INTERACTION OF SLOW IONS I.A.E.A. Vienna CRP 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

SURFACES INVESTIGATED AIM Studies 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 eV room temperature, scattering kinematics - tungsten surfaces (15 – 45 eV) room-temperature and heated - beryllium surfaces (15 – 45 eV PROJECTILE IONS small 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

ION SURVIVAL PROBABILITY 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

CD5+ (HOPG) MASS SPECTRA OF PRODUCTS ΦS = 300 CD5+ : close-shell, non-reactive projectile ion Observation: -only decreasing product dissociation with decreasing incident energy CD5+→ CD3+ + D2 (ΔE ~ 2 eV)

ANGULAR DISTRIBUTIONS OF PRODUCTS CD5+ (HOPG) ANGULAR DISTRIBUTIONS OF PRODUCTS Einc(CD5+) = 10.45 eV CD5+ CD3+

TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS CD5+ (HOPG) TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS Einc(CD5+) = 10.45 eV CD5+ CD3+

KINEMATICS: VELOCITY SCATTERING DIAGRAM CD5+ (HOPG) KINEMATICS: VELOCITY SCATTERING DIAGRAM Einc(CD5+) = 10.45 eV

KINEMATICS: EVALUATION CD5+ (HOPG) KINEMATICS: EVALUATION Einc(CD5+) = 10.45 eV Same peak velocity of CD5+ and CD3+: fragmentation AFTER interaction with surface 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

ANGULAR DISTRIBUTIONS OF PRODUCTS CD3+ (HOPG) ANGULAR DISTRIBUTIONS OF PRODUCTS Einc(CD3+) = 8.3 eV background fast deflected inelastic scattering

TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS CD3+ (HOPG) TRANSLATIONAL ENERGY DISTRIBUTIONS OF PRODUCTS Einc(CD3+) = 8.3 eV

KINEMATICS: EVALUATION CD3+ (HOPG) KINEMATICS: EVALUATION Einc(CD3+) = 8.3 eV No fragmentation of the projectile CD3+. 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

CD4+ • (HOPG) MASS SPECTRA OF PRODUCTS Φs = 300 open-shell, reactive projectile radical ion 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

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).

2. COLLISIONS WITH TUNGSTEN 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 Fresh sample: Islands of insulating matter (presumably tungsten oxides) cause part of projectile ions to be deflected by surface charge Heating decreases the amount of surface oxides and strongly increases the amount of non-insulating tungsten carbide (collisions mostly with WC on the surface) Room-temperature sample after heating: hydrocarbon layer of mostly WC surface

ION SURVIVAL PROBABILITY, Sa (%) SURFACE 15.4 eV 30.9 eV 45.4 eV ROOM-TEMP CD4+• W Be HOPG 0.05 0.37±0.1 0.34±0.2 0.12 0.27±0.2 CD5+ 5.8 2.1 12.5±5 0.8 12±5 1.2 (18±7) C2D4+• 0.17 0.4 1.0±0.5 0.7 1.0±0.4 0.19 0.9±0.2 C2H5+ 2.7 1.1±0.03 1.6 1.0±0.1 0.85 0.3±0.03 HEATED 0.03 0.5 0.02 0.08 0.23 1.1 0.15 (23) 0.35 0.1 0.4±0.05 0.56 0.32 0.24 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

COLLISIONS OF SMALL HYDROCARBON IONS: TUNGSTEN VS. CARBON SURFACES SUMMARY COLLISIONS OF SMALL HYDROCARBON IONS: TUNGSTEN VS. CARBON SURFACES 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 Survival probability: on W-surfaces up to 10-times smaller than on C-surfaces (HOPG) 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 surfaces heated: 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 Fresh sample: Islands of insulating matter cause part of projectile ions to be deflected by surface charge Heating decreases the amount of insulating material on the surface and strongly increases the amount of Be-carbide. Room-temperature sample after heating: hydrocarbon layer on at least part of the surface (BeC).

ANGULAR DISTRIBUTION OF PRODUCTS CD5+ - Be ANGULAR DISTRIBUTION OF PRODUCTS ΦS = 300, room-temperature surface after heating CD3+ Einc = 45.4 eV CD3+ Einc = 30.9 eV

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS CD5+ - Be TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS ΦS = 300, room-temperature surface after heating Einc = 30.9 eV Einc = 45.4 eV Energy losses (21%, 38%,56%) recalculated to CD5+

ANGULAR DISTRIBUTION OF PRODUCTS C2D4+• - Be ANGULAR DISTRIBUTION OF PRODUCTS Einc = 15.8 eV, ΦS = 300, room-temperature surface after heating C2D3+ C2D4+→ C2D3+ + D → C2D4H+ → C2D3+ + HD 1. Simple dissociation 2. chemical reaction + dissociation C2D2H+ C2D4+ → C2D4H+ → C2D2H+ + D2 chemical reaction + dissociation

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS C2D4+• - Be TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS ΦS = 300, room-temperature surface after heating C2D3+ C2D4+→ C2D3+ + D → C2D4H+ → C2D3+ + HD 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

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS C2D4+• - Be TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS ΦS = 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)

ANGULAR DISTRIBUTION OF PRODUCTS C2D4+• - Be ANGULAR DISTRIBUTION OF PRODUCTS Einc = 15.8 eV, ΦS = 300, heated surface to 6000C C2D3+

TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS C2D4+• - Be TRANSLATIONAL ENERGY DISTRIBUTION OF PRODUCTS Einc = 15.8 eV, ΦS = 300, heated surface to 600oC

KINEMATICS: EVALUATION C2D4+• - Be KINEMATICS: EVALUATION Einc = 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

SCATTERING ON BERYLLIUM SURFACES SUMMARY SCATTERING ON BERYLLIUM SURFACES Survival probability comparable to that on W-surfaces (lower than on carbon) On fresh room-temperature surfaces several % of incident ions deflected without collision, heating and after heating this fraction decreases to ~0.1 % or less. 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). 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)

DEPENDENCE ON INCIDENT ANGLE 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