  The generation of high harmonics in laser-irradiated light homonuclear
diatomics (H2+, N2 and O2) compared to that in atomic counterparts (of nearly
identical binding energy) is studied within the velocity gauge version of
conventional strong-field approximation. The applied strong-field approach (we
alternatively developed earlier to incorporate rescattering effects beyond the
conventional saddle-point approximation) is currently extended to molecular
case by means of supplement the standard linear combination of atomic orbitals}
and molecular orbitals method. The associated model proved to adequately
reproduce a general shape and detailed structure of molecular harmonic spectra,
which demonstrate a number of remarkable distinctive differences from
respective atomic spectra calculated under the same laser pulses. The revealed
differences are found to be strongly dependent on internuclear separation and
also very sensitive to the orbital and bonding symmetry of contributing
molecular valence shell. In particular, the model correctly predicts the
behavior of high-frequency plateau (both for its extent and even details of
structure) in molecular harmonic spectra at small (nearly equilibrium) and
large internuclear separations. In addition, for some group of harmonics, the
harmonic emission rates were ascertained to dominate by contribution from inner
molecular shells of higher binding energy and different orbital symmetry
compared to the outermost molecular orbital normally predominantly
contributing.
