1. Chemical kinetics Lecture IV Jenny Frodelius & Jonas Lauridsen

2. References Principles of Plasma Discharges and Materials Processing, 2nd Edition M. A. Lieberman & A. J. Lichtenberg Surface Chemistry and Catalysis G. A. Somorjai

3. Outline Kinetics – –What’s our interest? –Terms, Basic kinetics (rate constants) Surface Processes - Surface Kinetics -

4. Kinetics The study of how fast processes occur

5. We are interested in......kinetics for the gas phase, between the gas- phase and surface and on the surface.

6. Kinetics Elementary reactions – a reaction that proceeds in one step in a simultaneous collision of all reactant molecules. Cl 2 + H 2 2HCl O + + O 2 O + O 2 + Which one is elementary?

7. Kinetics The most important elementary reactions –Unimolecular Aproducts –BimolecularA+Bproducts In low-pressure termolecular are complex

8. Kinetics The reaction rate (R) for gas-phase reaction n j = volume density (m -3 ) of molecules of the j:th substance α j = stoichiometric coefficient

9. Kinetics Aproducts  A+Bproducts  A+B+Cproducts  The quantities K 1 (s -1 ), K 2 (m 3 /s) and K 3 (m 6 /s) are the first-, second- and third- order rate constants K1K1 K2K2 K3K3

10. Kinetics Relation to equilibrium constants A+B C+D K 2 n A n B =K -2 n C n D Thermal equilibrium  K2K2 K -2

11. First order consecutive reactions ABCABC K A K B

12. Outline Kinetics Surface Processes - –Positive ion neutralization & secondary electron emission –Atom/Ion impact on surface Adsorption Desorption Fragmentation Sputtering Implantation Surface Kinetics

13. Surface processes

14. Neutralization in the gas-phase is a forbidden reaction, but on the surface a fast three-body reaction occurs. A + + e -  A A + + e - + S  A + S S - surface

15. Neutralization of positive ions and secondary electron emission Case 1: Ion pick up electron, becomes excited and release an photon to reach atomic state Case 2: Ion pick up electron, becomes an atom, release excess energy that will cause the surface to release an Auger electron. Case 3: Upon ion impact kinetic energy is transfered to surface so that the surface can release an Auger electron

16. Case 2 Condition for emission:  iz > 2   Kinetic energy:  max =  iz - 2    min =  iz - 2   - 2  F  iz - ionization energy   - work function  F - fermi energy

17. Atom/Ion impact on surface Low ~10 V~100 V ~1000V energies Sputtering Adsorption, desorption Molecules split into atoms Implantation

18. Adsorption: Lennard-Jones diagram Dissociative chemisorption Physisorption Molecular chemisorption

19. Adsorption Physisorption Van der Waal -  H ads < 35 kJ/mol No energy barrier Never dissociative Many monolayers Not dependent on surface coverage Chemisorption Chemical bond -  H ads > 35kJ/mol Varying energy barriers Often dissociative One monolayer Dependent on surface coverage

20. Desorption Reverse reaction to adsorption Must be in balance with adsorption at thermal equilibrium First order desorption rate constant K 0 ~ the number of attempted escapes per second from the adsorption well Associative desorption Opposite to associative adsorption K 0 ~ the number of collisions per second per unit area on the surface between two adsorbed atoms

21. Equations Surface coverage (Täckningsgrad) : Langmuirs adsorption isotherm : Equilibrium surface coverage as a function of pressure at fixed temperature. Sticking coefficient :

22. Fragmentation (~10 V) Ionic and neutral molecules fragment into atoms Atoms are reflected or adsorbed Energy must be in the range of atomic bonds (1-10 V) When energy is 4-5 times higher than the threshold energy, over half of the molecules split into atoms

23. Sputtering (~100 V) Heavy particles, usually ions, bombard and transfer energy to many target atoms which collide with other atoms in the solid. Most atoms are trapped in solid but one or several escape from the surface. Threshold energy (  thr ) 20-50 V Distribution of atom energies is isotropic with mean energy (  t ) of the surface binding energy.

24. Sputtering yield Z t – Average atomic number of target  thr – Threshold energy for an ion to sputter target  t – Average energy for atoms in collision  i  N (  t ) (  i ) N – Number of atoms in the collision cascade having  t (  i ) – Range of ion penetration into the target (  t ) – Surface depth from where atoms can escape  t   i 

25. Sputtering yield as a function of angle of incidence for different metals

26. Outline Kinetics Surface Processes Surface Kinetics –Equations...and more equations...

27. Surface kinetics

28. KaKa KbKb KfKf KgKg KhKh KdKd KeKe KcKc Ka – flow to the surface Kb – adsorbation rate Kc – reaction rate Kd – desorption rate Ke – flow rate into gas phase Kf – normal desorption Kg – associative desorption Kh – adsorption of products