chemical kinetics lecture iv jenny frodelius & jonas lauridsen
Post on 21-Dec-2015
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TRANSCRIPT
References
• Principles of Plasma Discharges and Materials Processing, 2nd Edition
M. A. Lieberman & A. J. Lichtenberg
• Surface Chemistry and Catalysis
G. A. Somorjai
Outline
• Kinetics – – What’s our interest? – Terms, Basic kinetics (rate constants)
• Surface Processes -
• Surface Kinetics -
We are interested in...
...kinetics for
the gas phase , between the gas-phase and surface
and on the surface.
Kinetics
• Elementary reactions – a reaction that proceeds in one step in a simultaneous collision of all reactant molecules.
Cl2 + H2 2HCl
O+ + O2 O + O2+
Which one is elementary?
Kinetics
• The most important elementary reactions– Unimolecular A products– Bimolecular A+B products
• In low-pressure termolecular are complex
Kinetics
• The reaction rate (R) for gas-phase reaction
nj= volume density (m-3) of molecules of the j:th substance
αj= stoichiometric coefficient
dt
dnR j
j1
Kinetics
A products
A+B products
A+B+C products
The quantities K1 (s-1), K2 (m3/s) and K3 (m6/s) are the first-, second- and third- order rate constants
AA nKdt
dnR 1
BABA nnKdt
dn
dt
dnR 2
CBACBA nnnKdt
dn
dt
dn
dt
dnR 3
K1
K2
K3
Kinetics
• Relation to equilibrium constants
A+B C+D
K2nAnB=K-2nCnD
Thermal equilibrium
K2
K-2
)(nn
nn
2
2
BA
DC TK
K
Outline
• Kinetics• Surface Processes -
– Positive ion neutralization & secondary electron emission
– Atom/Ion impact on surface• Adsorption Desorption• Fragmentation• Sputtering• Implantation
• Surface Kinetics
Surface processes
Neutralization in the gas-phase is a
forbidden reaction, but on the surface a
fast three-body reaction occurs.
AA++ + e + e-- A A
AA++ + e + e-- + S + SA + SA + S
S - surfaceS - surface
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
Case 2
Condition for emission:
iz > 2
Kinetic energy:
max = iz - 2
min = iz - 2 - 2F
iz - ionization energy
- work function
F - fermi energy
Atom/Ion impact on surface
Low ~10 V ~100 V ~1000V energies
Sputtering
Adsorption,desorption
Molecules splitinto atoms
Implantation
Adsorption
Physisorption• Van der Waal
• -Hads < 35 kJ/mol
• No energy barrier• Never dissociative• Many monolayers• Not dependent on surface
coverage
Chemisorption• Chemical bond
• -Hads > 35kJ/mol
• Varying energy barriers• Often dissociative• One monolayer• Dependent on surface
coverage
Desorption
Desorption• Reverse reaction to adsorption• Must be in balance with
adsorption at thermal equilibrium
• First order desorption rate constant
K0 ~ the number of attempted escapes per second from the adsorption well
Associative desorption• Opposite to associative adsorption
K0 ~ the number of collisions per second per unit area on the surface between two adsorbed atoms
Tdesor
chemorphys
eKK
0
Equations
Surface coverage (Täckningsgrad) :
Langmuirs adsorption isotherm :Equilibrium surface coverageas a function of pressure at fixed temperature.
Sticking coefficient : )1(0 Tk
E
BeSS
SAads
SAads
n
n
:
:
1
sitesofnumberTotal
speciesadsorbedofNumber
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
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.
Sputtering yield
Zt – Average atomic number of target thr – Threshold energy for an ion to sputter targett – Average energy for atoms in collisioni 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
)(0.06
sput thritt
Z
ti
Outline
• Kinetics
• Surface Processes
• Surface Kinetics– Equations– Equations
• ...and more equations...