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| Physical Review A, 61, 13402 (2000) |
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| Comment in J. Physics B, 33 1279 (2000) |
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Laser induced resonance states as a cause for dynamic suppression of
ionization in high frequency short pulses
Danny
Barash and Ann. E. Orel
Dept. of Applied Science, University of California, Davis and
Lawrence National Laboratories, Livermore, California 94550
and Roi Baer·
Dept. of Physical Chemistry and The Lise Meitner Minerva-Center for Computational Quantum
Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
An adiabatic-Floquet
formalism is used to study the suppression of ionization in short laser pulses. In the
high frequency limit we derive adiabatic equations involving only the pulse envelope where
transitions between adiabatic states are purely ramp effects. For a short-ranged potential
having a single bound state we show that ionization suppression is caused by the
appearance of a laser induced resonance state, which is coupled by the pulse ramp to the
ground state and traps ionizing flux.
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When very strong pulses are shot at atoms an
electron may be ejected with a high probability (ionization). However, at high
frequencies and ultra high intensities the ionization may be suppressed. This may happen
for CW pulses or for short pulses (FIG 1). What is interesting is that there is a high
frequency limit and it has structure!« FIG. 1:
Total ionization probability vs. ao, for a short pulse with Ton=94.25 au, Tflat=251.3
au, and w = 0.25, 0.5, 1.0, 2.0 au. Thick line: the high
frequency limit (visually indistinguishable from the w = 2.0 au
line).
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We analyzed the ionization in an adiabatic picture
and showed that the interplay between the adiabatic ground state and the first continuum
state determines the suppression of ionization.FIG 2: Population of lowest 5
adiabatic states as a function of pulse rise and decay time for 4 pulses with the shown
maximal displacement ao. Pulse ramp forms are shown as a dotted line in the ao=6 au figure. In all cases the n=0 state starts
with |C0|2 = 1 at t=0 au and looses population to excited states
which are all positive energy states.
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It was found that the n=1 state becomes a resonance
state once the field intensity is strong enough (FIG 3). Thus a field induced resonance is
the cause of ionization supprssion: it traps the ionized flux!FIG. 3: The
shape of the dressed potential and the first 3 even adiabatic states at a = 0 au and a = 12.5 au. It
is seen that while the n=0 state is bound and n=2 is a continuum state in both cases, the
n=1 state changes character from a continuum state to a localized resonance.
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