Photochemistry on surfaces

Collision induced migration

Patterning and lithography

Surface diffusion

Kinetics of surface reactions

Simulation of a diffusion from a coverage grating
attractive interactions

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^*

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 K_x-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)

23. Zhao^a, G. Kerner, M. Asscher^a* , M. Wilde^b, K. Al-Shamery^b, H.-J. Freund^b , V. Staemmler^c and M. Wieszbowskac

a- Department of Physical Chemistry and the Farkas Center for Light Induced Processes,
The Hebrew University, Jerusalem 91904, Israel

2. The Fritz Haber Institute der Max-Planck Gesellschaft, Faradayweg 4-6, Berlin

1. Lehrstul Fü r Theoretical Chemie, Ruhr Universitä t Bochum, D-44780, Germany

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 D_o 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.

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.

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 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
 
  The effects of adsorbate lateral interactions on the kinetics of surface diffusion and desorption are studied by means of kinetic and thermodynamic Monte Carlo simulations. This study is motivated by recent diffusion and desorption experiments on the NH3/Re(001) system, which show that the activation energies of these processes decrease ( in different fashions) with increasing surface coverage, the interactions between the adsorbates are thus assumed to be repulsive. A long range dipole-dipole like potential is used to simulate both the diffusion and desorption processes. Most calculations are carried out with the interaction range extending up to fourth order neighbors. Longer ranges are found to barely affect the kinetic behavior. On the other hand, shorter ranges of interaction result in qualitatively and quantitatively different structural (thermodynamic phase) behaviors and, consequently, in very different kinetics of diffusion and desorption. The model used to calculate diffusion kinetics assumes that the activation barrier to particle diffusion depends, simultaneously, on the local environments of both the initial and the final sites involved in the elementary event of particle jumps. The chemical diffusion coefficient is evaluated based on thermodynamic and kinetic Monte Carlo simulations. It is found to increase with surface coverage, reflecting the repulsive nature of the interactions. Yet, unlike the experimental results, the increase is non- monotonic but, rather, somewhat oscillatory - reflecting the structural phase transitions of the adsorbed layer. The activation energy of desorption is found to decrease by about 15kcal/mole as the coverage increases from zero to one, showing steeper slopes around the coverages corresponding to a perfectly ordered adlayer phase. These results are in satisfactory qualitative and quantitative agreement with experiment. Finally, it is shown that the coverage dependence of the activation barrier for diffusion can be reasonably well evaluated from equilibrium thermodynamic desorption data.

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.