  Thomas-Fermi model is applied to describe some basic properties of a negative
hydrogen ion in a strong electric field. The equilibrium ionic size, energy and
polarizability of the ion are calculated. Collective modes of the dipole
oscillations are regarded. A barrier, due to which the ion is in a stable
state, is studied. The barrier vanishes at some large value of the electric
field, which is defined as a critical value. The dependence of the critical
field on frequency is studied. At high frequencies a stripping mechanism for
instability arises. At the resonant frequency comparatively low amplitude of
the electric field causes the stripping instability.
