneously with a constant ηe. Reasonable agreement with the experimental data can be achieved if we assume an empirical dependence of ηe on electron temperature, ηe(Te)= ηe0Te2 with 1.8×105 K-2 ps-1.
Indirect Excitation Mechanism
Since the electronic structure of O on Pt(111) does not support a temperature dependence of the electronic friction coefficient, we suggest an indirect excitation mechanism for the pathway of the energy transfer to the diffusive motion. This is more complicated than a direct coupling of the electronic excitation to the frustrated translation mode. This excitation mechanism requires an effective dependence of ηe on electron temperature within in the description of the friction model. It assumes a primary electronic excitation of the O-Pt stretch vibration, which indirectly excites the frustrated translation via anharmonic coupling. The energy of the stretch vibration (60 meV) is only slightly higher than the energy of frustrated O-Pt translations (50 meV). Therefore, many quanta of the stretch vibration mode need to be excited by repetitive electronic excitation cycles before the vibrational motion can couple efficiently to the lateral mode and before the diffusion barrier of 0.8 eV can be overcome.
- K. Klass, G. Mette, J. Güdde, M. Dürr, and U. Höfer
Second-harmonic microscopy for fluence-dependent investigation of laser-induced surface reactions
Phys. Rev. B 83, 125116 (2011).
- M. Lawrenz, K. Stépán, J. Güdde, and U. Höfer
Time-domain investigation of laser-induced diffusion of CO on a vicinal Pt(111)-surface
Phys. Rev. B 80, 075429 (2009).
- J. Güdde and U. Höfer
Dynamics of femtosecond-laser-induced lateral motion of an adsorbate: O on vicinal Pt(111)
J. Phys.: Condens. Matter 18, S1409 (2006).
- K. Stépán, M. Dürr, J. Güdde, and U. Höfer
Laser-induced diffusion of oxygen on a stepped Pt(111) surface
Surf. Sci. 593, 54-66 (2005).
- K. Stépán, J. Güdde, and U. Höfer
Time-Resolved Measurement of Surface Diffusion Induced by Femtosecond Laser Pulses
Phys. Rev. Lett. 94, 236103 (2005).