  We perform a detailed analysis of high-order harmonic generation in diatomic
molecules within the strong-field approximation, with emphasis on
quantum-interference effects. Specifically, we investigate how the different
types of electron orbits, involving one or two centers, affect the interference
patterns in the spectra. We also briefly address the influence of the choice of
gauge, and of the initial and final electronic bound states on such patterns.
For the length-gauge SFA and undressed bound states, there exist additional
terms, which can be interpreted as potential energy shifts. If, on the one
hand, such shifts alter the potential barriers through which the electron
initially tunnels, and may lead to a questionable physical interpretation of
the features encountered, on the other hand they seem to be necessary in order
to reproduce the overall maxima and minima in the spectra. Indeed, for dressed
electronic bound states in the length gauge, or undressed bound states in the
velocity gauge, for which such shifts are absent, there is a breakdown of the
interference patterns. In order to avoid such a problem, we provide an
alternative pathway for the electron to reach the continuum, by means of an
additional attosecond-pulse train. A comparison of the purely monochromatic
case with the situation for which the attosecond pulses are present suggests
that the patterns are due to the interference between the electron orbits which
finish at different centers, regardless of whether one or two centers are
involved.
