1. Fragmentation mechanisms for Methane induced by electron impact
魏宝仁
王新成、张煜、路迪、Roger Hutton、邹亚明
Fudan University

2. Outline Motivation: Why we study e--molecule interaction
The recoil-ion momentum spectroscopy at Fudan University
Interaction between the low energy electron with Methane
Summery and Outlook

3. Motivation Target Projectile Free Electron Recoil Ion
e--molecule collisions could be an ideal model system to study the atomic many-body systems.

4. Plasma-Wall Interactions (PWI)

5. Molecular Processes Issues in Edge and Divertor Plasma Region
e- + H2  H2+ + e- +e-  H+ + H + e- + e-
 H+ + H+ + 3e-
e- + H2  H + H + e-
e- + CH4  CH4+ + e- + e-
 CH3+ + H + e- + e-
 CH e-  CH3+ + H+ + 3e-
· · · · · · ·
--- R. K. Janev, Contemporary Physics 46 (2005) 121

6. Outline Motivation: Why we study e--molecule interaction
The recoil-ion momentum spectroscopy at Fudan University
Interaction between the low energy electron with Methane
Summery and Outlook

7. Principle of momentum image
-- from Frankfurt University

8. Principle of momentum image
x, y
TOF
x, y  Transverse Momentum
TOF  Longitudinal Momentum x
trajectory z y
Recoil Ion
Detector
Extraction Field
Beam

9. Experimental setup
Electrostatic field & Pulse field

10. Experimental setup
Time of Flight
Detector
Electronics
VＭＥ

11. Background Vacuum (Torr)
Supersonic Gas-jet
gas in
Skimmer 1
Skimmer 2
Skimmer 3
Gas-jet P1 P2 P4 P3
Pump 5
Pump 4
Pump 3
Pump 2
Pump 1 P5 P0
stage
Diameter (mm)
Turbomolecular
pumps (l/s)
Background Vacuum (Torr)
Vacuum
(4 bar gas injection) 1
0.03
1000
1.6×10-6
1×10-5 2
0.1
300
7.6×10-8
2.1×10-7 3
0.3 80
2.7×10-9
1.9×10-8 4
1.5
1.2×10-10 5
2.0×10-11
1.2×10-9 11

12. Low Energy Electron Gun
Electron Energy: 1~2000 eV
Energy spread : 0.4 eV
Current: 1nA ~ 10 μA
ELG-2 / EGPS-1022,
Kimball Physics

13. Experimental setup
Y. Zhang, et al, NIMB 337 (2014) 39–44

14. The energy spread of the pulsed electron beam
The first ionization energy of He is eV
The energy spread of the pulsed beam is about 8.5 eV
capacitance
Y. Zhang, et al, NIMB 337 (2014) 39–44

15. The Relative Cross Section of Dissociation Process
Electron induced dissociation of methane has been measured. The relative partial cross section. Ions of fragments intensity compared with methane ions. The data is agreement with previous measurement. Same measurement has been performed for Ethylene (C2H4).
B. Wei, et al, J. Phys. B 46 (2013)

16. Outline Motivation: Why we study e--molecule interaction
The recoil-ion momentum spectroscopy at Fudan University
Interaction between the low energy electron with Methane
Summery and Outlook

17. Molecular Processes Issues
Synchronous concerted
CH42+  CH2+ + H+ + H
 CH3+* + H+  CH2+ + H+ + H
 CH2+ + H2+*  CH2+ + H+ + H
 CH32+ + H  CH2+ + H+ + H
Two Step
No CH32+ in TOF Spectrum.
Then no reaction related CH32+.

18. Fragmentation mechanism of CH42+

19. Fragmentation mechanism of CH42+

20. Fragmentation mechanism of CH42+
Reaction Channel
55 eV
75 eV
100 eV
4 keV*
10 keV**
CH3+ + H+
± 0.01
± 0.17
± 0.02
CH2+ + H+ + H
± 0.02
± 0.01
± 0.18
± 0.04
CH+ + H+ + 2H
± 0.05
± 0.03
± 0.02
± 0.16
± 0.04
C+ + H+ + 3H
± 0.10
± 0.05
± 0.03
± 0.16
± 0.06
CH2+ + H2+
± 0.17
CH+ + H2+ + H
± 0.10
± 0.10
± 0.08
± 0.08
C+ + H2+ + 2H
± 0.11
± 0.10
± 0.10
± 0.08
* R. Flammini, et al, New Journal of Physics 11 (2009)
** R. Singh, et al, Phys. Rev. A 87 (2013)

21. CH42+→ CH2+ + H+ + H Fragmentation mechanism of CH42+
Synchronous concerted reaction

22. CH42+→ CH3+* + H+ → CH2+ + H + H+ Fragmentation mechanism of CH42+
Two-step reaction

23. CH42+→ CH2+ + H2+* → CH2+ + H + H+ Fragmentation mechanism of CH42+
Two-step reaction

24. CH42+→CH2+ + H+ + H Fragmentation mechanism of CH42+
The momentum of H depends on
the fragmentation mechanism.

25. Fragmentation mechanism of CH42+
𝜋 𝑖 = 𝑝 𝑖 2 / 𝑝 𝑗 2
H2+*
CH3+*
The distances of a given data point from the three edges are equal to the relative squared momenta
The neutral fragments produced in the two-step fragmentation shared the momentum with the charged fragments.
B. Wei, et al, J. Chem. Phys. 140 (2014)

26. CH42+→CH+ + H+ + 2H/H2 Fragmentation mechanism of CH42+
B. Wei, et al, J. Chem. Phys. 140 (2014)

27. CH42+→C+ + H+ + 3H Fragmentation mechanism of CH42+
B. Wei, et al, J. Chem. Phys. 140 (2014)

28. CH42+→CH2+ + H+ + H CH42+→CH+ + H+ + 2H/H2 CH42+→C+ + H+ + 3H
Fragmentation mechanism of CH42+
CH42+→CH2+ + H+ + H
CH42+→CH+ + H+ + 2H/H2
CH42+→C+ + H+ + 3H

29. Two-step: Asynchronous concerted reaction
Fragmentation mechanism of CH42+
Momentum distribution of neutral particle
CHn+ H+
R. Flammini, NJP 11 (2009)
Two-step: Asynchronous concerted reaction

30. Fragmentation mechanism of CH42+
kinetic energy release

31. Fragmentation mechanism of CO22+
Significant contribution from two-step channel
CO22+  CO+ + O+  C+ +O+ + O was distiguished.
X. Wang, et al, Phys. Rev. A (Accepted)

32. Molecular Processes Issues
e- + CH4  CH4+ + 2e-
 CH3+ + H + 2e-
 CH2+ + 2H/H2 + 2e-
 CH+ + 3H + 2e-
……………….

33. The kinetic energy distribution of fragment ions
E0 = 70 eV
CH4 + → CH3+ + H eV
CH4 + → CH2+ + H eV
→ CH2+ + 2H eV

34. The kinetic energy of fragment ions
The average kinetic energy for recoil ion produce in collision of e - Methane as a function of incident energy.
B. Wei, et al, J. Phys. B 46 (2013)

35. The kinetic energy of fragment ions
CH4+
E0, m0
CH3+
E1, m1 + H
𝐸 𝑘 =( 𝐸 1 − 𝐸 0 ∙ 𝑚 1 𝑚 0 )× 𝑚 0 𝑚 0 − 𝑚 1
E0 ~ 14 meV, E1 ~ 25 meV
The kinetic energy release Ek = 185 meV
B. Wei, et al, J. Phys. B 46 (2013)

36. Outline Motivation: Why we study e--molecule interaction
The recoil-ion momentum spectroscopy at Fudan University
Interaction between the low energy electron with Methane
Summery and Outlook

37. Summery & Outlook
A COLTRIMS combined with a low energy pulsed electron beam has been constructed and tested.
The electron impact ionization and dissociation process of CH4 has been studied.
By measuring the momentum of the recoil ions, the fragmentation mechanism for the double charged methane ion has been studied.
Studying of the interaction between the low energy electron and molecule (N2, H2 and CxHy).

38. Thanks for your attention !