Showing papers on "Photoacoustic spectroscopy published in 1995"

Photoacoustic frequency heat‐transmission technique: Thermal and carrier transport parameters measurements in silicon

Abstract: Photoacoustic frequency heat‐transmission technique is used to study thermal and carrier transport properties in low‐doped silicon wafers. Amplitude and phase photoacoustic signals as a function of modulation frequency of incident optical beam are measured using different experimental conditions. The thermal diffusivity, coefficient of excess carrier diffusion, carrier lifetime, and the surface recombination velocity were determined by comparing experimental results and calculated theoretical photoacoustic signals. The suitability of the photoacoustic frequency heat‐transmission technique as a contactless diagnostic method is assessed in comparison with the more conventional photothermal deflection and photothermal modulated reflection techniques.

Novel design of a resonant photoacoustic spectrophone for elemental carbon mass monitoring

Abstract: An improved design of a resonant photoacoustic spectrophone is reported for aerosol absorption measurements by using a near‐infrared laser diode (λ=802 nm; P=450 mW) in combination with a double‐pass configuration for excitation of azimuthal modes and right‐angle mounting of two electret microphones for signal detection. As a result of the new spectrophone setup a small‐sized portable sensor system for real‐time in situ elemental carbon monitoring is presented. The detection limit is estimated to 4.5×10−8 cm−1 or 1.5 μg elemental carbon per m 3.

Near and mid-infrared photoacoustic analysis of principal components of foodstuffs

Abstract: Photoacoustic spectroscopy, especially in the near infrared, for principal food component analysis (protein, fat, carbohydrate and moisture contents) is discussed. The thermal diffusion length is discussed as is the problem of signal saturation. An illustrative comparison of photoacoustic spectroscopy and Fourier transform mid-infrared-attenuated total reflectance spectroscopy (FTIR-ATR) as applied to cheese analysis is given.

Quantitative determination of the composition of individual pea seeds by fourier transform infrared photoacoustic spectroscopy

Abstract: Photoacoustic detection used in conjunction with multivariate partial least squares calibration showed a good predictive ability for the major components (starch, protein, lipid) of a single pea seed. Minimal sample preparation was required. The error of prediction achieved by photoacoustic spectroscopy was, for most components, close to the standard error of wet chemical methods but the technique had the advantage of a shorter analysis time and retained seed viability.

Thermoacoustic enhancement of photoacoustic spectroscopy: Theory and measurements of the signal to noise ratioa)

Abstract: Recent developments in thermoacoustics have shown that the quality factor, Q, of an acoustic resonator can be controlled by establishing a temperature gradient across properly positioned thermoacoustic elements. Quite separate from thermoacoustics, acoustic resonators are used in photoacoustic spectroscopy, where a laser beam, modulated at the acoustic resonance frequency, is partially absorbed, thereby producing sound. The photoacoustic signal is typically measured with a microphone, and is proportional to the laser power, to the absorption coefficient, and to the resonator Q, among other factors. The acoustic signal to noise ratio is proportional to Q1/2. Thermoacoustics can be used to enhance the signal to noise ratio of photoacoustic spectrometers by increasing Q. Measurements and theory are reported for the signal to noise ratio of a photoacoustic cell, with thermoacoustic enhancement, as a function of both resonator Q and the bandwidth of the lock‐in amplifier. Regimes where thermoacoustic enhanceme...

Laser photoacoustics for gas analysis and materials testing

Abstract: The application of laser photoacoustics to two different areas is discussed. First, laser-induced spallation and interferometric detection of transient surface displacements is proposed as a powerful noncontact tool for the investigation of adhesion properties of solid surface coatings. Results for nickel and plasma-sprayed ceramic coatings are presented. Delamination processes at the interface between substrate and coating could be detected with excellent spatial and temporal resolution and adhesion strengths in the 0.2 to 2 GPa range be determined. Second, laser photoacoustic spectroscopy is applied to trace gas monitoring. An automated mobile CO 2 laser photoacoustic system is employed for in situ air monitoring with parts per billion sensitivity in industrial, urban, and rural environments. An improvement in detection selectivity for multicomponent gas mixtures is achieved with a continuously tunable highpressure CO 2 laser with a narrow linewidth of 0.017 cm -1 . A CO laser photoacoustic system previously used for the analysis of motor vehicle exhausts is now employed for studying dimerization phenomena in fatty acid vapors. Finally, emphasis is put on the development of widely tunable, narrow-band, mid-IR laser sources based on optical parametric oscillation or difference frequency generation employing tunable diode lasers and AgGaSe 2 as nonlinear material.

Photoacoustic spectroscopy use in the analysis of ion‐implanted CuInSe2 single crystals

Abstract: A high‐resolution near‐infrared photoacoustic spectrometer of the gas–microphone type is used for room‐temperature analysis of the defect states in ion‐implanted CuInSe2 single crystals. A wide range of ions differing in their masses as well as in their electrical activities in the host crystal (O+, Ne+, Cu+, Xe+, and Li+) have been implanted at various energies and doses to assess the technique sensitivity to detect changes affecting the compound subgap absorption spectrum, which is characteristic of impurities. The detected changes (either by the appearance of new peaks or by a change in the impurity or defect concentrations) are shown to differ from one ion species to another. The depth profiling capability of the technique in analyzing implanted samples is also discussed. The results obtained here are correlated to existing published data. It is shown that photoacoustic spectrometry can be very useful if used to appraise the complex defect structure of ion‐implanted CuInSe2.

Photoacoustic drug delivery: the effect of laser parameters on the spatial distribution of delivered drug

Abstract: Photoacoustic drug delivery is a technique for delivering drugs to localized areas by timing laser-induced pressure transients to coincide with a bolus of drug. This study explores the effects of target material, laser energy, absorption coefficient, fiber size, repetition rate, and number of pulses on the spatial distribution of delivered drug. A microsecond flash-lamp pumped dye laser delivered 30-100 mJ pulses through optical fibers with diameters of 300-1000 micrometers . Vapor bubbles were created 1-5 mm above clear gelatin targets submerged in mineral oil containing a hydrophobic dye (D&C Red#17). The absorption coefficient of the oil-dye solution was varied from 50-300 cm-1. Spatially unconfined geometry was investigated. We have found that while the dye can be driven a few millimeters into the gels in both the axial and radial directions, the penetration was less than 500 micrometers when the gel surface remained macroscopically undamaged. Increasing the distance between the fiber tip and target, or decreasing the pulse energy reduced the extend of the delivery.

Photoacoustic Spectroscopy of Some Energetic Materials

Abstract: To find an effective laser source to ignite energetic materials, the absorption spectra of some energetic materials are obtained by means of a photoacoustic spectroscopy (PAS). In this experiment, PAS covers the wavelength region of 400 nm-1600 nm in which no other conventional method can take absorption spectra for powdered energetic materials. Photoacoustic spectra of 18 energetic materials are reported. In general, energetic materials tested showed peaks in 600 nm–800 nm and 1400 nm–1600 nm ranges. It is found that the energy required to initiate explosives in the case of ruby laser initiation were correlated with their photoacoustic signal intensities.

Analysis of pollutant chemistry in combustion by in situ pulsed photoacoustic laser diagnostics

Abstract: A technique for gas analysis based on pulsed-laser-induced photoacoustic spectroscopy in the UV and the visible is presented. The laser-based technique and the associated analysis probe have been developed for the analysis of pollutant chemistry in fluidized beds and other combustion environments with limited or no optical access. The photoacoustic-absorption spectrum of the analyzed gas is measured in a test cell located at the end of a tubular probe. This test cell is subject to the prevailing temperature and pressure in the combustion process. The instrument response has been calibrated for N2O, NO, NO2, NH3, SO2, and H2S at atmospheric pressure between 20 and 910 °C. The response of the probe was found to increase with pressure for N2O, NO, NH3, and NO2 up to 1.2 MPa pressure. The method and the probe have been used for detection and ranging of gas concentrations in a premixed methane flame. Some preliminary tests in a large 12-MW circulating bed boiler have also been done.

Optical band gap of the α−mercuric iodide

Abstract: We investigate by photoacoustic spectroscopy the optical band‐gap energy of mercuric iodide, α‐HgI2, grown by sublimation in a sealed ampoule. Due to its importance as a detector material operating at ambient temperature, the physical properties of α‐HgI2 have been recently studied. We found, by two different methods, the band‐gap energies EG=2.32 and 2.39 eV, respectively. These results are in good agreement with recent measurements based on reflection and absorption spectra.

Theory of X-ray photoacoustic spectroscopy

Abstract: PhotoAcoustic Spectroscopy (PAS) in the X-ray region is becoming a new field in PAS research and poses some new problems, such as heat production mechanisms and non-exponential heat distribution due to multi-excitation processes, additional signals caused by escaped fluorescence and electrons, special backing and fronting setup for comparison with absorption experiment, etc. In this paper we treat these problems and include them in an extended theoretical model which can be reduced to the RG model in a special case. The results indicate that the non-exponential heat distribution has no effect on the PA phase. The contribution by gas heating of the escaped electrons can be neglected. The thermally thin gas layers between the sample and the Be windows are sensitive factors to both amplitude and phase. The PA signal generated in the backing gas layer by the backing surface of the sample should be considered to understand the frequency dependence of the PA signal. The expansion of the backing gas layer is the reason for the phase change in PA-EXAFS.

Photoacoustic Spectroscopy of Polyaniline Films

Abstract: Photoacoustic spectra as well as optical absorption spectra were measured to study the heat generation by nonradiative processes in electrodeposited polyaniline films in the wavelength range of 310 to 800 nm. The shapes of both spectra depend on the pH for polymerization of electrodeposited polyaniline films. The shapes of infrared spectra also show the pH dependence. Photoacoustic spectra show that the changes of signal intensity are different from those of optical absorption spectra, suggesting the character of lattice relaxation of a low-dimensional system.

Optical fiber interferometry for photoacoustic spectroscopy in liquids.

Abstract: Optical fiber interferometry was used for measurement of the subnanometer surface and bulk displacements associated with photoacoustic pressure waves in methanol and water. The measurement system is both broadband and noncontacting, giving a pressure sensitivity of 0.1 Pa/ radicalHz.

Monitoring electron transfer by photoacoustic spectroscopy

Abstract: The electron transfer process between octaethylporphin and quinone molecules dispersed in a polymeric matrix was studied by the photoacoustic technique. It was observed that there was an enhancement of the octaethylporphin photoacoustic signal with an increase of the quinone concentration in the films. This increase appeared to be complementary to octaethylporphin fluorescence quenching and was associated with the electron transfer process. The data were analyzed according to the theory developed by Kaneko for fluorescence data (Kaneko 1992).

Photoacoustic FT-IR Spectroscopy of Natural Copper Patina

Abstract: Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) has proved to be a useful tool for nondestructive testing of copper corrosion layer (patina) formed in the atmosphere. The samples cut from a piece of the roof of the Stockholm City Hall were examined without any additional pretreatment. The components of the patina—brochantite Cu4(OH)6SO4, antlerite Cu3(OH)4SO4, and basic cupric carbonate Cu2CO3(OH)6·H2O—as well as some other minerals were identified. The photothermal beam deflection (PBD) method was used for independent photoacoustic characterization of the samples. The depth profiling capability of FT-IR/PAS was used to determine the degree of photoacoustic saturation of the spectral bands and to evaluate the depth distribution of the main patina components. The technique thus compares favorably with more common approaches of patina examination which are more expensive and require special sample preparation.

Photoacoustic methods for real‐time measurement of laser beam diameter and position, and for laser cavity alignment

Abstract: Photoacoustic signals generated in absorbing media can be used for real‐time alignment of laser cavity components, for measurements of positions and diameters of pulsed laser beams, and for control of the degree of overlap of two or more, focused and unfocused pulsed laser beams. The developed techniques require an absorbing medium and a piezoelectric detector. Measurements and control can be performed by real‐time inspection of the acoustic signals in an oscilloscope. These methods are convenient for experiments in nonlinear optics in which the power requirements lead to focusing and superposition of the beams, and for alignment of laser cavities in the liquid phase as well as gas and solid state infrared lasers.

Evaluation of an optical fiber probe for in vivo measurement of the photoacoustic response of tissues

Abstract: A miniature (1 mm diameter) all-optical photoacoustic probe for generating and detecting ultrasonic thermoelastic waves in biological media at the tip of an optical fiber has been developed. The probe provides a compact and convenient means of performing pulsed photoacoustic spectroscopy for the characterization of biological tissue. The device is based upon a transparent Fabry Perot polymer film ultrasound sensor mounted directly over the end of a multimode optical fiber. The optical fiber is used to deliver nanosecond laser pulses to the tissue producing thermoelastic waves which are then detected by the sensor. Detection sensitivities of 53 mv/MPa and a 10 kPa acoustic noise floor have been demonstrated giving excellent signal to noise ratios in a strong liquid absorber. Lower, but clearly detectable, signals in post mortem human aorta have also been observed. The performance and small physical size of the device suggest that it has the potential to perform remote in situ photoacoustic measurements in tissue.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Characteristics of photoacoustic displacement for silicon damaged by ion implantation

Abstract: A decrease in the thermal conductivity and the optical absorption length of a silicon wafer damaged by ion implantation causes an increase in the amplitude of photoacoustic displacement (PAD) on the sample surface. The behaviors of the PAD were investigated by thermoelastic analysis. The theoretical results indicate that the amplitude of PAD is approximately proportional to the square of the thickness of the damaged layer and is characterized by the ratio of the optical‐absorption coefficient to the thermal conductivity of the damaged layer. Experimental results, which were quantitatively measured by a highly sensitive PAD interferometer, can be explained well by the theoretical predictions. The thermal conductivity of the damaged layer by implantation above critical amorphization dose can be estimated to be 1–3 W/mK by comparison of the experimental data with the theoretical results.

A Two-Layer Model for the Laser Generation of Ultrasound in Graphite-Epoxy Laminates

Abstract: We previously reported the performances of a numerical simulation model [1] that calculates the mechanical displacements induced within a sample by the absorption of a laser pulse This model solves the heat diffusion and acoustic wave propagation equations over an orthotropic slab of finite thickness with the help of temporal Laplace and spatial 2D Fourier transformations The parallel and normal displacements predicted by the model were found to be in generally very good agreement with experimental data obtained on various samples in various excitation conditions Among these experiments, one consisted in the CO2 laser excitation of a graphite-epoxy sample We performed an optical study of the graphite-epoxy composite using FTIR photoacoustic spectroscopy [2] to determine the optical penetration depth spectrum of this material This study revealed that a thin (≈ 30 μm thick) epoxy layer covered the top graphite fiber sheet of the composite, and that the optical penetration depth of the CO2 radiation in the epoxy was about 20 μm Consequently, when a CO2 laser pulse impinges on the composite, all the radiation is absorbed in the epoxy layer, and it is easy to simulate this situation with the model, using the rigidity-expansion tensor [λ] of the epoxy for the generation and the rigidity tensor [C] of the composite for the propagation (see [1])

Photoacoustic determination of the absorption spectra in Hg1−xZnxTe alloys

Abstract: Hg1-ZnxTe alloys were prepared by inter-diffusion between the binary compounds at high temperatures. Photoacoustic spectroscopy is used to determine the optical absorption coefficient in the region of fundamental absorption. The energy bandgap is determined in the composition range 0.40

Optoacoustic detection of a kinetic-cooling effect in the liquid phase.

Abstract: A kinetic-cooling effect in the liquid phase has been observed by the use of pulsed optoacoustic spectroscopy. The magnitude of the optoacoustic signal in an aqueous Eu3+ solution, as measured from the absorption of pulsed light, was found to decrease when the excitation-laser wavelength was tuned to the transition from the thermally excited 7F1 state to the upper electronic 5D0 state of Eu3+ ion. This anomalous optoacoustic phenomenon is attributed to the kinetic-cooling effect.