High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases HI, D2. Nt and 0, using a linearly polarized picosecond dye laser (594 nm) and a linearly polarired 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: ( I ) 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 polm'zability is an important factor,
We observe the kinetic energy of ionic fragments that result from the Coulomb explosion of molecular iodine ions produced by rapid multiphoton ionization. Because the ion energies produce a clear signature of the molecular ion state from which they originate, we are able to determine the intensity dependence of the molecular multiphoton ionization. Generalizations of atomiclike concepts are used to model the experimental results. Measurements of angular distributions indicate the importance of the molecular polarizability.
Time-resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at lOI W/cm', is used to study the basic processes which control the x-ray pulse duration. Short x-ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time-dependent calculations of the atomic physics [Whys. Fluids B 4, 2007, 19921 coupled to a Fokker-Planck code [Phys. Rev. Lett. 53, 1461, 19841 indicate that it is essential to take into account the non-Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.
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