Showing papers on "Photoacoustic spectroscopy published in 1998"

Characterization of porous GaP by photoacoustic spectroscopy: The relation between band-gap widening and visible photoluminescence

Abstract: Photoacoustic and Raman spectroscopy as well as photoluminescence (PL) measurements have been used to investigate the effect of band-gap widening in porous GaP. A correlation between a blueshift of the absorption edge observed during photoacoustic measurements, accompanied by PL emissions in the blue and ultraviolet regions, and the occurance of a vibrational mode related to surface phonons is shown. Based on a simple scattering model, the structure of porous GaP is assumed to have a cylinderlike shape.

Thermal diffusivity of lead iodide

Abstract: The thermal diffusivity and thermal conductivity have been determined for lead iodide PbI2, at room temperature, using the photoacoustic spectroscopy. The result shows a thermal diffusivity αs=(25.0±0.4)×10−3 cm2/s, a value very close to other semiconductors of current technological importance. The electrical conductivity is also measured and discussed.

A new angle into time-resolved photoacoustic spectroscopy: A layered prism cell increases experimental flexibility

Abstract: A new pulsed photoacoustic calorimetry cell that uses transmission of light through a pair of dovetail prisms is discussed. The layered prism cell (LPC) combines the enhanced time-resolution capabilities of the “layered” front-face irradiation geometry with the zero-background and broadband flexibility of the classical cuvette geometry. This work provides a phenomenological description of photoinduced pressure changes to yield an analytical expression to calculate the magnitude of the photoinduced acoustic pressure wave in a series of solvents. The mechanical to electrical conversion efficiency for an ultrasonic transducer coupled to the LPC is presented to provide a comparison of the experimentally observed photoinduced acoustic signal amplitudes to the empirically calculated acoustic signal amplitudes. An analysis of the background signals due to absorption and electrostriction of the media provides insight into the issues of sensitivity and limitations of pulsed photoacoustic experiments. The LPC provides several benefits to increase the flexibility of the photoacoustic spectroscopy: (1) greater sensitivity, (2) enhanced time resolution, and (3) the ability to obtain kinetic data in complex solvent mixtures. Under optically dilute conditions in the layered cell geometry, the acoustic transient time, τa, approaches zero because the photoinduced acoustic wave homogeneously expands against the walls of the photoacoustic cell. To demonstrate the unique capabilities of the LPC, rates of hydrogen abstraction by tert-butoxyl radical from solvent mixtures containing ethyl and methyl alcohol are presented.

Laser-based photoacoustic sensor and method for trace detection and differentiantion of atmospheric NO and NO2

Abstract: A laser-based photoacoustic sensor performs trace detection and differentiation of atmospheric NO and NO2 in order to obtain respective concentrations for NO and NO2 using photoacoustic spectroscopy. The device of the invention employs a single, pulsed laser having two outputs near 454 nm and 227 nm, respectively, to irradiate a sample vapor, the sample vapor being collected by a hand-held wand. A photoacoustic response from both NO and NO2 is detected using microphone detectors, is converted into electrical signals, and is analyzed using a microcomputer. The results of the analysis--the respective concentrations of NO and NO2 in the sample vapor--are displayed on a digital oscilloscope. Preferably, the electrical signals are amplified prior to analysis, and a boxcar integrator is used to sample and average the electrical signals during the analysis operation.

Advances in Grating-Based Photoacoustic Spectroscopy for the Study of Protein Dynamics

Abstract: Recent advances in transient grating spectroscopy are described in relation to extracting photoacoustic parameters. The time resolution for measurement of dynamically driven acoustics has been extended to the picosecond level. This improvement was achieved by adopting counter-propagating beam geometries and resolving the acoustic phase shift in analogy to phase modulation spectroscopy. At the other extreme, the dynamic range of this technique has been extended to milliseconds in order to follow dynamical processes central to biological functions. In addition, diffractive optics were used for generating the necessary excitation and probe beam geometries. A novel optical setup was developed which permits both the rapid exploration of fringe spacing dependencies in separating thermal from nonthermal contributions to the observed signal as well as heterodyne detection without active feedback. The latter capability significantly increases the signal-to-noise ratio and permits separation of the real and imaginary components from the nonlinear four-wave-mixing signal. Applications of these new methods are demonstrated by following the functionally relevant structural relaxation processes of heme proteins over 10 decades in time.

Production and detection theory of pulsed photoacoustic wave with maximum amplitude and minimum distortion in absorbing liquid

Abstract: The photoacoustic effect produced by laser-induced stress waves in liquids has found a variety of scientific and practical applications in industry, medicine, and environmental protection. The photoacoustic technique has the natural advantage of being less sensitive to scattering than traditional optical tools, thus offering increased accuracy and sensitivity. This paper aims to give a systematic outline of photoacoustic techniques, starting with the production and extending to the propagation and detection of photoacoustic waves. The focal point is the production of acoustic waves with maximum amplitude and minimum distortion. Maximum amplitude plays an essential role in photoacoustic spectroscopy and minimum distortion is the key to the determination of optical distribution and imaging in turbid media.

Effects of Bi2O3 Impurities in Ceramic ZnO on Photoacoustic Spectra and Current-Voltage Characteristics

Abstract: Photoacoustic (PA) spectroscopy is applied to study the optical absorption of ceramic ZnO doped with 1 mol% Bi2O3 sintered at various temperatures (700–1100°C). PA signal intensities plotted semilogarithmically reveal two individual regions below 3.0 eV (A: 3.0–2.7 eV; B: 2.6–2.4 eV). The steepness factor (slope of exponential optical absorption) obtained at region A decreases as the sintering temperature increases, indicating an increase of Bi2O3 segregation at particle surfaces and grain boundaries which might contribute to the interface states. The steepness factor at region B increases rapidly as the sintering temperature increases, indicating an occurrence of structural ordering by the Bi2O3 liquid phase sintering effect. The varistor effect (highly nonohmic behavior in the current-voltage characteristics) of ceramic ZnO doped with 1 mol% Bi2O3 is correlated with the characterization obtained by the PA method.

Photoacoustic blood cell detection and imaging of blood vessels in phantom tissue

Abstract: Photoacoustic (PA) signals were used to detect and image optically absorbing structures in highly scattering media. Tissue phantoms were constructed from 10 μm diameter carbon fibres and nylon vessels containing blood or a variable dilution of Evens blue that were situated within a I% dilution oflntralipid A PVdF piezoelectric hydrophone with a bandwidth of 70 MHZ was used to detect the PA signals. A Q-switched frequency doubled Nd: Y AG laser provided 1 O ns pulses at 1 O Hz repetition rate and 532 nm wavelength. The sensitivity of the PA technique was demonstrated by the photoacoustic detection of single red blood cells that were situated on a glass plate in a PBS buffer. PA images of the samples were constructed from scanned array detection of the acoustic signals. The imaging algorithm is based on scanning synthetic delay-and-sum focus forming. The transducer directivity was simulated, verified and used to determine the weight factors of the algorithm. Threads and capillaries with various diameters situated at various depths have been imaged. The lateral resolution is limited by the diameter (200 μm) of the transducer. In the experimental images is shown that the depth resolution can be less than 10 μm. The accuracy of the source location reconstruction depends both on the method of signal processing and the PA signal duration. The PA images of larger blood vessels in phantom tissue show only the front and back side of these vessels due to the relatively low optical penetration depth in whole blood, acoustic interference and acoustic reflection at the walls of the vessels. The photoacoustic image of optically absorbing characters located at a depth of 1.8 mm and illuminated through 2 mm Intralipid dilution is presented. The results of this study are promising for the development ofphotoacoustic tomography. Keywords: Photoacoustics, imaging, phantom tissue, tomography, blood, piezoelectric detection.

Direct measurement of carbonate content in soil samples by means of co laser infrared photoacoustic spectroscopy

Abstract: CO laser infrared (IR) photoacoustic spectroscopy (PAS) at 1801 cm-1 was used to determine the content of carbonate in various soils. The PA signal is linearly proportional to the content of carbonate within the 0-30% (w/w) range; the 2% (w/w) limit of detection (LOD) is adequate for the application in practice.

On the Use of the Photoacoustic Technique for Monitoring the Doping Concentration Dependence of the Surface Recombination Velocity

Abstract: In this paper we study the surface recombination velocity of GaAs by means of the photoacoustic technique (PA). By fitting the experimental frequency dependence of the PA signal phase we were able to evaluate this parameter. The experiment was carried out in GaAs samples prepared by liquid phase epitaxy and doped with different concentrations of Ge. It is shown that the PA technique is sensitive to changes of the surface states originated by the doping concentration levels and it is demonstrated that this technique gives a promised non-invasive and non-destructive method to measure and evaluate the surface recombination velocity.

Deuterium Isotope Effects on the CH Stretching Overtone Spectrum of Toluene-α-d1

Abstract: The room-temperature vapor phase overtone spectrum of toluene-α-d1 has been recorded in the CH stretching regions corresponding to ΔvCH = 2−7 The vibrational overtone spectra are recorded by conventional near-infrared spectroscopy and by intracavity titanium:sapphire and dye laser photoacoustic spectroscopy Absolute oscillator strengths are obtained from the conventional spectra, and relative oscillator strengths within a given overtone, from both the conventional and photoacoustic spectra The aryl region of the spectrum is nearly identical to the aryl region of the spectrum of toluene-d0 and can be understood on the basis of two nonequivalent aryl local modes The methyl band differs markedly from the methyl band in toluene-d0 in relative intensity, in line width, and in structure We use an anharmonic oscillator local mode model and an ab initio dipole moment function to calculate oscillator strengths for the aryl and methyl transitions As was the case for toluene-d0, these simple calculations show

DSP technique for photoacoustic spectroscopy (PAS) sample pulse response for depth profiling

Abstract: A technique for extracting the impulse response of a sample of interest includes corresponding measurements made with the sample of interest and a reference sample. At each of a series of steps in an FT-IR spectrometer, the sample of interest is illuminated with an excitation pulse of infrared radiation, acoustic signals having a time dependence oS (t) arising from the excitation pulse are captured, and a Fourier transform OS of oS (t) is computed. At each of a series of steps in the FT-IR spectrometer, the reference sample is illuminated with an excitation pulse of analytic radiation, acoustic signals having a time dependence OR (t) arising from the excitation pulse are captured, and a Fourier transform OR of oR (t) is computed. For each step, an inverse Fourier transform of the ratio OS /OR is computed to provide a series of values s(ti) for a series of times ti. These values s(ti) represent the impulse response s(t) of the sample of interest for the mix of optical frequencies for that retardation value. Interferograms are processed to provide photoacoustic spectra.

Multicomponent trace-gas analysis by three intracavity photoacoustic cells in a CO laser: observation of anaerobic and postanaerobic emission of acetaldehyde and ethanol in cherry tomatoes.

Abstract: Three serial photoacoustic cells are employed within the cavity of a liquid-nitrogen-cooled CO laser to monitor on-line trace-gas concentrations. Multicomponent gas analysis is performed on sequential repetitive measurements of ethylene, acetaldehyde, CO2, ethanol, and H2O. To demonstrate the high sensitivity of the laser photoacoustic detector for the biologically interesting gases, acetaldehyde (0.1-parts per billion in volume detection limit) and ethanol (10 parts per billion in volume), we follow the time-dependent release by cherry tomatoes during changing aerobic-anaerobic conditions.

A pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water

Abstract: The development of a pulsed photoacoustic instrument for the detection of crude oil concentrations in produced water is reported. The instrument utilizes fiber-optic delivery of pulsed diode laser sources to an in-line sensor head, incorporating a gradient-index lens for optical delivery into the flow line and a piezoelectric transducer for acoustic detection. A crude oil concentration detection range of 5–8000 parts per million in water is demonstrated. The sensitivity of response was confirmed during preliminary field testing and is independent of various interfering analytes, flow rate, and droplet size.

Photoacoustic spectroscopy of layered samples: Phase-detection technique

Abstract: Depth profiles of double layer biological samples, obtained by photoacoustic spectroscopy, have been studied using the phase-resolved method. The application of the method was demonstrated by insulating out the spectra of the cuticle and the pigment layers of a lettuce leaf. Using this method, we were able to monitor the phase shift between the cuticle and the pigment layers as a function of the time elapsed after herbicide spraying. The temporal changes occurring in lettuce leaves under the action of dehydration were also investigated.

Fourier transform infrared photoacoustic spectroscopy of polymeric laminates

Abstract: Photoacoustic (PA) magnitude and phase spectra of, respectively, polyethyleneterephthalate/polyethyleneterephthalate (PET/PET), polypropylene/polyethyleneterephthalate (PP/PET), and polyethyleneterephthalate/polypropylene (PET/PP) laminates are presented. The extension of the Rosencwaig–Gersho theory to a double-layered system introduced by N. C. Fernelius [J. Opt. Soc. Am. 70, 480 (1980) and J. Appl. Phys. 51, 650 (1980)] is applied to compare the experimental data with simulated PA magnitudes and phases. The simulations for the double-layered systems indicate that the absorption coefficient β at the optically transparent region of the single layers—estimated from photoacoustic data—strongly influences the phase spectra of the polymeric laminate. In particular, the relative magnitude of these residual absorptions in the two layers is important. The optical absorption spectra of the polypropylene and polyethyleneterephthalate single layers (accessible from infrared absorbance and infrared photoacoustic sp...

Biomass monitoring using the photoacoustic effect

Abstract: Photoacoustic spectroscopy was evaluated as useful for the determination of the number of cells in a fermentation. The influence of scattering particles and gas bubbles on the measurement was investigated. The photoacoustic method is much less influenced by scattering particles than the turbidity method. A fermentation of cyanobacteria was used to compare the photoacoustic method with turbidity, fluorescence and cell counting. The number of cells is represented by the photoacoustic data especially in the exponential and stationary phase with greater accuracy than the turbidity. The necessary optical set-up at the fermenter vessel is much simpler. It requires no probes incorporated in the biological system. By choosing a specific light wavelength specific cell products can be monitored.

Step-Scan FT-IR Photoacoustic Studies of a Double-Layered Polymer Film on Metal Substrates

Abstract: The influence of various metal substrates on the photoacoustic response of a double-layered polymer coating (polypropylene on a polybutylacrylate-adhesive) is demonstrated. The extended Rosencwaig-Gersho theory by Fernelius for a double-layered sample is used to discuss the step-scan Fourier transform infrared photoacoustic spectroscopy (FT-IR PAS) experimental results. A clear dependence of the photoacoustic phase phi on the thermal diffusivity alpha of the metal substrate is found. Therefore, the quality of the thermal contact between the polymer coating and the metal substrate influences the photoacoustic phase spectrum.

Photoacoustic spectroscopy sample cells and methods of photoacoustic spectroscopy

Abstract: The invention encompasses photoacoustic apparatuses and photoacoustic spectrometry methods. The invention also encompasses sample cells for photoacoustic spectrometry, and sample cell/transducer constructions. In one aspect, the invention encompasses a photoacoustic spectroscopy apparatus, comprising: a) a sample reservoir and an acoustically-stimulable transducer acoustically coupled with the sample reservoir, the transducer comprising a detector surface having a substantially planar portion; and b) a beam of light configured to be directed through the sample at an angle oblique relative to the substantially planar portion of the detector surface to generate sound waves in the sample. In another aspect, the invention encompasses a photoacoustic spectroscopy sample cell, comprising: a) a first block of material having opposing front and back surfaces, the front surface comprising a substantially planar portion configured to be against a sample and the back surface comprising a substantially planar portion configured to be joined to a transducer, the back surface being parallel to the front surface; and b) a pair of opposing side surfaces joined to opposite ends of the front and back surfaces, one of the opposing side surfaces being configured for passage of light therethrough and extending at a first oblique angle relative to a plane containing the substantially planar portion of the front surface.

Open-ended photoacoustic cells: application to two-layer samples using pulse laser-induced photoacoustics.

Abstract: To measure the difference between samples in situ, two types of open-ended photoacoustic cells were constructed. One type was a differential open-ended photoacoustic cell with a differential microphone, which connected two equivalent open-ended cell chambers through two waveguides. The other design used single open-ended photoacoustic cell chambers, each with an electric condenser microphone. A pair of these was used to suppress background signal by subtraction of the signals. The performance of these cells was compared to that of a conventional photoacoustic apparatus with a chopper. Detection limits of Sudan red for those cells were about 1.29 and 2.35 ng for a differential open-ended cell and a single open-ended cell, respectively. The photoacoustic cells were then applied to the pulse mode operation, using a nitrogen laser as a pulse light source. The laser beams were led to the photoacoustic cells through quartz optical fibers. Using this instrument setup, depth profiling analysis was carried out for...

Photosynthetic energy storage and oxygen evolution determined through an open photoacoustic cell technique

Abstract: Recently, a new open photoacoustic cell (OPC) has been developed for the determination of both optical and thermal properties of the sample. The OPC can also be applied for the determination of photosynthetic parameters as energy storage and oxygen evolution for in vivo and in situ plant leaves. However, the membrane of the microphone used in this cell inserts a spurious component in the photoacoustic signal of non-opaque samples such as plant leaves. The membrane signal is added to the sample signal, making the analysis more complicated. In the present work, we discuss this problem and propose a simple method to eliminate it.

Influence of thermal and optical properties on the visibility of liquid-crystal phase transitions in photoacoustic spectroscopy

Abstract: The thermal, optical light absorption and light scattering properties of a liquid crystal can be distinguished and separately studied by a single technique, pulsed photoacoustic spectroscopy (PPAS), by using different excitation/detection configurations. The advantages of performing complementary thermal and optical measurements without modifying the sample environment are demonstrated in the case of a typical liquid crystal: 4-octyl-4′-cyanobiphenyl (8CB). Results show that light scattering can strongly influence the PPAS response signal through the light absorption profile, especially at phase transitions subjected to small changes in thermal properties, such as the Nematic/isotrope phase transition.

Photoacoustic spectroscopy of corrosion products of copper formed in tropical environments

Abstract: The evolution of thermal properties of copper samples, as a function of exposure time to a corroding atmosphere, is investigated in detail. The samples were exposed to a natural marine environment of the Yucatan Peninsula in Mexico and were analyzed without any additional pretreatment. The study is mainly based upon the measurements of thermal and optical properties of the sample, using the non-destructive photoacoustic technique. The thermal diffusivity was measured with a conventional open photoacoustic cell arrangement, whereas the thermal effusivity was determined using a new experimental configuration.

A new method of reconstruction of thermal conductivity-depth profiles from photoacoustic or photothermal measurements

Abstract: A new method to reconstruct the continuously inhomogeneous thermal conductivity-depth profiles of solid samples from photoacoustic or photothermal measurements by employing a hybrid of a Newton-like iterative method and a regularization method is presented. Three different error functions are defined in order to seek the proper regularization parameters automatically. Generally, the photoacoustic or photothermal signal is proportional to the temperature variation of the sample's surface, therefore the profile's reconstruction requires only measurements of the photoacoustic or photothermal signal over a sufficiently wide frequency range. The numerical experiments demonstrate that this algorithm is effective and stable for the reconstruction of profiles from the measured data with random noise.

Processing of weak photothermal signals from the optothermal window (ow) cell; application to the determination of ammonium ion in water

Abstract: The optothermal window (OW) cell is a compact photothermal device similar to an open photoacoustic cell. Using the c.w. He-Ne laser for excitation, and a piezoelectric transducer, the concentrations of ammonium ion (range 2 to 400 mmol/m3) in water were determined via the measurement of the amplitude of the OW signal. Although theoretical signal phase saturates in case of weak absorption, it was demonstrated here that at specific modulation frequencies the uncorrected signal phase carries spectrophotometric information as well. This is due to the fact that the phase of the background signal is shifted relative to that of the absorption signal. Simpler calibration procedure is an advantage in phase measurements.

Laser Induced Photoacoustic Spectroscopy Applied to a Study on Colloid Formation Processes

Abstract: Laser induced photoacoustic spectroscopy (LPAS) was applied to determination of the concentration and size of colloid particles in aqueous solutions. The intensity of photoacoustic signals for mono-dispersed polystyrene particles in different sizes was linearly increased with an increase of the polystyrene concentration, while the proportionality was dependent on the particle size. Furthermore, dynamic measurements of colloid-particle growing of gold sols produced by the reaction of aurate ions (AuCLr) with sodium citrate was made by means of LPAS. The intensity of photoacoustic signals for polystyrene and gold particles was successfully calculated as a product of the number of particles and the absorption cross section per particle based on the Mie's light scattering theory. With this technique, the coagulation of Tc(IV) colloids was observed successfully.