Nanosecond UV Laser Research Articles

Study of proton transfer processes in solution using the laser induced pH-jump: A new experimental setup and an improved data analysis based on genetic algorithms

We present an experimental setup for studying proton transfer reactions in solution and an innovative data analysis based on a genetic algorithm. We have used o-nitrobenzaldehyde to irreversibly release protons in an aqueous solution upon photolysis with an intense UV nanosecond laser pulse. Reactions of the photodetached protons with pH indicators have been followed by monitoring the changes in absorbance at 632.8 nm with a He–Ne laser and a Si avalanche photodiode. The setup has laser-limited time resolution and single shot sensitivity. The pH jump established with the laser shot lasts ∼30 ms, during which proton transfer reactions can be followed. The proposed methodology represents a simplification in the kinetic schemes with respect to the previous protocols based on the use of reversible caged protons.

Crosslinking of progesterone receptor to DNA using tuneable nanosecond, picosecond and femtosecond UV laser pulses.

UV laser crosslinking is a potentially powerful tool to investigate transient DNA-protein interactions and binding kinetics in intact cells. As the processes underlying UV laser crosslinking are not fully understood, we have performed a study of the influence of laser pulses with different physical parameters on crosslinking of the progesterone receptor to an oligonucleotide containing a hormone-responsive element. We also studied the influence of the various parameters on the amount of laser-irradiated DNA that can be correctly primer extended as an operational measurement of DNA integrity. A strong influence of pulse intensity and pulse length on the crosslink yield was found, likely due to a change in the 'two photon' processes responsible for crosslinking. The highest efficiency of protein crosslinking to DNA was achieved with femtosecond pulses and should be sufficient to enable use of this technique for in vivo studies.

Photoinduced Molecular Reorientation in Optical Nonlinear Langmuir−Blodgett Films

Photoinduced optical anisotropy due to molecular reorientation in hemicyanine Langmuir−Blodgett multilayers has been investigated by polarized UV−visible absorption and rotation-angle second harmonic generation measurements. Under polarized UV nanosecond laser illumination, the hemicyanine molecules reorient and align their chromophore axes along the direction of the UV beam polarization. Annealing of the films leads to randomization of the molecules and possibly dissociation of aggregate species.

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

On the mechanism of C60 thin film laser-induced desorption

The mechanism of thin film C60 laser desorption has been investigated using nanosecond and picosecond UV laser pulses. The desorption experiments were performed under ultrahigh-vacuum conditions using reflectron time-of-flight mass spectroscopy from which the velocity distributions of the desorbed ions and the dependence of the ion yield on the laser fluence were obtained. A strong nonlinear dependence of the desorption yield on laser fluence in the threshold region, indicative of a thermal mechanism, was found for both ns and ps pulses. Typically, the C+60 velocity distributions were bimodal and could be fitted by modified Maxwell–Boltzmann distributions. The fits to the slow contributions gave translational temperatures consistent with surface temperatures due to laser heating with ns pulses as estimated by solving the one-dimensional heat equation. In contrast, translational temperatures which are much too high to be consistent with purely thermal processes were obtained for the fast contributions. These fast contributions originate from different desorption mechanisms for the two different laser pulse lengths.

Preferential vaporization and plasma shielding during nano-second laser ablation

Preferential removal of components from mixed material targets and plasma shielding are studied by using inductively coupled plasma-atomic emission spectrometry (ICP-AES) during UV nano-second laser ablation. The ICP emission intensity for Cu and Zn during ablation of brass samples varies versus laser power density. A model using thermal evaporation and inverse Bremsstrahlung processes is presented. The model shows that plasma shielding occurs at approximately 0.3 GW/cm2, in agreement with experimental data for change in the mass ablation rate. The good agreement of model and experimental data suggest that thermal effects are important during nano-second laser ablation for power density less than 0.3 GW/cm2.

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

The influence of melting on the excitation of Surface Acoustic Wave (SAW) pulses in silicon is studied both theoretically and experimentally. The developed theory of Rayleigh-type SAW laser-induced thermoelastic excitation in a structure composed of a liquid layer on a solid substrate predicts that the SAW is predominantly generated in the solid phase due to the absence of shear rigidity in a liquid. The characteristic changes in the SAW pulse shape as well as the saturation and even the decrease of the SAW pulse amplitude observed above the melting threshold are explained theoretically to be a result of the decrease of the heat flux into the solid phase as well as due to the decrease of the volume of the solid phase caused by melting. Although the heat flux into the solid phase is decreased both as a consequence of the reflectivity increase and the additional energy losses (latent heat of melting) at the phase transition, it is demonstrated that the influence of reflectivity changes on the SAW pulse is negligible in comparison with the effect of melt-front motion. For laser pulses of 7 ns duration at 355 nm, the threshold value of laser fluence for meltingFm=0.23±0.04 J/cm2 and for the ablationFa=1.3±0.2 J/cm2 were determined experimentally as the points of characteristic changes in the observed SAW pulses.

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

Effect of melting on the excitation of surface acoustic wave pulses by UV nanosecond laser pulses in silicon

Laser Melting and Recrystallization of Bulk Si by Nanosecond UV Laser Pulses

ABSTRACTLaser ablation of semiconductors presents an increasing interest for both thin film growth and surface modification. We present herein a study of the damage produced in bulk silicon by nanoseconds UV laser pulses with energy above the melting threshold. This study is carried out with a Raman microprobe. Polarized microRaman was used to reveal the main changes in the melted and recrystallized volume. These changes were observed in the liquid/solid boundaries, where tensile stress due to the induced thermal wave is more important. The morphology of the melted region evidences matter accumulation at such a boundary.

Heat transfer and material removal in pulsed excimer-laser-induced ablation: Pulsewidth dependence

The pulsewidth-dependent ablation of polyimide, poly(methylmethacrylate), poly (etheretherketone), poly(ethylene terepthalate), and poly(ethersulphone) exposed to 248 and 308 nm, nanosecond UV laser pulses was modeled assuming one-dimensional heat transfer and a published model [G. H. Pettit and R. Sauerbrey, Appl. Phys. A 56, 51 (1993)] for photon absorption. The polymers were assumed to degrade/ablate after reaching a threshold temperature determined either from published temperature calculations of the ablating surface or the ceiling temperature. Since heat transfer calculations suggest that this temperature is reached before the end of the laser pulse, it was assumed that the degraded/ablated material continues to attenuate the incoming laser energy for the remaining duration of the laser pulse. Since the fluence-dependent absorption coefficient of this degraded material is unknown, it was obtained by fitting the experimental pulsewidth dependent ablation rate data of Schmidt, Ihlemann, and Wolff-Rottke (unpublished). The resulting values are consistent with mass spectrometric analysis of the ablation products and with the absence or occurrence of incubation. The threshold fluences and ablation rates predicted by this model are in good agreement with reported literature values; however, the use of a well-defined threshold temperature in the model leads to a different limiting etch rate dependence on the fluence at the threshold.

Laser electrochemistry of short-lived intermediate species

The specific features of photoelectron emission from metal into electrolyte solution, induced by nanosecond uv lasers, are considered. The application of this phenomenon to studies on electrode reactions of short-lived intermediates (hydrogen atoms and simple organic radicals) are briefly reviewed.

Melting kinetics of Sb under nanosecond UV laser irradiation

Real time reflectivity measurements at the HeNe laser wavelength (λ=632.8 nm) have been performed in crystalline Sb under pulsed UV laser irradiation (λ=193 nm). For energy densities above 100 mJ cm−2, a transient low-reflectivity phase is observed, which is related to surface melting. The change of reflectivity of the solid material upon heating is small, and a maximum variation of −0.02 relative to the room-temperature value (RSb,rt) is determined for the solid material at the melting point. The value of the reflectivity of the liquid material is determined to be 0.94RSb,rt(632.8 nm)=0.67, which is lower than that of the solid and it is associated with a decrease of the absorption coefficient upon phase change. The analysis of reflectivity transients at energy densities just above the melting threshold suggests that melting nucleation proceeds inhomogeneously.

Nanosecond and Femtosecond UV Laser Ablation of CdTe (100): Time-Of-Flight and Angular Distributions

AbstractAngularly resolved time-of-flight (TOF) measurements have been used to probe the velocity and angular distributions of Cd atoms and Te2 molecules ejected from CdTe (100) substrates under irradiation by 248 nm nanosecond and sub-picosecond laser pulses. These experiments employ a dye laser TOF mass spectrometer with resonance enhanced multiphoton ionization for sensitive, high resolution detection of the desorbed products. The velocity distributions are well described by Maxwell-Boltzmann distributions for low fluence nanosecond (<60 mJ/cm2) and sub-picosecond (<3.3 mJ/cm2) pulses. Angular flux distributions for nanosecond irradiation are observed to be highly forward peaked about the surface normal, whereas, for sub-picosecond irradiation the distribution approaches cos3θ.

Grain boundary triggering of diffusion in laser melted SbGe bilayer films and surface ripples

Thin SbGe bilayer films having either the Sb or Ge layer at the surface are irradiated with nanosecond UV laser single pulses. Diffusion of the elements is interfacially initiated and it is shown that the grain boundaries of the Sb crystals in the as-grown material act as nucleation sites for the reaction. Films whose surface layer is Ge exhibit a characteristics ripple pattern which follows the radial growth lines of the crystals in the non-transformed part of the Sb layer.

Ultrafast electrocyclic ring opening in previtamin D photochemistry

Photochemical reactions of 7-dehydrocholesterol and lumisterol were studied by nanosecond and picosecond UV laser excitation. The resulting production of previtamin D and tachyslerol was analysed by chromatography. With picosecond excitation, the formation of the secondary product tachysterol was significantly reduced. Model calculations gave reactive and non-reactive rate constants for 7-dehydrocholesterol and lumisterol.

Oxygen-Trapping and Oxidation Induced by Laser Irradiation in Silicon

ABSTRACTRapid oxidation in silicon induced by nanosecond UV laser pulses has been recently reported [1]. A significant amount of oxygen was observed to be incorporated in theregrown silicon layer when irradiation took place in air or in oxygen ambient. The fundamental interaction and transport kinetics involved include: incorporation of impurity during surface melting, trapping of solute during resolidification, and rapid reaction to form chemical bonds. The present study has investigated the mechanism of oxygen incorporation and trapping under various regrowth conditions. Oxygen depth-concentration distributions were analyzed using secondary-ion-mass spectroscopy (SIMS). The oxide formed was studied using differential Fourier-Transformed IR (FT-IR) spectroscopy.

Nanosecond spectroscopy: kinetics of state selected species

OH is chemically produced by photolysis of ozone in a O 3/H 2O mixture with a nanosecond uv laser pulse. The OH-radicals were observed immediately after their formation nearly collision free by measuring the absorption intensities of the X 2¶ 3 2 (v'' = O, K'') → A 2σ +(v' = O, K'') transitions. The scanning of the rotational spectrum is performed by pressure tuning of the dye laser. The population of the Λ - doublets can be described by two different rotational temperatures. In the four lowest rotational states we observe inversion and in the higher rotational states antiinversion in the Λ - doublets. For K''⩾ 7 there is a strong deviation from these thermal distributions.