MODEL-CALCULATIONS OF HIGH-HARMONIC GENERATION IN MOLECULAR-IONS

One electron bound by a three-dimensional two-center zero-range potent ial is embedded in an electric field with sinusoidal time dependence a nd arbitrary polarization and orientation with respect to the axis of the two-center potential. In the absence of the field, the model suppo rts up to two bound states, which have a large transition dipole momen t. Hence, the physical systems best described by the model are molecul ar ions such as H-2(+). Rates for high-harmonic emission are calculate d analytically up to one final quadrature. In terms of the rescatterin g picture, harmonic emission can be attributed to two different mechan isms: electrons recombine either at the center they started from or at the other one. The latter case allows for three topologically differe nt classes of orbits, which lead to different spectral ranges of harmo nics. Two of them are similar to atomic (one-center) harmonic generati on, but have different cutoff laws that are no longer proportional to the ponderomotive potential. In the third the electron moves directly from one center to the other. This leads to strong harmonic emission a t comparatively low frequencies similar to emission from a two-level a tom with the cutoff proportional to the field amplitude rather than th e intensity. The molecular dipole phase in this case is almost indepen dent of the field intensity and, at constant intensity, the phases of neighboring harmonics are locked. Different orientations of the two-ce nter system with respect to the field with various polarization config urations are investigated. Most of the observed features lend themselv es to interpretation in terms of the simple man's model. [S1050-2947(9 8)09111-2].