I.A.E.A. ViennaCRP Atomic and Molecular Data for Plasma Modelling

Coordination Meeting 18-20 June, 2007

INTERACTION OF SLOW IONS

WITH SURFACES:

COLLISIONS OF SMALL HYDROCARBON IONS WITH CARBON, TUNGSTEN AND BERYLLIUM SURFACES

ZDENEK HERMAN, JAN ŽABKA, ANDRIY PYSANENKO

J. Heyrovský Institute of Physical Chemistry, v.v.i.

Academy of Sciences of the Czech Republic,

Prague

IAEA, Vienna, 18-22 June, 2007

AIMStudies of polyatomic ions in scattering experiments:

Ion survival probability, energy transfer at surfaces, fragmentation and chemical reactions at surfaces

SURFACES INVESTIGATED- (carbon surfaces (15 – 45 eV)) HOPG (highly-oriented pyrolytic graphite), Tokamak tiles

a) room-temperature (covered with hydrocarbons)b) heated to 6000 C 1(“clean”)

- carbon (HOPG) surfaces at 3 – 10 eVroom temperature, scattering kinematics

- tungsten surfaces (15 – 45 eV)room-temperature and heated

- beryllium surfaces (15 – 45 eVroom-temperature and heated

PROJECTILE IONSsmall hydrocarbon ions : CH3

+, CH4+•, CH5

+ (D, 13C); C2Hx+

(x=2-5), C3Hx+ (x=2-8),

cations and dications C7Hn+/2+ (n=6-8)

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)

•scattering kinematics

1.COLLISIONS OF CDn+ (n=3-5) WITH CARBON (HOPG),

ROOM TEMPERATURE, Φs = 300

VERY LOW ENERGY 3 – 11 eV

ION SURVIVAL PROBABILITY

Sa(%)

SA decreases below Einc. = 10 eV to zero

Observation:

-only decreasing product dissociation with decreasing incident energy

CD5+→ CD3

+ + D2 (ΔE ~ 2 eV)

CD5+ (HOPG)

MASS SPECTRA OF PRODUCTSΦS = 300

CD5+ : close-shell, non-reactive projectile ion

CD5+ (HOPG)

ANGULAR DISTRIBUTIONS OF PRODUCTS

CD5+

CD3+

Einc(CD5+) = 10.45 eV

CD5+ (HOPG)

TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS

CD5+ CD3

+

0

20

40

60

80

0

50

100

150

0

50

100

150

200

0

50

100

150

0 1 2 3 4 5 6 7 80

50

100

150

0 1 2 3 4 5 6 7 80

50

100

35o40o

CD+

5

45o 49o

P(

E' tr )

[a

.u.]

P(

E' tr )

[a

.u.]

P(

E' tr )

[a

.u.]

E'tr [V]

50o

E'tr [V]

60o

Einc(CD5+) = 10.45 eV

CD5+ (HOPG)

KINEMATICS: VELOCITY SCATTERING DIAGRAM

Einc(CD5+) = 10.45 eV

CD5+ (HOPG)

KINEMATICS: EVALUATION

1. Same peak velocity of CD5+ and

CD3+: fragmentation AFTER

interaction with surface

2. Effective surface mass for collisions of CD5

+:

meff = 62 m.u.

(2xC2H5-, 4xCH3-)

3. Inelastic collisions: inelasticity in C.M.

T = 62/84 Einc = 7.7 eV

T’ = 0.29 T

ΔT = T – T’ = 5.49 eV

Einc(CD5+) = 10.45 eV

CD3+ (HOPG)

ANGULAR DISTRIBUTIONS OF PRODUCTS

Einc(CD3+) = 8.3 eV

background

fast deflected

inelastic scattering

CD3+ (HOPG)

TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS

Einc(CD3+) = 8.3 eV

CD3+ (HOPG)

KINEMATICS: EVALUATION

1. No fragmentation of the projectile CD3

+.

2. Effective surface mass for collisions of CD3

+:

meff = 29 m.u.

(C2H5-, 2xCH3-)

3. Inelastic collisions: inelasticity in C.M.

T = 29/47 Einc = 5.12 eV

T’ = 0.51 T

ΔT = T – T’ =2.62 eV

Einc(CD3+) = 8.3 eV

Observation:

- simple fragmentation of projectile ions

CD4+•→ CD3

+ + D• (ΔE = 1.8 eV)

- chemical reaction with surface material

CD4+• + H-S → CD4H+ → CD3

+ +HD

→ CD2H+ + D2

- fast deflected CD4+ projectile ions of

incident energy

CD4+ • (HOPG)

MASS SPECTRA OF PRODUCTS

Φs = 300

open-shell, reactive projectile radical ion

SUMMARY

VERY LOW ENERGY (3-10 eV) SCATTERING ON ROOM-TEMPERATURE CARBON SURFACE

1. Ion survival probability decreases below 10 eV towards zero

2. Non-reactive ions (CD5+, CD3

+): only inelastic collisions, fragmentation indicates dissociation AFTER interaction with the surface (CD5

+)

3. Kinematic analysis: determination of effective mass of the surface involved in the inelastic collision (different for different ions).

4. Reactive ions (CD4+•): both fragmentation and chemical reaction with surface

material (H-atom transfer from surface hydrocarbons: a very sensitive reaction tracing hydrocarbon on the surface).

Sample Material: 99.9% w-sheet (0.05 mm) cleaned mechanically or chemically to remove surface impurities

Observation

Unheated fresh sample: about 5 % of projectile ions deflected with full incident energy (evidently not hitting the surface at all)

Heated sample: heating decreases the amount of deflected ions to 0.05 % or less

Room-temperature sample after heating: the amount of deflected ions remains under 0.1%

XPS analysis of the sample

Unheated fresh sample: tungsten oxides and small amount of tungsten carbide + hydrocarbon C-H groups on the surface

Sample after heating: decrease of tungsten oxides, sharp increase of tungsten carbides (2.5-times: evidently degradation of surface hydrocarbons)

CONCLUSION

1. Fresh sample: Islands of insulating matter (presumably tungsten oxides) cause part of projectile ions to be deflected by surface charge

2. Heating decreases the amount of surface oxides and strongly increases the amount of non-insulating tungsten carbide (collisions mostly with WC on the surface)

3. Room-temperature sample after heating: hydrocarbon layer of mostly WC surface

2. COLLISIONS WITH TUNGSTEN SURFACE

SURFACE 15.4 eV 30.9 eV 45.4 eV

ROOM-TEMP

CD4+• W

BeHOPG

0.050.05

0.37±0.1

0.05

0.34±0.2

0.120.05

0.27±0.2

CD5+ W

BeHOPG

5.82.1

12.5±5

0.82.1

12±5

1.21.2

(18±7)

C2D4+• W

BeHOPG

0.170.4

1.0±0.5

0.170.7

1.0±0.4

0.19

0.9±0.2

C2H5+ W

HOPG2.7

1.1±0.031.6

1.0±0.10.85

0.3±0.03HEATED

CD4+• W

BeHOPG

0.03

0.5

0.020.080.23

0.02

CD5+ W

BeHOPG

1.1 0.5 0.50.15(23)

C2D4+• W

BeHOPG 0.35

0.10.4

0.4±0.05

C2H5+ W 0.56 0.32 0.24

ION SURVIVAL PROBABILITY, Sa (%)

CONCLUSION: survival probability on W or Be usually about 5-10x smaller than on HOPG

MASS SPECTRA OF PRODUCT IONS

TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS --------- room temperature --------- heated to 6000C

SUMMARYCOLLISIONS OF SMALL HYDROCARBON IONS:

TUNGSTEN VS. CARBON SURFACES

1. W-surface: fraction of projectile ions deflected by surface charges (up to 5 % on fresh room-temp surface), decreases with or after heating of the surface. Probable reason: islands of W-oxides on the surfaces

2. Survival probability: on W-surfaces up to 10-times smaller than on C-surfaces (HOPG)

3. W-surface at room–temperature covered with hydrocarbons: analogous to C- surfaces - fragmentation and chemical reactions of radical projectile ions - CH4

+: H-atom transfer, formation of C2- and C3- hydrocarbons; - C2D4

+: H-atom transfer, formation of C3- hydrocarbons

W-surface heated: only fragmentation of projectile ions: analogous to C-surfaces

4. Inelasticity of surface collisions (from product ion translational energy distributions):- similar on W-surfaces to that on C-surfaces

room-temperature: collisions with hydrocarbons on the surfacesheated: collision with WC on the surface(?)

- heated surfaces usually less inelastic (similarly as on C-surfaces)- for C1-projectile ions: less inelastic with increasing incident energy, i.e.

fraction of energy in translation slightly increases

3. COLLISIONS WITH BERYLLIUM SURFACES

Sample Material: Be-foil, 0.5 mm, >99% Be (Goodfellow), cleaned mechanically to remove surface impurities

Observation

Unheated fresh sample: several % of projectile ions deflected with full incident energy (evidently not hitting the surface at all)

Heated sample: heating decreases the amount of deflected ions to 0.05 % or less

Room-temperature sample after heating: the amount of deflected ions remains under 0.1%

XPS analysis of the sample

Unheated fresh sample: on the surface Be-oxides, Be-carbides, 42 % Be as metal; 78% hydrocarbon C-H groups on the surface, small amount of carbon (~10%) also in C=O and COOH groups

Sample after heating: Be as metal decreases to 9 %, sharp increase of Be-carbides (to 18% - 32%) carbidic phase covered with hydrocarbons on room-temperature surface; surface also contains Be-oxides (67% of Be in oxides)

CONCLUSION

1. Fresh sample: Islands of insulating matter cause part of projectile ions to be deflected by surface charge

2. Heating decreases the amount of insulating material on the surface and strongly increases the amount of Be-carbide.

3. Room-temperature sample after heating: hydrocarbon layer on at least part of the surface (BeC).

CD5+ - Be

ANGULAR DISTRIBUTION OF PRODUCTSΦS = 300, room-temperature surface after heating

CD3+

Einc = 45.4 eV

CD3+

Einc = 30.9 eV

CD5+ - Be

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTSΦS = 300, room-temperature surface after heating

Einc = 30.9 eV Einc = 45.4 eV

Energy losses (21%, 38%,56%) recalculated to CD5+

C2D4+• - Be

ANGULAR DISTRIBUTION OF PRODUCTSEinc = 15.8 eV, ΦS = 300, room-temperature surface after heating

C2D3+

C2D4+→ C2D3

+ + D → C2D4H+ → C2D3

+ + HD

1. Simple dissociation2. chemical reaction + dissociation

C2D2H+

C2D4+ → C2D4H+ → C2D2H+ + D2

chemical reaction + dissociation

C2D4+• - Be

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTSΦS = 300, room-temperature surface after heating

C2D3+

C2D4+→ C2D3

+ + D

→ C2D4H+ → C2D3+ + HD

1. Simple dissociation

2. chemical reaction + dissociation

Einc = 15.8 eV

Observation:

Three different energy losses to 23%, 37%, and 57% of Einc, probability angle-dependent; presumably scattering from different surface material

C2D4+• - Be

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTSΦS = 300, room-temperature surface after heating

C2D2H+

C2D4+ → C2D4H+ → C2D2H+ + D2

(chemical reaction + dissociation)

Einc = 15.8 eV

Observation:Surface chemical reaction:

Two different energy losses to 25% and 39% of Einc, probability angle-dependent; presumably scattering from different surface material.Evidently, high-energy (59% Einc) and low-angle scattering is non-reactive (only fragmentation)

C2D4+• - Be

ANGULAR DISTRIBUTION OF PRODUCTSEinc = 15.8 eV, ΦS = 300, heated surface to 6000C

C2D3+

C2D4+• - Be

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTSEinc = 15.8 eV, ΦS = 300, heated surface to 600oC

C2D4+• - Be

KINEMATICS: EVALUATIONEinc = 15.8 eV, ΦS = 300, room-temperature and heated

A: meff = 59 m.u. (2BeO)

B: meff = 47 m.u. (3 CH3, C3H7)

C: meff = 30 m.u. (2CH3, C2H5),

No chemical reaction in low-angle scattering

SUMMARYSCATTERING ON BERYLLIUM SURFACES

1. Survival probability comparable to that on W-surfaces (lower than on carbon)

2. On fresh room-temperature surfaces several % of incident ions deflected without collision, heating and after heating this fraction decreases to ~0.1 % or less.

3. Reactive ions (CD4+, C2D4

+): on room-temperature surfaces both simple dissociation and chemical reaction (+ dissociation). Main reaction : H-atom transfer from reaction with surface hydrocarbons (similarly as on C or W).

4. Scattering on Be-surfaces more complex than on C or W: structures both in angular distributions and translational energy distributions. Presumably connected with various materials on the surface (oxides, carbides, only partially covered with hydrocarbons on room-temperature surfaces).

POSITIONS OF PEAKS

IN TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS

(MEAN INELASTICITY OF SURFACE COLLISIONS)

PROBABILITY OF ION SURVIVAL DEPENDENCE ON INCIDENT ANGLE

IONS FROM ETHANOL (SS SURFACE COVERED BY HYDROCARBONS)

C2H5OH+•

C2H5OH2+, C2H5O+

CONCLUSIONS

- survival probability depends strongly on incident angle: lower for steep collisions

- survival much higher for ions of low ionization energy (usually closed-shell ions), for ions of IE> ~10.5 eV about an order of magnitude lower