ENERGY TRANSFER, DISSOCIATION AND CHEMICAL REACTIONS IN COLLISIONS OF SLOW POLYATOMIC IONS WITH SURFACES ZDENEK HERMAN V. Čermák Laboratory J. Heyrovský Institute of Physical Chemistry Academy of Sciences, Prague Collaborators Prague: 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) Funding EURATOM,, I.A.E.A., Grant Agency of the Czech Republic, GA Academy of Sciences, CZ-A cooperation programs

AIM Studies 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-(CF 2 ) 10 -CF 3 CH-SAM: -S-(CH 2 ) 11 -CH 3 COOH-SAM: -S-(CH 2 ) 11 -COOH - stainless steel (covered by hydrocarbons) - carbon surfaces HOPG (highly-oriented pyrolytic graphite) a) covered by hydrocarbons b) at 1000K (“clean”) Tokamak bricks PROJECTILE IONS ethanol polyatomic ions : C 2 H 5 OH +, C 2 H 5 O +, C 2 H 5 OH 2 +; toluene ions small hydrocarbon ions : CH 3 +, CH 4 +, CH 5 + (D, 13 C); C 2 H x + (x=2-5), C 3 H n + (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 E TOT = E tr + (E int ) = 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

ENERGY PARTITIONING EXAMPLE OF EVALUATION FOR CF-SAM, E tr = 21.1 eV mass spectraP(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

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

P(E’ int ): INTERNAL ENERGY DISTRIBUTIONS OF SURFACE-EXCITED IONS Projectile ion: CH 4 + Heated and non-heated HOPG surface Mean internal energy ~ 5-6% E inc

ENERGY PARTITIONING INCIDENT ANGLE DEPENDENCE INCIDENT ENERGY DEPENDENCE CF-SAM, E inc = 22 eV SS-hydrocarbons,  N =60 0

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 (E peak ~6% E tr ), only for CF-SAM about 3-times higher (~18%)

PERCENTAGE OF SURVIVING IONS, S a (%)

C2Xn+C2Xn+ C2Xn+C2Xn+ C2Xn+C2Xn+ C3C3 C3C3 C3C3 MASS SPECTRA CD X +, HOPG, E inc = 51.6eV,  N =60 o HEATEDNON HEATED

C 2 X 3 + (X=H,D) formation : interaction of the projectile with terminal CH 3 - group CD 4 H + formation : interaction of CD 4 +. with terminal H-atom, direct H-atom transfer C group formation : C 3 H 3 + mainly reaction, C 3 H 5 + (C 3 H 7 + ) mainly sputtering SUGGESTED MECHANISMS  (CD 4 H + ) :  (diss) :  (C 2 H 3 + ) = 10 : 3: 1

PRODUCT ION TRANSLATION ENERGY DISTRIBUTION 51.6eV CD 5 +, HOPG,  N =60 o HEATEDNON 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% E inc non-heated ~ 40-50% E inc

HEATEDNON HEATED MASS SPECTRA

C3Xn+C3Xn+ MASS SPECTRA C 2 D 4 + : FORMATION OF C 3 - GROUP

PRODUCT ION TRANSLATION ENERGY DISTRIBUTIONS C 2 H 3 +, C 2 H 5 +, HOPG,  N =60 o

CH 4 + (R) CH 4 + (NR) INITIAL INTERNAL ENERGY OF PROJECTILES: EFFECT ON DISSOCIATION Projectile ion preparation (R): relaxed ions (Colutron source) (NR): non-relaxed ions (Nier source) CH 4 + Estimated internal energy (from differences in crossings and thresholds of CERMS curves) (E int ) max < eV Estimated (E int ) max of CH 4 + (from break-down pattern and photoelectron spectra: <1.8 eV CONCLUSION Initial E int fully available for dissociation

C 2 H 4 + (C 2 H 4 ) (NR) C 2 H 4 + (R) C 2 H 4 + (C 2 H 6 ) (NR) INITIAL INTERNAL ENERGY OF PROJECTILES: EFFECT ON DISSOCIATION Projectile ion preparation (R): relaxed ions (Colutron source) (NR): non-relaxed ions (Nier source) C 2 H 4 + Estimated internal energy (from differences in crossings and thresholds of CRMS curves) ~ 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, - C 2 from C 1 or C 3 from C 2 : interaction of the projectile with the terminal CH 3 - 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 eV NEW DEMAND Lower plasma temperatures important, too: surface interactions of hyperthermal particles of energies eV