Nanosecond and femtosecond ablation of La0.6Ca0.4CoO3: a comparison between plume dynamics and composition of the films

Thin films of La_0.6Ca_0.4CoO₃ were grown by pulsed laser ablation with nanosecond and femtosecond pulses. The films deposited with femtosecond pulses (248 nm, 500 fs pulse duration) exhibit a higher surface roughness and deficiency in the cobalt content compared to the films deposited with nanosecond pulses (248 nm, 20 ns pulse duration). The origin of these pronounced differences between the films grown by ns and fs ablation has been studied in detail by time-resolved optical emission spectroscopy and imaging. The plumes generated by nanosecond and femtosecond ablation were analyzed in vacuum and in a background pressure of 60 Pa of oxygen. The ns-induced plume in vacuum exhibits a spherical shape, while for femtosecond ablation the plume is more elongated along the expansion direction, but with similar velocities for ns and fs laser ablation. In the case of ablation in the background gas similar velocities of the plume species are observed for fs and ns laser ablation. The different film compositions are therefore not related to different kinetic energies and different distributions of various species in the plasma plume which has been identified as the origin of the deficiency of species for other materials.     

Laser ablation of a silicon carbide target: a time-of-flight quadrupole mass spectrometric study

Laser ablation of a SiC target was attempted to investigate the formation mechanisms and expansion dynamics of ionic species including silicon carbide cluster ions in the plume. Time-of-arrival distributions of ions were analysed to obtain their mean velocities and spatial distributions. Laser-plume interaction was examined by illuminating the plume with a UV laser pulse. Si₂C was found to be the major photodissociation product of silicon carbide cluster ions. PACS 82.40.Ra; 79.20.Ds; 79.20.La     

Carbon-plasma produced in vacuum by 532 nm-3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C_n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm², produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.     

Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV

The mechanisms of laser ablation of CdS targets at different laser wavelengths have been investigated. (CdS)_n⁺ cluster formation is only observed upon 532 nm ablation. The time and energy distributions of neutral S, S₂, Cd and CdS show significant dependence on laser wavelength. Bimodal distributions are observed at 266 and 308 nm. For the former, the average kinetic energy increases significantly with mass, taking values in the range of 0.3-1.7 eV. At 308 nm the slow component of the time distribution disappears at distances above the target larger than 1 cm. At this wavelength, the observed time distribution appears to reflect mainly the dynamics of the expansion. At 532 nm the time distribution is monomodal and the average kinetic energies are below 0.2 eV. Clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm) have been found. It is tentatively concluded that the cluster formation observed upon ablation at 532 nm can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and their similar velocities favor the aggregation processes.     

Nanospallation induced by an ultrashort laser pulse

A femtosecond laser pulse with power density of 10¹³ to 10¹⁴ W/cm² incident on a metal target causes ablation and ejection of the surface layer. The ejected laser plume has a complicated structure. At the leading front of the plume, there is a spall layer where the material is in a molten state. The spall layer is a remarkable part of the plume in that the liquid-phase density does not decrease with time elapsed. This paper reports theoretical and experimental studies of the formation, structure, and ejection of the laser plume. The results of molecular dynamics simulations and a theoretical survey of plume structure based on these results are presented. It is shown that the plume has no spall layer when the pulse fluence exceeds an evaporation threshold F _ev. As the fluence increases from the ablation threshold F _a to F _ev, the spall-layer thickness for gold decreases from 100 nm to a few lattice constants. Experimental results support theoretical calculations. Microinterferometry combined with a pump-probe technique is used to obtain new quantitative data on spallation dynamics for gold. The ablation threshold is evaluated, the characteristic crater shape and depth are determined, and the evaporation threshold is estimated.     

Characterization of aerosol plumes in nanosecond laser ablation of molecular solids at atmospheric pressure

Ablation of molecular solids with pulsed ultraviolet lasers at atmospheric pressure is an important process in (bio-)organic mass spectrometry. Of practical importance for analytical sampling and analysis are the plume formation and expansion. Plumes formed by atmospheric-pressure laser ablation of anthracene and 2,5-dihydroxybenzoic acid (2,5-DHB) were studied by light scattering imaging, which showed significant material release in the form of aerosols. The monitored plume expansion dynamics could be fitted to the drag-force model, yielding initial plume velocities of 150 m/s for anthracene and 43 m/s for DHB. While the angle of incidence does not affect the plume direction and propagation, a large dependence of the plume-expansion velocity on the laser pulse energy could be found, which is limited at atmospheric pressure by the onset of plasma shielding. With respect to analytical applications, the efficiency of sampling of the laser ablation products by a capillary could be experimentally visualized.     

Temporal and spatial TaO emission generated from UV laser ablation of Ta and Ta2O5 in oxygen ambient

2 O₅ targets in oxygen ambient are presented. Line assignments indicate the presence of the excited Ta(I), Ta(II), and TaO in the plume. At higher oxygen pressure, a single peak appears in the TaO emission spectrum from the laser ablation of Ta while two peaks corresponding to a fast and a slow component of TaO emission are observed from the laser ablation of the Ta₂O₅ target by time-resolved emission spectroscopy. The delay times after laser pulse corresponding to two components of TaO emission from the laser ablation of Ta₂O₅ have been investigated as a function of oxygen pressure, laser fluence, and observation distance from the target surface. The two components of TaO emission could be attributed to different pathways for the generation of excited TaO molecules. A blast wave model is proposed to describe the behavior of the excited TaO in the plume of laser ablation of Ta₂O₅. Received: 1 February 1997/Accepted: 12 March 1997     

Plasma dynamics from laser ablated solid lithium

Emission plasma plume generated by pulsed laser ablation of a lithium solid target by a ruby laser (694 nm, 20 ns, 3 J) was subjected to optical emission spectroscopy: time and space resolved optical emission was characterised as a function of distance from the target surface. Propagation of the plume was studied through ambient background of argon gas. Spectroscopic observations can, in general, be used to analyse plume structure with respect to an appropriate theoretical plasma model. The plume expansion dynamics in this case could be explained through a shock wave propagation model wherein, the experimental observations made were seen to fit well with the theoretical predictions. Spectral information derived from measurement of peak intensity and line width determined the parameters, electron temperature (T _e) and electron number density (n _e), typically used to characterise laser produced plasma plume emission. These measurements were also used to validate the assumptions underlying the local thermodynamic equilibrium (LTE) model, invoked for the high density laser plasma under study. Some interesting results pertaining to the analysis of plume structure and spatio-temporal behaviour of T _e and n _e along the plume length will be presented and discussed.     

Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Nanostructured high valence silver oxide produced by pulsed laser deposition

Among silver oxides, Ag₄O₄, i.e. high valence Ag(I)Ag(III) oxide, is interesting for applications in high energy batteries and for the development of antimicrobial coatings. We here show that ns UV pulsed laser deposition (PLD) in an oxygen containing atmosphere allows the synthesis of pure Ag₄O₄ nanocrystalline thin films, permitting at the same time to control the morphology of the material at the sub-micrometer scale. Ag₄O₄ films with a crystalline domain size of the order of tens of nm can be deposited provided the deposition pressure is above a threshold (roughly 4 Pa pure O₂ or 20 Pa synthetic air). The formation of this particular high valence silver oxide is explained in terms of the reactions occurring during the expansion of the ablated species in the reactive atmosphere. In particular, expansion of the PLD plasma plume is accompanied by formation of low stability Ag-O dimers and atomic oxygen, providing reactive species at the substrate where the film grows. Evidence of reactive collisions in the expanding ablation plume is obtained by analysis of the plume visible shape in inert and reactive atmospheres. In addition, we show how the dimensionless deposition parameter L, relating the target-to-substrate distance to the ablation plume maximum expansion length, can be used to classify different growth regimes. It is thus possible to vary the stoichiometry and the morphology of the films, from compact and columnar to foam-like, by controlling both the gas pressure and the target-to-substrate distance.     

Cluster growth in an ablation plume propagating through a buffer gas

A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced. Selected experiments including LaMnO₃ and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy.     

Spectroscopic measurements of excited particles in a N2 gas RF plasma-assisted carbon laser ablation

In this work, we investigated a carbon plasma plume produced by laser ablation of a graphite target in a nitrogen gas environment. The spatial distributions of C and N atoms were measured by time-resolved absorption spectroscopy. The spatial distributions of the relative densities of CN radicals, C₂, and C₃ molecules were measured using time-resolved emission spectroscopy. We determined that nitrogen gas produced an increase in carbon atom and molecule densities in the ablation plume. It was observed that the addition of RF plasma to the plume increased the CN radicals and C atom densities, and decreased the C₂ and C₃ molecule densities. The RF plasma changed the evolution of various fractional species of C, N, CN, C₂, and C₃ in the ablation plume. The chemical reactions with and without RF plasma were explained using the evolution and density of the fractional species of C, N, CN, C₂, and C₃in the plume. PACS 52.38.Mf; 42.62.Fi; 33.20.-t; 81.05.Uw     

Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm² Si, SiO and Si₂ have comparable intensity and Si_n (n≤7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, u_T²∝T_S, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Si_n clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Si_n provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered. PACS 82.30.Nr; 81.05.Gc; 78.70.-g     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Hydrogenation of carbon fragments after femtosecond laser ablation of solid C60

We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C₆₀. The observation indicates that the carbon fragments produced just after laser ablation of the C₆₀ molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of C_nH₅⁺ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA.     

Plume propagation through a buffer gas and cluster size prediction

A mixed-propagation picture for the expansion of an ablation plume through a buffer gas at different pressures is proposed after a brief discussion of the available analytical expansion models. Using parameter values deduced by the model the size of carbon clusters formed during plume propagation in helium is evaluated and compared with experimental data. Starting from experiments on silver plume propagation through molecular oxygen the size of silver clusters is predicted.     

Propagation of LaMnO3 laser ablation plume in oxygen gas

The propagation of LaMnO₃ laser ablation plume in oxygen background has been investigated using fast photography of overall visible plume emission and time-resolved optical emission spectroscopy. The plume expansion was studied with ambient oxygen pressures ranging from vacuum level to 100 Pa. Free-expansion, splitting, sharpening and stopping of the plume were observed at different pressures and time delays after the laser pulse. Time-resolved optical emission spectroscopy showed that oxides are mainly formed through reaction of the atomic species ablated from the target with oxygen in the gas-phase. These reactions mainly affect the content of lanthanum oxide in the plume, while emission of manganese oxide is barely observed in all the range of pressure investigated.