The photochemistry of NO2

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The photochemistry of NO2
School of Chemistry FACULTY OF MATHEMATICS
PHYSICAL SCIENCES

• Iain Wilkinson Benjamin J Whitaker

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Why study NO2?
22B2

• Photodissociation of NO2 has been studied
  extensively as a model for unimolecular
  decomposition and non-adiabatic coupling
• Most previous studies have focused on single
  photon dissociation via the first dissociation
  limit (3.11 eV)

v2
v1
12B2
NO(2?)O(1D2)
v0
12A2
v4
v3
v2
12B1

• By understanding the high energy dissociation
  dynamics of NO2 we then hope to employ phase
  shaping techniques to control the branching
  ratios of the product channels

• We have studied the dissociation dynamics via
  single photon excitation in the UV and also using
  multiphoton visible-UV excitation

NO(2?)O(3PJ)
12A1
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Velocity map imaging
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Velocity map imaging
e
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Velocity map imaging
Eppink Parker, Rev. Sci. Inst. 68 (1997) 3477
To Pump
To Pump
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DC slice imaging
Townsend, Minitti Suits, Rev. Sci. Inst. 74
(2003) 2530
Velocity map imaging
To Pump
To Pump
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Single photon energetics

• 2 competing dissociation pathways below 244 nm

• At 226 nm, O(3PJ) can be produced in coincidence
  with NO in v 0-11
• At 226 nm, O(1D2) can be produced in coincidence
  with NO in v 0 or 1

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School of something FACULTY OF OTHER
O(3PJ) fragment slice imaging

• O(3PJ) distributions peak in coincidence with
  vibrationally cold, rotationally excited NO

e

• Anisotropies suggest 2 competing dissociation
  pathways

O(3P0)
(1)2B2
(2)2B2
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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

O(1D2) slice imaging

• Orbital angular momentum alignment studies yield
  information regarding the symmetry of
  dissociative states

• REMPI probe schemes provide sensitivity to such
  alignment effects

• By probing the O(1D2) distribution with various
  polarisation combinations, the dissociative state
  of this channel can be identified

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

O(1D2) energy distribution

• At 226 nm, O(1D2) is produced in coincidence with
  a vibrationally inverted NO distribution

• Vibrationally excited NO fragments are formed
  with a bimodal rotational distribution

• Rotationally and vibrationally excited NO
  fragments are produced with strongly aligned
  O(1D2) fragments

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

O(1D2) alignment

• ß values indicate dissociation via different
  geometries and/or on different timescales

• ml populations implicate dissociation via the
  (1)2A2 surface

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

Visible excitation of NO2

• Excitation below the 1st dissociation threshold
  primarily occurs to the (1)2B2 state which is
  strongly coupled to the (1)2A1 ground state

• Within 500 fs, the excited state population has
  crossed to perturbed vibrationally excited levels
  of the ground electronic state non-adiabatically

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

Multiphoton excitation of NO2

• A number of dissociative, multiphoton excitation
  pathways have been observed on excitation to the
  (1)2B2 manifold

• Time-resolved experiments (400 nm pump 266 nm
  probe) implicate an initial (31) dissociative
  ionisation process via the (1)2B2 state and a
  bent valence state of the cation (b3A2)

• At delays greater than 200 fs, a (12)
  dissociative ionisation process occurs via
  vibrationally excited levels of the (1)2A1 state
  and the 3pp2?u Rydberg state of the neutral

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

Multiphoton excitation of NO2

• Nanosecond experiments have suggested the
  production of neutral NO, O(3PJ) and O(1D2) via
  multiphoton excitation at visible wavelengths
  well below the first dissociation limit (Grant
  and co-workers, Shibuya et al. and Crowley and
  co-workers ?pump 420-532 nm)

• Experiments in our laboratory have ruled out the
  production of neutral O(1D2) via such pathways in
  the weak field regime

• Neutral O(3PJ) has been observed with low kinetic
  energies at excitation energies between 430 and
  460 nm although the dissociation pathway of these
  fragments is as yet unconfirmed

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

Conclusions

• Single photon dissociation dynamics occur via a
  number of non-adiabatic surface crossings
  resulting in a number of different dissociation
  pathways

• Multiphoton dissociation dynamics primarily occur
  via Rydberg states at pump energies close to the
  first dissociation limit

• At lower excitation energies multiphoton visible
  excitation produces low kinetic energy, neutral
  O(3PJ)

• The highly coupled nature of the electronic
  manifolds of NO2 results in a range of distinct
  product channels making the system amenable to
  time-domain pulse shaping experiments

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• REMPI spectra have been recorded for NO fragments
  in v 0-3 here both dissociation channels are
  probed

Future work acknowledgements

• Time-resolved imaging of the atomic fragments
  following UV or visible pumping
• Pulse-shaping experiments with fs and ns probe
  fields
• Thanks,
• Professor Benjamin J Whitaker
• Nick Form, Panagiotis Kapetanopoulos and Dr Ivan
  Anton Garcia
• Dr Valerie Blanchet, Professor Godfrey Beddard,
  Dr Marcelo Miranda, Dr Andrew Goddard and
  Dr Mark Blitz
• EPSRC for funding

This work has been sponsored by the