HIGH HARMONIC-GENERATION IN ATOMS AND DIATOMIC-MOLECULES USING ULTRASHORT LASER-PULSES IN THE MULTIPHOTON REGIME

High harmonic generation has been investigated in the diatomic molecul ar gases H-2, D-2, N-2 and O-2 as well as the atomic gases Ar and Xe u sing a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). It is s hown that in the cutoff region, the ionization potential and mass are not sensitive parameters for harmonic efficiency but the polarizabilit y is an important factor. A two-step model is suggested to explain the experimental observation in the multiphoton regime. The correlation o f the high harmonic yield and the static polarizability for different systems with nearly the same ionisation potentials is discussed. Compa rison of theoretical estimation with experimental data shows a rather good agreement.