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Simulation of a diffusion
from a coverage grating
repulsive interactions
The
Diffusion of Gold Nanoclusters on the Ru(100) and the p(1*2)-Oxidized
Phase Gabriel Kerner, Yonatan Horovitz and Micha Asscher* Dept. of Physical Chemistry, The
Farkas Center for Light induced processes,
The Hebrew COVERAGE GRATING TEMPLATE FOR THE STUDY OF SURFACE DIFFUSION: K COADSORBED WITH CO ON Re(001) W. Zhao and M. Asscher Department of Physical Chemistry and the Farkas Center for Light Induced Processes The Hebrew University, Jerusalem 91904, Israel Coverage grating method as a template for coadsorbates is introduced for the study of surface diffusion. The effect of coadsorbed CO inside coverage troughs formed by laser induced thermal desorption (LITD), on potassium surface diffusion on Re(001) has been investigated using the coverage grating - optical second harmonic diffraction method. Enhancement of the first order diffraction peak at a certain CO coadsorption coverage, observed for the first time, demonstrates the very strong electronic interaction between these coadsorbates. The activation energy for K surface diffusion and the preexponential factor significantly increase with CO coverage. At initial potassium coverage of 1.0 ML, the activation energy for potassium diffusion increases from 5.0 kcal/mole on the clean rhenium surface up to 15.0 kcal/mole in the presence of 0.065 ML CO and the preexponential factor increases from 5.6x10-3 to 2.0x102 cm2/sec. The activation energy doubles for potassium coverages of 0.9 ML and 0.8 ML as a result of the same CO coverage. TPD measurements indicate that strong attractive interactions exist between CO and K on Re(001). Coadsorbed CO and K stabilize each other while forming Kx-CO surface complexes which slow down the potassium surface diffusion predominantly by site-blocking effect. This site-blocking effect increases as the number of potassium atoms (x) interacting with a single CO coadsorbate increases at higher initial K coverages. The nature of this site blocking is discussed.
INTERACTION AND DIFFUSION OF POTASSIUM ON Cr2O3(0001)/Cr(110)
a- Department of Physical Chemistry and the Farkas Center for
Light Induced Processes,
The interaction of potassium atoms on top of Cr2O3(0001)/Cr(110) has been studied using work function, TPD and optical second harmonic generation (SHG) measurements. Upon adsorption at temperatures in the range 100 -300K, the potassium atoms grow by a uniform first monolayer, followed by a three dimensional islands at higher coverages, as indicated from work function and SHG data. SHG signal measured during the desorption of potassium, goes through three peaks which identify 3D islands, second layer potassium atoms and the complete first monolayer. The diffusion of alkali metal atoms on the surface of an oxide, is reported here for the first time. This was performed using optical SH diffraction technique from laser induced desorption coverage grating. The activation energy for surface diffusion has been determined to be 11 kcal/mol with a preexponential factor D0 = 105 (cm2 / sec) at potassium coverage of 1ML. From the size of the barrier for diffusion it is concluded
that the diffusion of partially ionized potassium ad-atoms is
monitored. The degree of charge transfer from potassium to the surface
has been calculated to be 0.6± 0.1 of
unit charge by employing an extensive electrostatic model which
considers a semi-infinite oxide lattice. COVERAGE DEPENDENT DIFFUSION OF AMMONIA ON Re(001) Z. Rosenzweig, I.Farbman and M.Asscher Department of Physical Chemistry and the Farkas Center for Light Induced Processes The Hebrew University, Jerusalem 91904, Israel Diffusion of ammonia was studied over the smooth Re(001) surface utilizing optical second harmonic diffraction from surface coverage grating. The decay of the first order diffraction peak as a function of surface temperature for different initial coverages was measured and then simulated by numerical solution of Fick's second diffusion equation, employing a coverage dependent diffusion constant. For the first time the second order diffracted second harmonic signal was monitored during the diffusion process, supporting the calculated initial coverage profile and the diffusion model. The resulting diffusivity D(theta)= D0exp[-Em(theta)/RT], is defined by a barrier for diffusion Em(theta)=E0- Z , with activation energy at zero coverage E0=3.4±0.6 kcal/mol, D0=2.8ª10-3 cm2sec-1. The repulsion energy between a pair of nearest neighbor molecules =0.4±0.06 kcal/mol is obtained if an average of three nearest neighbors is assumed at the edge of the grating. The coverage effect on the diffusivity was found to correlate quantitatively with dipole-dipole repulsion ( = 2/r3) between nearest neighbors. A coverage independent "corrugation ratio" Ediff/Edes = 0.22±0.1 is deduced from the above diffusion model.
EFFECT OF LATERAL INTERACTIONS ON COVERAGE GRATING
FORMATION ON SURFACES R. W. Verhoef and M. Asscher Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University, Jerusalem 91904, Israel The effect of lateral interactions among surface adsorbates on the formation of coverage gratings by laser induced thermal desorption (LITD) has been simulated. It was found that measurements of diffracted second harmonic generation (SHG) signals from these gratings as a function of laser power and initial surface coverage can give information on the type and strength of interactions among adsorbates on the surface, i.e., attractive, repulsive, or none. The ratios of the second to the first order diffracted SH signals were found to increase with initial coverage for attractive interactions and to decrease for repulsive interactions.
DIFFUSION ON SURFACES STUDIED BY LASER DIFFRACTION: EFFECT OF ADSORBATE LATERAL INTERACTIONS R. W. Verhoef and M. Asscher Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University, Jerusalem 91904, Israel The importance of lateral interactions in surface diffusion is discussed by means of optical second harmonic diffraction from monolayer grating using model simulations. Measurements of high order diffraction signals during surface diffusion at various laser powers and initial surface coverages provide information on the type of interactions between adsorbates on the surface, i.e., attractive, repulsive, or none. Strong interactions between surface adsorbates are predicted to show unusual behavior of the higher order (n>1) diffraction signals, with the signal strength sometimes oscillating with time during diffusion. These results are applicable for linear diffraction as well.
DIFFUSION OF K ON Re(001) W. Zhao, R.W. Verhoef and M. Asscher Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University, Jerusalem 91904, Israel Using the coverage grating-optical second-harmonic diffraction
method, we have measured the diffusion rate of potassium on Re(001) in
the coverage range of 0.6-1 ML. We observe a minimum in both the
activation energy for diffusion and the diffusion preexponential at an
initial potassium coverage of 0.75 ML. For potassium coverages of 0.93,
0.84, 0.73, and 0.59 ML, the activation energies for diffusion are
determined to be 5.0 ± 0.2, 3.9 ± 0.7, 3.6 ±
0.2, and 5.0 ± 1.0 kcal/mol, and Do
is 5.6x10-3± 0.3, 3.5x10-4± 1.1, 5.8x10-4± 0.3, and 3.7x10-1± 1.9 cm2/sec,
respectively. The results are discussed in terms of electrostatic
dipole-dipole repulsive interactions among neighboring adsorbates.
R. W. Verhoef and M. Asscher Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University, Jerusalem 91904, Israel The importance of lateral interactions in surface diffusion is discussed by means of optical second harmonic diffraction from monolayer grating using model simulations. Measurements of high order diffraction signals during surface diffusion at various laser powers and initial surface coverages provide information on the type of interactions between adsorbates on the surface, i.e., attractive, repulsive, or none. Strong interactions between surface adsorbates are predicted to show unusual behavior of the higher order (n>1) diffraction signals, with the signal strength sometimes oscillating with time during diffusion. These results are applicable for linear diffraction as well.
Z. Rosenzweig, I.Farbman and M.Asscher Department of Physical Chemistry and the Farkas Center for Light Induced Processes The Hebrew University, Jerusalem 91904, Israel Diffusion of ammonia was studied over the smooth Re(001)
surface utilizing optical second harmonic diffraction from surface
coverage grating. The decay of the first order diffraction peak as a
function of surface temperature for different initial coverages was
measured and then simulated by numerical solution of Fick's second
diffusion equation, employing a coverage dependent diffusion constant.
For the first time the second order diffracted second harmonic signal
was monitored during the diffusion process, supporting the calculated
initial coverage profile and the diffusion model. The resulting
diffusivity D(theta)= D0exp[-Em(theta)/RT], is
defined by a barrier for diffusion Em(theta)=E0- Z , with activation
energy at zero coverage E0=3.4±0.6 kcal/mol, D0=2.8ª10-3
cm2sec-1. The repulsion energy between a pair of nearest neighbor
molecules =0.4±0.06 kcal/mol is obtained if an average of three
nearest neighbors is assumed at the edge of the grating. The coverage
effect on the diffusivity was found to correlate quantitatively with
dipole-dipole repulsion ( = 2/r3) between nearest neighbors. A coverage
independent "corrugation ratio" Ediff/Edes = 0.22±0.1 is deduced
from the above diffusion model. EFFECT OF LATERAL REPULSION ON DESORPTION AND DIFFUSION KINETICS STUDIED BY MONTE - CARLO SIMULATIONS I. Farbman, M. Asscher and A. Ben-Shaul Department of Physical Chemistry and the Fritz Haber Center for Molecular Dynamics The Hebrew University, Jerusalem 91904, Israel
EFFECT OF LATERAL REPULSION ON DESORPTION AND DIFFUSION KINETICS: SHG EXPERIMENTS AND MC SIMULATIONS W. Zhao , R. Verhoef and M. Asscher* Department of Physical Chemistry and the Farkas Center for Light Induced Processes The Hebrew University, Jerusalem 91904, Israel and I. Farbman and A. Ben-Shaul Department of Physical Chemistry and the Fritz Haber Center for Reaction Dynamics The Hebrew University, Jerusalem 91904, Israel Activation energies for desorption and for diffusion were experimentally determined as a function of surface coverage for the system of ammonia on Re(001) utilizing optical second harmonic generation techniques. For the first time coverage grating with up to 5th order SH-diffraction is reported for K atoms on Re(001). Preliminary diffusion measurements were performed on this system as well. These systems may be considered as ideal model to study the effect of very strong lateral repulsion on the kinetics of desorption and diffusion. A MC study on the ammonia - Re(001) system is presented, which examines the significance of long range repulsive dipole-dipole interactions on the outcome desorption and diffusion kinetics. We found that a single set of parameters, within the dipole-dipole like (1/r3 ) dependence on adsorbates separation distance, explains qualitatively and in certain cases quantitatively the experimental observations. Interaction range up to 4torder neighbors must be computed in order to properly account for the results.
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