Pengqian Wang

1
Dissociation of molecular ions studied
by coincidence mass spectroscopy
Pengqian Wang Department of Physics Western
Illinois University February 13, 2008
2
In this talk

• The dissociation of molecular ions
  Pathway-specific measurements
• Coincidence mass spectroscopy,
• Three-dimensional momentum imaging
• Dynamics of molecular dissociation
• 1) Electron impact ionization
• 2) Interaction with intense laser fields

3
Garching, GERMANY
4
Manhattan, KANSAS
5
Molecular orbitals and potential energy curves
6
Time-of-flight mass spectrometer
7
Time-of-flight mass spectrum
8
Dissociation of molecular ions Pathway-specific
measurements

• Challenge 1
• Distinguishing different channels.
• CO ? C O
• CO2 ? C O
• Solution Coincidence measurement
• 1 breakup per pulse.
• Record the fragments for each pulse.
• Challenge 2
• Pathway-specific measurements Identifying
  different pathways.
• CO2 ? C O Ek1
• CO2 ? C O Ek2
• Solution Momentum imaging
• Measure the momentum of each fragment.

Dissociative ionization of CO ____________________
___________ Parent ion Dissociation
channel _______________________________ CO ?
CO ? CO ? C O ? C O ?
CO2 ? CO2 ? C2 O ? C O2 ? C
O ? CO3 ? ______________________________
_
9
Coincidence time-of-flight mass spectrum
Principle
10
Coincidence time-of-flight mass spectrum Example
n-butane
11
Time- and position-sensitive detectors
Microchannel plate
Resolution 0.5 ns, 250 mm
Delay-line anode
12
Molecular ions in intense laser fieldsH2 n
hn ? H H
13
Coincidence three-dimensional momentum imaging
(x1, y1, t1, x2, y2, t2) ? (p1x, p1y, p1z, p2x,
p2y, p2z) p1 p2 0
Example H2 nhn ? H H
14
Cross section data for the different dissociation
channels
Example Electron impact ionization of isocyanic
acid
HNCO3 ? X Y Z n
HNCO2 ? X Y n
15
Dissociation sequence
Example Sequential three-body dissociation,
second step out-of Coulomb field of A
1) HNCO2? HN CO CO? C O
-arctan(CO/C) -66.8? (Exp. -66.3?3?) 2)
HNCO2 ? HNC O HNC ? C HN
-arctan(HNC/C) -66.0? (Exp. -66.7?3?)
16
Ionization pathways of H2
17
Summary

• Coincidence mass spectroscopy distinguishes
  different dissociations channels and performs
  pathway-specific measurements.
• Cross section data and dynamics for electron
  impact dissociative ionization
• Ionization mechanisms of hydrogen molecular ions
  in intense laser fields

18
THANK YOU