Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Effect of pressure relaxation during the laser heating and electron–ion relaxation stages

The multi-phase equation of state by Bushman et al. (Sov. Tech. Rev. 5:1–44, 2008) is modified to describe states with different electron and ion temperatures and it is applied to the non-equilibrium evolution of an aluminum sample heated by a subpicosecond laser pulse. The sample evolution is described by the two-temperature model for the electron and ion temperatures, while the pressure and density are described by a simplified relaxation equation. The pressure relaxation in the heating stage reduces the binding energy and facilitates the electron-driven ablation. The model is applied to estimate the ablation depth of an Al target irradiated by a subpicosecond laser pulse. It improves the agreement with the experimental data and provides a new explanation of the ablation process.     

SIMS study of plumes generated from laser ablation of polymers

Plumes generated by ablation of polymer targets using a third-harmonic Nd:YAG laser under different atmospheres (air, N₂ and He) were deposited on a H-terminated silicon substrate. The chemical composition and distribution of deposited ablation debris were measured using time-of-flight secondary-ion mass spectrometry. Mass-resolved images show that the size and shape of the plume is dependent on the laser pulse energy and atmosphere in which the plume expanded. In air and nitrogen, plumes are hemispherical with distinct borders. In He they are mushroom-shaped without sharp borders. Since all experiments were carried out at atmospheric pressure, these differences can be related to the reactivity and molecular weight of the gas. Nitrogen-containing compounds (NCCs) and oxygen-containing compounds (OCCs) were found in plumes ablated in air but not in N₂ and He environments. We suggest that the formation of NCCs and OCCs is due to the interaction of the hot plume with air, initiating thermal dissociation of O₂ and oxygen-assisted dissociation of N₂. PACS 52.25.Kn     

Plume splitting in pico-second laser-material interaction under the influence of shock wave

In this work, molecular dynamics simulations are conducted to study the physics of plume splitting in pico-second laser material interaction in background gas. The velocity distribution shows a clear split into two distinctive components. Detailed atom trajectory track reveals the behavior of atoms within the peaks and uncovers the mechanisms of peak formation. The observed plume velocity splitting emerges from two distinguished parts of the plume. The front peak of the plume is from the faster moving atoms and smaller particles during laser-material ablation. This region experiences strong constraint from the ambient gas and has substantial velocity attenuation. The second (rear) peak of the plume velocity originates from the larger and slower clusters in laser-material ablation. These larger clusters/particles experience very little constraint from the background, but are affected by the relaxation dynamics of plume and appear almost as a standing wave during the evolution. Density splitting only appears at the beginning of laser-material ablation and quickly disappears due to spread-out of the slower moving clusters. It is found that higher ambient pressure and stronger laser fluence favor earlier plume splitting.     

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

Expansion dynamics of the plasma plume created by laser ablation in a background gas

The dynamics of the expansion of the plasma plume induced by laser ablation of a copper target at a fluence of 17 J/cm² was investigated theoretically by means of a Monte Carlo simulation. When the expansion occurs under a relatively high pressure, the ambient gas particles may be involved in the collective motion of the plume. The simulation allows the study of the simultaneous collective motion of different species, such as the laser-ablated and the ambient gas particles. The influence of the background gas nature and pressure on the laser-induced plasma plume expansion behavior was studied. The expansion dynamics were found to be different in the case of the expansion in ambient gases of different molecular weight. The dynamics of the plume expansion under an argon pressure of 200 Pa seem to be strongly related to the equilibration of the pressure gradients in the gas phase, and evidence of the oscillatory behavior of the plume expansion was shown from the evolution over time of the pressure profiles in the plume. This behavior has also been observed in similar conditions for a krypton atmosphere, but for a lower pressure than for argon. The vortical flow formation at the plume periphery, involving both the laser-ablated and the argon particles at moderate pressure, was also predicted from the Monte Carlo simulation.     

Angular distributions of plume components in ultrafast laser ablation of metal targets

In view of its fundamental interest and relevance to nanoparticle film production, we have characterised the nanoparticle component of the ablation plume generated in femtosecond laser irradiation of metals. The results are compared to those of the ion plume, which is considered as representative of the atomic component. At moderate laser fluences, the angular distributions of both nanoparticle and ionic components were studied by measuring the spatial distribution of deposition on a transparent substrate and with a planar Langmuir probe, respectively. Our results show that both angular profiles of the plume components can be described by Anisimov model of isentropic expansion. As the laser fluence is increased above a value of several times the ablation threshold, the shape of the nanoparticle angular distribution progressively differs from the Anisimov prediction, contrary to what is observed for the ion component. This effect is interpreted in terms of the influence of the pressure exerted by the nascent atomic plasma plume on the initial hydrodynamic evolution of nanoparticle material.     

环境气压对激光烧蚀Cu诱导发光机理的影响

利用时间分辨的光谱测量技术,测定了不同氪气压强下脉冲激光烧蚀金属Cu诱导等离子体发光羽的发射光谱及其强度随时间的分布。利用快速同步照相的方法,拍摄了不同氪气压强下的等离子体发光羽的照片。实验结果表明,等离子体发光羽的光谱主要由原子谱线构成,发光羽颜色随环境气压而变化。结合实验结果探讨了环境气压对脉冲激光烧蚀Cu诱导等离子体发光羽的发光机理的影响,认为不同环境气压下等离子体发光羽的发光机理不同,低压下以电子碰撞传能激发辐射为主,中压下以电子与原子碰撞传能激发和电子与一价离子的复合激发辐射为主,高压下以电子与一价离子的复合激发辐射为主,并用此机理定性地解释了所观察到的实验现象。     

Laser-produced plasma ion characteristics in laser ablation of lithium manganate

Laser ablation is widely used to assist in the fabrication of prototype lithium manganate (LiMn₂O₄) thin film structures for Li-ion battery electrodes via the pulsed laser deposition technique. However, films can be considerably Li and/or O deficient, depending the deposition conditions used. Here we present data on the ionic component of laser-produced plasma in laser ablation of lithium manganate with ns excimer laser. Plasma was monitored using an electrical Langmuir ion probe, in time-of-flight mode in conjunction with mass spectrometry to identify the dominant ionic species. Ablation in vacuum at ∼2.5 J cm⁻² revealed the plasma's ionic component was composed primarily of singly charged Li and Mn ions. The time-of-flight data indicates significant deceleration of the plasma when ablation is carried out in an oxygen background gas pressure of the order of 10 Pa. The implications for thin film growth are considered in terms of the possible gas phase interactions and/or thin film re-sputtering yield.     

Detailed Characteristics of Expansion Velocity of Si from Laser Ablated SiC

Optical emission of plasma is used to investigate the characteristics of dynamics distribution in the plume gen- erated by ablation of a SiC sample using Nd：YAG laser. The plume expansion dynamics is characterized by time-of-flight measurement. We find that the profiles of Si （I） （390.55 nm） split into two components and the Si （1I） （634. 71 nm） spectra show two distinct expansion dynamics regions. The time-of-flight measurement of Si（ll） （634. 71 nm） under different laser irradianee conditions, from 0.236 G W/cm^2 to 1.667 G W/cm^2, are presented and discussed.     

Temporal evolution of laser-ablated Co ions probed by time-of-flight mass spectrometry

A pulsed-field time-of-flight mass spectrometric (TOFMS) technique was used to investigate the expansion dynamics of the ionic species ejected from the visible (λ=532 nm) laser ablation of cobalt target at low laser fluence less than 1 J/cm². The temporal evolution of Co⁺ ions was studied by varying the delay time of the ion repelling pulse with respect to the laser irradiation, which provides significant information on the ablated plume characterization. The obtained TOF mass spectra were well fitted by shifted Maxwell–Boltzmann distributions on a stream velocity, commonly used to describe the measured velocity distributions. The TOF distribution of Co⁺ ions showed a bimodal distribution with fast and slow velocities. These velocities show a decreasing tendency with delay time, which is attributed to the gas collisions between the plume ejecta and to the related gas dynamics. The present results suggest that the in situ measurements of the most probable velocity of ablated ions along the normal to the solid target can be accomplished by the simple technique of a laser ablation/TOFMS.     

Laser ablation of YBa2Cu3O7-x: the wavelength dependence

Formation mechanisms of atomic and oxide ions in a plume (laser-induced plasma) produced by the laser ablation of a YBa₂Cu₃O_7-x (YBCO) target was studied by time-of-flight (TOF) analysis. The dependence of the TOF spectra on the laser wavelengths (266 nm and 1064 nm) was examined to elucidate photochemical phenomena in the plume. Significant changes in the relative enrichment of metal oxide and oxygen ions as well as their TOF distributions were observed by the injection of an oxygen jet to the plume, which induces reactive scattering of the YBCO plume and the oxygen jet.     

Study of the plume generated by Nd:YAG laser ablation of a hydroxyapatite target

The plume generated by Nd:YAG laser ablation of a hydroxyapatite target has been investigated in vacuum and at 0.1 and 0.2 mbar of water vapor. The investigation has been carried out by means of fast intensified CCD imaging with the aid of bandpass interferential filters that allow the following single species to be isolated: neutral calcium, calcium oxide radicals and neutral oxygen. Results obtained in vacuum reveal that expansion takes place at a constant velocity of about 2᎒⁴ m/s for the atomic species and about 3᎒³ m/s for the molecular ones and that emission is completely dominated by emissive neutral calcium. When ablation is carried out in a water atmosphere, the background gas confines the species in the leading edge of the plume, which results in the formation of a planar shock wave at 0.1 mbar and a spherical shock wave at 0.2 mbar. Comparison of the images with those obtained at 0.1 mbar of Ne has revealed the existence of chemical reactions between the plume and the water atmosphere, leading to the formation of calcium oxide radicals. In that case, plume emission is dominated by these molecular species.     

Laser ablation of lithium and lithium/cadmium alloy studied by time-of-flight mass spectrometry

Ablation of solid lithium and lithium/cadmium alloy was performed by a 308-nm, nanosecond excimer laser. Analysis of the atomic and molecular composition of the plume in vacuum and in nitrogen atmosphere was performed by means of a linear time-of-flight mass spectrometer. Several ionic masses were observed and systematically studied with respect to the laser fluence, laser beam spot size, background pressure, and target composition. PACS 52.38.Mf; 52.50.Jm; 82.80.Rt     

Spectroscopy of laser-produced cerium plasma for remote isotope analysis of nuclear fuel

A cerium oxide sample was ablated by 2nd harmonic radiation of Nd:YAG laser at a power density of 0.1 GW/cm². Time evolution of the ablation plume was investigated by laser absorption time-of-flight (TOF) measurement. It was found that the ablated ionic plume in vacuum consisted of two components having different velocities whereas the ablated neutral atoms had mainly a single component. The flow velocity perpendicular to the sample surface in vacuum was determined to be 3.5 km/s for neutral atoms, and 4.7 km/s and 9.3 km/s for singly charged ions. From the detailed plume evolution in ambient atmosphere with several pressures we obtained some experimental conditions suitable for isotope analysis of atomic cerium.     

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⁻.     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

Phase explosion and recoil-induced ejection in resonant-infrared laser ablation of polystyrene

We investigate the phenomenon of resonant-infrared laser ablation of polymers using polystyrene as a model material. Ablation is carried out using various mid-IR laser wavelengths that are resonant with vibrational modes of a polystyrene target. Time-resolved plume imaging coupled with etch-depth measurements and thermal calculations indicate that ablation begins after a superheated surface layer reaches a temperature of ∼1000°C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area.     

Pulsed laser ablation of SiC in a nitrogen atmosphere: formation of CN

Optical emission lines from the plasma generated by a laser ablation process have been investigated to gather information on the nature of the chemical species present. In particular, the experiments were carried out during the laser ablation of a ceramic sintered SiC target, both in vacuum and in presence of controlled nitrogen atmosphere. Time integrated and spatially resolved emission spectra are dominated by the atomic emission lines from silicon and carbon species, either neutral, or singly ionized. When the ablation process was carried out in a nitrogen gas background direct evidence of the formation of the CN molecular specie was found. Fast photography imaging of the expanding plume revealed the formation of a shock wave at nitrogen pressure above 13.3 Pa, with the consequent heating of the shocked region and enhancement of the kinetics of ionization and excitation. Since the C₂ specie was absent, a CN formation mechanism involving atomic carbon and nitrogen in the presence of a shock wave is suggested. PACS 52.38.Mf; 52.50.Jm, 47.40.-x