ENERGY TRANSFER, DISSOCIATION AND CHEMICAL REACTIONS IN COLLISIONS

OF SLOW POLYATOMIC IONS WITH SURFACES

ZDENEK HERMANV. Čermák Laboratory

J. Heyrovský Institute of Physical Chemistry Academy of Sciences, Prague

CollaboratorsPrague: J. Žabka, J. Roithová, J. Jašík, Z. Dolejšek, J. Kubišta

Innsbruck: T.D. Märk,, L. Feketeová

(A. Qayyum, T. Tepnual, C. Mair, P.Scheier, S. Matt-Leubner)

FundingEURATOM,, I.A.E.A., Grant Agency of the Czech Republic, GA Academy of Sciences, CZ-A cooperation programs

AIMStudies of polyatomic ions in scattering experiments:

(i) Energy transfer at surfaces (ion activation in MS)

(ii) Surface-induced dissociation- chemical reactions at surfaces

(iii) Survival probability in surface collisions

SURFACES INVESTIGATED- self-assembled monolayers (SAM surfaces)

CF-SAM: -S-(CF2)10-CF3

CH-SAM: -S-(CH2)11-CH3

COOH-SAM: -S-(CH2)11-COOH

- stainless steel (covered by hydrocarbons)

- carbon surfaces HOPG (highly-oriented pyrolytic graphite)

a) covered by hydrocarbonsb) at 1000K (“clean”)

Tokamak bricks

PROJECTILE IONS

ethanol polyatomic ions : C2H5OH+•, C2H5O+, C2H5OH2+; toluene ions

small hydrocarbon ions : CH3+, CH4

+•, CH5+ (D, 13C); C2Hx

+ (x=2-5), C3Hn

+ (n=3-8)

MEASUREMENTS- mass spectra of ion products- translational energy distributions of ion products- angular distributions of ion products

EXPERIMENT

PROCESSES OBSERVED

•neutralization of ions (survival pobability)

•surface-induced dissociations (energy partitioning)

•chemical reactions at surfaces (H-atom, CHn-transfer)

•quasi-elastic scattering of projectiles

ENERGY PARTITIONING IN POLYATOMIC ION-SURFACE COLLISIONS

BASIC EQUATION

ETOT = Etr + (Eint) = E’int + E’tr + E’surf

EVALUATION

P(E’int) from the extent of fragmentation (mass spectra) + break-down pattern

P(E’tr) from direct measurements

P(E’surf) from the difference

DEPENDENCE ON - incident ion energy

- incident angle

- type of surface

- surface temperature

20 25 30 35 40 45 50

0

20

40

60

80

100

m/z

c)COOH-SAM

20 25 30 35 40 45 50

0

20

40

60

80

100

b)CH-SAM

20 25 30 35 40 45 50

0

20

40

60

80

100

a)CF-SAM

32.0 eV

21.1 eV

11.1 eV

0 2 4 6 8 10

C2H5

+

C2H4

+

C2H5OH+

C2H5O+

CH2OH+

CH3

+

C2H3

+

COH+

E int

( eV )

ENERGY PARTITIONING EXAMPLE OF EVALUATION FOR CF-SAM, Etr= 21.1 eV

mass spectra P(E’int) P(E’tr) P(v’)

CONCLUSIONS - strongly inelastic collisions - practically the same velocity distributions for product ions: dissociation after the interaction with the surface in a unimolecular way

32.0 eV

21.1 eV

11.1 eV

0 2 4 6 8 10

C2H5

+

C2H

4

+

C2H

5OH

+

C2H

5O

+

CH2OH

+

CH3

+

C2H

3

+

COH+

E int

( eV )

0 2 4 6 8

(d)CH/SS

E´int (eV)

N=60o, (22.3 V)

(c)COOH-SAM

N=60o, (22.3V)

(b)CH-SAM

N=80o, (22.4 V)

N=60

o, (21.8 V)

P(E

' int) (a)

CF-SAM

N=80

o, (22.0 V)

N=60

o, (21.1 V)

N=40

o, (21.0 V)

P(E’int):

INTERNAL ENERGY OF SURFACE-EXCITED IONS P(E’int) P( E’ int) incident energy dependence (CF-SAM) various surfaces

0 1 2 3 4 5

Eint(CH4

+)

P(E'int

)

E'int

[eV]

CH2

+

CH3

+CH4

+

51.6 eV H NH exp. exp. calc.

CD4

+ 5.0 10.3 10.3

CD3

+ 54.3 50.9 67.8

CD2

+ 40.8 38.8 21.9

H NH exp. exp. calc.

CD4

+ 28.5 34.8 31.3

CD3

+ 49.5 48.7 55.7

CD2

+ 22.0 16.5 13.0

31.6 eV

NON-HEATED(room temperature)HEATED

~600oC

16.6eV H NH exp. calc. exp. calc.

CD4

+ 61.0 61.0 45.7 45.6

CD3

+ 29.9 34.7 48.3 48.5

CD2

+ 9.1 4.3 5.9 5.9

P(E’int): INTERNAL ENERGY DISTRIBUTIONS OF SURFACE-EXCITED IONS

Projectile ion: CH4+

Heated and non-heated HOPG surface

Mean internal energy ~ 5-6% Einc

0 5 10 15 20 25

0

20

40

60

80

100

P(E')

P(E')

N= 80o

E' [eV]

P(E')

0.520.18

0.30

0 5 10 15 20 250

20

40

60

80

100

E'int

E'tr

E'int

E'trE'

surf

E'surf

E'int E'

trE'

surf

N= 60

o

0.46

0.37

0.17

0 5 10 15 20 250

20

40

60

80

100

N= 40

o

0.66

0.16

0.18

ENERGY PARTITIONING

INCIDENT ANGLE DEPENDENCE INCIDENT ENERGY DEPENDENCE

CF-SAM, Einc = 22 eV SS-hydrocarbons, N=600

ENERGY PARTITIONING

DIFFERENT SURFACES

CONCLUSIONS

- P(E’int) does not depend on incident angle

- relative fractions of P(E’int), P(E’tr), P(E’surf) practically independent of collision energy

- P(E’int) practically the same for all studied surfaces (Epeak~6% Etr), only for CF-SAM about 3-times higher (~18%)

0 5 10 15 20 250

20

40

60

80

100SS/CHP(E')

E'int

E'tr

E'surf

0.620.32

0.06

0 5 10 15 20 250

20

40

60

80

100

P(E')

P(E')

CH-SAMP(E')

0.30

0.06

0.64

0 5 10 15 20 250

20

40

60

80

100

E'int

E'tr

E'int E'tr

E'surf

E'surf

E'intE'tr

E'surf

COOH-SAM

0.68

0.27

0.05

0 5 10 15 20 250

20

40

60

80

100

CF-SAM

0.46

0.370.17

0 5 10 15 20 250

20

40

60

80

100SS/CHP(E')

E'int

E'tr

E'surf

0.620.32

0.06

0 5 10 15 20 250

20

40

60

80

100

P(E')

P(E')

CH-SAMP(E')

0.30

0.06

0.64

0 5 10 15 20 250

20

40

60

80

100

E'int

E'tr

E'int E'tr

E'surf

E'surf

E'intE'tr

E'surf

COOH-SAM

0.68

0.27

0.05

0 5 10 15 20 250

20

40

60

80

100

CF-SAM

0.46

0.370.17

PERCENTAGE OF SURVIVING IONS, Sa(%)

PERCENTAGE OF SURVIVING IONS, Sa(%)

TABLE 2: Survival probability, Sa (%) of ions in collision of C7H82+/+,

C7H72+/+ and C7H6

2+/+ with HOPG surface at room temperature*

Einc= 10.3 eV Einc= 25.3 eV

projectile Seff 10-6 Sa (%) Seff 10-6 Sa (%)

C7H82+ 3.6 1.4 6.4 2.5 11 4 20 7

C7H72+ 14 4 25 9 13 2 23 4

C7H62+ 4.0 1.6 7.2 2.5 18 4 32 7

C7H8+ 6.3 1.0 11 2

C7H7+ 7.7 2.0 14 4

* Incident angle N = 60 .

PERCENTAGE OF SURVIVING IONS, Sa(%)

0

5

5x

0

5

10

CD5

+CD

5

+

CD4

+ CD4

+

CD3

+CD

3

+

0

50

8x

0

25

50

5x

15 20 25 30 35 40 450

50

100

150 60x

15 20 25 30 35 40 450

50

100

10x

0

5

10

0

5

10

0

1

2

5

10

m/z

0.5

1.0

(b) 51.2 eV(a) 31.2 eV

15 20 25 30 35 40 450

100

200

I (a.u.)

CD5

+

CD5

+

(c)

x20

m/z

0

5

10

CD4

+

CD4

+

(b)

0

5

CD3

+

(a)CD

3

+

C2Xn+

C2Xn+

C2Xn+

C3

C3

C3

MASS SPECTRA

CDX+, HOPG, Einc = 51.6eV, N=60o

HEATED NON HEATED

• C2X3+ (X=H,D) formation :

interaction of the projectile with terminal CH3 - group

• CD4H+ formation :interaction of CD4

+. with terminalH-atom, direct H-atom transfer

• C3+ - group formation : C3H3

+ mainly reaction, C3H5+ (C3H7

+) mainly sputtering

SUGGESTED MECHANISMS

(CD4H+) : (diss) : (C2H3+) = 10 : 3: 1

15 16 17 18 19 20

(b)CD3

+CD5

+

CD3

+CD5

+

v' [km/s]

E'tr [eV]

20 25 30 35 40 45

(a)

P(v')

P(Etr')

CD5

+

CD3

+

16.6 eV

CD5

+

CD3

+

(d)16.6 eV

(b)31.6 eV

(a)

(e)31.6 eV

0 5 10 15 20 25 30 35

(c)51.6 eV

0 5 10 15 20

P[v']

(f)

P[E'tr]

51.6 eV

E'tr [eV] v' [km/s]

PRODUCT ION TRANSLATION ENERGY DISTRIBUTION

51.6eV

CD5+, HOPG, N=60o

HEATED NON HEATED

CONCLUSIONS:

1.Practically the same velocity of product ion species - dissociation after surface interaction

2. Inelastic collisions:

E’tr of product ions: heated ~ 75% Einc

non-heated ~ 40-50% Einc

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 440

20

40

60

80

100

C2H

3

+, E

inc = 46.3 eV

I [a

rb.

un

its]

m/z [ thomson ]

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 440

20

40

60

80

100

C2H

2

+, E

inc = 46.3 eV

I [a

rb.

un

its]

m/z [ thomson ]

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 440

10

20

30

40

50

C2H

3

+, Einc

= 46.3 eV

I [a

rb.

un

its]

m/z [ thomson ]

HEATED NON HEATED

24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 440

10

20

30

40

50

C2H

2

+, E

inc = 46.3 eV

I [a

rb.

un

its]

m/z [ thomson ]

MASS SPECTRA

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 450

1

2

3

4

5

6

7

8

9

C2D

3

+

C2D

2

+

C2D

4

+

I [a

rb. u

nits

]

m/z [ thomson ]

HEATED NON-HEATED chemical reaction sputtering background

Ek = 46.3 eV

37 38 39 40 41 42 43 440.00

0.05

0.10

0.15

0.20

0.25

0.30 39:40:41:42=0.03:0.35:0.4:0.14

39 0.03 0.03

40 0.34 0.38

41 0.51 0.43

42 0.12 0.15

C3Xn+

MASS SPECTRA C2D4+: FORMATION OF C3 - GROUP

0 5 10 15 20 25 30 35

46.3eV

E'tr [eV]

31.3eV

20oC

600oC

C2H

3

+ + HOPG -> C2H

3

+

16.6eV

P[E

' tr]

0 5 10 15 20 25 30 35

Calculated from product C2H

3

+

46.3eV

E'tr [eV]

21.3eV

20oC

600oC

C2H

5

+ + HOPG -> C2H

5

+

11.6eV

P[E

' tr]

PRODUCT ION TRANSLATION ENERGY DISTRIBUTIONS

C2H3+, C2H5

+, HOPG, N=60o

CH4+(R)

CH4+ (NR)

INITIAL INTERNAL ENERGY

OF PROJECTILES:

EFFECT ON DISSOCIATION

Projectile ion preparation

(R): relaxed ions (Colutron source)

(NR): non-relaxed ions (Nier source)

CH4+

Estimated internal energy (from differences in crossings and thresholds of CERMS curves)

(Eint)max < 1.8-2.1 eV

Estimated (Eint)max of CH4+ (from break-down pattern and photoelectron spectra:

<1.8 eV

CONCLUSION

Initial Eint fully available for dissociation

C2H4+(C2H4) (NR)

C2H4+(R)

C2H4+(C2H6) (NR)

INITIAL INTERNAL ENERGY

OF PROJECTILES:

EFFECT ON DISSOCIATION

Projectile ion preparation

(R): relaxed ions (Colutron source)

(NR): non-relaxed ions (Nier source)

C2H4+

Estimated internal energy (from differences in crossings and thresholds of CRMS curves)

<Eint> ~ 1.5 eV

CONCLUSIONS

1. Energy transfer in collisions of polyatomic ions with surfaces:

- unimolecular decomposition after surface interaction

- internal excitation of projectiles: independent of incident angle,

mean value mostly about 6% of incident E (CF-SAM: about 18%)

- translational energy of products: decreases with incident angle

- relative fractions (P(E’int), P(E’tr), P(E’surf)) do not change with incident energy

2. Initial internal energy of projectile ions:

fully available in dissociative processes

3. Survival probability on C/hydrocabons:

- about 0.1 % for cations of RE>10.5 eV,

- about 1-10 % for cations of RE<10.5 eV (closed-shell ions)

4. Chemical reactions of radical hydrocarbon ions on C/hydrocarbons:

- H-atom transfer,

- C2 from C1 or C3 from C2: interaction of the projectile with the terminal CH3- group

MOTIVATION FROM FUSION RESEARCH

1.Collisions of hyperthermal particles with solid surfaces (limiters, divertors) leads to erosion of material by physical and chemical processes

2. Products of these collsions (ionic, neutral) again interact with plasma and solid surfaces

3. Importance of molecular species

EARLIER DEMAND

Interaction of atomic and molecular species with exposed surfaces of fusion vessels (carbon, tungsten): from plasma temperature energy range 10 - 50 eV

NEW DEMAND

Lower plasma temperatures important, too:

surface interactions of hyperthermal particles of energies 1 - 10 eV