TITLE: Quasistationary stabilization and atomic dichotomy at low frequencies

{1} Institute for Theoretical Atomic and Molecular Physics, at Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
{2} Department of Physics, University of Connecticut, Storrs, CT 06269
{3} Department of Chemistry, University of Bucharest, Bucharest, Romania}

ABSTRACT:

{\it Stabilization} is one of the highlights of superintense laser-atom
interactions, actively studied in recent years. In its ``quasistationary''
(QS) form of manifestation, it characterizes the fact that the rate of
ionization of a light atom (e.g., hydrogen) in a monochromatic field \omega
of amplitude E_{0}, starts to {\it decrease} beyond some high value of
E_{0}, which indicates a growing stability of the atom in the field.
This counter-intuitive phenomenon has been found consistently only at
\omega ,high with respect to typical internal atomic frequencies.
A related high-frequency phenomenon is that of {\it atomic dichotomy}, i.e.,
the splitting of the electronic charge of the atom, in a field of linear
polarization, into two non-overlapping lobes. We now show that {\it QS and
atomic dichotomy occur quite generally also at low frequencies}, if the
field E_{0} is strong enough. We consider in the oscillating (KH) frame of
reference, the large \alpha _{0}\equiv E_{0}\,\omega ^{-2} behavior at
some fixed low \omega  of the solution of the Floquet system of
differential equations. We treat a realistic 3 potential V\left( {\bf
r}\right), e.g., Coulomb. We show that, only one Floquet component can
survive in the limit \alpha _{0}\rightarrow \infty, and that this goes
over into an eigenfunction of the time-independent Schroedinger
equation for the ``dressed potential'' V_{0}\left( \alpha_{0},{\bf r}\right).
The latter is defined as the time average of the oscillating potential
V\left( {\bf r+}\alpha _{0}{\bf e}\cos \omega t\right), where \alpha
_{0}{\bf e}\cos \omega t is the motion of the classical electron in the
linearly polarized field. We also show that the corresponding quasienergy
{\cal E} becomes real, while tending to zero as \alpha _{0}^{-2/3}.
This situation is necessarily coupled to a``dichotomous'' atomic structure,
the two non-overlapping lobes the wave function being centered on the
turning points \pm \alpha _{0}{\bf e} of the classical motion. The
ionization rate \Gamma  is a decreasing function of \alpha _{0} (and
E_{0}) and therefore manifests QS. We illustrate numerically our general
analytic conclusions on a 1D potential model. We present: 1. The map of
Re{\cal E} for the Floquet states of the system at \omega =0.24 a.u. and
\alpha _{0}\leq 100, showing the predicted clustering of the energies to
zero; 2. The decrease to zero (albeit in an oscillating manner) of \Gamma
for the Floquet states (i.e., QS); 3. The dichotomy of the closed channel
Floquet components, and the dominance of one of them.

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