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 molecular gases H-2, D-2, N-2 and O-2 as well as the atomic gases Ar and Xe using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). It is shown that in the cutoff region, the ionization potential and mass are not sensitive parameters for harmonic efficiency but the polarizability is an important factor. A two-step model is suggested to explain the experimental observation in the multiphoton regime. The correlation of the high harmonic yield and the static polarizability for different systems with nearly the same ionisation potentials is discussed. Comparison of theoretical estimation with experimental data shows a rather good agreement.