New perspectives on surface kinetics

Daniil Marinov

19th International Summer School 2014 Bad Honnef

O

N

O

N

N

N

Time, Aug.28,1950Can man learn to control the atmosphere he lives in? 1

Contribution of Irving Langmuir

Plasma physicsL. probesL. wavesL. paradox

Surface chemistryL. monolayerL. mechanismL. unit of exposure

Outdoor scienceFirst work on cloud

seeding

Air 1 mbar

TiO2

J-P. Wolf / University of GenevaO. Guaitella/ LPP

2

Why to study plasma-surface interactions?

Etching &deposition

ITER

Plasma assisted catalysis

Plasma medicineSurface modification

Astrochemistry

3

What are the main challenges?

O

surface

plasma

ions+

electrons

photons

radicalsO

Surfaceis modified

Sink of speciesSink of energy

Production of new species

Plasma is modified

Adsorption/desorption/chemical reactions

Creation of active sitesEtching/deposition

products

Not well-defined surfacesSynergy between ions/photons/neutrals

Role of energy distributions 4

Focus of today's lecture

Surface-catalyzed reactions in plasmas

• Low pressure plasmas• Re-entry• Plasma-catalysis• Astrochemistry

What we know and what we don’t know?

First observations H+H→ H2

Robert W Wood 1920 Proc. R. Soc.

5

Agenda

� Introduction to surface processes in plasmas� Surface kinetics in N2/O2 plasmas

- Adsorption and reactivity of O- Adsorption and reactivity of N

� Vibrational relaxation on surfaces� Surface in contact with plasma: a sink or a source?

6

Surface recombination of atoms: plasma scientist viewpoint

nDvnJ thin ∇−=2

1

4

1

nDvnJ thout ∇+=2

1

4

1

inout JJ )1( γ−=2/14

1

γγ

−= thwalllost vnJ

Surface

Volume

vD th

42/12

γγτ −+Λ=

Flux lost at the surfaceBoundary condition

The lifetime of atoms in the reactor (approx)

Surface

Volume

vth

42/1

1

γγτ

γ−=

<< Surface limited

Diffusion Surface losses

D

2

1

Λ=

≈

τ

γ Diffusion limited

wal

l

γ – loss probability [0..1]

7

Typical experiments for determination of γ

x

Diffusion tube experiment

ln[O

(x)/

O(0

)]γΟ=1.1·10-4

J. Marschall, SRI labs, 2007

Modulated discharge

Quartz

γΟ=8.6·10-5

Cartry, J Phys. D. 32 1999

t

[O]

Plasma ON Plasma OFF

00

8

Complex dependence of γΟ on the wall temperature

Macko et al. PSST 13 (2004)

A model is required to explain this behavior

Evidence of two mechanisms

9

Surface recombination of atoms: surface scientist viewpoint

Interaction of atoms with SiO 2 surface

Temperature-activated processes

Desorption

νd=ν0exp(-Ed/kT) ; ν0 ~1015s-1 ; Ed ~ 0.5 eV

DiffusionνD=ν01exp(-ED/kT); ν01 ~1013s-1 ; ED ~ Ed/2

Kim&Boudart,Langmuir ,1991, 7

physisorption

chemisorption

EDEd

νd

νD

Van der Waals interaction

Chemical bond

Surface parameters

Density of physisorption sites [F] ~ 1015 cm-2

Density of chemisorption sites [S] ~ 1013 cm-2

Chemisorption energy Echem ~ 3 – 5 eV

Sticking coefficient for physisorption s ~ 1

Guerra, IEEE, 35, 5, 2007 10

recombination

Eley-Rideal

Langmuir-HinshelwoodSiO2

Surface recombination of atoms: surface scientist viewpoint

Reaction parameters

Probability of recombination on occupied

chemisorption site:

kER=k0ERexp(-EER/kT); k0ER ~1 ; EER ~ 0.2 eV

kLH=k0LHexp(-ELH/kT) ;k0LH ~1 ; ELH ~ 0.2 eVKim&Boudart,Langmuir ,1991, 7

Guerra, IEEE, 35, 5, 2007 11

Formulation of a mesoscopic surface model

1. Fractional coverage of sites [occupied]/[total] θf = [Af]/[F], θs = [As]/[S]

Guerra, IEEE, 35, 5, 2007

2. Differential equations for θ: dθ/dt = (Ads) – (Des) – (Rec)For one type of sites:

3. Input parameters: [S], [F], νd, νD, Ed, ED, EER,..

12

low Thigh T

1) High T

θs =1no physisorptionE-R recombinationγER~exp(-EER/ kT)

Eley-Rideal

2) Low T

Langmuir-Hinshelwood

θs =1Surface diffusion L-H recombination

Results of the mesoscopic model

• Chemisorbed atoms are the main sites for recombination

• A lot of (unknown) parameters

• Is the model true and unique?

• How to limit the number of unknown parameters?

13

What we don’t know: effect of plasma exposure

Cartry J. Phys.D 32 1999t

O2 plasma

treatment

Plasma exposure increases γΟ by an order of magnitude

How can plasma modify the surface?• Creation of active sites by ion bombardment.• Cleaning of occupied sites.• Creation of defects by UV.

A B C

[O] decay in silica discharge tube

Shortpulse

14

What we don’t know: recombination in complex plasmas

N N

N OO

Example: N2/O2 plasma Possible channels:

N+N |wall → N2 (γN2)O+O |wall → O2 (γO2)N+O |wall → NO (γNO)

On SiO2 in N2/O2 afterglow(‡)

γN2 ~ γO2 ~ γNO ~ 10-5

(less than in pure O2)

(‡) Pejakovic et al. Journal of Thermophysics and Heat Transfer, 22, 2, 2008

γNO

• Do different atoms compete for adsorption sites?• Are there different types of adsorption sites?• What is the coverage of adsorbed atoms?• How do adsorbed atoms catalyze production of new molecules?

15

Surface sciencePlasma science

Possible approaches to the problem

Model surfacesSurface diagnostics in ultra high vacuum

Real surfacesComplex plasmasDiagnostics of gasphase species

Sticking coefficientsAdsorption energySurface coverage

Effective reactionprobabilities (γ)

Not detailed enough Not applicable to real conditions

Challenge: obtain detailed information about surface processes in real plasmas

16

Langmuir-Hinshelwood recombination:spinning wall experiments

0.4 0.6 0.8 1.010-10

10-5

100

resi

denc

e tim

e [s

]

Ed [eV]

1 ms

τd=νd-1exp(Ed/kT)

νd ~ 1015 s-1

Residence time of atoms on the surface as a function of Ed

Donnelly group papersStafford et al. Pure Appl. Chem. 82, 6, 2010

up to 40000 rpm => 1 ms betweenthe exposure and the analysis

17

Recombination of atomic oxygen on the spinning wall

O2 desorption flux as a function of rotation time

Only O2 is desorbed, no O or O3detected by the mass spec.

γΟ is independent of O and O2 flux

18

Mesoscopic model of the LH recombination

fast

fast

kd.i

Model with one typeof active sites fails toexplain experimentalobservations

Donnelly et al., J. Vac. Sci. Thechnol. A 29(1) 2011

anodized aluminum

Complex non-exponential decay of Drec

19

Mesoscopic model of the LH recombination

• Active sites on disordered surfaces always exhibit adistribution of reactivity

• Weakly bonded species are more reactive

Groups of sites with different reactivity

20

Auger electron spectroscopy of the spinning wall: anodized aluminum in Cl 2 plasma

Donnelly et al., J. Vac. Sci. Thechnol. A 29(1) 2011

Surface density of Clatoms is independent of

the rotation speed

In-situ surface analysis of the spinning wall

21

Auger electron spectroscopy of the spinning wall: anodized aluminum in Cl 2 plasma

Desorption flux of Cl 2strongly depends on the rpm

• Most of Cl ads are useless for recombination• A small fraction of atoms is reactive• Surface diagnostics provides information on the densitybut not on the reactivity of species

Cl coverage is constant

22

SAQ:What happens if we stop the spinning wall?

Only O2 is desorbed, no O detected by the mass spec.

=> Mobile O atoms don’t leave the surface unless they recombine into O2.

What value of γO one can expect at f 0=0?

23

Surface kinetics in N 2/O2 plasmas

Q: What is the coverage and reactivity of chemisorbed atoms?

Q: How do chemisorbed atoms react and how contribute to recombination?

24

How to probe the reactivity of chemisorbed atoms?

tPretreatment Probing

plasma

adsorption

Step 1 Step 2

few minutes

Stable (strongly bonded) atoms are detected

Probing

25

tuneable laser

detector

Experimental setup

Gas phase diagnostics:Mass-spectrometryIR lasersOptical emission/absorption

discharge tube –the surface under

investigation

26

Adsorption and reactivity of O on surfaces under plasma exposure

30min t

Plasma ON

O2

stop

Pumping

Buffer volume fill in

NO

NO2

Matching unit

pump gas inlet

buffer volume

RF gen.

NO

10min

Reactivity of O adsorbed on the reactor walls

laser 1

laser 2 Detector

NO2

NO

• detection limit 10 12 cm -3 (equivalent to 0.1% of a monolayer)• millisecond time resolution

Experimental sequence

Discharge tube

Evolution in closed reactor

27

N N

O

Pyrex

O OO

NO is converted into NO 2 on the surface

0 60 120

0

1x1013

2x1013

3x1013

conc

entr

atio

ns /c

m-3

t/ s

NO NO2 NO+NO2

• NO is fully oxidized into NO 2• Stable and reactive O ads are grafted to the surface by O 2 plasma

Guerra et al. J.Phys.D: Appl Phys. 47 (2014) 28

Coverage of O ads can be estimated by introducing a saturating amount of NO

0 1000 2000 30000

5x1014

1x1015

conc

entr

atio

ns (

cm-3)

t /s

NO NO2 NO+NO2

40x more NO

takes 40x longer

Oads ≈ 2·1014 cm -2

θΟ ~ 0.1

Guerra et al. J.Phys.D: Appl Phys. 47 (2014) 29

Mesoscopic description of NO oxidation requires a distribution of reactivity of O ads

Don’t miss the talk of V. Guerra

Activation energy of NO+Oads

← reactivity increases

Distribution of active sitesModel

30

The multi-site model gives a good description of the experiment on short and long time scales

• Real surfaces exhibit a distribution of reactivity• Discrete distribution is an effective way to approximate the real distribution which is probably continuous

31

Reactivity of O ads depends on the surface material and on the target molecule

0 500 1000 1500 20000

2x1014

4x1014

6x1014

TiO2

C2H

2 [cm

-3]

t [s]

Pyrex

Pyrex

O OO

C2H2

TiO2

O OO

C2H2products

30min t

Plasma ONO2 Pumping

C2H2introducing 32

Adsorption and reactivity of N on silica surface under plasma exposure

33

gas N2

Pressure 0.5 mbar

Power 17 W

Flow 10 sccm

Duration 1-360 min RUB, Bochum

ex-situ XPS

Matching unitRF gen

dischargeactive speciesN2

+, N2*, N, N2(v)

post-dischargeactive speciesN, N2(v)

Surface analysis (XPS) of SiO 2 samples exposed to N2 plasma

34

400 200

0

6

12

75 min postdischarge

360 min discharge

Si2pSi2sC1s

O1s

XP

S in

tens

ity [a

.u.]

binding energy [eV]

N1s

25 min discharge405 400 395

0

1

2

3

4

5

6

Inte

nsity

(a.

u.)

binding energy (eV)

N1s

Si≡N

Si-NO2

N are grafted to the surface only under direct plasma exposure

Surface analysis (XPS) of SiO 2 samples exposed to N2 plasma

35

0 60 120 180 240 300 360

0

15

30

45

60

conc

entr

atio

n [c

m-2]

O

conc

entr

atio

n [a

t %]

treatment time (min)

Si

N

0

2x1015

4x1015

6x1015

8x1015

1x1016

Evidence for nitridation of SiO2

N replace O

[N]max ≈ 5·1015 cm-2

~ monolayer

NN

N

[ Seino et al. 2002]

~3 nmThe density of N ads from XPS measurements

36

t

Matching unit

pump gas inlet

RF gen.

N2

N2 plasmaNN

N

N

N

Studied surface – the wall of silica disharge tube

Plasma ON28N2 0.5 mbar, P=17W

Ar plasmacleaning

N2 plasmaN

N

N

Probing the reactivity of N ads using isotopic exchange 15N - 14Nads

pretreatment

37

NN

Matching unit

pump gas inlet

RF gen.

N2

Plasma ON28N2 0.5 mbar, P=17W Pumping

Ar plasmacleaning

NN

N

Probing the reactivity of N ads using isotopic exchange 15N - 14Nads

pumping

38

30N2

Matching unit

pump gas inlet

RF gen.

Tube fill in

Plasma ON28N2 0.5 mbar, P=17W Pumping

Ar plasmacleaning

NN

N

Probing the reactivity of N ads using isotopic exchange 15N - 14Nads

30N2 injection

39

30N2

Matching unit

pump gas inlet

RF gen.

N2

1514

15

[14N]desorbed =[14N]gas·Volume/Surface

Plasma ON28N2 0.5 mbar, P=17W Pumping

Plasma ON30N2 0.5 mbar, P=17W

Ar plasmacleaning

NN

N

Probing the reactivity of N ads using isotopic exchange 15N - 14Nads

probe discharge in 30N2

40

0,1 1 10 100 1000

1E14

1E15

1E16

14N

des [c

m-2]

30N2 plasma duration [s]

Nads are continuouslyexchanged under N2 plasma exposure

Plasma ON28N2 0.5 mbar, P=17W

3600 s 0.05 - 1000 s

PumpingPlasma ON

30N2 0.5 mbar, P=17W

1514

15

NN

N

14

Kinetics of isotopic exchange under 30N2plasma exposure

41

0,1 1 10 100 1000

1E14

1E15

1E16

14N

des [c

m-2]

τ=316sτ=6.7s

30N2 plasma duration [s]

τ=0.42s

Plasma ON28N2 0.5 mbar, P=17W

3600 s 0.05 - 1000 s

PumpingPlasma ON

30N2 0.5 mbar, P=17W

∑=

−−=3

1

/des

14Ni

ti

ieaa τ

Distribution of reactivity

i ai [cm-2] τι [s]1 5.5·1014 0.422 1.1·1015 6.73 3.6·1015 316

Kinetics of isotopic exchange under 30N2plasma exposure: distribution of reactivity

42

Distribution of adsorption energy

In depth distribution

NN

N

NN

N

zN

Distribution of reactivity

∑=

−−=3

1

/des

14Ni

ti

ieaa τ

i ai [cm-2] τι [s]1 5.5·1014 0.422 1.1·1015 6.73 3.6·1015 316

43

1E14 1E15 1E16

1E14

1E15

14N

des

orbe

d [c

m-2]

15N lost [cm-2]

15 14 15

lost 15N atomsdesorbed 14N atoms

Plasma ON28N2 0.5 mbar, P=17W

3600 s 5 ms - 100 s

PumpingPulsed discharge30N2 0.5 mbar

Only 10% of 15N lostrecombined with 14Nads

NN

N

Recombination with Nadsdoesn’t explain losses of 15N

Are Nads the main sites for N recombination on the surface?

44

Conclusions

• Demonstration of adsorption of stable O and N on

surfaces under plasma exposure

• Adsorbed atoms exhibit a distribution of reactivity

• Nads are not the main sites for surface recombination

45

Relaxation of N2 vibrational energy on the surface

N

N

N

N

γΝ2γΝ2

γΝ2 – probability of vibrational quantum loss on the surface in

one collision

N2(ν)N2(ν−1)

Vibrational relaxation on surfaces

46

Role of surface relaxation in N2 plasmas

e- ↔ N2(ν) ↔ wallγΝ2

gas phase quenchingis slow

X

At low ( ~mbar) pressures vibrational relaxation on the surface controls the global energy balance

0 10 20 30 40

1E11

1E12

1E13

1E14

1E15

1E16

N2(

v)/ c

m-3

vibrational quantum number

γ=0.002 γ=1

N2, 1.3 mbar, 50 mA(For more details don’t miss the

talk of V. Guerra)

By changing γN2 we can change vibrational distribution

Energy balance in N 2 discharge:

calculation

47

[Morgan and Shiff 1963 Can. J. Chem.][Egorov et al. 1973 Chem. Phys. Lett.] [Black et al. 1974 J.Chem.Phys.][Parish and Yaney 1994 GEC] N2(v) flow plasma

What we know: γN2 in the post-discharge

γN2 on SiO 2 10-4 – 10-3

Previous works

What we want to know: • γN2 in the discharge• pathways of energy accommodation on the surface

Proposed quenching mechanism:

N

N

N2(ν−1)phononexcitation

N

N

N

N

N2(ν)

hω

physisorptiondesorption

48

fastexchange

IR X

CO2 – “image” of N2(ν)

no absorptiononly Raman

Technique for sensitive in- situ detection of N 2(v): titration with CO 2

Marinov et al. J. Phys. D: Appl. Phys. 47 (2014) 49

tO2 plasma N2 + (0.05 -1)% Admix

single pulse

afterglow

e-

excitation quenching

wall wall

N2(1)+CO2 (0000) ↔N2(0)+CO2 (0001)

N

N

N

N

Experimental procedure

50

R

QCL 1

Detector

HV

+Ext.

Trig.

QCL 3 QCL 2

CO2

2325 cm-1

CO /N2O

2209 cm-1

TRIPLE Q spectrometer

INP Greifswald

Experimental setup

• Sensitive CO2 detection → we can use small CO2 admixtures• µs time resolution → we can capture fast processes• Entire relaxation can be recorder in a single plasma pulse

51

0 100 200

0

2x1013

4x1013

6x10130.2 % CO2

CO

2 con

cent

ratio

n [c

m-3]

t [ms]

plasma pulse

vibrational excitation of CO2

Vibrational relaxation in N 2-CO2 mixture

52

0 100 200

0

2x1013

4x1013

6x10130.2 % CO2

CO

2 con

cent

ratio

n [c

m-3]

t [ms]

plasma pulse

Vibrational relaxation in N 2-CO2 mixture: N2(v) is the energy reservoir

Relaxation time of CO 2 = relaxation time of N 2(v)

CO2 in Ar

CO2 in N2

53

0 50 100 150 2000

1x1014

2x1014

0.066 % CO2

0.1 % CO2

0.2 % CO2

0.33 % CO2

CO

2 co

ncen

trat

ion

t [ms]

plasma pulse

0.5 % CO2CO2

0.0 5.0x1013 1.0x10140

10

20

30

1/τ ef

f [s-1

]

CO2 [cm-3]

relaxation in "pure" N2

silica surface

Influence of CO 2 admixture concentration

effect of CO2

CO2 admixture accelerates the relaxation

54

Probability of surface relaxation – the ONLY tuning parameter of the model

0 5 10 15 20 251E9

1E11

1E13

1E15

60

40

20

5 1 ms

N2(

v) [c

m-3]

vibrational levels

0.1 ms

0 50 100 150 2001E11

1E12

1E13

1E14

CO2(100)

eff

CO2(100)

eff

CO2(0002)

CO2(0001)

CO2(0110)

CO

2 [c

m-3]

t [ms]

CO2(0000)

dN2(i)dt = dN2(i)dt

e-V+dN2(i)dt

VV

N2-N2+dN2(i)dt

VT+dN2(i)dt

R1

N2-CO2+ dN2(i)dt

W

dCO2(k)dt = dCO2(k)dt

R1

CO2-N2+dCO2(k)dt

VT+ dCO2(k)dt

intra

CO2-N2+dCO2(k)dt

W+ dCO2(k)dt

RD

Kinetic model of the N 2 – CO2 relaxation

55

γN2 is obtained from the best fit of the experiment

0.0 5.0x1013 1.0x1014 1.5x1014 2.0x10140

1x10-3

2x10-3

3x10-3

γ Ν2

CO2 [cm-3]

Surface relaxation of N 2(v) is enhanced by CO 2

N

N

N2(ν)

New mechanism:

Vibrational energy transfer to adsorbed molecules

56

0 5x1013 1x10140

10

20

30

40

50

O2 plasma

Ar plasma

1/τ e

ff [s-1

]

CO2 [cm-3]

N2 plasma pretreatment

γ1=1.5·10-3

γ1=1·10-3

γ1=6·10-4

γΝ2 depends on surface pretreatment by plasma

SiO2

NN N

> SiO2 SiO2>O O

ArAr

Influence of plasma pretreatment of silica surface

57

Surface Plasma pretr γN2 this work literature values

Silica O2 5.7·10-4

(1.8-7) ·10-4Silica N2 10.5·10-4

Silica Ar 8.2·10-4

Pyrex O2 6·10-4

(2.3-10) ·10-4Pyrex N2 11 ·10-4

Al 2O3 O2 15·10-4 (11-14) ·10-4

Anodized Al O2 29·10-4 no

TiO2 sol gel film all 19·10-4 no

TiO2 with nanoparticles

>4·10-2 no

Summary of titration measurements

58

Conclusions

• New diagnostics for in-situ determination of γN2

• Single pulse experiments

• New mechanism of vibrational energy transfer to adsorbed molecules

• Plasma exposure can vibrational quenching on the surface

59

Surface: sink or source?

Nd:YAG laser

THG λ=355 nm

Dye laser

λ=450 nm

λ=225 nm

f=300 mm

BBO

PMT Oscillo PC

Recombination of oxygen on SiO 2 surface can produce O 3

Marinov et al.J. Phys. D: Appl. Phys. 46 (2013)

Rb

HV+

iCC

DS

ha

mro

ck

R3

03

iUV

lampUV absorption for O3

TALIF for O

60

0 50 1000.1

1 empty discharge tubeτ

Ο=65 ms

TA

LIF

O [

a.u.

]

t [ms]

0 50 100 150 200 250 3000

1x1014

2x1014

3x1014

O3

[cm

-3]

t [ms]

empty discharge tubeτ

O3 = 63 ms

p=6.7 mbarE=0.16 J/pulse

Recombination of oxygen on SiO 2 surface can produce O 3

Marinov et al.J. Phys. D: Appl. Phys. 46 (2013)

O2, 6.7 mbar, 1 ms pulse

In bare tube O 3 is formed in the volume (V. Guerra)

61

SiO2

OO

OO

O O

0 50 1000.1

1

catalytic surfaceτΟ=5.5 ms

O [

a.u.

]

t [ms]

0 50 100 150 200 250 3000

1x1014

2x1014

3x1014

catalyticτO3 = 9 ms

O3

[cm

-3]

t [ms]

p=6.7 mbarE=0.16 J/pulse

Surface production

Recombination of oxygen on SiO 2 surface can produce O 3

With SiO 2 catalyst, 30 % of O surface recombination yields ozone!

62

Is the surface always a sink of energetic species?

O2 potential curves

Up to 5 eV is available! Excitation of multiplelevels is possible

(O+O)s → O2*

63

N2/N

NOtitrationN+NO→ N2+O

N2

N2/N/O

Recent experiments evidence O 2(∆) formation from O recombination on SiO 2

O2(∆) yield >10%

O2(∆) detection

White et al. 52nd Aerospace Sciences Meeting 2014 64

M. Foucher, J-P Booth ESCAMPIG 2014

Highly excited O 2 (v) have been observed in low pressure O 2 plasmas

O2(v>10)

Further plasma modeling is needed to confirm surface mechanism

65

First principle simulation of surface processes

66

Ab-initio calculations of surface processes:N recombination on SiO 2

Quantum simulations of the solid-gas system:Potential energy surfacePhonon spectrum

N - Si7O14H14

Rutigliano Surface Science 600 (2006) 4239–4246

Molecular dynamics simulation of the collision process

addcleff VHH +=∧

classical motionof atoms

coupling to quantum internal states

• Reaction mechanism and pathways• State-to-state surface coefficients• Energy distributions of products• Energy accommodation

67

N2(v): product distribution

Ab-initio calculations of surface processes:N recombination on SiO 2

Energy partitioning

Vibrationally excited N 2

Pros: • Full picture of the process• No adjustable parameters

Cons:• Simulation is limited to nanometer and picosecond scale• Computationally expensive

EphEtr

Evib

Erot

Rutigliano Surface Science 600 (2006) 4239–4246 68

Multiscale approach for simulations of plasma-surface interactions

Mesoscale

Interaction potentialsReaction mechanisms

Timescale: 10 – 100 ps

Quantum mechanics10 – 100 atoms

Molecular dynamics104-105 atoms

Surface coverageGlobal surface reactionprobabilities Surface modification

Timescale: seconds

Elementary reaction coefficientsSynergetic effects

Timescale: 10 – 100 ns

Neyts&Bogaerts J.Phys.D 47 2014 Bedra et al. Langmuir 2006, 22

SiO2

69

Closing remarks

• Surface processes are very important

• Energetic and reactive species can be produced on the surface (and not only lost)

• New experiments and first principle calculations are needed for fundamental understanding of surface reaction mechanisms

• There is no database for rates of surface reactions

• If you need reliable surface data for your particular plasma the best is to measure it

70