ALBERT

Notes: The plasma plume induced by pulsed CO2 laser irradiation of a Ti target at power densities up to 4×108 W cm−2 was studied by emission spectroscopy. Time- and space-resolved measurements were performed by varying laser intensity, laser temporal pulse shape, ambient gas pressure, and the nature of the ambient gas. Experimental results are discussed by comparison with usual models. We show that shock wave and plasma propagation depend critically on the ratio Ivap/Ii, Ivap being the intensity threshold for surface vaporization and Ii the plasma ignition threshold of the ambient gas. Spectroscopic diagnostics of the helium breakdown plasma show maximum values of electron temperature and electron density in the order of kTe∼10 eV and ne=1018 cm−3, respectively. The plasma cannot be described by local thermodynamic equilibrium modeling. Nevertheless, excited metal atoms appear to be in equilibrium with electrons, hence, they can be used like a probe to measure the electron temperature. In order to get information on the role of the plasma in the laser-surface interaction, Ti surfaces were investigated by microscopy after irradiation. Thus an enhanced momentum transfer from the plasma to the target due to the recoil pressure of the breakdown plasma could be evidenced.

Notes: (1+1) resonant multiphoton ionization of several metal atoms was investigated in the 285–301-nm-wavelength range. Fe, Mg, Cr, Ti, and Ni free atoms were produced using Ar+-ion sputtering of different metal samples in ultrahigh-vacuum conditions. The photoionization yields, measured as function of the laser fluence, exhibit saturation or near saturation at the highest laser energy density available in the experiment (100 mJ/cm2). From these measurements the ionization cross sections of the intermediate excited states are deduced, assuming saturation of the excitation step. In this way values ranging between 0.7 and 4×10−17 cm2 are obtained. One can take advantage of the relatively large cross sections to detect these sputtered metal atoms by resonance ionization mass spectroscopy with great sensitivity. Some examples are given, demonstrating, on a single-laser-shot basis, the real analyses of Mg and Ti atoms in materials at the ppm level.

Notes: Pulsed CO2 laser-induced ablation of solid lithium is studied in the low-energy-density regime where no plasma forms on the surface. Li atoms emitted from the surface are characterized using laser-induced fluorescence and absorption spectroscopy. Atom densities measured as a function of time for different distances from the surface are well described by a full-range Maxwellian in a center-of-mass coordinate system. For 0.9 J/cm2 incident energy density (a fraction being absorbed), the beam velocity and the characteristic temperature are 3×105 cm/s and 8500–10 000 K, respectively. Under these conditions, the number of ablated atoms is about 5×1011 per laser shot. The determined effective beam temperature is much higher than the boiling point of pure lithium. This could be explained considering that a film of oxide with greater vaporization temperature is always present on the surface even in relatively good vacuum conditions.

Notes: New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (λ=10.6 μm) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space- and time-resolved emission spectroscopy, while sample vaporization is probed by laser-induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization-initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two-stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam-metal surface energy transfer. Laser irradiation always causes an important vaporization when performed in He, while in the case of Ar or N2, the interaction is reduced in heating and vaporization of some surface defects and impurities.

Notes: This work deals with the study of laser-induced surface vaporization in the presence of an ambient gas, in the conditions where a plasma develops at the gas-material interface. In the experiment, a pulsed CO2 laser is focused onto a titanium target in a cell containing He, Ar, or N2 from a few tenths of Torr up to atmospheric pressure. The temporal and spatial distributions of Ti atoms vaporized from the surface are measured by laser-induced fluorescence spectroscopy. Strong evaporation is observed in helium up to several hundred Torr. In this gas, the Ti atom plume expands according to the propagation of a blast-wave. On the contrary, a very small quantity of material is vaporized in the presence of Ar or N2, because a highly absorbing breakdown plasma develops in this case, as soon as the surface vaporization threshold is reached, shielding the target from subsequent laser heating. These results corroborate our previous analysis based on the spectroscopic observation of the plasma.

Notes: We have studied the soft laser sputtering of (100)GaAs with 337 nm photons, starting from the threshold for particle emission (a few tens of mJ/cm2) to some 300 mJ/cm2 fluences. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured, and their time of flight determined using laser resonant ionization mass spectrometry. The surface after laser irradiation is examined by scanning electron microscopy and electron microprobe analysis.One observes a significant preferential emission of arsenic in the form of As2. This leads to the formation of perturbed Ga-rich surface structure which appears even at low fluence and after a few tens of laser shots on the same spot. This initial transformation seems to determine the further evolution of the irradiated surface. First, Ga atoms aggregate to form Ga islands on the surface; after a sufficient number of shots, micrometric structures are produced which finally behave as pure Ga metal. This evolution of the surface state after multipulse irradiation appears practically the same for low and medium laser fluences, the only difference being in the number of shots required to obtain the same microscopic structure. The velocity distribution of Ga atoms and As2 molecules is well fitted by half-space Maxwellian distributions. The kinetic temperatures are in broad agreement with the results obtained from a model of laser heating of the surface. The gross features of the experimental results can be interpreted from the particular thermodynamics properties of GaAs which exhibits very large As2 pressure above the solid as soon as the temperature exceeds 950 K. After a few laser shots, corresponding to particle emission from defect sites, the thermodynamics of GaAs appears to govern the further evolution of the laser-sputtered surface. Two sputtering regimes are evidenced: In the low-fluence regime (from threshold to 90 mJ/cm2) sputtering appears to be dominated by surface defect emission, whereas for higher fluences emission is more characteristic of thermal process accompanied by preferential sputtering of arsenide. According to these experimental results, a simple analytical model was developed which relates the quantitative surface to the quantitative sputtered cloud compositions. © 1995 American Institute of Physics.

Notes: Laser sputtering of InP(100) surface with 337 nm photons was investigated for fluences ranging from the threshold for particle emission up to about 370 mJ/cm2. Sputtered atoms and molecules are detected during their flight using resonant laser post-ionization and mass spectrometry. From the shot number and the energy dependencies of the sputtering yield, it is shown that two sputtering regimes exist. For low fluence values (〈190 mJ/cm2), the sputtering results mainly from absorption and excitation of defect sites. Conversely, at higher fluences, interband transitions in the whole absorption volume lead after relaxation to a process similar to thermal evaporation. This thermal-like process induces the preferential emission of phosphorus in the form of atoms and molecules and the quite different velocities of phosphorus and indium populations which in absence of collisions separate during their flight. The limit between the two regimes might correspond to the point where melting of the surface occurs. © 1994 American Institute of Physics.

Notes: The initial step of particulate growth in a dust forming low pressure radio-frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser-particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.

Notes: Time- and space-resolved emission and laser-induced fluorescence spectroscopic measurements were performed to investigate vaporization and plasma formation resulting from excimer laser irradiation of titanium targets in a low-pressure nitrogen atmosphere. Measurement series have been done by varying the laser intensity from the vaporization threshold at 25 MW cm−2 up to values of about 500 MW cm−2 typically applied in pulsed laser deposition processing of titanium nitride films. Thus, the transition from thermal evaporation to the high-density plasma formation process, leading to the production of reactive species and high-energy ions, was evidenced. An interesting result for the comprehension of the reactive deposition process was the observation of a quantity of dissociated and ionized nitrogen, which is transported with the plasma front in the direction of the substrate. © 1995 American Institute of Physics.

Notes: Successful carbidation of Ti in a layer forming on the surface of a Ti sample submitted to multipulse excimer (λ=308 nm) laser treatment in CH4 at a slightly superatmospheric pressure is reported. The layer is only surface contaminated with oxygen while its main part consists of fcc TiC. The layer apparently ends with a tail of carbides with low C content, extending deeper into the sample's bulk. The characteristics of the synthesized layer are suggested to be related to the peculiarities of the chemical synthesis which are enhanced by gas propulsion into a melted layer under the recoil action of a plasma evolving in front of the sample. A cavitation mechanism inside the melted surface layer in order to account for plasma initiation is proposed. This mechanism also facilitates the strong substance propulsion into the sample's bulk.