High harmonic generation beyond the electric dipole approximation

A generalization of the analytical theory of high harmonic generation in th e long wavelength limit and in the single active electron approximation is; developed taking into account the magnetic dipole and electric quadrupole interaction. Quantum mechanical and classical theories are found to be in e xcellent agreement, which allows one to explain the influence of multipole effects in terms of an intuitive picture. For Ti:S lasers (0.8 mum) multipo le contributions are found to be small below an intensity of about 10(17) W /cm(2), at which harmonic radiation with photon energies of several keV is generated. This promises the extension of high harmonic generation well int o the sub-nm wavelength regime.