The interaction between radiation and a two-level molecular system in the p
resence of permanent dipole moments is analyzed. It is found that the nonze
ro difference between permanent dipoles induces a polarization that oscilla
tes at frequencies omega, 2 omega, 3 omega, etc., where omega is the freque
ncy of the electromagnetic field resonant with the two-level system. An ana
lysis of the propagation equation of the harmonics shows that even harmonic
s are amplified while odd harmonics are absorbed.