M. Armenante

Thermal and nonthermal ion emission during high-fluence femtosecond laser ablation of metallic targets

We have investigated the emission of positive ions from metallic targets irradiated with intense, ultrashort laser pulses (≈120 fs) at 780 nm, in both S and P polarized states. The measured energy spectra show the presence of a nonthermal, high-energy (several keV) ion component accompanying low-energy ions (tens of eV) produced by a thermal mechanism. The yield of the high-energy component shows a strong dependence on both laser fluence and light polarization. For the low-energy component a higher ablation efficiency was observed for P polarization, and ascribed to a more effective absorption mechanism active during the laser–target interaction.

Emission of prompt electrons during excimer laser ablation of aluminum targets

We report on prompt emission of fast electrons occurring during xenon–fluoride (351 nm) laser ablation of aluminum targets in the nanosecond regime. We have measured both the kinetic energy distribution and the energy-integrated time-of-flight distribution of these electrons. Experimental data evidence that the energetic electrons are produced during the laser pulse as a consequence of two-photon processes, and that space-charge effects influence the photoemitted electron kinetic energy, leading to prompt electrons kinetic energy distributions extending up to ≈15 eV.

High fluence laser ablation of aluminum targets: Time-of-flight mass analysis of plasmas produced at wavelengths 532 and 355 nm

We report on Time-of-Flight Mass Spectrometry (TOFMS) analysis of plasmas produced in laser ablation of Al targets. We used both the second (532 nm) and third (355 nm) harmonic of a Nd: YAG laser system, carrying out the investigation in a regime of relatively high laser fluence (up to 70 J/cm2), where the production of ionized species in the plume is maximized. We present TOF mass spectra of ions in the laser-produced plasma, and a detailed analysis of the relative abundance of different charged species as a function of the laser fluence. The presence of single, doubly and triply ionized Al atoms has been observed and the fluence threshold for their production is reported. We also studied the total ion and electron yield at different laser fluences, its saturation above specific energy densities, and singly ionized cluster-ions produced in the laser plasma.

Chargel species analysis as a diagnostic tool for laser produced plasma characterization

In this work we report on a study of the plasma produced by UV (λ = 351 nm) laser ablation of an aluminum target. In particular, we have directly measured the flux velocity of ablated ions along the normal to the metal target, the corresponding ion temperature, and the average kinetic energy by using an electrostatic energy analyzer. Moreover, by interpreting the measured ion energy distributions within the framework of the hydrodynamical model, we have also inferred an estimate of plasma electron density and temperature, which provide significant information on the laser plasma characterization in a number of applications.

Analysis of polycyclic aromatic hydrocarbon sequences in a premixed laminar flame by on-line time-of-flight mass spectrometry.

A time-of-flight mass spectrometer in reflectron configuration has been used for the real-time detection of combustion products. The products of a premixed laminar C2H4/O2 flame at atmospheric pressure were sampled along its axis, diluted with inert gas and carried to the ion source as a molecular beam under minimal perturbation. Electron ionization and different optical ionization sources are compared. Photoionization was achieved with laser radiation from a Nd:YAG nanosecond pulsed laser at two different wavelengths in the UV range (266 and 355 nm). The mass spectra obtained using laser wavelength of 355 nm and electron ionization present a series of ions regularly spaced by 18 m/z units up to m/z 2000. This series allowed precise calibration of the instrument for compounds of high molecular weight. Information on the chemical nature of the analyzed species has been obtained by comparing mass spectra produced with different ionization methods. In order to better understand the growth mechanisms, polycyclic aromatic hydrocarbon sequences have been analyzed by fast Fourier transform of the mass spectra.

Kinetic-energy distributions of charged fragments from CO2 dissociative ionization

The electron impact ionization of CO2 molecules has been studied in order to determine the dissociative paths leading to charged fragments. The electron impact energy (EIE) ranged in the interval 40-130 eV. Time-of-flight spectroscopy was used to obtain the kinetic-energy distribution (KED) extending down to thermal kinetic-energy fragments. We have evaluated the contribution of indirect processes leading to charged fragments through CO2 autoionization. The threshold for C2+ production from CO2 has been measured and a possible production channel is analysed.

High fluence visible and ultraviolet laser ablation of metallic targets

Abstract Time of flight (TOF) distributions of positive ions in nanosecond laser produced plasmas of Al targets at 532 nm and 355 nm, are reported. The investigation has been carried out in vacuum at relatively high laser fluence (up to ∼60 J cm−2). From TOF analysis, a direct characterization of both ions kinetic energy distribution and yield as a function of laser fluence has been derived. The data indicate average translational kinetic energy of the emitted ions up to ∼100 eV and that all the investigated parameters strongly depend on the laser fluence until a plateau, whose onset depends on the laser wavelength, is reached. For our conditions, this phenomenon has been analysed discussing the main laser photon absorption and ionization mechanisms active in the Al plasma, at both laser wavelengths.

Pulsed laser ablation of borocarbide targets probed by time-of-flight mass spectrometry

Abstract Excimer laser ablation of superconductive borocarbide material (YNi2B2C) in typical conditions for the deposition of superconductive thin films has been investigated using time-of-flight mass spectrometry. The mass spectra show the presence of all the target elemental ionized atoms as well as diatomics. The ablation yield of the metal ions is a strongly increasing function of the laser fluence, while the contrary is true for non-metal ions. The dependence of non-metal light mass diatomic ions on laser fluence indicates the presence of aggregation processes as the laser fluence is increased. Moreover, evidence of aggregation processes involving metallic ions at high laser fluence is also obtained by the mass spectra. An interesting aspect of our results is the observation of an ion spatial distribution characterized by the presence of the lighter species at the plume edges, while the heavier ones are concentrated at the plume center.

Charged species analysis in YNi2B2C laser ablation by time-of-flight mass spectrometry

We report an experimental characterization of the charged species produced in excimer laser ablation of a borocarbide target by using time-of-flight mass spectrometry. The observed TOF distribution show a double peak structure. A very interesting aspect of our experimental results is the observation of an ion spatial distribution characterized by the presence of heavier ions in the plume center and the lighter ones at plume edges.

Time of flight mass spectrometry and covariance mapping technique investigation of charged specie evolution in Pb(Ti0.48Zr0.52)O3 laser ablation

Abstract Time of flight mass spectrometry and covariance mapping technique have been used to investigate the formation of charged species in the laser ablation of Pb(Ti 0.48 Zr 0.52 )O 3 (PZT) targets. The mass spectral analysis has been carried out as a function of distance h from the target, both in high vacuum condition (10 −7 mbar) and in oxygen environment (0.2 mbar background pressure). The relative abundance of atomic, monoxide and heavy ions has been measured and discussed. In high vacuum condition the relative abundance of heavy ions is a decreasing function of the distance from the target and becomes very low, about 8%, at the deposition distances. On the contrary, the relative abundance of monatomic ions and simple oxides increases with the distance from the target. In oxygen environment the relative abundance of heavy ions does not show a monotonic trend. This is likely due to the contemporary presence of fragmentation and aggregation processes. However, at the deposition distance a significant part of the plume, about 40%, is composed of large aggregates containing Pb. Moreover, the covariance mapping analysis shows that the light species are mainly produced by dissociation of the heavy aggregates.