Photochemistry on surfaces

Collision induced migration

Patterning and lithography

Surface diffusion

Kinetics of surface reactions

COLLISION INDUCED DESORPTION OF WATER FROM Ru(001)

L. Romm, T. Livneh and M. Asscher

Department of Physical Chemistry and the Farkas Center for Light Induced Proceses

The Hebrew University, Jerusalem 91904, Israel

The effect of energetic rare-gas colliders in a supersonic beam on the interaction of H2O and D2O with Ru(001) is discussed. It was found that molecules which are thermally desorbing near 180K, the A2 state for both H2O and D2O undergo selective collision induced desorption (CIDE) following collisions with energetic Kr. Threshold energies for CIDE are 3.4 eV for H2O and 3.8 eV for D2O, consistent with the known isotope effect in desorption rate of these molecules. Ice features which are thermally less stable than the A2 molecules ( desorbing near 160K) are not affected by collisions at energies up to 5.0 eV. This observation indicates that kinetic energy dissipation by the hydrogen bonded network - is extremely efficient.

DISSOCIATIVE CHEMISORPTION OF N₂ ON RU(001) ENHANCED BY VIBRATIONAL AND KINETIC ENERGY: MOLECULAR BEAM EXPERIMENTS AND QUANTUM MECHANICAL CALCULATIONS

L. Romm^a , G. Katz^b, R. Kosloff^b and M. Asscher^a *

Department of Physical Chemistry and the Farkas Center for Light-Induced Processes^a and the Fritz Haber Center for Molecular Dynamics^b , The Hebrew University, Jerusalem 91904, Israel

The dissociation probability of N₂ on Ru(001) increases from 5× 10^-7 at kinetic energy of 0.15eV to 10^-2 at 4.0eV. Vibrational excitation of the impinging nitrogen molecules enhances the dissociation more than the equivalent energy in translation. Its relative importance increases as the incident kinetic energy grows. The dissociation was found to be surface temperature independent at all incident kinetic energies, in agreement with theoretical predictions based on quantum mechanical nonadiabatic calculations. These simulations reproduce accurately the kinetic energy dependence of S₀ over the entire energy range, suggesting that N₂ tunnels from the molecular to the adsorbed atomic state through an effective barrier of 2.2 eV.

MOLECULAR DYNAMICS SIMULATIONS OF COLLISION INDUCED DESORPTION OF N₂ FROM Ru(001): I. LOW COVERAGE

Leonid Romm and Micha Asscher

Department of Physical Chemistry and Farkas Center for Light Induced Processes, The Hebrew University, Jerusalem, 991904, Israel

Yehuda Zeiri

Department of Chemistry, Nuclear Research Center, Negev, P.O.B. 9001, Beer-Sheva, Israel

Classical molecular dynamics (MD) simulations have been performed to study the details of collision induced desorption (CID) of nitrogen molecules adsorbed at low coverages on Ru(001). Semi-empirical potential energy surfaces were used to describe the movable two layers of 56 ruthenium metal atoms each, the nitrogen adsorbate, the Ar and Kr colliders and the interactions between them. An experimentally measured threshold energy for the CID process of 0.5 eV and the dependence of the cross section s_des on incidence energy and angle of incidence have been precisely reproduced in the energy range of 0.5-2.5 eV. Strong enhancement of the s_des is predicted as the angle of incidence increases. Kinetic energy and angular distributions of the scattered rare gas and the desorbing nitrogen were determined as a function of the dynamical variables of the collider. It is predicted that half of the collision energy is transferred to the solid and the other half is shared among the two scattered species. While no vibrational excitation is observed, efficient rotational energy excitation is predicted which depends on both incident energy and angle of incidence. Polar and azimuthal angular distributions were found to be strongly dependent on the incidence angle and energy of the colliders.

These results suggest a new CID mechanism for the weakly chemisorbed nitrogen molecules on Ru(001), based on extensive analysis of individual trajectories. According to this mechanism, the CID event is driven by an impact excitation of frustrated rotation or tilt motion of the adsorbed molecule as a result of collision with the energetic rare gas atom. In addition, lateral motion along the surface is also excited. Strong coupling of these two modes with the motion in the direction normal and away from the surface eventually leads to desorption and completes the CID process. The efficiency of this coupling is dictated by the details of the corrugation of the Ru- N₂ PES.

It is concluded that simple hard cube-hard sphere model, frequently used to analyze CID processes, is insufficient for the description of this system. While reasonably well predicting threshold energy, it cannot explain the full dynamical picture of the CID event.
 

COLLISION INDUCED MIGRATION OF ADSORBED N₂ on Ru(001): MD SIMULATIONS

L. Romm¹ , Y. Zeiri² and M. Asscher¹

1) Department of Physical Chemistry and Farkas Center for Light Induced Processes, The Hebrew University, Jerusalem, 91904, Israel 2)Department of Chemistry, Nuclear Research Center, Negev, P.O.B. 9001, Beer-Sheva, Israel

Collision induced migration (CIM) has been identified as a new surface phenomenon and has been studied for the first time using molecular dynamics simulations. The CIM process is initiated by an energetic gas phase argon atom, striking an adsorbed nitrogen molecule on Ru(001). The efficiency of CIM was investigated as a function of the collider initial kinetic energy and angle of incidence. It was found that at low coverages an adsorbed molecule can migrate more than 150Å following collisions at high energies and grazing angles of incidence. As coverage increases, inter-adsorbate collisions result in significant reduction of migration distances. At high energies, the competing process of collision induced desorption, becomes dominant, leaving behind molecules which migrate shorter distances. These competing channels lead to a collision energy for which CIM is maximized. For the N₂/Ru system, the CIM process is most effective near collider energy of 2.0 eV.

This new surface phenomenon of CIM has to be considered for better understanding the full range of surface processes which govern industrial high pressure catalysis. At the tail of the thermal kinetic energy distribution, energetic colliders from the gas phase lead to CIM and generate high energy inter-acollisions, sometimes discussed in terms of “hot-particle” chemistry.  

A REMARKABLE HEAVY ATOM ISOTOPE EFFECT IN THE DISSOCIATIVE CHEMISORPTION OF N₂ on Ru(001)

L. Romm, O. Citri, R. Kosloff and M. Asscher

Department of Physical Chemistry and The Fritz Haber and Farkas Research Centers

The Hebrew University, Jerusalem 91904, Israel

A gigantic isotope effect (I_eff = P_diss(¹⁵N₂)/P_diss(¹⁴N₂)) has been measured in the dissociative chemisorption of nitrogen molecules over Ru(001), changing from 0.2 at E_K=1.4 eV to unity at kinetic energies above 2 eV. These observations are consistent with a barrier for dissociation of 1.8 eV, in agreement with previous experiments and recent ab-initio DFT calculations. It supports earlier studies that proposed tunneling as the dissociation dynamics mechanism.