Slow Coulomb Explosions

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Slow Coulomb Explosions
A.E. SLATTERY, T.A. FIELD, M. AHMAD, R.I. HALL,
P. LABLANQUIE, J. LAMBOURNE, F. PENENT, J.H.D.
ELAND
DIAM, Université Pierre et Marie Curie, 75252
Paris 5, France LURE, Université Paris Sud,
91898 Orsay, France Physical and Theoretical
Chemistry Laboratory, Oxford, OX1 3QZ
Department of Pure and Applied Physics, Queens
University Belfast, BT7 1NN t.field_at_qub.ac.uk
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CONTENTS

• Introduction
• Experiment
• Data Processing
• Results
• Conclusions
• Acknowledgements

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DICATIONS e.g. CO22
Relevance

• Interstellar medium PAH2
• Planetary atmospheres Mars CO22

a potential well due to chemical bonding. ß
coulombic repulsion between fragment ions, A and
B. Metastable states can be supported by
potential wells
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METASTABLE STATES

• Observed in Mass spectrometry for many years
• Show a range of lifetimes e.g. CO22

• TPEsCO ion coincidence technique ? lifetimes of
  vibrational states of CO2
• Aim investigate larger energy selected
  metastable dications (CO22, C6X62 - X H,D,F)
  with TPEsCO ion coincidence

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DICATION ENERGY SELECTION
The formation of molecular dications by
ionisation with a photon, h? proceeds as M
hv ? M2 e1- e2- The internal
energy of the molecular dication, E(M2), is
given by, E(M2) hv fDIP E(e1-)
E(e2-) Where fDIP - double ionisation
energy E(e1-), E(e2-) Energy of electrons For
two threshold electrons E(M2) hv
fDIP The internal energy of the M2 ion can be
selected by tuning the photon energy.
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APPARATUS
Penent F., Hall R.I., Panajotovic R., Eland
J.H.D., Chaplier G., Lablanquie P., Phys. Rev.
Lett., 81, 1998, 3619
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METASTABLE SIGNATURE
peak instant fragmentation (lt10 ns). tail
fragmentation in the source and acceleration
regions. V fragmentation in drift region
CO
TOF 2nd ion
CO22
TOF 1st ion
O
CO22
2d TOF mass spectrum for pairs of CO and O ions
detected from the ionization of CO22 by 40 eV
photons.
Schematic diagram of TOF spectrometer
Data include true and false coincidences.
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DATA PROCESSING 1D
1e- 1 ion coincidence spectra
CO22
O
Raw data spectra
CO
C
Subtraction spectra
ion yield
Corrected spectra
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DATA PROCESSING 2D
2e- 2 ion spectra of CO22 39.8 eV
CO
CO22
O
CO22
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Removal of false coincidences from 2e- 2 ion
coincidence spectra.
False coincidences, 2e- 2 ion where (1) All
false (2) 1 electron false
(3) 1 ion false (4) 2 independent pairs of true
coincidences
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CO22 BRANCHING RATIOS
CO O (t lt10 ns)
CO O (10lt t lt2300ns)
normalized counts
CO22 (t gt2300ns)
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LIFETIME CALCULATION
fraction 0.322
Iteration of Monte-Carlo simulation to match
experimental data and find lifetime
?1 1000 ns
fraction 0.417
CO22 at 40.4 eV
Fraction 0.50 ? 0.08
?2 639 ns
fraction 0.50346
?final 463 ns
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RESULTS CO22
Lifetimes of CO22 vs. photon energy.
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COMPARISON WITH THEORY
Hochlaf M., Bennett F.R., Chambaud G., Rosmus P.,
J. Phys. B, 31, 1998, 2163
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C6H62 AND C6D62
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LIFETIMES

• Energy Shift due to
• Zero point energy (isotope effect )
• or
• Different tunnelling rates for H and D

Statistical mechanism supported by new
experiments on 2,4-hexadiyne - C6H6

• Decrease in lifetime as energy increases due to
• dissociation due to a tunnelling mechanism.
• or
• dissociation due to a statistical mechanism.

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C6F62
29.60 eV
30.20 eV
29.80 eV
30.40 eV
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CONCLUSIONS

• Lifetimes of energy selected molecular dications
  CO22, C6H62, C6D62 and C6F62 have been
  determined.
• CO22 - metastability due to a spin forbidden
  intersystem crossing.
• Isotope effect in the lifetimes of C6H62 and
  C6D62.
• C6X62 metastability due to statistical
  dissociation (XH,D,F)

Further information on the potential energy
surfaces of these molecular dications would help.
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ACKNOWLEDGEMENTS

• LURE, Univ. Paris Sud, Orsay, France
• The Access to Research Infrastructures
  activity of EC.
• EPSRC