Observation of Multiphoton Process in Liquid CS2 Using Pulsed Photoacoustic Technique

Pulsed photoacoustic measurements have been carried out in liquid CS2 using 532 nm radiation from a frequency doubled Nd:YAG laser. Variation of signal amplitude with laser fluence clearly indicates the role of multiphoton processes in the generation of photoacoustic effect. It is also shown that four photon induced dissociation and five photon induced ionization are likely processes in CS2 and 532 nm radiation.

High resolution optogalvanic study in nitrogen discharge

Doppler limited high resolution spectrum in the wavelength region 17224 to 17236 cm−1 of the first positive system (B 3Π g −A 3Σ u + ) of the N2 molecule is recorded by optogalvanic spectroscopic technique using a single mode ring dye laser. It is observed that the intensity and line width of the rotational line increase with the discharge current. Dependence of the collision broadening coefficient on the current was also evaluated.

NIR Spectroscopic and Laser Induced Fluorescence studies of some organic molecules

Vibrational overtone spectroscopy of molecules containing X-H oscillators (X = C, N, O...) has become an effective tool for the study of molecular structure, dynamics, inter and intramolecular interactions, conformational aspects and substituent effects in aliphatic and aromatic compounds. In the present work, the author studied the NIR overtone spectra of some liquid phase organic compounds. The analysis of the CH, NH and OH overtones yielded important structural information about these systems. In an attempt to get information on electronic energy levels, we studied the pulsed Nd:YAG laser induced fluorescence spectra of certain organic compounds. The pulsed laser Raman spectra of some organic compounds are also studied. The novel high resolution technique of near infrared tunable diode laser absorption spectroscopy (TDLAS) is used to record the rotational structure of the second OH overtone spectrum of 2-propanol. The spectral features corresponding to the different molecular conformations could be identified from the high resolution spectrum. The whole work described in this thesis is divided into five chapters.

Studies on laser induced optogalvanic effect in certain gas discharges

Electrical gas discharges have been the subject of numerous investigations from the last century due to their growing interest in technological and fundamental applications. Absorption of electromagnetic radiation by a gas discharge result into a change in electrical impedance due to a significant perturbations in the steady state population of excited levels and the degree of ionization. This change in impedance produced by resonant absorption of radiation is known as optogalvanic COG) effect. where as that is produced by injecting electrons in to the discharge by photoelectric emission is usually known as photoemission optogalvanic (FOG) effect. With the development of lasers and sophisticated electronic equipment. these effects have established their importance in analytical and spectroscopic measurements. The present thesis deals with the work carried out by the author in the field optogalvanic effect during the past few years at the Department of Physics in Cochin University of Science| and "Fechnology. The results and the observation are summarized in nine chapters and the references to the literature is made at the end of each chapter

The Surface Plasmon Resonance of Supported Noble Metal Nanoparticles: Characterization, Laser Tailoring, and SERS Application

This work deals with the optical properties of supported noble metal nanoparticles, which are dominated by the so-called Mie resonance and are strongly dependent on the particles’ morphology. For this reason, characterization and control of the dimension of these systems are desired in order to optimize their applications. Gold and silver nanoparticles have been produced on dielectric supports like quartz glass, sapphire and rutile, by the technique of vapor deposition under ultra-high vacuum conditions. During the preparation, coalescence is observed as an important mechanism of cluster growth. The particles have been studied in situ by optical transmission spectroscopy and ex situ by atomic force microscopy. It is shown that the morphology of the aggregates can be regarded as oblate spheroids. A theoretical treatment of their optical properties, based on the quasistatic approximation, and its combination with results obtained by atomic force microscopy give a detailed characterization of the nanoparticles. This method has been compared with transmission electron microscopy and the results are in excellent agreement. Tailoring of the clusters’ dimensions by irradiation with nanosecond-pulsed laser light has been investigated. Selected particles are heated within the ensemble by excitation of the Mie resonance under irradiation with a tunable laser source. Laser-induced coalescence prevents strongly tailoring of the particle size. Nevertheless, control of the particle shape is possible. Laser-tailored ensembles have been tested as substrates for surface-enhanced Raman spectroscopy (SERS), leading to an improvement of the results. Moreover, they constitute reproducible, robust and tunable SERS-substrates with a high potential for specific applications, in the present case focused on environmental protection. Thereby, these SERS-substrates are ideally suited for routine measurements.

Room-temperature continuous-wave operation of GaInNAs/GaAs quantum dot laser with GaAsN barrier grown by solid source molecular beam epitaxy

We present the results of GaInNAs/GaAs quantum dot structures with GaAsN barrier layers grown by solid source molecular beam epitaxy. Extension of the emission wavelength of GaInNAs quantum dots by ~170nm was observed in samples with GaAsN barriers in place of GaAs. However, optimization of the GaAsN barrier layer thickness is necessary to avoid degradation in luminescence intensity and structural property of the GaInNAs dots. Lasers with GaInNAs quantum dots as active layer were fabricated and room-temperature continuous-wave lasing was observed for the first time. Lasing occurs via the ground state at ~1.2μm, with threshold current density of 2.1kA/cm[superscript 2] and maximum output power of 16mW. These results are significantly better than previously reported values for this quantum-dot system.

Reactions of silylene with unreactive molecules. 2. nitrogen: gas-phase kinetic and theoretical studies

Time-resolved studies of the reaction of silylene, SiH2, with N-2 have been attempted at 296, 417, and 484 K, using laser flash photolysis to generate and monitor SiH2. No conclusive evidence for reaction could be found even with pressures of N-2 of 500 Torr. This enables us to set upper limits of ca. 3 x 10(-15) cm(3) molecule(-1) s(-1) for the second-order rate constants. A lower limit for the activation energy, E-a, of ca. 47 kJ mol(-1) is also derived. Ab initio calculations at the G3 level indicate that the only SiH2N2 species of lower energy than the separated reactants is the H2Si...N-2 donor-acceptor (ylid) species with a relative enthalpy of -26 kJ mol(-1), insufficient for observation of reaction under the experimental conditions. Ten bound species on the SiH2N2 surface were found and their energies calculated as well as those of the potential dissociation products: HSiN + NH((3)Sigma(-)) and HNSi + NH((3)Sigma(-)). Additionally two of the transition states involving cyclic-SiH2N2 (siladiazirine) were explored. It appears that siladiazirine is neither thermodynamically nor kinetically stable. The findings indicate that Si-N-d bonds (where N-d is double-bonded nitrogen) are not particularly strong. An unexpected cyclic intermediate was found in the isomerization of silaisocyanamide to silacyanamide.

New-on-line method for water isotope analysis of speleothem fluid inclusions using laser absorption spectroscopy (WS-CRDS)

A new online method to analyse water isotopes of speleothem fluid inclusions using a wavelength scanned cavity ring down spectroscopy (WS-CRDS) instrument is presented. This novel technique allows us simultaneously to measure hydrogen and oxygen isotopes for a released aliquot of water. To do so, we designed a new simple line that allows the online water extraction and isotope analysis of speleothem samples. The specificity of the method lies in the fact that fluid inclusions release is made on a standard water background, which mainly improves the δ D robustness. To saturate the line, a peristaltic pump continuously injects standard water into the line that is permanently heated to 140 °C and flushed with dry nitrogen gas. This permits instantaneous and complete vaporisation of the standard water, resulting in an artificial water background with well-known δ D and δ18O values. The speleothem sample is placed in a copper tube, attached to the line, and after system stabilisation it is crushed using a simple hydraulic device to liberate speleothem fluid inclusions water. The released water is carried by the nitrogen/standard water gas stream directly to a Picarro L1102-i for isotope determination. To test the accuracy and reproducibility of the line and to measure standard water during speleothem measurements, a syringe injection unit was added to the line. Peak evaluation is done similarly as in gas chromatography to obtain &delta D; and δ18O isotopic compositions of measured water aliquots. Precision is better than 1.5 ‰ for δ D and 0.4 ‰ for δ18O for water measurements for an extended range (−210 to 0 ‰ for δ D and −27 to 0 ‰ for δ18O) primarily dependent on the amount of water released from speleothem fluid inclusions and secondarily on the isotopic composition of the sample. The results show that WS-CRDS technology is suitable for speleothem fluid inclusion measurements and gives results that are comparable to the isotope ratio mass spectrometry (IRMS) technique.

Low-level Laser Therapy Improves Skeletal Muscle Performance, Decreases Skeletal Muscle Damage and Modulates mRNA Expression of COX-1 and COX-2 in a Dose-dependent Manner

We tested if modulation in mRNA expression of cyclooxygenase isoforms (COX-1 and COX-2) can be related to protective effects of phototherapy in skeletal muscle. Thirty male Wistar rats were divided into five groups receiving either one of four laser doses (0.1, 0.3, 1.0 and 3.0 J) or a no-treatment control group. Laser irradiation (904 nm, 15 mW average power) was performed immediately before the first contraction for treated groups. Electrical stimulation was used to induce six tetanic tibial anterior muscle contractions. Immediately after sixth contraction, blood samples were collected to evaluate creatine kinase activity and muscles were dissected and frozen in liquid nitrogen to evaluate mRNA expression of COX-1 and COX-2. The 1.0 and 3.0 J groups showed significant enhancement (P < 0.01) in total work performed in six tetanic contractions compared with control group. All laser groups, except the 3.0 J group, presented significantly lower post-exercise CK activity than control group. Additionally, 1.0 J group showed increased COX-1 and decreased COX-2 mRNA expression compared with control group and 0.1, 0.3 and 3.0 J laser groups (P < 0.01). We conclude that pre-exercise infrared laser irradiation with dose of 1.0 J enhances skeletal muscle performance and decreases post-exercise skeletal muscle damage and inflammation.

A Study of the Effect of Nitrogen and Air Plasma Immersion Ion Implantation Treatments on the Properties of Carbon Fiber

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

New Terahertz Laser Lines From (13)CD(3)OH Pumped by Regular and Hot Bands CO(2) Laser

We have used our new pulsed CO(2) laser, operating both on regular and hot bands, to excite the (13)CD(3)OH methanol isotopomer. This has lead to the observation of 13 new high-threshold far-infrared laser emissions (also identified as terahertz laser lines), with frequencies in the range between 24.11 and 102.56 cm(-1) (0.72-3.07 THz). The absorption transitions leading to these new FIR laser emissions have been located by observing the optoacoustic absorption spectra around the CO(2) emissions. Here, we present these new far-infrared laser lines, characterized in wavelength, polarization, offset relative to the center of the pumping CO(2) laser transition, relative intensity, and optimum operation pressure.

Root surface treatment using diode laser in delayed tooth replantation: radiographic and histomorphometric analyses in rats

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Influência da energia de soldagem em uniões de lâminas finas através de laser pulsado de Nd:YAG

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

One-dimensional analytical model for oxide thin film growth on Ti metal layers during laser heating in air

This paper presents the theoretical and experimental results for oxide thin film growth on titanium films previously deposited over glass substrate. Ti films of thickness 0.1 μm were heated by Nd:YAG laser pulses in air. The oxide tracks were created by moving the samples with a constant speed of 2 mm/s, under the laser action. The micro-topographic analysis of the tracks was performed by a microprofiler. The results taken along a straight line perpendicular to the track axis revealed a Gaussian profile that closely matches the laser's spatial mode profile, indicating the effectiveness of the surface temperature gradient on the film's growth process. The sample's micro-Raman spectra showed two strong bands at 447 and 612 cm -1 associated with the TiO 2 structure. This is a strong indication that thermo-oxidation reactions took place at the Ti film surface that reached an estimated temperature of 1160 K just due to the action of the first pulse. The results obtained from the numerical integration of the analytical equation which describes the oxidation rate (Wagner equation) are in agreement with the experimental data for film thickness in the high laser intensity region. This shows the partial accuracy of the one-dimensional model adopted for describing the film growth rate. © 2001 Elsevier Science B.V.

Laser ultrasonics system for measurement of speed of sound in gases

In this work we developed a setup to measure the speed of sound in gases using a laser ultrasonics system. The mentioned setup is an all optical system composed by a Q-switched Nd:YAG laser to generate the sound waves, and a fiber optical microphone to detect them. The Nd:YAG provided a laser pulse of approximately 420 mJ energy and 9 ns of pulse width, at the wavelength of 1064 nm. The pulsed laser beam, focused by a positive lens, was used to generate an electrical breakdown (in the gas) which, in turn, generates an sound wave that traveled through a determined distance and reached the fiber optical microphone. The resulting signal was acquired in an oscilloscope and the time difference between the optical pulse and the arrival of the sound waves was used to calculate the speed of sound, since the distance was known. The system was initially tested to measure the speed of sound in air, at room pressure and temperature and it presented results in agreement with the theory, showing to be suitable to measure the speed of sound in gases. © 2012 American Institute of Physics.