^{1,2,3}, P. Larrégaray

^{2,3,a)}, C. Crespos

^{2,3}, L. Martin-Gondre

^{4,5}, J. Rubayo-Soneira

^{1}and J.-C. Rayez

^{2,3}

### Abstract

The scattering of atomic nitrogen over a N-pre-adsorbed W(100) surface is theoretically described in the case of normal incidence off a single adsorbate. Dynamical reaction mechanisms, in particular Eley-Rideal (ER) abstraction, are scrutinized in the 0.1–3.0 eV collision energy range and the influence of temperature on reactivity is considered between 300 and 1500 K. Dynamics simulations suggest that, though non-activated reaction pathways exist, the abstraction process exhibits a significant collision energy threshold (0.5 eV). Such a feature, which has not been reported so far in the literature, is the consequence of a repulsive interaction between the impinging and the pre-adsorbed nitrogens along with a strong attraction towards the tungsten atoms. Above threshold, the cross section for ER reaction is found one order of magnitude lower than the one for hot-atoms formation. The abstraction process involves the collision of the impinging atom with the surface prior to reaction but temperature effects, when modeled via a generalized Langevin oscillator model, do not affect significantly reactivity.

The authors acknowledge R. Diez-Muino, M. Alducin, and A. Cadi for rewarding discussions and advices on density functional theory computations. E.Q.S. acknowledges the Bordeaux1-InSTEC inter university agreement and the French embassy in Cuba for fundings.

I. INTRODUCTION

II. METHODOLOGY

III. RESULTS AND DISCUSSION

A. Rigid surface model

B. Moving surface model

IV. CONCLUSIONS

### Key Topics

- Surface dynamics
- 40.0
- Atom surface collisions
- 34.0
- Interatomic potential energy surfaces
- 19.0
- Atom surface interactions
- 18.0
- Surface reactions
- 18.0

## Figures

Top left: Coordinate system and W(100) unitary cell. The Cartesian reference frame is originated on a tungsten top surface atom (grey circles). Black circles represent nitrogen atoms. The plane perpendicular to the surface and containing the diagonal of the unit cell is highlighted. Top right and bottom: 2D cuts of the PES as a function of projectile altitude (Zp) and impact parameter (b) within the diagonal plane. The FPLEPS model and DFT (PW91,RPBE) calculations are compared. The thick black line indicates the zero energy level, taken as the N atom adsorption energy. Full lines (dashed lines) are positive (negative) isovalues, separated by 0.1 eV (0.2 eV), δ = 3.175 Å.

Top left: Coordinate system and W(100) unitary cell. The Cartesian reference frame is originated on a tungsten top surface atom (grey circles). Black circles represent nitrogen atoms. The plane perpendicular to the surface and containing the diagonal of the unit cell is highlighted. Top right and bottom: 2D cuts of the PES as a function of projectile altitude (Zp) and impact parameter (b) within the diagonal plane. The FPLEPS model and DFT (PW91,RPBE) calculations are compared. The thick black line indicates the zero energy level, taken as the N atom adsorption energy. Full lines (dashed lines) are positive (negative) isovalues, separated by 0.1 eV (0.2 eV), δ = 3.175 Å.

2D cut of the FPLEPS PES as a function of the altitude of both the target (Zt) and projectile (Zp) for *b* = 1.09 Å impact parameter in the diagonal direction. The thick black line indicates the zero energy level, taken as the N atomic adsorption energy. Full lines (dashed lines) are positive (negative) and separated by 0.8 eV. Inset: Variation of the potential along the red line. Distances are in Å.

2D cut of the FPLEPS PES as a function of the altitude of both the target (Zt) and projectile (Zp) for *b* = 1.09 Å impact parameter in the diagonal direction. The thick black line indicates the zero energy level, taken as the N atomic adsorption energy. Full lines (dashed lines) are positive (negative) and separated by 0.8 eV. Inset: Variation of the potential along the red line. Distances are in Å.

Opacity maps of initial coordinates (*x*, *y*) of the projectile leading to the different exit channels: reflection (magenta dots), absorption (black dots), bound (cyan dots) and metastable (blue dots) HA formation, trapped molecules (green dots), and ER reactions (red dots) for 0.3 eV (left), 1.0 eV (center), and 2.6 eV (right) collision energy. The square represents the unitary cell in the center of which the nitrogen adatom is initially adsorbed. Distances are given in units of δ = 3.175 Å.

Opacity maps of initial coordinates (*x*, *y*) of the projectile leading to the different exit channels: reflection (magenta dots), absorption (black dots), bound (cyan dots) and metastable (blue dots) HA formation, trapped molecules (green dots), and ER reactions (red dots) for 0.3 eV (left), 1.0 eV (center), and 2.6 eV (right) collision energy. The square represents the unitary cell in the center of which the nitrogen adatom is initially adsorbed. Distances are given in units of δ = 3.175 Å.

Left: Cross sections for bound (bHA, circles), metastable (mHA, triangles) HA formation and ER reaction (squares). Right: Cross sections for trapped molecule formation (TM, circles), reflection (REF, triangles) and absorption (ABS, squares) as a function of the projectile energy.

Left: Cross sections for bound (bHA, circles), metastable (mHA, triangles) HA formation and ER reaction (squares). Right: Cross sections for trapped molecule formation (TM, circles), reflection (REF, triangles) and absorption (ABS, squares) as a function of the projectile energy.

Eley–Rideal contributions. Left: Opacity map of initial coordinates (*x*, *y*) of the projectile leading to ER1 (dark yellow dots), located along the diagonal, and ER2 (dark cyan dots). The square represents the unitary cell. Right: Eley–Rideal recombination cross section (squares) decomposed in its ER1 (circles) and ER2 (triangles) contributions as a function of projectile energy.

Eley–Rideal contributions. Left: Opacity map of initial coordinates (*x*, *y*) of the projectile leading to ER1 (dark yellow dots), located along the diagonal, and ER2 (dark cyan dots). The square represents the unitary cell. Right: Eley–Rideal recombination cross section (squares) decomposed in its ER1 (circles) and ER2 (triangles) contributions as a function of projectile energy.

Opacity map of (*x*, *y*) positions of the target and projectile (left) and their altitude distribution (right) at first projectile's rebound for the ER1 (up) and ER2 (bottom) mechanisms. Projectile (target) contribution is displayed in color red (black). Distances are in Å.

Opacity map of (*x*, *y*) positions of the target and projectile (left) and their altitude distribution (right) at first projectile's rebound for the ER1 (up) and ER2 (bottom) mechanisms. Projectile (target) contribution is displayed in color red (black). Distances are in Å.

Overlapping of trajectories on the 2D representation of the PES of Fig. 1 for different collision energies: 0.1 eV (top), 0.5 eV (middle), and 0.6 eV (bottom). Impact parameters are uniformly sampled within the diagonal plane. All trajectories are integrated until the first bounce. Trajectories in red indicate the zones where ER reactions take place.

Overlapping of trajectories on the 2D representation of the PES of Fig. 1 for different collision energies: 0.1 eV (top), 0.5 eV (middle), and 0.6 eV (bottom). Impact parameters are uniformly sampled within the diagonal plane. All trajectories are integrated until the first bounce. Trajectories in red indicate the zones where ER reactions take place.

Opacity maps of initial coordinates (*x*; *y*) of the projectile leading to the different exit channels: reflection (magenta dots), absorption (black dots), bound (cyan dots) and metastable (blue dots) HA formation, trapped molecules (green dots), and ER reactions (red dots) at different energies—1.0 eV (upper figures) and 2.6 eV (lower figures)—and temperatures: 300 K (left), 800 K (center), and 1500 K (right). The square represents the unitary cell the center of which the Nitrogen adatom is initially adsorbed. Distances are given in units of δ = 3.175 Å.

Opacity maps of initial coordinates (*x*; *y*) of the projectile leading to the different exit channels: reflection (magenta dots), absorption (black dots), bound (cyan dots) and metastable (blue dots) HA formation, trapped molecules (green dots), and ER reactions (red dots) at different energies—1.0 eV (upper figures) and 2.6 eV (lower figures)—and temperatures: 300 K (left), 800 K (center), and 1500 K (right). The square represents the unitary cell the center of which the Nitrogen adatom is initially adsorbed. Distances are given in units of δ = 3.175 Å.

Cross section for ER abstraction within the rigid surface model and the moving surface model at *T* _{ s } = 300, 800, and 1500 K as a function of collision energy.

Cross section for ER abstraction within the rigid surface model and the moving surface model at *T* _{ s } = 300, 800, and 1500 K as a function of collision energy.

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