In situ investigation of laser ablation

The complex system of in situ diagnostic methods such as SEM, ballistic microbalance, electric probe and high speed photography is necessary for measurement of basic parameters of laser target interaction. Only the knowledge of these basic parameters allows the discussion of microscopical processes on the target surface by laser irradiation. The results are discussed for our synchronised double laser system and for aluminium target. The mass relation of atoms, ions and droplets emitted by the target was determined. This relation is the base of a complete energy balance of laser target interaction.     

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.     

Rotational averaging and optimization of laser-induced population transfer in molecules

The dynamics of a molecule subject to a short laser pulse is investigated, with focus on the averaging over initial rotational states and on the optimization of laser parameters for the efficient population transfer between vibrational and electronic states. A relation is established between final-state populations obtained with a fixed orientation and those based on a full treatment of the rotational degrees of freedom. In the short-pulse approximation, rotational averaging amounts to integrating the fixed molecule results over all orientations. The theory is applied to a variety of model systems and verified with numerical calculations using Gaussian pulses. We calculate target state populations with three procedures, optimizing the laser pulse for a fixed orientation without orientational averaging, averaging without changing the laser parameters, and reoptimizing the parameters after averaging. The analysis of the two-level system provides a reference for the order of magnitude of the effects of averaging. The three-level system brings out the relevant role of the geometry of polarization vectors and transition dipoles. The multiphoton excitation of a Morse oscillator shows the importance of taking into account the dependence of resonance frequencies on the laser intensity. Within a proton transfer model we discuss the results obtained with and without chirping and we show that "optimizing after averaging" can be as effective as choosing a more refined pulse shape.     

Absolute asymmetric synthesis from an isotropic racemic mixture of chiral molecules with the help of their laser orientation-dependent selection

We analyzed the absolute asymmetric synthesis (AAS) of enantiomers from an isotropic racemic mixture of chiral molecules, which employs the laser electrodipole interaction, and revealed a set of basic symmetry-based conditions on the parameters of field-molecule interaction. Using these conditions, we developed a novel scenario of the AAS (through selective photodestruction of the enantiomers of a given type) based on the joint action of the strong multicomponent femtosecond and picosecond laser pulses. Key mechanism of this scenario is the partly modified scheme of laser orientation-dependent selection of molecules proposed by us earlier [D. V. Zhdanov et al., JETP 130, 387 (2006)]. Calculations made on example of chiral molecule SiHNaClF show rather high efficiency and stability of the proposed AAS scenario with respect to the parameters of the incident laser pulses and even feasibility of its realization at room temperature.     

CN emission spectroscopy study of carbon plasma in nitrogen environment

Spectroscopic emission diagnostics of a carbon plasma created by an excimer KrF laser pulse at three laser fluences (12, 25 and 32 J/cm²) is performed under nitrogen ambient at pressures of 0.5 and 1 mbar. By following the time evolution of the radical CN spectral emission profiles, we notice, at a certain distance from the target surface, the existence of twin peaks for the time of flight distribution. This double structure depends on laser fluence and gas pressure parameters. The first peak moves forward in relation with the plasma expansion whereas the second peak moves backward and it is attributed to CN species undergoing oscillations or reflected shocks.     

Micro- and nanosecond laser TiN coating/steel modification: Morphology studies

Morphology effects induced during interaction of μs- (Transversely Excited Atmospheric (TEA) CO₂ laser) or ns- (HF laser) pulses with titanium nitride (TiN) coating, deposited on austenitic stainless steel AISI 316, were studied. Experiments were carried out in regime of focused laser beam in air at atmospheric pressure. The used laser fluences were found to be sufficient for inducing intensive surface modifications of the target. The energy absorbed from the CO₂ as well as HF laser beam is mainly converted into thermal energy, causing different effects like ablation, appearance of hydrodynamic features, etc. Morphology characteristics obtained during ns-pulses irradiation (HF laser) were different to those initiated by μs-pulses (TEA CO₂ laser). The changes on the target surface in form of massive resolidifed droplets and crown-like structures were observed only for ns- (HF laser) pulses. It was found that these effects are a consequence of higher temperature and better coupling of the HF laser radiation with the target. Recent investigations of ps-Nd:YAG laser interaction with the same TiN coating showed that morphology picture is quite different including the reduction of thermal effect. The article is published in the original.     

Immobilization of individual cells by local photo-polymerization on a chip

A novel separation method for random screening of target cells from a large heterogeneous population by using a local photo-polymerization is developed. A photo-crosslinkable resin solution is mixed with the sample liquid and we controlled the state from sol to gel by irradiating the near ultraviolet (UV) light with the mercury lamp and He-Cd laser near the target cell. We applied three types of immobilization methods such as direct immobilization method, caging method, and direct immobilization with position control method. The selected cell is immobilized in the cured resin directly or inside the cage of the cured resin. In the position control method, laser tweezers are employed to manipulate the target cell indirectly by using the droplet of the resin as a microtool. The cell is positioned properly by the laser manipulation system and is immobilized in the polymerized resin. After the selected cells are immobilized we can easily remove the other objects by the cleaning flow in the microchannel since the polymerized resin strongly binds with the cover glass and resists more than 466 mm s(-1) flow speed in the microchannel (microchannel size: width is 500 micron and depth is 100 micron). We tested the mercury lamp as well as the He-Cd laser for UV-light irradiation at the local area and confirmed improvement of resolution of the cured area by using the He-Cd laser (from 7 micron to 5 micron). Based on this method, we succeeded in single cell immobilization and basic experiments such as culture and fluorescent dyeing of immobilized yeast cells.     

Corrélation entre les conditions de traitement thermique par laser co2 de puissance et les modifications structurales de surface d'alliage de titane TA6V

The transfer of electromagnetic wave energy to the metallic material is done by an exchange between the laser photons and the lattice electrons via the inverse bremstrahlung. This process induces the passage of an electron from the valence to the conduction band in which it becomes free and energetic and it provokes, by collision with others electrons of the crystal lattice, the heating of the sample surfaces. The laser energy is then transformed, at the surface, into thermal energy (heat) which diffuses into the sample.For the same kind of materials with surfaces prepared in the same conditions, only laser beam parameters vary, following the relation: Qs = P τ / S, where Qs is the specific energy at the lighted surface, P the power of the laser, τ the interaction time and S the surface lighted by the laser beam. This factor indicates if the laser treatments are done without microstructural modification of the samples or with melting of the material surface.The martensitic phase α' obtained after the laser treatment is metastable, with a small grain size compared to those obtained with the classical thermal treatments. The size of α' grains depends on the energy density (Qs = P τ / S ) received by the specimen from the laser wave.     

Correlated electron dynamics: how aromaticity can be controlled

In this Article, we show that the aromaticity of a molecule can be turned off by controlling the electron dynamics. We present a controlled switching from the aromatic ground state of benzene to two different nonaromatic states, using a laser pulse. The propagation of the molecular wave function is carried out with the time-dependent configuration interaction method. The laser pulse for switching between the ground and excited states is optimized using optimal control theory. Bond orders and Mulliken charges serve as an aromaticity criterion. The nonaromatic target states exhibit localized bonds and partial charges on the carbon atoms; these localized electrons circulate on an attosecond time scale in the ring system.     

Adsorption preferentielle dans le systeme: Polystyrene-acetone-cyclohexane

The co-solvent system polystyrene-cyclohexane-acetone has been studied by the light-scattering technique. The selective adsorption parameter exhibits a dependence on solvent composition with the inversion point at u₂ = 0.470. The co-solvent behaviour is discussed in relation to the binary liquid interaction parameters χ_ij the ternary interaction parameter being negligible for this system.     

Studies on Fundamental Processes During Laser Ablation using Time-of-Flight Mass-Spectrometry (TOF-MS)

When a short pulsed, high power laser is focused on any solid target,a portion of the material is instantaneously exploded into its vapor. Laser ablation is a term to describe this explosive laser material interaction. Various processes like ejection of ions,atoms and clusters, thermal evaporation, plasma initiation and expansion, interaction between the plasma and the target, may result.     

Born-Floquet theory of electron-atom collisions in the presence of a laser field

We present a theory of fast electron-atom collisions in the presence of a strong laser field, which treats the interaction of the laser field with both the projectile electron and the target atom in a fully non-perturbative way. The theory is illustrated by considering the laser-assisted “elastic” scattering of fast electrons by atomic hydrogen, for non-resonant as well as resonant cases.     

Laser ablation of gall bladder stones.

Study of laser interaction with calculi is presented. A system of Nd-Yag and Ho-Yag pulsed lasers were used to produce fluorescence and plasma signals at the stone surface surrounded by saline and bile fluids. Fourth harmonic from Nd-Yag laser was transmitted to the samples by graded UV optical fibres. Gall bladder stones of various compositions were subjected to the high power Ho-Yag laser. Temporal transients and spectral evolution of plasma and fluorescence signals were monitored by a streak camera. A profile of acoustic pressures generated by shock waves was recorded with sensitive hydrophones placed in the surrounding fluids. Ablation threshold, cavitation process and fluorescence dependence on the laser parameters were studied in detail. Potential of stone identification by fluorescence and possible hydrodynamic model for ablation of biological samples is discussed.     

The Morse oscillator under time-dependent external fields

A method to solve the equations for the Morse oscillator under intense time-dependent external fields is presented. Exact analytical formulas for the dipole matrix elements are calculated by the use of the hypergeometric algebra. The continuum is described by an expansion using Laguerre functions. The full algorithm for the calculation of wave functions can be controlled by the convergence of series and by the errors of a first order integration method. We apply our technique to the selective preparation of high overtones by femtosecond laser pulses. The population of the target state is optimized as a function of the intensity and frequency. Introducing a second simultaneous laser, we study the effects of relative frequency and phase over the target state population and dissociation channels. The calculations exhibit a rich interference pattern showing the enhancement and the suppression of the target population by varying the laser parameters.     

Isolation and extraction of target microbes using thermal sol-gel transformation

We developed a novel separation method for random screening of target microorganisms from a large heterogeneous population by using a local viscosity control. A thermal sol-gel transformation material is mixed with the sample liquid and we controlled the state from sol to gel and gel to sol reversibly based on the temperature change controlled by heating of the microelectrode with the electric current and focused laser irradiation near the target. The selected microorganisms are fixed on the bottom plate by gel, since the viscosity around the target is temporally increased by the local heating by the focused laser. The other objects are easily washed away by the cleaning flow in the microchamber. Process of fixation, cleaning, isolation and extraction of the target microbe was possible in very short time. Based on this method, two separation systems are developed and basic experimental results of fixation and isolation of targets are shown.     

Phosphoramidates: Features of the formation mechanism and the relationship structure-bioaction

Mechanism of the synthesis of phosphoramidates by Todd-Atherton reaction is based on the primary interaction of a polyhaloalkane with the highly basic amine to form a 1:1 associate. The subsequent attack by the associate on the hydrophosphoryl compound of ??symmetric?? structure leads to the formation of the target compounds in high yields. The test of the effect of dialkyl hexamethylene- and dialkyl(pyridin-2-yl)-phosphoramidates in vitro against the strains of a number of the producents of pathogenic bacteria and mycotoxins showed that the high level of biological activity of the target compounds is correlated well with the physicochemical parameters characterizing their structure.     

Double pulse laser ablation and laser induced breakdown spectroscopy: A modeling investigation

A numerical model, describing laser–solid interaction (i.e., metal target heating, melting and vaporization), vapor plume expansion, plasma formation and laser–plasma interaction, is applied to describe the effects of double pulse (DP) laser ablation and laser induced breakdown spectroscopy (LIBS). Because the model is limited to plume expansion times in the order of (a few) 100 ns in order to produce realistic results, the interpulse delay times are varied between 10 and 100 ns, and the results are compared to the behavior of a single pulse (SP) with the same total energy. It is found that the surface temperature at the maximum is a bit lower in the DP configuration, because of the lower irradiance of one laser pulse, but it remains high during a longer time, because it rises again upon the second laser pulse. Consequently, the target remains for a longer time in the molten state, which suggests that laser ablation in the DP configuration might be more efficient, through the mechanism of splashing of the molten target. The total laser absorption in the plasma is also calculated to be clearly lower in the DP configuration, so that more laser energy can reach the target and give rise to laser ablation. Finally, it is observed that the plume expansion dynamics is characterized by two separate waves, the first one originating from the first laser pulse, and the second (higher) one as a result of the second laser pulse. Initially, the plasma temperature and electron density are somewhat lower than in the SP case, due to the lower energy of one laser pulse. However, they rise again upon the second laser pulse, and after 200 ns, they are therefore somewhat higher than in the SP case. This is especially true for the longer interpulse delay times, and it is expected that these trends will be continued for longer delay times in the μs-range, which are most typically used in DP LIBS, resulting in more intense emission intensities.     

Laser-assisted (e, 2e) collisions in helium

The influence of a strong laser field on the dynamics of fast (e, 2e) collisions in helium is analyzed in the asymmetric, coplanar geometry. The interaction of the laser field with the incident, scattered and ejected electrons is treated in a non-perturbative way, while the remaining interactions are treated by using first order perturbation theory. Detailed calculations are performed for an incident electron energyE _{k i}=600 eV, an ejected electron energyE _{k B}=5 eV and a scattering angle θ _A =4°. The influence of the laser parameters (photon energy, intensity and direction of polarization) on the angular distribution of the ejected electron is analyzed. We find that in general the triple differential cross sections are strongly dependent on the dressing of the projectile and the target by the laser field.     

激光烧蚀La~2O~3产生的等离子体的发光光谱研究

用发光光谱法研究了355nm脉冲激光在真空和O~2气氛中烧蚀La~2O~3产生的等离子体的组成和形成过程。对等离子体中La^+离子和LaO的空间和时间分辨发光光谱分析表明,在O~2气氛中LaO有两个生成通道:一是在靶附近的等离子体内直接生成的,另一是由La,La^+与O^2发生氧化反应而生成的。测定了激光能量密度,离靶表面的距离和O~2压力对产物发光的延迟时间和发光强度的影响。此外,还讨论了激光烧蚀La~2O~3诱导产生等离子体的形成和演化机理。     

Laser-assisted electron impact rovibrational excitations in HF molecule

Electron impact rovibrational excitation of the HF molecule (in the electronic ground state) is studied in the presence of an infra-red laser beam taken in the dipole approximation. The non-perturbative quasi-energy method is used to describe the laser-molecule interaction while the electron-molecule interaction is treated within the first Born approximation. The utility of our formalism is illustrated by computing the differential cross section (DCS) for vibration-rotation transitions in HF molecule due to joint collisions with electrons and photons. We also discuss the influence of various laser parameters (polarization, frequency and intensity) that strongly affect the dynamics of the collisions. It is found that laser fields can produce significant changes in the DCS.