1

Pengqian Wang

Department of PhysicsDepartment of Physics

Western Illinois UniversityWestern Illinois University

March 4, 2013

2

Garching, GERMANYGarching, GERMANY

3

Manhattan, KANSASManhattan, KANSAS

4

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

6

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.