Laser-induced vibrational motion through impulsive ionization

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Laser-induced vibrational motion through
impulsive ionization
Grad students Li Fang, Brad Moser Funding NSF-AM
O
George N. Gibson University of Connecticut Departm
ent of Physics
October 19, 2007 University of New
Mexico Albuquerque, NM
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Motivation

• Excitation of molecules by strong laser fields is
  not well-studied.
• Excitation can have positive benefits, such as
  producing inversions in the VUV and providing
  spectroscopy of highly excited states of
  molecules. Excited states of H2 have never been
  studied before!
• Can be detrimental to certain applications, such
  as quantum tomography of molecular orbitals.

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How to detect excitation

• TOF experiments are very common, but are not
  sensitive to excitation, except in one case
  Charge Asymmetric Dissociation.
• I22 ? I2 I0 has 8 eV more energy than I22
  ? I1 I1
• Also see N26 ? N4 N2, which has more than 30
  eV energy than the symmetric channel.

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Pump-probe experiment with fixed wavelengths.
In these experiments we used a standard TiSapphir
e laser 800 nm 23 fs pulse duration 1 kHz rep.
rate Used 80 ?J pump and 20 ?J probe.
Probe
Pump
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Pump-probe spectroscopy on I22
Enhanced Excitation
Enhanced Ionization at Rc
Internuclear separation of dissociating molecule
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Lots of vibrational structure in pump-probe
experiments
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Vibrational structure

• Depends on wavelength (800 vs 400 nm).
• Depends on relative intensity of pump and probe.
• Depends on polarization of pump and probe.
• Depends on dissociation channel.
• Will focus on one example the (2,0) channel with
  400 nm pump and probe.

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Laser System

• TiSapphire 800 nm Oscillator
• Multipass Amplifier
• 750 ?J pulses _at_ 1 KHz
• Transform Limited, 25 fs pulses
• Can double to 400 nm
• Have a pump-probe setup

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Ion Time-of-Flight Spectrometer
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I2 pump-probe data
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(2,0) vibrational signal

• Final state is electronically excited.
• See very large amplitude motion, can measure
  amplitude and phase modulation.
• Know final state want to identify intermediate
  state.

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I2 potential energy curves
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Simulation of A state
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Simulation results
From simulations - Vibrational period-
Wavepacket structure- (2,0) state
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(2,0) potential curve retrieval
It appears that I22 has a truly bound potential
well, as opposed to the quasi-bound ground state
curves. This is an excimer-like system bound
in the excited state, dissociating in the ground
state. Perhaps, we can form a UV laser out of
this.
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What about the dynamics?

• How are the states populated?
• I2 ? I2 ? (I2) - resonant excitation?
• I2 ? (I2) directly innershell ionization?
• No resonant transition from X to A state in I2.

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Ionization geometry
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Ionization geometry
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From polarization studies

• The A state is only produced with the field
  perpendicular to the molecular axis. This is
  opposite to all other examples of strong field
  ionization in molecules.
• The A state only ionizes to the (2,0)
  state!?Usually, there is a branching ratio
  between the (1,1) and (2,0) states, but what is
  the orbital structure of (2,0)?
• Ionization of A to (2,0) stronger with parallel
  polarization.

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Conclusions from I2

• Can identify excited molecular states from
  vibrational signature.
• Can perform novel molecular spectroscopy.
• Can learn about the strong-field tunneling
  ionization process, especially details about the
  angular dependence.
• Could be a major problem for quantum tomography.

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Ground state vibrations
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Lochfrass J. Ullrich A. Saenz
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TOF Data
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Phase lag
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Phase lag
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Simulations
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Thermal effects
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Conclusions

• We see large amplitude ground oscillations in
  neutral iodine molecules.
• We believe them to result from Lochfrass or
  R-dependent ionization of the vibrational
  wavefunction.
• From simulations, we conclude that the amplitude
  of the coherent vibrations is larger for larger
  temperature.
• This is very different from all other coherent
  control schemes that we are aware of.