The angular distribution of electron temperature and density in a laser-ablation plume has been studied for the first time. The electron temperature ranges from 0.1 to 0.5 eV and is only weakly dependent on the angle in the low-intensity range studied here. In contrast, the typical ion energy is about 2 orders of magnitude larger, and its angular distribution is more peaked about the target normal. The derived values of the electron density are in agreement with the measured values of ion density.PACS numbers: 79.20. Ds, 81.15.Fg Laser ablation of solids with nanosecond pulses of high intensity leads to complicated interactions of the laser beam with both the solid and the ablated material. Some of the fundamental physical features, such as the nature of the laser absorption in the vaporized material and acceleration mechanism for the ions, are not yet fully understood [1][2][3][4]. Nevertheless, the processing of solids by intense laser light and the production of thin films by pulsed laser deposition (PLD) are widely used techniques for a variety of materials [5,6].When a nanosecond laser pulse strikes a solid surface the rapid rise in temperature leads to intense evaporation of atoms and molecules from the solid. Even at relatively low intensities near the threshold for ablation ͑0.2 1.0 GW cm 22 ͒, it is observed that the ablated material is significantly ionized [7,8], and the ions in the plasma plume have energies ranging up to several hundred eV [9]. At the end of the nanosecond laser pulse, the ablated material exists as a thin layer of plasma on the target surface. Initially the expansion of the plume is primarily driven by the plasma pressure gradients [2], but there may be an additional contribution from Coulomb repulsion between the ions if there is significant net loss of the more mobile electrons [3,10]. In any case, when the plume has propagated more than a few hundred mm from the target surface, the major part of the initial thermal energy in the plasma is converted to the directed kinetic energy of the ions, which are much more massive than the electrons [11,12].The energy distribution of the ions has been measured using time-of-flight (TOF) optical spectroscopy [13,14] and ion probes [4,[7][8][9][15][16][17]. Mostly these ion probe measurements have been for the plasma flow close to the normal of the target surface. Recently, we measured the ion energy distribution for angles up to more than 60 ± , and it was observed that both the number and the average energy of the ions are strongly peaked about the target normal [8,9]. The electron component of the laser-induced plasma plume has been less widely studied, and to our knowledge only the electron temperature and density in the plasma flow perpendicular to the target surface have been reported [17][18][19][20]. This is somewhat surprising, since in PLD the temperature and density of the electrons in the plasma will have a major influence on both the gas phase chemistry, between the target and the substrate, and the surface chemistry on the gr...

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Combined diagnostic measurements of deposition rates and ion time-of-flight signals have been employed to study the expansion of a laser ablation plume into a background gas. With increasing gas pressure the angular distribution of the collected ablated atoms becomes broader, while the total collected yield decreases. The total collected yield shows three separate regimes with increasing pressure, a vacuumlike regime, a transition regime with increasing plume broadening and splitting of the ion signal, and at the highest pressure a diffusionlike regime with a broad angular distribution. In the high-pressure regime the expansion can be described by a simple model based on diffusion from a confined plume.

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The dynamics of the ablation plume of a partially ionized plasma produced by a nanosecond UV laser with different irradiation spot geometries has been explored. We have used an ensemble of quartz crystal microbalances to make the first systematic and quantitative study of how the shape of the plume varies as the aspect ratio (b/a) of the elliptical laser spot is varied by about a factor of ten. The flip-over effect can be described by the adiabatic expansion model of Anisimov et al. using a value of the adiabatic constant of about ␥ ϭ1.4. We have also studied the forward peaking of the ablation plume for a large number of metals at the same laser fluence. Contrary to earlier reports, we find that the more refractory metals have the broader angular distributions.