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TRANSCRIPT
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