We consider two barriers enclosing a well that possesses a bound state. In
the presence of an oscillating field, this bound state is transformed into
a resonance with a width reflecting the escape process by absorption of qua
nta from the field. We show that a particle incident on this device with an
energy just one quantum above the bound-state energy has a transmissivity
approaching unity as the amplitude of the field is reduced. The conditions
for efficient transmission at this energy are examined.