The creation of laser-induced inner shell vacancies in doubly ionized Ar atoms is reported. First, theoretical calculations are performed for different laser systems and as much as 30% of doubly ionized Ar atoms in the [Ne]3s3p5 configuration with a 3s shell vacancy at optical and near-infrared frequencies are predicted. Analysis of the results shows that the mechanism for creation of the inner shell vacancy is a non-sequential double ionization process. Finally, the radiation spectrum of Ar in the field of an intense Ti:sapphire/Nd:glass laser system at 1053 nm has been measured, and the line corresponding to the fluorescence emission due to radiative filling of the created inner vacancy in the 3s shell is observed.

High harmonic conversion efficiency has been experimentally compared in the atomic gas Ar and the diatomic molecular gases and . It is shown that, in the saturation region, if the atomic and diatomic molecular gases have similar polarizabilities, the one with the higher ionization potential has the higher harmonic efficiency. If the atomic and molecular gases have similar polarizabilities and ionization potentials, the atomic gas will have a higher harmonic efficiency than the molecular one.

Using intense 200 fs Ti:sapphire laser pulses, multiphoton multiple ionization of Xe and Kr has been studied over a dynamic range of up to nine orders of magnitude. Huge enhancements in the probability of double ionization of both gases and triple ionization of Xe were observed and attributed to the occurrence of non-sequential (NS) ionization. It was found that the probability of NS ionization of any charge state is a linear combination of the tunnelling probability of the previous charge states.

Using very stable intense fs Ti:sapphire laser pulses, new structures in the ion versus intensity plot of are observed which can be attributed to electron trapping in excited electronic vibrational levels. This is further studied by a new and more sensitive technique based on high-order harmonic generation.

In the multiphoton regime, the correlation between high-harmonic generation and multiphoton ionization is investigated when the laser polarization changes from linear to elliptical. The ellipticity dependence of high harmonics and ion signal is measured experimentally. Both high harmonics and ion signal decrease with increased ellipticity at fixed laser intensity, but the decrease in ion signal is much slower compared to that of the harmonics. This phenomenon is due to the fact that high-harmonic generation is more strongly affected by the selection rules than ionization, when polarization changes.

High-harmonic generation processes have been experimentally investigated in molecular hydrogen gas as both molecular dissociation and ionization occur in the gas. The high harmonics observed in our experiment can be mainly attributed to neutral hydrogen molecules. Besides the harmonic spectra, two strong Lyman lines from the dissociated hydrogen atoms and the emission from the neutral hydrogen molecules are observed. Several possible excitation and dissociation channels of hydrogen molecules are proposed. It is found that resonance effects are important for both high-harmonic generation and molecular dissociation. Theoretical calculations using the time-dependent Hartree-Fock (TDHF) method for H-2 reproduce some of the resonance effects.

High-order harmonic generation is observed in an argon jet using elliptically polarized picosecond dye laser pulses. The experimental data appear to be in near agreement with the perturbation theory. However, there is a general trend of small deviation from the theory. We speculate that such a deviation could be due to the laser induced coupling of upper states in or near the continuum which relaxes the selection rules leading to an enhancement of harmonic emission.

High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases H2, D2, N2 and O2 using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability. It is shown that the ionization potential and mass are not sensitive parameters for harmonic efficiency, but the polarizability is an important factor.

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 ionization potentials is discussed. Comparison of theoretical estimation with experimental data shows a rather good agreement.

Experiments on the tunnel ionization of simple molecules in the ground state were performed using intense CO(2) laser pulses of 2-ns duration. The results showed that in the nonresonant cases, CO, NO, O(2), H(2), N(2), and CO(2), the molecules were ionized as if they were structureless particles; the tunnel ionization depended only on the ionization potential of the molecule. Fragmentation always followed ionization. In the resonant case (D(2)O), vibrational excitation and field dressing of the electronic surfaces led to the tunnel ionization of a laser-induced electronically excited molecule. This occurred at a lower intensity than would be required for ionizing a ground state, nonresonant molecule with an ionization potential equal to that of D(2)O.

High-harmonic generation is investigated in Ar, N2 and H2 with dye laser pulses. The diatomic molecules are proposed to act as atoms in harmonic generation processes.

The non-resonant interaction of an intense nanosecond CO2 laser pulse with Hg2, N2, and H2 shows that the molecules are tunnel ionized as if they were atoms with the same ionization potentials as the molecules. Moreover, dissociation of the neutral molecule appears to be much less probable even though the dissociation energies are lower than the ionization potentials.

Second harmonic generation (SHG) is observed experimentally in static gaseous argon, nitrogen and helium as well as in an argon gas jet using tightly focused 10.6 mu m CO2 laser pulses with peak intensities up to 7*1012 W cm-2. Under the electric dipole approximation, SHG in isotropic media is forbidden. The SHG in the authors' experiments can be traced to the non-uniformity of the plasma, with the density gradient providing the necessary anisotropy. SHG efficiencies of up to 1*10-5 were obtained. A theoretical analysis as well as an investigation of the phase-matching condition are given. Numerical calculations show that the proposed theoretical model fits the experimental data very well.