QUANTUM EFFECTS AND THE ROLE OF THE POTENTIAL RIDGEIN
                         IONIZATION THRESHOLD LAW

Naoyuki Miyashita$^{*\dagger}$, Daiji Kato$^\sharp$ , Shinichi
Watanabe$^\dagger$,
 Michio Matsuzawa$^\dagger$, and J.~H.~Macek$^\ddagger$

$^\dagger$Department of Applied Physics and Chemistry, U. of
Electro-Communications,
 1-5-1 ~Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan
$^\sharp$Japan Science and Technology Corporation, International
Project ``COLD TRAPPED IONS'' AxisChofu Bldg 3F, 1-40-2, Fuda,
Chofu-shi, Tokyo 182-0024, Japan
$^\ddagger$Department of Physics and Astronomy, University of
Tennessee,Knoxville, Tennessee 37996, U.S.A.
and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A.

Do quantum effects distinct from the classical counterpart show up in
electron-atom ionization
near the threshold?We investigate the threshold behavior of two model
electron-atom ionization
problems, one in which the target atom interacts with the
incidentelectron only by monopole (Temkin-Poet model) and the other in
which the nuclear charge is 1/4. These models help us identify
purely quantum mechanical effects as well as the limitation of the
semi-quantal treatment based on the expansion of the Hamiltonian near
the ridge.