A two-center zero-range potential in the presence of an external field sinu
soidal in time is used to describe high-harmonic generation by a molecular
ion. The resulting properties of the harmonic spectrum are analyzed in term
s of the semiclassical three-step or simple man's model. Very good agreemen
t is observed. A time-frequency analysis of the harmonic spectrum points to
the significance of the semiclassical return times for the temporal struct
ure of the harmonic output, In simple cases, the return time is directly re
lated to the phase difference between adjacent harmonics.