Why I never let go of my Ph.D. thesis research!

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The Physics of Charge-Asymmetric Molecular States

• Why I never let go of my Ph.D. thesis research!

Rhodes Scholars Symposium University of Illinois,
Chicago March 28, 2012
Supported by National Science Foundation Research
Corporation
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The story
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The review
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Major result Inner-shell ionization

• Common assumption only the least bound electron
  is ionized by tunneling in a strong field and the
  resulting ion is left in the ground state.
• Our (Gibson, Rhodes, et al.) result showed
  inner-shell ionization and, consequently,
  excitation of the ion by the strong laser field.
  In fact, excitation led to fluorescence of a
  previously unobserved state of N22.
• Results met with some resistance!
• I continued to pursue this question in different
  ways as a postdoc and a professor.

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Postdoc work at Bell Labs
Could ionize the 1pu and 2sg electrons, as well.
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Dissociation Channels
? N2 N0 (15.1 eV) ? N3 N1 (17.8 eV) ? N4
N2 (30.1 eV)

• N2 ? N21 ? N22 ? N1 N1 ? N23 ?
  N1 N2 ? N24 ? N2 N2 ?
  N25 ? N3 N2 ? N26 ? N3 N3
  ? N27 ? N4 N3

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Conclusions from VUV SpectraCoffee and Gibson,
PRA 69 (2004)

• Nitrogen shows many fluorescence lines generated
  from direct strong field excitation.
• In all cases, the excitation involves one or two
  2s holes.
• Some upper states consist of multiply excited
  states. One is at 25 eV above the ground state.
  N2 2s2p2 2p3.
• Direct lines identified from N4 - a state not
  seen in ion TOF data, until recently.

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Theory of Multiphoton Coupling in Molecules PRL
89 263001, PRA 67 043401

• Atoms do not show signs of multiphoton excitation
  when exposed to strong laser fields at
  intensities high enough to drive multiphoton
  transitions, the ac Stark shift detunes the laser
  and ionization sets in.
• So, what is so special about ionized diatomic
  molecules?
• They have an excited state structure that is
  highly susceptible to multiphoton coupling.

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2 electrons in a double well.
Ground state is a far off-resonant covalent
state. Above this is a pair of strongly coupled
ionic states. Only a weak coupling between them.
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3-Level Model System
This system can be solved exactly for the
n-photon Rabi frequency!
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N-photon Rabi Frequency
2-level frequency from Duvall (or Shirley), et
al.
In the 3-level system, multiphoton coupling
depends on R23 while the AC Stark shift depends
on R12. In the 2-level system, both effects come
from the same coupling.
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Perfect Floquet Ladder of States
The pair of ionic states are strongly modulated
by the laser field and create a complete Floquet
ladder of states with no ac Stark shift! The
ground state couples to this through a 1-photon
process which only produces a small Stark shift.
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Example Population transfer in a model system
A24.
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Again, a Floquet Ladder of States
The pair of strongly coupled ionic states is so
effective, it can assist a high-order multiphoton
transition to a regular covalent state! Verified
through a 5-level calculation. Transition
requires R23 to be large.
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Can even get adiabatic transfer on a 10-photon
transition!
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Pump-probe experiments in I2
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Iodine potential curves
Many time-resolved pump-probe experiments are
possible. Right now, we are specifically
interested in the I2 I0 states. The (2,0)
and (1,1) curves form an excimer-type system in
the dication! (2,0) is strictly bound while the
(1,1) is at best quasi-bound. Wanted to see if
we could populate the (2,0) states.
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Populating the (2,0) state
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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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Asymmetric channels can show spatial asymmetry in
a 1?2? field

• An asymmetric channel like (2,0) actually
  consists of two states with gerade and ungerade
  symmetry. Then one can form(2,0)R (2,0)g
  (2,0)u(2,0)L (2,0)g - (2,0)uwhere R and L
  refer to the 2 ion going to the right or the
  left.
• Of course, the (2,0)g and (2,0)u states must be
  populated coherently.

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I2 TOF Region with 1?2? fields
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Experimental results
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1-D 2-electron model
From the asymmetry measurements, we can show that
the ionization projects the molecules into the
field-induced states. This has not really been
considered before and suggests a new form of
strong-field control.
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Conclusions

• Strong laser fields do a lot more than just
  ionize the least bound electron and leave the ion
  in its ground state.
• Diatomic molecules have a structure that is
  highly susceptible to strong field excitation.
• High levels of excitation are seen through the
  dissociation channels and direct fluorescence
  from the excited molecule.
• Ionization occurs within the electronic structure
  induced by the strong laser field.