1. Electron Impact Ionization of selected hydrocarbon molecules
Štefan Matejčík
Comenius University
Bratislava,
Slovakia

2. Coworkers Dušan Kubala Jaroslav Kočíšek Peter Papp Pavel Mach
Jan Urban
Stephan Denifl
Fabio Zappa
Univ. Innsbruck

3. Electron impact ionisation - EII
e + M → M+ + 2e
e + M → X+ + (M-X) + 2e
Appearance energies
Partial cross sections for EII

4. Crossed beams apparatus

5. Electron impact ionization
high Tg
low Tg σ
The EII is characterised from point of view of the kinetics by the cross section
The cross section has a monotonic character with maximum at about 100 eV
The EII is endothermic reaction with a threshold, called appearance energy of the ions
In present experimet we have focused on the estimation of the threshold behaviour of the CS, estimation of the AE
At elevated temperatures there are changes in the ion yield in the vicinity of the threshold E

6. Molecules studied in 2007 and 2008
Experiment
C2D6
CH3D
CD4
C3H8
C3H6D2
Theory
BenHm

7. Ar and Ne cross sections
R. Rejoub, B.G. Lindsay, R.F. Stebbings, Phys. Rev. A, 65 (2002)

8. Cross section EII C2D6 low temperature

9. Cross section EII C2D6 high temperature

10. Comparison total cs C2D6

11. Comparison partial cs C2D6

12. Comparison partial cs C2D6

13. Comparison partial cs C2D6

14. Comparison partial cs C2D6

15. Comparison partial cs C2D6

16. Threshold cross sections C2D6

17. Threshold cross sections C2D6

18. Ion
Mass (amu)
AE (100°C)
AE (420°C)
C2D6+ 36
11.72
11.48
C2D5+ 34
12.64
12.33
C2D4+ 32
12.00
11.54
C2D3+ 30
14.92
14.94
C2D2+ 28
14.95
14.42
CD+ 16
22.25
22.17
C2D5++ 17
35.71
34.12
CD2+ 18
15.69
15.66
C2+ 24
23.54
28.54
23.17
28.11
C2D+ 26
25.22
24.84

19. e + C2H6
Experiment
Theory
AE [eV]
293 K
Thermochemistry
[eV]
G3B3
AIE [eV]
C2H6+
11.46±0.04
11.55
C2H5+ + H-
C2H5+ + H
12.06±0.06
12.7±0.1
11.72
12.47±0.03
12.03
12.79
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
14.61±0.01
14.61
C2H++2H2+H
C2H++H2+3H
25.7±0.3
20.5±0.15
25.1±0.15
21.65
26.12
C2+ + 3H2
22.6±0.3
20.95±0.3
21.73

20. EII to CD4 cross sections
Tarnovsky et al. J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 139

21. CD4+/CD4

22. CD4+/CD4

23. CD3+/CD4

24. CD2+/CD4

25. CD+/CD4

26. Tarnovsky et al. J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 139

27. EII to CH3D cs

28. EII to C3H8

29. EII to C3H8

30. EII to C3H6D2

31. VIE
VIE
Now I would like to explain some basic concepts, probably trivial for most of the audience, concerning ionization
In case of atoms we have simple situation, the atom in the ground state and the ground state of the ion. The ionization energy is defined as a difference between this two states.
I the molecules is the situation more difficuls. We have to work with potential curves or potential energy surfaces and thus the IE depends on the initial geometry of the molecules. For moelcule usually two ionization anrgies are given, vertical IE and adiabatic IE.
Vertical IE is defined as a energy necessary to ionise a molecule in the equilibrium geometry
Adiabativ IE is defined as a energy difference between the energies os the ion and molecule in its ground states
The experiments are sensitive on the lowest energy necessary to ionize molecule and this is colled appearance energy. This corresponds to the ionization energy at low distancies. As the geometry changes in the moelcule and molecular ion are only moderate, offen is the AE very close to the AIE.

32. EII to C3H6D2 e + M → C3H8 AE (eV) 290 K C3H6D2 (C3H8)+ v (C3H6D2)+
10.51 ± 0.05
10.96 ± 0.05
10.81 ± 0.05
11.13 ± 0.05

33. Ionization cross sections calculations
Kim-s BEB (binary encounter Bethe) theory
[W. Hwang, Y.-K. Kim, M. E. Rudd, J. Chem. Phys. 104, 2956 (1996)],
HF/6-31++G(2df,2pd) orbital kinetic energies
OVGF/6-31++G(2df,2pd) ionization potentials for ionization from valence orbitals
For core orbitals IP the HF values were used (their contribution to total cross section in studied region is very small).

34. Ionization potential of Be5Hn clusters as a function of “n”.
Most stable cluster of given stechiometry.
Method:
MP4(SDTQ)/6-31++G(2df,2pd)
Geometry:
MP2/6-31++G(2df,2pd)
Graf je pre najstabilnejsi konformer o danej stechiometrii, ktory sa nasiel. Zatial sme sli pre neradikalove klastre od n=10 po n=2, radikalove klastre (n=1,3) uz su spracovane,
Na dalsich (n=5,7,9) sa pracuje… Aspon na tych najmensich je vidiet, ze radikalove a neradikalove sa hodne lisia, radikalove maju nizsi IP (pochopitelne, idu na closed-shell kation)

35. Total ionization cross sections for Be5Hn clusters
Increasing hydrogen content in mixed clusters increases IP
and decrease cross section for ionization

36. Conclusions Measured partial cs for EII to Appearance energy – C3H6D2
CD4
C2D6
C3H8
Appearance energy – C3H6D2
Calculated
ionization energies Be5Hn
EII total cross sections Be5Hn