#
Above Threshold Ionization photoelectron spectra look the way that they
do because of single-electron effects!

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Below you will find pairs of graphs, the top of which is experimental data
obtained by The
Van Woerkom Group and the bottom of which is calculation performed
by Harm Muller at FOM. They are photoelectron
spectra of Argon atoms in the presence of ultra-short, high-intensity laser
light. We use 800 nm, 120 fs laser pulses at a 1 kHz repetition rate, focused
into an argon-backfilled vacuum chamber, with pressures low enough to assure
that there are no space-charge effects. We employ a restricted volume technique
of time-of-flight spectroscopy to determine the kinetic energy (KE) of
the electrons. Our electronics allow us a resolution of better than 40
ps and our KE resolution is approximately 20 meV, even for energies greater
than 30 eV. The restricted volume (allowing only photoelectrons ejected
from a 500 µm radius surrounding the focus) technique allows for
facile spatial deconvolution of the data. A l/2
wave plate/polarizing cube pair are used to manipulate the intensity of
the light; we obtained photoelectron spectra from 35 different intensities,
ranging from 3 - 10.5 * 10$13W/cm2$,
and a wide range of kinetic energies, which run from 0.4 eV to out past
45 eV. The calculation is a numerical integration of the Schrdinger equation
for a model argon atom in the SAE approximation. The atom is represented
by a three-dimensional potential well with a repulsive core, fitted to
reproduce the bound-state spectrum of argon. The time-dependant Schrdinger
equation is integrated in the velocity gauge, on a radial-position, angular
momentum grid. The propagation uses a half-implicit split-operator scheme
accurate to second order in the time step. Fourth-order implicit finite-difference
expressions are used to approximate the radial derivatives. The pulse is
modeled as a 45-cycle flat top pulse with a 1/2-cycle turn-on and turn-off.
To account for the spin-orbit splitting of the core, the J=3/2 and J=1/2
ionization states are each modeled as separate species, and then added
in a two-to-one (respectively) ratio.

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The graphs below each have photoelectron spectra from various intensities
(represented by different colors). The intensity is given in units of 10$13W/cm2$.

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Note that for all intensities and all kinetic energies, that there is an
amazing similarity between the data and calculation! Keep in mind that
the calculation only takes into account single-electron, single-ionization
effects.

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Highest Intensities

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Higher Intensities

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Middle Intensities

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Lower Intensities

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Lowest Intensities

*Page created by LVW Group. Last modified 1-Feb-99*