Intense Laser Interactions with Trapped Molecular Ions

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Intense Laser Interactions with Trapped Molecular
Ions

• Jason Greenwood

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Outline

• Dissociation of H2 in an Intense Laser Field
• Electrostatic Ion Cavity
• Stability and loss
• Preliminary results of D2, HD dissociation
• Manipulating trapped ions
• Planned Experiments
• ASTRA laser
• Future

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Photon Induced Dissociation of H2

• TiSapphire laser pulse
• 800 nm
• 1.55 eV
• 2.5 fs period
• Two states
• 1s?g (ground)
• 2p?u
• H2(?9) 1? (800nm) ? H H 0.77eV

www.mpq.mpg.de/haensch/h2/introduction.htm
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Intense Field Dressed States

• System represented by superposition of states
  dressed by large number of photons, separated by
  1?
• Diabatic Curves

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Avoided Crossings

• Coupling between statesproduces avoided crossing
• Coupling E field dependent
• Vibrational period ? 15 fs
• Adiabatic dressed states
• long pulses ? gtgt 15 fs
• avoided crossing at 1? or 3?

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H2 Dissociation

• Field induces electric dipolealong internuclear
  axis
• Bond Softening
• nett 1 photon absorptionfor ? ? 5
• nett 2 photon absorptionfor ?? 4

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Experimental Studies

• Neutral Target
• H2 laser ? H H non-sequential ionization
• ? H2(?) laser ? H H0 dissociation
  ? H H ionization (SI, EI)
• Ion Target
• H2 e? ? H2(?) laser ? H H0
  dissociation ? H H ionization (SI,
  EI)
• Advantages of ion target
• first ionization process unrelated to laser
  interaction
• fast products, hence H0 detectable (pure
  dissociation)

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H2 Vibrational Distibutions
e- impact
3 x 1013 W cm-2

• Posthumus et al., PRL, 92, 163004 (2004)

4.8 x 1013 W cm-2
1.5 x 1014 W cm-2
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Theory vs Experiment

• Comparison difficult due to strong dependence on
• Vibrational population
• Intensity (varies spatially, temporally)
• Experimental energy resolution

1? vib. release (dashed)
2? vib. release (dotted)
Numerical solution of time dependent Schrödinger
equation for 2 state model for H2 with
Franck-Condon vibrational population.
L-Y Peng et al. J. Phys. B 38, 1727 (2005)
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H neutrals from H2 50fs pulses Ian
Williams(QUB), Roy Newell(UCL) et al.
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Transition to Ultra-short Pulses

• ? ? 15 fs inclusion of quantum nuclear motion
  important
• Dramatic change from adiabatic dissociation (long
  pulse) to frozen nuclei (short pulse)
• To verify theoretical predictions experiment
  needs well defined vibrational states!

H2 (? 0) Dissociation
5 fs
10 fs
20 fs
L-Y Peng et al. J. Phys. B 38, 1727 (2005)
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Why Trap Ions?

• Extended period for ion manipulation
• Internal pumping, de-excitation
• External manipulation of ion phase space
• Extended period of measurement
• Metrology, mass spectrometry
• Lifetime measurements

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Linear Electrostatic Trap (Ion Cavity)

• Ion Cavity (Dahan et al., Rev. Sci. Instrum., 69,
  76 (1997)
• For ions, refractive index ? V1/2
• Symmetric Laser cavity stable when

R 2f
L
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QUB Trap

• 2 planar mirrors, 2 lenses
• Trapped filled by pulsing entrance mirror
• Features
• Electrostatic, trapping mass independent
• keV ions stored, fast neutrals detected
• ToF measurements possible due to linear motion of
  ions

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Potential Energy Surface
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Ion Loss Mechanisms

• Population of Unstable Trajectories
• most ions lost by 1 ms
• Neutralisation in background gas
• X A ? X0 A
• X? A ? X0 A e-
• cross section can vary for excited states giving
  multi-component decay
• Elastic scattering in background gas
• Ion lost if scattered outside region of stable
  phase space
• May undergo multiple collisions
• Ion-ion scattering
• Density dependent
• Most significant at early trapping times
• External Perturbations
• Ions pushed outside stable phase space

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H2
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Dissociation Analysis by ToF

• Detect neutral D fragments
• 2 ToF peaks

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Laser Intensity lt 1011 Wcm-2
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Producing HD (? 0)

• Passive Cooling
• use HD with dipole moment
• ? 1 to ? 0 lifetime 80 ms
• gt 95 ? 0 after 300 ms

Vibrational decay of HD from initial
Franck-Condon distribution
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Ion Velocity Cooling

• Routinely performed at MeV, GeV energies
• Not directly demonstrated at keV energies
• Results in improved ToF resolution

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V
V
V
t
?E
??E
V
V
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Manipulation of Ion Phase Space
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Ion Cooling
Al? Si?
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Summary of Progress

• Positive and negative ions trapped for seconds
• Molecular dissociation energy release measured
  for different trapping times
• Ion cooling / bunch stabilisation demonstrated
• Energy spread reduced from 16 eV to 8 eV

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Potential Applications

• High resolution ToF studies of internally and
  externally cold molecular ions, e.g. HD, CH,
  CO
• ASTRA Run June,July 2006
• HD (? 0) passive cooling 300 ms
• Dissociation using 30 fs and 10 fs pulses
• Future ASTRA work with CEP stabilised ultrashort
  pulses. Control of
• HD (? 0) ? H D
• HD (? 0) ? H D
• Potential for studies of biomolecular
  fragmentation

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Acknowledgements

• CCLRC, EPSRC
• EU Framework 6 Integrated Infrastructure
  Initiative
• Ion trapping
• Higher Education Authority (Rep. of Ireland)
• Plasma and Ion Beam Network (Dublin City
  University/QUB)

Ian Williams Jarlath McKenna Jofre
Pedregosa-Gutierrez Philip Orr John
Alexander Wesley Walter (Denison University, Ohio)