Electron impact ionization and dissociation of molecular ions

Transcription

1 Electron impact ionization and dissociation of molecular ions P. DEFRANCE 1 J. LECOINTRE 1, J.J. JURETA 1.2, D.S. BELIC 3 R.K. JANEV 4 1 Département de Physique-PAMO, Université Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium 2 Institute of Physics, Belgrade, P.O. Box 68, Belgrade, Serbia 3 Faculty of Physics, P.O. Box 386, Belgrade, Serbia 4 Macedonia Academy of Sciences and Art IAEA-November 2009 pierre.defrance@uclouvain.be

2 OUTLINE Introduction Experimental set-up Measurements and results: Absolute Cross sections for dissociative excitation (DE) and for dissociative ionization (DI) Kinetic Energy Release Distributions (KERD) Conclusion

3 Introduction Accurate values of electron impact ionization and dissociation cross sections of molecular ions are usefull: in laboratory plasmas [1]: Important as impurities in plasmas of fusion reactors Present in some plasma processing also in natural plasmas [2]: Modelling planetary or cometary's atmospheres Studying the evolution and composition of interstellar gas clouds [1] G.H.Dunn, 1992, Nucl. Fusion Suppl. 2, 25A. [2] Sternberg and A. Dalgarno, 1995, Astrophys. J., Suppl. Ser. 99, 565

4 Introduction Processes concerned with ion collection: Single Ionization (SI) AB + + e - AB e - Dissociative Ionization (DI) AB + + e - A + + B e - Dissociative Excitation (DE) AB + + e - A + + B + e - A + B + + e -

5 Introduction

6 Introduction Atmospheric ions: H 2+, D 2+, D 3+, CO +, N 2+, O 2+, CO 2+. Thermonuclear fusion plasmas: Hydro(deutero)-carbon ions: C 2 H +, C 2 D + ; C 2 H 2+, C 2 HD +, C 2 D 2+, CD n+ (n=1 4) Other ions: NeD + - Lecointre et al, J. Phys. B: At. Mol. Opt. Phys. 39 (2006) Lecointre et al, J. Phys. B: At. Mol. Opt. Phys. 40 (2007) 85

7 Introduction Absolute cross sections for electron impact single ionization (SI), dissociative excitation (DE) and dissociative ionization (DI), are measured from their respective thresholds up to 2.5 kev. The animated crossed beams method is applied contributions from different reaction channels (DE and DI) are separated kinetic energy release distribution (KERD) of the fragment ions are determined. Lecointre et al, J. Phys. B: At. Mol. Opt. Phys. 39 (2006) 3275

8 Introduction Particular difficulties affect electron impact experiments for molecular ions: Complex structure, many-body problem Excited or metastable targets High Kinetic Energy Release (KER) Difficulty to collect the fragments

9 Experimental set-up NeD + Example: NeD + + e - Ne Analysing magnet Fragments Ne + NeD + K A L E F S L E F f S C K: Cathode A: Anode L: Lens E: Collision region F,f: Wires S: Suppressor C: Collector

10 Kinetic Energy Release Distribution KERD Ee = 100 ev Ee = 100 ev dσ / dv (10-8 cm.s) DE dσ / dker (10-17 cm²/ev) DI DE DI backward: v<vc forward: v>vc v c Fragments velocity: v (10 5 cm/s) 2. The velocity distribution in the laboratory frame Kinetic Energy Release: KER (ev) 3. Estimate KERD via the following differentiation of the velocity distribution: dσ( EKER) -2µ v c d 1 dσ( v) = deker m² ( 1-ε 2) dv v dv

11 Example : the methane family CD + n Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CD n + (n=1-4). P. Defrance, J. Lecointre and R.K. Janev APID (2009) Analytic Representation of Cross Sections for Electron-Impact Dissociative Excitation and Ionization of CH y + (y = 1 4) Ions R.K. Janev, J. Lecointre, R.E.H.Clark, D. Humbert, P. Defrance and D. Reiter APID (2009)

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13 20 e + CD + (DE) Absolute Cross Section (10-17 cm²) Electron energy (ev) Present absolute dissociative excitation cross sections for C + (, σ 1 ) and for D + (, σ 2 ) versus electron energy.

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15 Potential energy curves for CH + ( ) and for CH (-----).

16 E KER Threshold (E th ) E KER DE CD + + e - Present results (ev) Other results (ev) Theoretical results (5), (6), (7) (± 0.1 ev) C ± 1.0 < 3 (1) 5.6 (4.4) D ± (2) Molecular state Dissociation limit b 3 Σ (v' 4) / c 3 Σ + C + ( 2 P) + D( 2 S) Theoretical results (7) (± 0.2 ev) 9.0 (7.8) c 3 Σ (5.3) b 3 Σ (v'>10) C( 3 P) + D (7.6) d 3 Π (7.0) 2 1 Σ + C( 1 D) + D (11.5) 2 1 Π (11.8) 3 1 Σ + C( 1 S) + D Present results (ev) ± 1 (3.5 ± 0.5) 0 9 ± 1 (3.8 ± 0.5) DI C + + D ± (4) 29.0 (3) 24.1 (22.9) 2 Σ + C + ( 2 P) + D ± 1 (5 7 ± 1) ADI C ± C ± ± 2 (2.0 ± 0.1) 0 40 ± 2 (15 ± 1) (1) Bannister et al (2003), (2) Djuric et al (1997), (3) Janev and Reiter (2002), (4) Kim et al (2000), (5) Saxon et al (1983), (6) Green et al (1972) and (7) Lorquet et al (1971). Note: Threshold energies in brackets are calculated from the metastable state a 3 Π (1.2 ev above the ground state X 1 Σ + ). E KER values in brackets represent the mean kinetic energy release for specific electron energies.

17 Total kinetic energy release distributions for C + fragments, for indicated electron energies.

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21 Total kinetic energy distribution for electron impact upon CD4+

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23 NeD + + e - NeD + 2e - Single Ionisation simple (SI) Ne + + D + e - Dissociative excitation (DE) (σ 1 ) Ne + D + + e - (σ 2 ) NeD + + e - (NeD )* + 2e - Ne + + D + + 2e - Dissociative Ionisation (DI) (σ 3 ) Ne + Asymmetric DI (ADI) (σ 4 ) Ne + (σ 5 ) 2008 Absolute cross sections and kinetic energy release distributions for electron impact dissociative excitation and ionization of NeD +. J. Lecointre, J.J. Jureta, J.B.A. Mitchell, V. Ngassam, A.E. Orel and P. Defrance, J. Phys. B: At. Mol. Opt. Phys. 41 (2008)

24 NeH + : potential energy of singlet states and of some states of NeH and of NeH (a) NeH + : Singlet states A 1 Π B 1 Σ + C 1 Σ + D 1 Π E 1 Σ + F 1 Π Potential energy (ev) NeH 2+ NeH + NeH** X 2 Π X 1 Σ + Ne* + (1s 2 2s2p 6, 2 S) + H(1s, 2 S) Ne + (1s 2 2s 2 2p 5, 2 P) + H + Ne + (1s 2 2s 2 2p 5, 2 P) + H*(3s, 2 S) Ne + (1s 2 2s 2 2p 5, 2 P) + H*(2s, 2 S) Ne*(1s 2 2s 2 2p 5 3s, 1,3 P) + H + Ne + (1s 2 2s 2 2p 5, 2 P) + H(1s, 2 S) Ne*(1s 2 2s 2 2p 5 3s, 1,3 P) + H(1s, 2 S) Ne(1s 2 2s 2 2p 6, 1 S) + H + 0 NeH A 2 Σ + Ne(1s 2 2s 2 2p 6, 1 S) + H*(3s, 2 S) Ne(1s 2 2s 2 2p 6, 1 S) + H*(2s, 2 S) -10 X 2 Σ + Ne(1s 2 2s 2 2p 6, 1 S) + H(1s, 2 S) R (a.u.)

25 NeH + : potential energy of triplet states and of some states of NeH and of NeH 2+. Potential energy (ev) NeH 2+ NeH + NeH** NeH (b) NeH + : Triplet states X 2 Π X 1 Σ + A 2 Σ + X 2 Σ + Ne* + (1s 2 2s2p 6, 2 S) + H(1s, 2 S) Ne + (1s 2 2s 2 2p 5, 2 P) + H + a 3 Σ + b 3 Π c 3 Σ + d 3 Π e 3 Σ + f 3 Π g 3 Σ + Ne + (1s 2 2s 2 2p 5, 2 P) + H*(3s, 2 S) Ne + (1s 2 2s 2 2p 5, 2 P) + H*(2s, 2 S) Ne*(1s 2 2s 2 2p 5 3s, 1,3 P) + H + Ne + (1s 2 2s 2 2p 5, 2 P) + H(1s, 2 S) Ne*(1s 2 2s 2 2p 5 3s, 1,3 P) + H(1s, 2 S) Ne(1s 2 2s 2 2p 6, 1 S) + H + Ne(1s 2 2s 2 2p 6, 1 S) + H*(3s, 2 S) Ne(1s 2 2s 2 2p 6, 1 S) + H*(2s, 2 S) Ne(1s 2 2s 2 2p 6, 1 S) + H(1s, 2 S) R (a.u.)

26 Absolute cross sections NeD + + e - Ne + + D + e - Production du Ne + 8 Total : σ 1.3 DE : σ 1 Section efficace absolue (10-17 cm 2 ) Energie des électrons (ev) σ = σ + σ = σ+ σ 1.3 DE DI 1 3

27 Absolute cross sections NeD + + e - Ne + D + + e - 12 Production du D + Section efficace absolue (10-17 cm 2 ) Total : σ 2.3 DE : σ Energie des électrons (ev) σ = σ + σ = σ + σ 2.3 DE DI 2 3

28 Resonances 8 Excitation dissociative Ne: Anneaux de stockage (Novotny, 2006) D + : Louvain-la-Neuve NeD + + e - Ne + D + + e - Section efficace absolue (10-17 cm 2 ) Energie des électrons (ev)

29 Dissociative Ionisation NeD + + e - Ne + + D + + 2e - 8 DI : σ 3 Section efficace absolue (10-17 cm 2 ) E th = 35.1 ev E th = 26.8 ev Energie des électrons (ev)

30 KERD Fragments Ne + and D + Production du Ne + Production du D + dσ / de KER (10-17 cm²/ev) ev 45.1 ev 95.1 ev ev dσ / de KER (10-17 cm²/ev) ev 25.1 ev 45.1 ev 95.1 ev ev 0 0 DE DI E KER (ev) E KER (ev)

31 Energy thresholds and E KER Thresholds (ev) E KER (ev) NeD + + e - Present results (± 0.5 ev) Theory Molecular state Dissociation limit D 0 (± 0.1 ev) Theory (± 0.2 ev) Present results 18.8 a 3 Σ a 3 Π 9.8 Ne + (1s 2 2s 2 2p 5, 2 P) + H(1s, 2 S) (10.5) 21.3 A 1 Π A 1 Σ DE Ne b 3 Π 30.3 B 1 Π c 3 Σ Ne + (1s 2 2s 2 2p 5, 2 P) + H*(2s, 2 S) (20.7) 32.4 C 1 Σ ± C 1 Π c 3 Π 13.3 D DI Ne + + D b 3 Σ + Ne*(1s 2 2s 2 2p 5 3s, 1, 3 P) + H + (19.2) B 1 Σ X 2 Π Ne + (1s 2 2s 2 2p 5, 2 P) + H + (24.1) ± ± ± 1

32 Dissociative ionisation Forme de Bethe NeD + + e - Ne + + D + + 2e - Section efficace Energie des électrons (10-17 cm².ev) σ a E = b EeI + I 3 ln e I: energy threshold (ev) Energie des électrons (ev)

33 (a) Ne 2+ Multicharged fragments NeD + + e - Ne 2+ + (σ 4 ) Ne 3+ + (σ 5 ) (b) Ne Section efficace absolue: σ 4 (10-18 cm 2 ) Section efficace absolue: σ 5 (10-20 cm 2 ) Energie des électrons (ev) Energie des électrons (ev)

34 NeD + : σ I = σ 3 + σ 4 + σ 5 Total ionization 8 NeD + : σ I Ne: Almeida et al (1995) Ne: Schram et al (1966) Section efficace absolue (10-17 cm²) 'isoelectronic effect': good agreement with Ne Almeida et al, J. Phys. B. 28 (1995) Schram et al, Physica 32 (1966) 185 Energie des électrons (ev)

35 Theory: cross sections ionization crossections are estimated by: Binary-Encounter Encounter-Bethe, BEB, Kim et al Deutsch-Märk Formalism, DM, Deutsch et al Semi-empirical model of R.K. Janev and D. Reiter for dissociative excitation (DE) and for dissociative ionization (DI) - Kim et al, J. Res. Nat. Stand. Technol. 105 (2000) Deutsch et al, Int. J. Mass Spectrom. Ion Processes 137 (1994) 77 - Janev RK and Reiter D, Phys. Plasmas 9 (2002) 4071

36 Conclusion Absolute cross sections for electron impact single ionization (SI), dissociative excitation (DE) and dissociative ionization (DI), are measured from the respective thresholds up to 2.5 kev for molecular ions. The animated crossed beams experiment is applied contributions from different reaction channels are separated, kinetic energy release distributions are determined (KERD) for the fragment ions. data are included in the data base: cross sections, threholds, KER