Ultrafast Laser Interactions with Atoms, Ions and Molecules

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Ultrafast Laser Interactions with Atoms, Ions and
Molecules
I.D.Williams B. Srigengan, M. Suresh, J. McKenna
W.R. Newell W.A. Bryan, S.L. Watson, E.M.L.
English
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Research Interest.
Strong field ionization of atoms (noble gases),
ions and small molecules (H2, H2, CO2, N2)
Tests of non-sequential ionization by comparing
linear to circular laser polarization effects
Pulse lengthening experiments on ionization of
small molecules, namely H2
Ionization of metastably produced ionic targets
(C, Ar, Xe)
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Strong Field Ionization.
?
Laser Intensity 1016 Wcm-2
Electric field 2x1011 Vm-1
Field Free
i.e. similar to atomic Coulomb field strength
Alternating electric field gradient lowers the
Coulomb barrier
Tunnelling
Barrier width decreases hence there is an
increased probability of the electron tunnelling
through it
Over-the-Barrier Ionization
If barrier height is lowered below electron
potential level, the electron escapes over the
barrier
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Sequential Ionization.
Walker et al PRL, 73, 1227 (1994)
An example for He
The normal Sequential ionization of an atom is
indicated by the blue curves
Initially the He atom is ionized through tunnel
or multiphoton ionization to form He
He ? He e-
The newly formed He ion is then further field
ionized to form He2
He ? He2 e-
This is a sequential stepwise process
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Nonsequential Ionization.
The observed double ionization yield is enhanced
(red curve) by orders over magnitude above the
sequential yield. This is Nonsequential
ionization.
Walker et al PRL, 73, 1227 (1994)
Both electrons are believed to be ejected in a
single step process
He ? He2 2e-
Several models have been proposed to explain this
enhanced ionization
Collective Tunneling Shake-up model
Rescattering model
S
NS
Rescattering model is generally most widely
accepted.
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Rescattering model.
Linear Polarized Field
The first ejected electron, before it escapes the
vicinity of the atom, is driven back by the
reversal of the laser field direction.
Over a number of optical cycles it may recollide
with a secondary electron.
This electron may be ejected directly or excited
to a high lying state from which it tunnels free
giving double ionization.
This model predicts a strong laser polarization
dependence.
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Rescattering model.
Circular Polarized Field
Unlike with linear polarization, with circular
polarization the electron will be driven around
the core region as opposed to back through it.
The probability of a recollision is therefore
much reduced for circular.
Experiments have shown that the Nonsequential
component is strongly suppressed for circular
polarization giving credence to the Rescattering
model.