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Pairs of 80 fs, 800 nm laser pulses were used to ablate Si, Cu, and CaF2 in air. The spectrally resolved plasma emission was measured as a function of laser fluence and pulse delay. After an initial dip, the fluorescence was found to increase monotonically with pulse delay, reaching a plateau after some tens of picoseconds, depending on the material and fluence. The enhancement ratio (defined as the ratio of the fluorescence produced by the pulse pair to that produced by a single pulse of the same total fluence) reaches a maximum value of 6 and 11 at a fluence of ∼6 J/cm2 for Si and Cu, respectively, and declines to a value below 2 at higher fluences. In contrast, the enhancement for CaF2 increases slowly from zero near threshold to a broad maximum value of 2 near 50 J/cm2. Using reflectivity and atomic force microscopy measurements as diagnostics, we interpret the Si and Cu behavior in terms of a two phase mechanism, in which the first pulse melts the surface of the crystal and the second pulse ablates the resulting liquid film. A qualitatively different mechanism initiated by multiphoton absorption is involved in CaF2 ablation. [ABSTRACT FROM AUTHOR]

Amplified spontaneous emission (ASE) is a process which competes with laser-induced fluorescence (LIF) in three level systems. This was demonstrated in the case of oxygen atoms, for which both ASE and LIF were observed at 845 nm. Ground state oxygen atoms were generated by photodissociating O2, SO2, and NO2 in a pump and probe experiment. Both nascent and thermalized O(3Pj‘) atoms were measured using two-photon LIF at 226 nm. The fine structure populations deduced from the 845 nm (3p3Pj’→3s3S) emission were considerably hotter than the distributions obtained from the 130 nm (3s3S→2p3P) transition. A kinetic model is developed showing that the distributions observed at 845 are distorted by ASE, which selectively depletes the populations of atoms originating from more highly populated multiplet levels. [ABSTRACT FROM AUTHOR]

Studies the partial pressure of vibrationally excited hydrogen molecules in a flowing gas using the technique of vacuum ultraviolet laser-induced fluorescence. Effects of vibrational energy on molecular reaction dynamics; Details on the experiment; Discussion on the results of the study.

Pairs of ultrafast laser pulses are used to ablate Si<111>. The fluorescence from Si atoms and ions was observed to increase by an order of magnitude as the delay between the pulses was increased. From the dependence of the fluorescence enhancement on the laser fluence and the pulse delay, it is deduced that the first pulse melts the surface and that the second pulse interacts more strongly with the liquid phase. [ABSTRACT FROM AUTHOR]