This thesis describes the interaction of intense optical and extreme-ultraviolet laser pulses with atoms and solids. The work was divided into two main parts: The firstudpart describes studies on laser-produced plasmas done at the National Centre for Plasma Science and Technology in Dublin City University using an intense optical laser system. The second part describes the interaction of intense EUV radiation with atoms, performed at the free-electron laser in Hamburg. When an intense optical laser impinges on a solid metal surface a high density, high temperature plasma is produced. If two such plasmas are produced within close proximity on flat (or wedge-shaped) targets, as they expand into vacuum they will collide. The collision of these two expanding plumes will eventually lead to a region in space where the counter-streaming plumes begin to stagnate. As plasma expansion continues the temperature and density of material at this interface increases significantly, a dense stagnation layer is formed and begins to glow. Plasma diagnostics, more specifically the extraction of plasma electron temperaturesudand densities and also plume expansion dynamics have been performed on this stagnation layer and the seed plasmas which feed it. The primary goal in the first part of this work was the study of colliding laser produced plasmas using an EKSPLATM 312p picosecond laser system. We study the evolution of the stagnation layer under different experimental conditions and discuss the suitability of thisudsystem for current and future applications, e.g., pulsed-laser deposition. The second part of this thesis deals specifically with the interaction of an intense extreme-ultraviolet free electron laser with gas jets. The primary goal in this study was the understanding of two-color above-threshold ionization in rare gases. Theudintense extreme-ultraviolet (EUV) radiation from FLASH was combined with an intense synchronized optical laser (800 nm, 4 ps). The photoelectron spectrum revealed one strong photoline attributed to the one-photon direct ionization of helium by FLASH pulses accompanied on either side by smaller sidebands originating from the two-color (FEL and optical laser) above-threshold ionization. These sidebandsudshow strong variations of their amplitudes as a function of both the intensity of the optical dressing field and the relative orientation of the linear polarization vectorsudof the two fields. We exploit this polarization dependence to (i) make the first interference-free study of the dynamics of the two-color above-threshold ionizationudprocess in rare gases, and (ii) to directly probe the branching ratios for continuumcontinuum transitions in helium.
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