Photodissociation of Small

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Photodissociation of Small Aromatic Molecules in
Molecular Beam Studied by Multimass Ion Image
Techniques Cheng-Liang Huang(???) (?????????)
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• Outline
• 1. Introduction
• 2. Multimass ion image techniques
• 3. Photodissociation of aromatic molecules
• Toluene, Xylene, Picoline
• Ethylbenzene, Propybenzene, Ethyltoluene
• Fluorobenzene
• 4. Conclusion

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Introduction ? Bond-selected dissociation by
laser biochemical method and physical
method ? Energy transfer intramolecular
electronic state coupling (internal conversion
and intersystem crossing) IVR (intramolecular
vibrational energy redistribution)
intermolecular collision quenching ?
Photodissociation in molecular beam
collision free, low internal energy, direction
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Photodissociation studied by Crossed beam
apparatus
Advantage general, angular distribution
Disadvantage ? Not COM detect ? Cracking (for
EI) ? Single channel ? Not for long-lived species
Photodissociation laser
Reaction angle
Molecular beam
Beam velocity
skimmer
Recoil velocity
Reaction Zone
Electron impact or VUV ionization
Flight length
Detector
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idea proposed by Y. T. Lee
In center of mass frame
Momentum match
Molecular beam
Product pair
mA mB mC mD mE mF mG mH
Skimmer
Velocity Axis
Mass Axis
UV light
VUV light
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Multimass Ion Image Detector (????????? )
Energy analyzer
Ion detector
nozzle
mA
mC
mB
mAgtmBgtmC EK,A lt EK,BltEK,C
Electro plate
Probe laser
Photolysis laser beam
Molecular beam
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Relationship of Image and Pump, Probe laser beam
Disc-like Image
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Photodissociation of Aromatic Compound Excited _at_
248, 193 and 157 nm
Previous work
Park, J. Bersohn, R. Oref, I. J. Chem. Phys.
1990, 93, 5700 Nakashima, N. Yoshihara, K. J.
Phys. Chem. 1989, 93, 7763 Luther, K. Troe,
J. Weitzel, K. L. J. Phys. Chem. 1990, 94, 6316
Shimada, T.et.al. J. Phys. Chem. 1992, 96,
6298-6302.
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Photodissociation of toluene _at_193nm
Previous studies

• Dissociation Mechanism hot molecule mechanism
• Direct H atom and CH3 elimination following the
  internal conversion
• Branching ratio H CH3 80 20
• Dissociation rate 2?106 s-1
• Fragment translational energy distribution
  small kinetic energy released, decreases
  monotonically with energy
• Internal conversion from S2 (pump by 200nm) to
  S1 and S0 is 50fs

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Photodissociation of d3-toluene _at_193nm
?
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C6H5CD3_at_193nm
Velocity Axis
Mass 15, CH3 Mass 16, CH2D Mass 17, CHD2 Mass
18, CD3
Delay 6µs
(a)
Mass Axis
Delay 30 µs
Mass 76 Mass 77, C6H5 Mass 78, C6H4D Mass 79,
C6H3D2 Mass 80, C6H2D3
(b)
Product pairs
Momentum match
In center of mass frame
J. Am. Chem. Soc. 124, 4068 (2002)
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1,3-C6H4CD3CD3 Photodissociation _at_ 193 nm
Velocity Axis
m/e 90 m/e 91 m/e 92 m/e 93 m/e 94 m/e
95 m/e 96 m/e 97
m/e 16 17 18
Product pairs
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Aromatic molecular isomerization
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Toluene Energy Diagram
B3LYP/6-31G
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Xylene energy diagram
B3LYP/6-31G
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25?4
20?6
Direct dissociation
Direct dissociation
75?4
80?6
Summary

• 25?4 of hot toluene molecules isomerize to
  cycloheptatriene
• 20?6 of hot xylene molecules isomerize to
  methylcycloheptatriene
• 3. The most significant difference of this
  particular isomerization
• from that of other aromatic isomerization is
  that alkyl carbon and hydrogen atoms are involved
  in the exchange with those atoms in the aromatic
  ring during this isomerization process. This is
  unlike the ring permutation isomerization, in
  which only the hydrogen and carbon atoms of the
  aromatic ring are involved in the exchange.

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Photodissociation of d3-1, 4-picoine _at_193nm
?
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d3-4-picolin photodissociation at 193 nm
d15us
77 78 79 80 81 mass 93 94 95 96
-CD3 or -ND2 -CD2H or -NDH -CDH2 or -NH2 -CH3
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d3-4-picolin photodissociation at 193 nm
Mass 15 16 17 18
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Photodissociation of Akylbenzene _at_193nm
Previous Studies
Benzene toluene ethyl-? , propyl-?,
... 193nm Diss. lifetime 10 ms 500 ns 30 ns
5 ms Diss. Channel H H, CH3 CH3, C2H5,
I.C. rate 100 fs 50 fs Trans.Energy
dis. Small Small 248nm Dissociation rate gt300
ms gt100 ms
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Photodissociation of C6H5C2H5 _at_193 nm
m89 m90 m91 m92
Two photon Two photon
-CH3
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Transnational energy distribution of C6H5C2H5 ?
C6H5CH2 CH3 _at_193 nm
One photon available energy
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Photodissociation of C6H5C2H5 _at_248 nm
delay 15ms
C6H5CH2
91 ? 92 ? 93 ?
delay 22ms
91 ? 92 ? 93 ?
delay 32ms
Mass Axis
91 ? 92 ? 93 ?
delay 41ms
91 ? 92 ? 93 ?
CCD pixel
J. Chem. Phys. 116, 7779 (2002)
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The experimental and simulated image intensity
profile of ethylbenzene dissociated _at_ 248 nm
Simulation1 T11us T210us
Exp. data
Simulation2 T15us T210us
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Transnational energy distribution of C6H5C2H5 ?
C6H5CH2 CH3 _at_248 nm
One photon available energy
J. Chem. Phys. 116, 7779 (2002)
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Photodissociation of propylbenzene _at_ 248 nm
20 ms
m/e 91
5 ms
m/e 29 m/e 30
Mass indicator NO
J. Chem. Phys. 117, 7034 (2002)
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Photodissociation of propylbenzene _at_ 193 nm
28ms
m/e 91
8ms
m/e 29
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photodissociation of 1,2-C6H4CH3C2H5 _at_ 248 nm and
_at_193 nm
193 nm pump
Delay 35 us
Delay 37us
248 nm pump
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Potential energy diagram of ethylbenzene and
Propylbenzene
B3LYP/6-31G
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• Summary
• Photodissociation of ethylbenzene, propylbenzene,
  
• ethyltoluene, at 193 nm is from electronic
  ground state, or hot molecule.
• 2. Photodissociation from the first triplet state
  is the major
• channel of ethylbenzene, propylbenzene,
  ethyltoluene
• at 248 nm.
• 3. This is the first time that dissociation of
  alkylbenzene from
• electronic excited state was observed.

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Conclusion
1.
Evaluation of multimass ion imaging techniques A.
More than one mass can be detected
simultaneously B. Measurement is in the center
of mass C. Fragment cracking is reduced D.
Molecule with slow dissociation rate can be
studied E. Dissociation lifetime between
10ns100?s can be measured
2.
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Acknowledgment
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