Strong-field physics revealed through time-domain spectroscopy

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Strong-field physics revealed through time-domain
spectroscopy
Grad student Li Fang now at LCLS Funding NSF-A
MO
George N. Gibson University of Connecticut Departm
ent of Physics
November 18, 2009 Stevens Institute of
Technology Hoboken, New Jersey
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Motivation

• Vibrational motion in pump-probe experiments
  reveals the role of electronically excited
  intermediate states.
• This raises questions about how the intermediate
  states are populated. Also, we can study how
  they couple to the final states that we detect.
• We observe inner-orbital ionization, which has
  important consequences for HHG and quantum
  tomography of molecular orbitals.

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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
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 (400 to 800 nm).
• relative intensity of pump and probe.
• polarization of pump and probe.
• dissociation channel.
• We learn something different from each signal.
• Will try to cover several examples of vibrational
  excitation.

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I2 pump-probe data
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(2,0) vibrational signal

• Amplitude of vibrations so large that we can
  measure changes in KER, besides the signal
  strength.
• 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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What about the dynamics?

• How is the A-state 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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From polarization studies

• The A state is only produced with the field
  perpendicular to the molecular axis. This is
  opposite to most 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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Implications for HHG and QT

• We can readily see ionization from orbitals
  besides the HOMO.
• Admixture of HOMO-1 depends on angle.
• Could be a major problem for quantum tomography,
  although this could explain some anomalous
  results.

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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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Wavelength-dependent pump probe scheme

• Change inner and outer turning points of the
  wave packet by tuning the coupling wavelength.
• Femtosecond laser pulses
• Pump pulse variable wavelength. (517 nm, 560 nm
  and 600 nm.) Probe pulse 800 nm.

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I2 spectrum vibrations in signal strength and
kinetic energy release (KER) for different pump
pulse wavelength 517nm, 560 nm and 600 nm
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Simulation trapped population in the (2,0)
potential well
The (2,0) potential curve measured from the A
state of I2 in our previous work
PRA 73, 023418 (2006)
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I2 In dissociation channels
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Neutral ground state vibrations in I2

• Oscillations in the data appear to come from the
  X state of neutral I2.
• Measured the vibrational frequency and the
  revival time.

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Revival structure

• Vibrational frequencyMeasured 211.0?0.7
  cm-1Known 215.1 cm-1 Finite temp 210.3 cm-1

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Raman scattering/Bond softening

• Raman transitions are made possible through
  coupling to an excited electronic state. This
  coupling also gives rise to bond softening, which
  is well known to occur in H2.

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Lochfrass

• New mechanism for vibrational excitation
  LochfrassR-dependent ionization distorts the
  ground state wavefunction creating vibrational
  motion.

• Seen by Ergler et al. PRL 97, 103004 (2006) in
  D2.

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Lochfrass vs. Bond softening

• Can distinguish these two effects through the
  phase of the signal.

• ?LF ?
• ?BS ?/2.

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Iodine vs. Deuterium

• Iodine better resolved 23 fs pulse/155 fs
  period 0.15 (iodine) 7 fs pulse/11 fs period
  0.64 (deuterium)
• Iodine signal huge

• DS/Save 0.10

• DS/Save 0.60

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Variations in kinetic energy

• Amplitude of the motions is so large we can see
  variations in KER or ltRgt.

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Temperature effects

• Deuterium vibrationally cold at room
  temperatureIodine vibrationally hot at room
  temperature
• Coherent control is supposed to get worse at high
  temperatures!!! But, we see a huge effect.
• Intensity dependence also unusual
• We fit ltRgt DRcos(wtj) RaveAs intensity
  increases, DR increases, Rave decreases.

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Intensity dependence

• Also, for Lochfrass signal strength should
  decrease with increasing intensity, as is seen.

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• But, Rave ? temperature

T decreases while DR increases!!!
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We have an incoherent sea of thermally populated
vibrational states in which we ionize a coherent
hole

• So, we need a density matrix approach.

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Density matrix for a 2-level model

• For a thermal system
• where p1(T) and p2(T) are the Boltzmann factors.
  This cannot be written as a superposition of
  state vectors.

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Time evolution of r

• We can write
• These we can evolve in time.

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Coherent interaction use p/2 pulse for maximum
coherence

• Off diagonal terms have opposite phases. This
  means that as the temperature increases, p1 and
  p2 will tend to cancel out and the coherence will
  decrease.

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R-dependent ionization assume only the right
well ionizes.

• yf (yg ye)/2
• Trace(r) ½ due to ionization

What about excited state?
NO TEMPERATUREDEPENDENCE!
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Expectation value of R, ltRgt
The expectation values are p/2 out of phase for
the two interactions as expected.
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Comparison of two interactions

• Coherent interactions
• Off diagonal terms are imaginary.
• Off diagonal terms of upper and lower states have
  opposite signs and tend to cancel out.

• R-dependent ionization
• Off-diagonal terms are real.
• No sign change, so population in the upper state
  not a problem.

Motion produced by coherent interactions and
Lochfrass are p/2 out of phase.
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Real (many level) molecular system

• Include electronic coupling to excited state.
• Use I(R) based on ADK rates. Probably not a good
  approximation but it gives R dependence.
• Include n 0 - 14

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Generalize equations
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Same conclusions

• For bond-softening
• Off-diagonal terms are imaginary and opposite in
  sign to next higher state. r12(1) ? -r12(2)
• DR decreases and ltngt increases with temperature.
• For Lochfrass
• Off diagonal terms are real and have the same
  sign. r12(1) ? r12(2)
• DR increases and ltngt decreases with temperature.

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• Excitation from Lochfrass will always yield real
  off diagonal elements with the same sign for
  excitation and deexcitation f(R) is the survival
  probablility

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DR and ltngt
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Density matrix elements
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Conclusions

• Coherent reversible interactions
• Off-diagonal elements are imaginary
• Excitation from one state to another is
  out-of-phase with the reverse process leading to
  a loss of coherence at high temperature
• Cooling not possible
• Irreversible dissipative interactions
• Off-diagonal elements are real
• Excitation and de-excitation are in phase leading
  to enhanced coherence at high temperature
• Cooling is possible

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Conclusions

• Excitation of the A-state of I2 through
  inner-orbital ionization
• Excitation of the B-state of I2 to populate the
  bound region of (2,0) state of I22
• Vibrational excitation through tunneling
  ionization.

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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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Phase lag
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Ionization geometry
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Ionization geometry
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I2 pump-probe data