  The tunneling model of ionization applies only to longitudinal fields:
quasistatic electric fields that do not propagate. Laser fields are transverse:
plane wave fields that possess the ability to propagate. Although there is an
approximate connection between the effects of longitudinal and transverse
fields in a useful range of frequencies, that equivalence fails completely at
very low frequencies. Insight into this breakdown is given by an examination of
radiation pressure, which is a unique transverse-field effect whose relative
importance increases rapidly as the frequency declines. Radiation pressure can
be ascribed to photon momentum, which does not exist for longitudinal fields.
Two major consequences are that the near-universal acceptance of a static
electric field as the zero frequency limit of a laser field is not correct; and
that the numerical solution of the dipole-approximate Schr\"{o}dinger equation
for laser effects is inapplicable as the frequency declines. These problems
occur because the magnetic component of the laser field is very important at
low frequencies, and hence the dipole approximation is not valid. Some
experiments already exist that demonstrate the failure of tunneling concepts at
low frequencies.
