<s>
Photoionization	B-Algorithm
is	O
the	O
physical	O
process	O
in	O
which	O
an	O
ion	O
is	O
formed	O
from	O
the	O
interaction	O
of	O
a	O
photon	B-Application
with	O
an	O
atom	B-Language
or	O
molecule	O
.	O
</s>
<s>
Not	O
every	O
interaction	O
between	O
a	O
photon	B-Application
and	O
an	O
atom	B-Language
,	O
or	O
molecule	O
,	O
will	O
result	O
in	O
photoionization	B-Algorithm
.	O
</s>
<s>
The	O
probability	O
of	O
photoionization	B-Algorithm
is	O
related	O
to	O
the	O
photoionization	B-Algorithm
cross	O
section	O
of	O
the	O
species	O
--	O
the	O
probability	O
of	O
an	O
ionization	O
event	O
conceptualized	O
as	O
a	O
hypothetical	O
cross-sectional	O
area	O
.	O
</s>
<s>
This	O
cross	O
section	O
depends	O
on	O
the	O
energy	O
of	O
the	O
photon	B-Application
(	O
proportional	O
to	O
its	O
wavenumber	O
)	O
and	O
the	O
species	O
being	O
considered	O
i.e.	O
</s>
<s>
In	O
the	O
case	O
of	O
molecules	O
,	O
the	O
photoionization	B-Algorithm
cross-section	O
can	O
be	O
estimated	O
by	O
examination	O
of	O
Franck-Condon	O
factors	O
between	O
a	O
ground-state	O
molecule	O
and	O
the	O
target	O
ion	O
.	O
</s>
<s>
For	O
photon	B-Application
energies	O
below	O
the	O
ionization	O
threshold	O
,	O
the	O
photoionization	B-Algorithm
cross-section	O
is	O
near	O
zero	O
.	O
</s>
<s>
But	O
with	O
the	O
development	O
of	O
pulsed	O
lasers	O
it	O
has	O
become	O
possible	O
to	O
create	O
extremely	O
intense	O
,	O
coherent	O
light	O
where	O
multi-photon	O
ionization	O
may	O
occur	O
via	O
sequences	O
of	O
excitations	O
and	O
relaxations	O
.	O
</s>
<s>
Several	O
photons	B-Application
of	O
energy	O
below	O
the	O
ionization	O
threshold	O
may	O
actually	O
combine	O
their	O
energies	O
to	O
ionize	O
an	O
atom	B-Language
.	O
</s>
<s>
This	O
probability	O
decreases	O
rapidly	O
with	O
the	O
number	O
of	O
photons	B-Application
required	O
,	O
but	O
the	O
development	O
of	O
very	O
intense	O
,	O
pulsed	O
lasers	O
still	O
makes	O
it	O
possible	O
.	O
</s>
<s>
In	O
the	O
perturbative	O
regime	O
(	O
below	O
about	O
1014	O
W/cm2	O
at	O
optical	O
frequencies	O
)	O
,	O
the	O
probability	O
of	O
absorbing	O
N	O
photons	B-Application
depends	O
on	O
the	O
laser-light	O
intensity	O
I	O
as	O
IN	O
.	O
</s>
<s>
Above-threshold	O
ionization	O
(	O
ATI	O
)	O
is	O
an	O
extension	O
of	O
multi-photon	O
ionization	O
where	O
even	O
more	O
photons	B-Application
are	O
absorbed	O
than	O
actually	O
would	O
be	O
necessary	O
to	O
ionize	O
the	O
atom	B-Language
.	O
</s>
<s>
More	O
precisely	O
,	O
The	O
system	O
will	O
have	O
multiple	O
peaks	O
in	O
its	O
photoelectron	O
spectrum	O
which	O
are	O
separated	O
by	O
the	O
photon	B-Application
energies	O
,	O
this	O
indicates	O
that	O
the	O
emitted	O
electron	O
has	O
more	O
kinetic	O
energy	O
than	O
in	O
the	O
normal	O
(	O
lowest	O
possible	O
number	O
of	O
photons	B-Application
)	O
ionization	O
case	O
.	O
</s>
<s>
The	O
electrons	O
released	O
from	O
the	O
target	O
will	O
have	O
approximately	O
an	O
integer	O
number	O
of	O
photon-energies	O
more	O
kinetic	O
energy	O
.	O
</s>
<s>
When	O
either	O
the	O
laser	O
intensity	O
is	O
further	O
increased	O
or	O
a	O
longer	O
wavelength	O
is	O
applied	O
as	O
compared	O
with	O
the	O
regime	O
in	O
which	O
multi-photon	O
ionization	O
takes	O
place	O
,	O
a	O
quasi-stationary	O
approach	O
can	O
be	O
used	O
and	O
results	O
in	O
the	O
distortion	O
of	O
the	O
atomic	O
potential	O
in	O
such	O
a	O
way	O
that	O
only	O
a	O
relatively	O
low	O
and	O
narrow	O
barrier	O
between	O
a	O
bound	O
state	O
and	O
the	O
continuum	O
states	O
remains	O
.	O
</s>
