fragmentation mechanisms for methane induced by electron impact fudan university 魏宝仁...

Fragmentation mechanisms for Methane

induced by electron impact

Fudan University

魏宝仁王新成、张煜、路迪、 Roger Hutton 、邹亚明

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

ProjectileTarget

Free Electron

Recoil Ion

e--molecule collisions could be an ideal model system to study the atomic many-body systems.

Motivation

Plasma-Wall Interactions (PWI)

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-

CH42+ + 3e- CH3

+ + H+ + 3e-

· · · · · · · --- R. K. Janev, Contemporary Physics 46 (2005) 121

Outline




Summery and Outlook

Principle of momentum image

-- from Frankfurt University

Principle of momentum image

Beam

Recoil Ion

trajectory

x, y

TOF

y

x

z

Extraction Field

Detector

x, y Transverse MomentumTOF Longitudinal Momentum

Experimental setup

Electrostatic field & Pulse field

Experimental setup

Time of Flight Detector Electronics V ＭＥ

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

stageDiameter

(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 1.5 300 1.2×10-10 1.2×10-10

5 3 300 2.0×10-11 1.2×10-9

Low Energy Electron Gun

Electron Energy: 1~2000 eV

Energy spread : 0.4 eV

Current: 1nA ~ 10 μA

ELG-2 / EGPS-1022,Kimball Physics

Experimental setup

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

The first ionization energy of He is 24.59 eV

The energy spread of the pulsed beam is about 8.5 eV

The energy spread of the pulsed electron beam

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

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e C

ross

Sec

tion

E0(eV)

CH3

+

CH2

+

CH+

C+

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e C

ross

Sec

tion

E0 (eV)

C2H

3

+

C2H

2

+

C2H+

C2

+

The Relative Cross Section of Dissociation Process

B. Wei, et al, J. Phys. B 46 (2013) 215205

Outline




Summery and Outlook

CH42+ CH2

+ + H+ + H

CH3+* + H+ CH2

+ + H+ + H

CH2+ + H2

+* CH2+ + H+ + H

CH32+ + H CH2

+ + H+ + H

Molecular Processes Issues

Synchronous concerted

Two S

tep

No CH32+ in TOF Spectrum.

Then no reaction related CH32+.

Fragmentation mechanism of CH42+


Reaction Channel 55 eV 75 eV 100 eV 4 keV* 10 keV**

CH3+ + H+ - 1.00 ± 0.01 - 1.00 ± 0.01 - 1.00 ± 0.01 - 1.03 ± 0.17 - 1.00 ± 0.02

CH2+ + H+ + H - 0.95 ± 0.02 - 0.95 ± 0.02 - 0.95 ± 0.01 - 1.11 ± 0.18 - 0.94 ± 0.04

CH+ + H+ + 2H - 0.87 ± 0.05 - 0.85 ± 0.03 - 0.81 ± 0.02 - 0.93 ± 0.16 - 0.90 ± 0.04

C+ + H+ + 3H - 0.80 ± 0.10 - 0.68 ± 0.05 - 0.61 ± 0.03 - 0.80 ± 0.16 - 0.68 ± 0.06

CH2+ + H2

+ - 1.00 ± 0.01 - 1.00 ± 0.01 - 1.00 ± 0.01 - 1.01 ± 0.17 - 1.00 ± 0.02

CH+ + H2+ + H - 0.85 ± 0.10 - 0.84 ± 0.10 - 0.78 ± 0.08 - 0.97 ± 0.08

C+ + H2+ + 2H - 0.69 ± 0.11 - 0.75 ± 0.10 - 0.74 ± 0.10 - 0.80 ± 0.08

* R. Flammini, et al, New Journal of Physics 11 (2009) 083006** R. Singh, et al, Phys. Rev. A 87 (2013) 062706

H+

+

CH42+→ CH2

+ + H+ + H


Synchronous concerted reaction

H+ H+

++


Two-step reaction

CH42+→ CH3

+* + H+ → CH2

+ + H + H+

H+

+

+

+


Two-step reaction

CH42+→ CH2

+ + H2+*

→ CH2+ + H + H+

CH42+→CH2

+ + H+ + H

The momentum of H depends on

the fragmentation mechanism.


The neutral fragments produced in the two-step fragmentation

shared the momentum with the charged fragments.

H2+*

CH3+*


B. Wei, et al, J. Chem. Phys. 140 (2014) 124303

𝜋 𝑖=𝑝𝑖2/∑ 𝑝 𝑗

2

CH42+→CH+ + H+ + 2H/H2



CH42+→C+ + H+ + 3H




CH42+→CH2

+ + H+ + H

CH42+→CH+ + H+ + 2H/H2

CH42+→C+ + H+ + 3H

-40 -30 -20 -10 0 10 20 30 40

Momuntum (a.u.)

H2H

3H


H+CHn+

Two-step: Asynchronous concerted reaction

Momentum distribution of neutral particle

R. Flammini, NJP 11 (2009) 083006


kinetic energy release

Fragmentation mechanism of CO22+

X. Wang, et al, Phys. Rev. A (Accepted)

Significant contribution from two-step channelCO2

2+ CO+ + O+ C+ +O+ + O was distiguished.

e- + CH4 CH4+ + 2e-

CH3+ + H + 2e-

CH2+ + 2H/H2 + 2e-

CH+ + 3H + 2e-

……………….

Molecular Processes Issues

CH4 + → CH3

+ + H 1.6 eV

CH4 + → CH2+ + H2 2.5 eV

→ CH2+ + 2H 7.0 eV

The kinetic energy distribution of fragment ions

E0 = 70 eV

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) 215205

The kinetic energy of fragment ions

𝐸𝑘=(𝐸1−𝐸0 ∙𝑚1

𝑚0

)×𝑚0

𝑚0−𝑚1

E0 ~ 14 meV, E1 ~ 25 meV

The kinetic energy release Ek = 185 meV

CH4+

E0, m0

CH3+

E1, m1

+ H

B. Wei, et al, J. Phys. B 46 (2013) 215205

Outline




Summery and 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.

Summery & Outlook

Studying of the interaction between the low energy electron

and molecule (N2, H2 and CxHy).

fragmentation mechanisms for methane induced by electron impact fudan university 魏宝仁...

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