1. Dissociation of Molecular Ions Studied by
Coincidence Mass Spectrometry
Pengqian Wang
Department of Physics
Western Illinois University
Presented at Physics Class
March 2, 2017

2. Macomb, ILLINOIS

3. In this talk
Introduction to the Problems 1) Dissociation of molecular ions 2) Challenges: Pathway-specific measurement
Experimental Methods 1) Coincidence mass spectrometry 2) Three-dimensional momentum imaging
Some Experimental Results 1) Electron impact dissociative ionization 2) Molecules interact with intense lasers

4. Motivations for studying the dissociation of molecular ions
Exploring the structure and energy states of molecules and molecular ions, which is a main goal in studying atomic and molecular physics.
Understanding the dynamics of molecules when interacting with particle beams and intense lasers.

5. Introduction: mass spectrometry
Mass spectrometry is the technique of measuring the spectrum of the masses of the constituent atoms or molecules in a material.
It is usually performed by ionizing the sample using an electron beam and measuring the resultant ion fragments by a mass spectrometer.
Mass spectrometers are also used to measure the initial momentum and kinetic energy distribution of the fragments, which are important in studying the molecular dissociation processes.

6. Dissociation of molecular ions:
Molecular orbitals and potential energy curves
Dissociative
ionization
Light or
electron beams
Ionization
The measurable quantities are the fragment species, their momentum and the kinetic energy release.
Dissociation
Excitation

7. Dissociation channels of molecular ions
Example: Dissociative ionization of CO
_________________________________
Parent ion Dissociation channel
CO  CO+  CO+
 C+ + O  C + O+
 CO2+  CO2+
 C2+ + O
 C + O2+
 C+ + O+
 CO3+  … _________________________________

8. Challenges in dissociation of molecular ions:
Pathway-specific measurements
Challenge #1:
Challenge #2:
Distinguishing different channels.
Pathway-specific measurement: Identifying different pathways.
CO+  C+ + O
CO2+  C+ + O+
CO2+  C+ + O+ + Ek1
CO3+  C+ + O2+
CO2+  C+ + O+ + Ek2
Solution: Coincidence mass spectrometry
Solution: Momentum imaging
~1 molecule dissociates per pulse.
Measure the three-dimensional momentum of each fragment.
Record the mass spectrum for each pulse.

9. Experimental Methods

10. Electron impact dissociative ionization of molecules: Experimental scheme
Ion Detector
Data
Acquisition
Mass
Spectrometer
Ions
Pulsed Electron Gun
Window
Vacuum
Chamber
Vacuum Gauge
Gas Valve

11. Electron impact dissociative ionization of molecules: apparatus
Gas Sample in
Turbo-molecular Pump
Mass Spectrometer
Electron Gun Control
Vacuum Gauge
Gas Valve Control

12. Time-of-flight mass spectrometer
Acceleration Region 1
Acceleration Region 2
Flight Tube
−V1
−V2
Backing Plate (Grounded)

13. Conventional non-coincidence time-of-flight mass spectrum: Example
Electron impact ionization of carbonyl sulfide (OCS), 200 eV

14. Coincidence time-of-flight mass spectrum: Principle
Single pulse mass spectra (~1 molecule/pulse)
Corresponding raw coincidence mass spectra
Accumulated raw coincidence mass spectra

15. Refining the data: Covariance mapping
Raw
HNCO
Time-of-flight
False
True
Covariance mapping: = –
Raw
False
True

16. Three-dimensional momentum imaging:
Time- and position-sensitive detectors
Microchannel plate
Resolution:
0.5 ns, 250 mm
Delay-line anode

17. Some Experimental Results

18. Distinguishing the dissociation channels
Example: Electron impact ionization of normal-butane, 200 eV
n-C4H10
n-butane

19. Measuring the cross section data for different dissociation channels
Example: Electron impact ionization of isocyanic acid (HNCO), 200 eV
HNCO2+ → X+ +Y+ + n
HNCO3+ → X+ +Y+ +Z+ + n

20. Determining the dissociation sequence
Example: Tree-body sequential dissociation p
- p A+ B +C
B + C B+ q
HNCO
HNCO2+ HN+ + CO+ CO+ C+ + O -arctan(CO/C)= -66.8 (Exp 3)
HNCO2+  HNC+ + O+ HNC+  C+ + NH -arctan(HNC/C)= -66.0 (Exp 3)

21. Three-dimensional momentum imagingq
laser field p1 p2 x
Example: H2+ + nhn (790 nm, 35 fs) H+ + H
(x1, y1, t1, x2, y2, t2)  (p1x, p1y, p1z, p2x, p2y, p2z)
Refined by p1 + p2 = 0
Laser field

22. Dissociation from different vibrational levels
Example: H2+ + nhn  H+ + H
Momentum distribution
Kinetic energy distribution
Laser field
1ssg

23. Summary
Coincidence mass spectrometry distinguishes different dissociation channels and pathways.
We measured the cross section data and studied the dynamics of the electron impact dissociative ionization of molecules.
We performed three-dimensional momentum imaging for the dissociation of molecular ions in intense laser fields.

24. THANK YOU!