Department of Experimental Physics, Department of Experimental Physics, Comenius University BratislavaComenius University Bratislava, , SlovakiaSlovakia

Formation of positive ions by electron impact:Formation of positive ions by electron impact:Temperature effectsTemperature effects

Š. MatejčíkŠ. Matejčík

Department of Experimental PhysicsDepartment of Experimental Physics

Division of plasma physicsDivision of plasma physics • llow temperature plasma ow temperature plasma technological plasmas, glow discharges ...

• high pressure discharges high pressure discharges corona discharge, barrier discharge ...

• reactions in the plasma reactions in the plasma electron impact ionization, electron attachment to the molecules ...

• Electron Electron interinteractionactionss

• Photon induced reactionsPhoton induced reactions

• Ion –Ion – molecule reactions molecule reactions

• Neutral reaction Neutral reaction

• Reactions on the surfacesReactions on the surfaces

Reactions in plasmaReactions in plasma

• Electron impact ionization Electron impact ionization

• Electron attachmentElectron attachment

• Excitation Excitation

• DissociationDissociation

• Electron reaction on the surfaces Electron reaction on the surfaces

Electron interactionsElectron interactions in plasma in plasma

Comenius universityComenius university

• Electron attachment Electron attachment

• Electron impact ionizationElectron impact ionization

• Electron Electron inducedinduced reactions on surfaces reactions on surfaces

PreparationPreparation

• Electron impact excitationElectron impact excitation

Electron impact ionization:Electron impact ionization:e + AB → AB+ + 2e

Dissociative electron impact ionization:Dissociative electron impact ionization:e + AB → A+ + B + 2e

Ion pair formation:Ion pair formation:e + AB → A+ + B- + e

Fusion related researchFusion related research

Molecules:Molecules:CHCH44, CH, CH33D, CDD, CD44, C, C22HH66, C, C33HH88

We have studiedWe have studied

- appearance energies (AE) of the positive ionsappearance energies (AE) of the positive ions

- gas temperature effects on the AEgas temperature effects on the AE

- isotopic effects on AEisotopic effects on AE

Theory:Theory:

Ionization energies (OVGF, CCSD(T))Ionization energies (OVGF, CCSD(T))

Appearance energies (G3, G3B3 and CBS-APNO, CBS-Q)Appearance energies (G3, G3B3 and CBS-APNO, CBS-Q)

Experimental setup:Experimental setup:

E = 0 – 150 eVE = 0 – 150 eVFWHM ~FWHM ~50-50-140 meV140 meVIe~Ie~20-20-100nA100nA

T = 293KT = 293K 693K693K

low Tghigh Tg

Electron impact ionizationElectron impact ionization

E

σ

low Tg

Rate coefficientRate coefficient

E

σ

dETEfTEETT ewe ),(),(),(0

f(E,Te)high Tg

A

A+

IE

En

erg

y

Q

M+

M

VIE

AIEAE

En

erg

y

Q

M+

M

AE1

AE2

E

f

ΔAE = AE2-AE1=

= Ei(T2)-Ei(T1)

Ei(T)=Ev(T)+Er(T)

σ

E

Sensitivity in present experiment:

σ ~10-26 - 10-25 m2

23,5 24,0 24,5 25,0 25,5

0

100

200

300

400

500

He+/Hed=1.127±0.005

Ion

yiel

d (a

rb.

units

)

E (eV)

Problem of thresholdProblem of threshold energy energy estimation estimation

Atoms:

Wannier theory:

w(E,AE,A,d) = 0 E<AE A(E-IE)d E>AEd=1.127

Threshold behaviour of the cross sections for EIIThreshold behaviour of the cross sections for EII

dEUEfAdAEEAdAEEI w ),(),,,(),,,(0

f(E,U) - EEDF

Fitting function:

w(E,AE,A,d)

23,5 24,0 24,5 25,0 25,5

0

100

200

300

400

500

He+/He

Ion

yiel

d (a

rb.

units

)

E (eV)

d=1.127±0.01

Threshold behaviour of the cross sectionsThreshold behaviour of the cross sections

Molecules - empirical formula:

w(E,AE1,AE2,d1,d2A) = 0 E<AE1

A(E-AE1)d1 E>AE1

Threshold behaviour of the cross sections for EIIThreshold behaviour of the cross sections for EII

dEUEfAddAEAEEAddAEAEEI w ),(),2,1,,,(),2,1,,,(0

2121

A(E-AE2)d2 + A(E-AE1)d1 E>AE2

...

Temperature effects on Appearance energies of Temperature effects on Appearance energies of the ionsthe ions

P. Plessis et al., Can. J. Chem. 65 (1987) 1424, J. Phys. B, 16 (1983) 1770

H. M. Rosenstock, Int. J. Mass Spectr. Ion Phys., 20 (1976)139

Predicted by:Predicted by:

Rosenstock – review article Rosenstock – review article

PlessisPlessis && Marmet – in CHMarmet – in CH44 and C and C22HH66 papers papers

CHCH44

e + CHe + CH44 → CH→ CH44++ + 2e + 2e

CHCH33++ + H + 2e + H + 2e

CHCH33++ + H + H-- + 2e + 2e

CHCH22++ + H + H22 + 2e + 2e

CHCH++ + H + H22 + + H H + + 2e2e

CC++ + + 22HH22 + 2e + 2e......

12,0 12,2 12,4 12,6 12,8 13,0 13,2 13,4

0

20

40

60

80

100Io

n y

ield

(a

rb. u

nits

)

Electron energy (eV)

293K

12,0 12,2 12,4 12,6 12,8 13,0 13,2 13,4

0

20

40

60

80

100Io

n y

ield

(a

rb. u

nits

)

Electron energy (eV)

CHCH44++/CH/CH44

693K

Experiment AE (eV)

T=293 K

AE (eV)

T=693 K

ΔAE

(eV)

Plessis et al. AE(eV)

T=293 K

CH4+/CH4 12.650.04 12.510.04 0.14 12.62 0.02

CH3+/CH4 13.580.1

14.340.1

13.450.1

14.200.1

0.14 - - -

14.01 0.08

EEii(T) - rotational and vibrational energy of the molecule at T(T) - rotational and vibrational energy of the molecule at T

EEii =E =Eii(693) – E(693) – Eii(293 K) ~ (293 K) ~ 0.12 eV0.12 eV

Stano et al., J. Phys. B: At. Mol. Opt. Phys. , 36 (2003) 261

CHCH33++/CH/CH44

13,0 13,5 14,0 14,5 15,0

0

20

40

60

80

100

120

140

Ion

yie

ld (

arb

. un

its)

Electron energy (eV)

13,0 13,5 14,0 14,5 15,0

0

20

40

60

80

100

120

140

Ion

yie

ld (

arb

. un

its)

Electron energy (eV)

693K

293K

CHCH33DD++/CH/CH33DD

12,0 12,5 13,0

0,0

0,6

1,2

1,8Io

n si

gnal

(ar

b. u

nits

)

Electron energy (eV)

AE1=12.75 eVAE1=12.62 eV

293K

693K

e+CH3D→ AE(293K) AE(693K) D(AE(693K)-AE(293K))

(CH3D)+ 12.75 ± 0.03 12.58 ± 0.05 0.17 ± 0.07

(CH2D)+ + H–

(CH2D)+ + H

13.66 ± 0.0714.42 ± 0.05

13.49 ± 0.0714.25 ± 0.05

0.17 ± 0.10.17 ± 0.06

CC22HH66++/C/C22HH66

11,0 11,5 12,0 12,5-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0Io

n y

ield

(a

rb. u

nits

)

Electron energies (eV)

11,0 11,5 12,0 12,5-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0Io

n y

ield

(a

rb. u

nits

)

Electron energies (eV)

693K

293K

e + C2H6 Experiment Theory

AE [eV]293 K

Thermochemistry[eV]

G3B3AIE [eV]

C2H6+ 11.46±0.04 11.55

C2H5+ + H-

C2H5+ + H

12.06±0.0612.7±0.1

11.7212.47±0.03

12.0312.79

C2H4+ + H2 11.90±0.04 11.92±0.0006 11.93

C2H3+ + H + H2 15.02±0.1 14.47±0.46 14.83

C2H2++2H2 15.02±0.1 14.61±0.01 14.61

C2H++2H2+H

C2H++H2+3H 25.7±0.3

20.5±0.1525.1±0.15

21.6526.12

C2+ + 3H2 22.6±0.3 20.95±0.3 21.73

CC22HH66

9,5 10,0 10,5 11,0 11,5

0,0

0,2

0,4

0,6

0,8

1,0

Ion

yie

ld (

arb

. un

its)

Electron energy (eV)

9,5 10,0 10,5 11,0 11,5

0,0

0,2

0,4

0,6

0,8

1,0

Ion

yie

ld (

arb

. un

its)

Electron energy (eV)

CC33HH88++/C/C33HH88

693K

293K

Molecule ΔAE (eV)

Exp.

ΔAE (eV)

Theory

CH4 0.12-0.14 0.12

CH3D 0.12-0.17 0.13

CD4 0.16-0.20 0.15

C2H6 0.19-0.3 0.31

C3H8 0.21-0.42 0.46

SummarySummary

Temperatures 293 and 693 K

En

erg

y

Q

M+

M

Isotopic effect on AEIsotopic effect on AE

Changes in vibrational energies in Changes in vibrational energies in isotopomers isotopomers

Heavy isotopomer – lower zero point Heavy isotopomer – lower zero point eenergy nergy ==> larger AE> larger AE

Comparison:Comparison:CHCH44, CD, CD44, CH, CH33DD

CH3D

AE (eV)293 K

CD4

AE (eV)293 K

CH4

AE (eV)293 K

CH3D+ 12.75 ± 0.03 12.89 ± 0.03 12.65 ± 0.04 CH4

+

CH2D+ + H–

CH2D+ + H

13.66 ± 0.07 14.42 ± 0.05

- 14.54 ± 0.05

13.58 ± 0.1 14.34 ± 0.1

CH3+ + H–

CH3+ + H

IIssotopic effect on AEotopic effect on AE

PresentmeV

a)meV

b)meV

(CH3D)+/CH3D 100 50

(CH2D)+/CH3D 80

(CH3)+/CH3D 200

(CHD)+/CH3D 110

(CD4)+/CD4 190 170 160

(CD3)+/CD4 200 130

(CD2)+/CD4 310 90

a) F. P. Lossing, A. W. Tickner, W. A. Bryce, J. Chem. Phys. 19, 1254 (1951).b) V. H. Dibeler, M. Krauss, R. M. Reese, F. N. Harlee J. Chem. Phys. 42, 3791 (1965).

Isotopic shiftIsotopic shift

ConclusionsConclusions

experimentally observed dependence of AE´s on the experimentally observed dependence of AE´s on the temperaturetemperature

magnitude of the shift magnitude of the shift ~~ vibrational and rotational vibrational and rotational excitation of the moleculeexcitation of the molecule

confirmed isotopic shift in the appearance energies of confirmed isotopic shift in the appearance energies of the moleculesthe molecules

Contribution to new CRPContribution to new CRP

Experiment:Experiment:The temperature effects:-estimation of the partial cross sections in C3H8 for particular

fragment ions- evaluation of the rate coefficients

Theory - QM:Theory - QM:- BenHm - electronic structure, binding energies, volatility, ionization

potentials and electron affinities - H2 reactions with Ben

CoworkersCoworkers Comenius University BratislavaComenius University BratislavaM. Stano, E. Vašeková, J.D. Skalný, D. Kubala,M. Stano, E. Vašeková, J.D. Skalný, D. Kubala, J. Kočíšek J. Kočíšek

TheoristTheorists:s: I. Hubac, I. Hubac, J. Urban, P. MachJ. Urban, P. Mach

S. Denifl, T.D. MärkS. Denifl, T.D. MärkUniversity InnsbruckUniversity Innsbruck

SupportSupportVW StiftungVW Stiftung, , EU (Euroatom-FUSION)EU (Euroatom-FUSION),, IAEA IAEA, APVT, APVT