1
Pengqian Wang
Department of PhysicsDepartment of Physics
Western Illinois UniversityWestern Illinois University
March 4, 2013
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Garching, GERMANYGarching, GERMANY
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Manhattan, KANSASManhattan, KANSAS
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Motivations for studying the Motivations for studying the dissociation of molecular ionsdissociation of molecular ions
• Exploring the structure and energy Exploring the structure and energy levels of molecules and molecular levels of molecules and molecular ions, which is a main goal of atomic ions, which is a main goal of atomic and molecular physics.and molecular physics.
• Understanding the dynamics of Understanding the dynamics of molecules when interacting with molecules when interacting with electron beams or intense lasers.electron beams or intense lasers.
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In this talkIn this talk• IntroductionIntroduction
1) Dissociation of molecular ions1) Dissociation of molecular ions2) Challenge: Pathway-specific measurement2) Challenge: Pathway-specific measurement
• MethodsMethods1) Coincidence mass spectroscopy1) Coincidence mass spectroscopy2) Three-dimensional momentum imaging2) Three-dimensional momentum imaging
• ResultsResults1) Electron impact dissociative ionization1) Electron impact dissociative ionization2) Molecules interact with intense lasers2) Molecules interact with intense lasers
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0 2 4 6 8 10
0
5
10
15 AB
AB+
.
.
.
.
Po
ten
tial
en
erg
y (e
V)
Inter-nuclear distance (a. u.)
Dissociation of molecular ions:Dissociation of molecular ions:Molecular orbitals and potential energy curvesMolecular orbitals and potential energy curves
Light orelectron beams
Excitation
Momentum and kinetic energyrelease
Dissociation
Ionization
Dissociativeionization
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Dissociation channels of molecular ionsDissociation 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+ … _________________________________
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Dissociation of molecular ions:Dissociation of molecular ions:Pathway-specific measurements are requiredPathway-specific measurements are required
9
Experimental MethodsExperimental Methods
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Time-of-flight mass spectrometerTime-of-flight mass spectrometer
DetectorAcceleration region Flight tube
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Conventional non-coincidence time-of-flight Conventional non-coincidence time-of-flight mass spectrum: mass spectrum: ExampleExample
200 400 600 800 1000 1200 1400
0
1x107
2x107
3x107 OCSOCS+
SO+
x100
CS+
S+
OCS2+
CO+CS2+
x100O+/S2+
CO2+
x100
C+
O2+
x100
C2+
x100
Co
unts
Time-of-flight (2ns/bin)
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Coincidence time-of-flight mass spectrum: Coincidence time-of-flight mass spectrum: PrinciplePrinciple
0
1
0
1
0
1
0 2 4 6 8 100
1
B+A+
C+A+
Co
un
ts
C+B+
A+C+B+
Time-of-flight (arb. unit)
Single pulsemass spectra
0 2 40
2
4
6
8
10
B++C+A++C+
A++B+
Tim
e-o
f-fl
igh
t (a
rb. u
nit
)
Time-of-flight (arb. unit)
Corresponding rawcoincidence mass spectra
0 2 40
2
4
6
8
10
B++C+A++C+
A++B+
Tim
e-o
f-fl
igh
t (a
rb. u
nit
)
Time-of-flight (arb. unit)
Accumulated rawcoincidence mass spectra
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Refining the data: Covariance mappingRefining the data: Covariance mapping
False
True
Covariance mapping:
=–Raw False True
– =
Raw
HNCO
Time-of-flight
14Delay-line anode
Resolution:0.5 ns, 250 m
Three-dimensional momentum imaging:Three-dimensional momentum imaging:Time- and position-sensitive detectorsTime- and position-sensitive detectors
Microchannel plate
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Experimental ResultsExperimental Results
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n-butane
Distinguishing the dissociation channels Distinguishing the dissociation channels
n-C4H10
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Measuring the cross section data for Measuring the cross section data for different dissociation channelsdifferent dissociation channels
HNCO2+ → X+ +Y+ + n HNCO3+ → X+ +Y+ +Z+ + n
Example: Electron impact ionization of isocyanic acid
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Determining the dissociation sequenceDetermining the dissociation sequence
1) HNCO2+ HN+ + CO+
CO+ C+ + O-arctan(CO/C)= -66.8 (Exp. -66.33)
2) HNCO2+ HNC+ + O+
HNC+ C+ + NH-arctan(HNC/C)= -66.0 (Exp. -66.73)
Example: Tree-body sequential dissociation
p- p
- p
A+ B +C
A+
A+
B +C
CB+ pBC
Cp
BC
B
B
BCarctan :)(B)(A
channel theof
xy
nOrientatio
HNCO
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Three-dimensional momentum imagingThree-dimensional momentum imaging
Example: H2+ + nh790nm H+ + H
-10 -5 0 5 10
-10
-5
0
5
10
P1y
(a. u.)P
1z (
a. u
.)
25112763150
-10 -5 0 5 10
-10
-5
0
5
10
P1x
(a. u.)
P1y
(a.
u.)
1392783250
Laser field Laser field
(x1, y1, t1, x2, y2, t2) (p1x, p1y, p1z, p2x, p2y, p2z)p1 + p2 = 0
C
laser field
p1
p2
x
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Dissociation from different vibrational levelsDissociation from different vibrational levels
Example: H2+ + nh H+ + H
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
500
1000
1500
11
10
9
87
v= 6
I0
33% I0
|cos | > 0.9
Co
un
ts
KER (eV )
1sg
-10 -5 0 5 10
-10
-5
0
5
10
T
P1 //
(a. u.)
P1
(a.
u.)
102248105229500
Laserfield
Momentum distribution Kinetic energy distribution
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SummarySummary
• Coincidence mass spectroscopy distinguishes Coincidence mass spectroscopy distinguishes different dissociation channels and pathways.different dissociation channels and pathways.
• We measured the cross section data and We measured the cross section data and studied the dissociation sequences of electron studied the dissociation sequences of electron impact dissociative ionization of molecules.impact dissociative ionization of molecules.
• We did three-dimensional momentum imaging We did three-dimensional momentum imaging for the dissociation of molecular ions in for the dissociation of molecular ions in intense laser fields.intense laser fields.