Showing papers on "Photoacoustic spectroscopy published in 1989"

The Determination of Water Contents in Various Powdered Foods by Photoacoustic Spectroscopy (PAS)

Abstract: 単光束型の光音響分光測定装置を作成し小麦粉(薄力粉,強力粉),コーンスターチ,サツマイモデンプンに含まれる水分量を測定した.照射光にはハロゲンランプ光,および2種類のフィルターを透過した光を使用した.水分量の実測に先立ち,光源光の変調周波数ωと光音響信号強度Sとの関係を求あた.この結果, S∝ω-1.35となり,理論関数とほぼ一致した.さらに粉体試料を容器に充填する条件を求めたが,充填圧力が3.12kPa以下では,充填条件によらず一定の音響信号強度を得た.本実験で使用した粉体試料においては,水分量と音響信号強度は粉体の種類によらず同じ傾きを持つ直線式として表示することが可能であった.従って,光音響信号の強度をもとに水分を定量をすることが可能である.

Photoacoustics, Photothermal and Photochemical Processes in Gases

Abstract: 1. Principles of Photoacoustic and Photothermal Analysis.- 1.1 Photoinduced Processes and Detection.- 1.1.1 Variety of Processes.- 1.1.2 Detection Methods.- 1.1.3 Literature Review.- 1.2 Principles of Photoacoustics.- 1.2.1 Basic Considerations.- 1.2.2 Time and Frequency Domain Analysis.- 1.2.3 Resonant and Nonresonant Operation.- 1.2.4 Detection Devices.- 1.3 Recent Advances and Developments.- 1.3.1 Theory.- 1.3.2 Fundamental Constants and Thermophysical Properties.- 1.3.3 Kinetic Processes.- 1.3.4 Applications in Trace Analysis and Pollution Monitoring.- 1.4 Summary and Outlook.- References.- 2. Theoretical Foundation of Photoacoustics in the Frequency and Time Domains.- 2.1 The Equations of Linear Gas Dynamics.- 2.2 Theory of the Cylindrical Optoacoustic Resonator.- 2.3 The Pulse Source Thermal Lens Effect.- 2.4 Thermal Recovery.- 2.5 Short Time-Scale Measurements.- 2.6 Improved Models for the Pulsed Source Thermal Lens.- 2.7 Optics of the Thermal Lens.- 2.8 Conclusions.- References.- 3. Thermal Lensing.- 3.1 Introduction.- 3.1.1 Survey of Approaches.- 3.1.2 Configurations.- 3.2 Experimental.- 3.2.1 Laser Sources.- 3.2.2 Probe Lasers.- 3.2.3 Laser Beam Tailoring.- 3.2.4 Laser Beam Alignment Techniques.- 3.2.5 Beam Splitting and Combining.- 3.2.6 Sample Cells.- 3.2.7 Signal Detection.- 3.2.8 Signal Retrieval.- 3.3 Theory.- 3.3.1 The Pulsed Source Experiment.- 3.3.2 The Modulated Source Experiment.- 3.3.3 The Transverse and Collinear Photothermal Lens.- 3.4 Applications.- 3.4.1 Determination of Energy Transfer Rate Constants.- 3.4.2 Transport Phenomena.- 3.4.3 Photochemistry.- 3.4.4 Surface Phenomena.- 3.4.5 Spectroscopies.- 3.4.6 Photoacoustic Diagnostics.- 3.4.7 Laser Physics.- 3.5 Summary.- References.- 4. Spherical Acoustic Resonators.- 4.1 Introduction.- 4.2 Basic Theory.- 4.3 Steady-State Response.- 4.4 Wave Modes.- 4.5 Thermal and Viscous Boundary Layers.- 4.6 Precoundensation Effects.- 4.7 Bulk Dissipation and Relaxation.- 4.8 Shell Motion.- 4.9 Imperfect Spherical Geometry.- 4.10 Ducts and Slits in the Shell Wall.- 4.11 Measurement of the Speed of Sound.- 4.12 Thermophysical Information from the Speed of Sound.- References.- 5. Laser Excitation of Acoustic Modes in Cylindrical and Spherical Resonators: Theory and Applications.- 5.1 Introduction.- 5.1.1 History.- 5.1.2 Recent Developments.- 5.1.3 Scope of Review.- 5.2 Optical Excitation of Acoustic Modes.- 5.2.1 General Considerations.- 5.2.2 Acoustical Resonances in a Cylinder.- 5.2.3 Acoustical Resonances in a Sphere.- 5.2.4 Accuracy of the Resonance Method.- 5.3 Experimental Method.- 5.3.1 Apparatus.- 5.3.2 Temperature Measurement.- 5.3.3 Computer Control.- 5.4 Theory.- 5.4.1 General Remarks.- 5.4.2 Basic Equations.- 5.4.3 Kinetics in the Frequency Domain.- 5.4.4 Helmholtz Equations.- 5.4.5 Boundary Conditions.- 5.4.6 Solutions.- 5.5 Applications.- 5.5.1 Chemical Reaction.- 5.5.2 Energy Transfer.- 5.5.3 Thermophysical Properties and Fundamental Constants.- 5.5.4 Condensation Effects.- 5.5.5 Intracavity Experiments.- 5.6 Conclusions.- References.- 6. Application of the Photoacoustic Effect to Studies of Gas Phase Chemical Kinetics.- 6.1 Pulsed Excitation.- 6.1.1 Signal Description.- 6.1.2 Experimental Results.- 6.2 Continuous Excitation.- 6.2.1 Theory.- 6.2.2 Experimental Results.- 6.3 Nonlinear Effects.- 6.4 Chemical Amplification.- 6.5 Unimolecular Reactions.- 6.6 Direct Detection of Reactants and Products.- 6.7 Flames, Combustion, and Other Applications.- References.- 7. Atmospheric and Exhaust Air Monitoring by Laser Photoacoustic Spectroscopy.- 7.1 Introduction.- 7.1.1 Air Pollution.- 7.1.2 Methods for Monitoring Gaseous Pollutants.- 7.2 Basic Principles of Trace Gas Detection by Laser Photoacoustic Spectroscopy.- 7.2.1 Generation of Photoacoustic Signal.- 7.2.2 Main Characteristics.- 7.3 Experimental Arrangements for Laser Photoacoustic Spectroscopy.- 7.3.1 Tunable Lasers.- 7.3.2 Modulation Techniques.- 7.3.3 Cell Design.- 7.3.4 Detection Schemes.- 7.4 Previous PA Studies on Trace Gases.- 7.4.1 Measurements on Certified Gases.- 7.4.2 Measurements on Real Air Samples.- 7.5 Stationary CO-Laser PA System.- 7.5.1 Spectral Range.- 7.5.2 Experimental Arrangement.- 7.5.3 PA Measurements on Vehicle Exhausts.- 7.6 Mobile CO2-Laser PA System.- 7.6.1 Spectral Range.- 7.6.2 Experimental Arrangement.- 7.6.3 Trace Gas Measurements.- 7.7 Conclusion.- References.- 8. Trace Detection in Agriculture and Biology.- 8.1 Photoacoustic Detection of Ethylene Production in Plants.- 8.1.1 The Choice of Ethylene.- 8.1.2 Experimental Setup.- 8.1.3 Ethylene Production During Senescence of Carnation and Orchid Flowers.- 8.1.4 Growth of Docks (Rumex Species) Under Flooded Conditions.- 8.2 Comparison of Chlorophyll Fluorescence and Photoacoustic Transients in Spinach Leaves.- 8.2.1 Photosynthetic Energy Conversion, Chlorophyll Fluorescence and Photoacoustic Transients in Spinach Leaves.- 8.2.2 Experimental Setup.- 8.2.3 Results and Discussion.- 8.3 Potentialities of Photoacoustic Sensing.- 8.3.1 Potential Use of Photoacoustic Sensing in Greenhouses, Stables, Fumigation Chambers, Storage Compartments and in Meteorological Studies.- 8.3.2 Leaf Chamber for Adsorption Studies.- 8.3.3 Olfactory Psychophysics.- 8.3.4 Aerobic Meat Spoilage.- 8.3.5 Heat Pipe Cell.- 8.3.6 Soilless Growth.- 8.4 Conclusion.- References.

Atmospheric and Exhaust Air Monitoring by Laser Photoacoustic Spectroscopy

Abstract: The application of laser photoacoustic spectroscopy to air pollution monitoring is reviewed and compared to other spectroscopic and nonspectroscopic techniques. Emphasis is put on sensitivity, selectivity and on temporal resolution of the detection scheme. The problem of interfering absorptions by different trace gases present in multi-component mixtures is studied in detail and a new mathematical procedure for the analysis of measured photoacoustic (PA) spectra is presented. The versatility of the PA method is demonstrated by our studies performed with computer-controlled PA systems on ambient air as well as on motor-vehicle and industrial exhausts.

Depth Profiling and Signal Saturation in Photoacoustic FT-IR Spectra Measured with a Step-Scan Interferometer

Abstract: A commercial step-scan FT-IR spectrometer has been used to obtain photoacoustic spectra of carbon black, coals, polymers, and clays. Variation of the external modulation frequency from 40 to 400 Hz shows that saturation affects many of the spectra. Nevertheless, comparison of spectra of oxidized coal with spectra of fresh coal obtained at the corresponding modulation frequencies yields results implying successful depth profiling. Several suggestions for further experiments are also presented.

Quantitative depth profiling with photoacoustic spectroscopy using a new approximation method based on inversion of the Laplace transform

Abstract: A new and simple approximation method for quantitative depth profiling using photoacoustic spectroscopy (PAS) is described. In the method, an approximate calculation of the inversion of the Laplace transform is adopted in order to simplify the calculation of the depth‐dependent optical‐absorption coefficient from photoacoustic frequency responses. Some results of simulations shown demonstrate the effectiveness of this method. The method has been applied to the depth profiling of the practical sample in which the optical‐absorption coefficient varies continuously with the depth from the surface. The result is qualitatively consistent with microscopic observations of the cross section of the specimen. This approximation could be useful not only for the depth profile with PAS but also for the general depth profile calculations based on the Laplace transformation.

Surface and depth-profile analysis using FTIR spectroscopy

Abstract: The introduction of Fourier Transform techniques and the increasing use of computers in infrared spectroscopy has made new techniques of investigation available to the spectroscopist, such as photoacoustic spectroscopy (PAS) and IR microscopy. These methods now complement the techniques of specular reflectance and attenuated total reflectance. Thin films on metals, sometimes with a thickness of much less than a micron, can be studied by various specular reflectance methods. The physical basis of the attenuated total reflectance technique (ATR) leads to a penetration of the radiation in the order of a few microns. It is, therefore, especially suitable for the investigation of surfaces and of layers close to the surface. By changing the modulation frequency of the IR radiation, i.e. the mirror velocity of the FTIR spectrometer, photoacoustic spectroscopy (PAS) can be employed to study layers at various depths below the surface of a sample. Therefore, this technique allows depth-profile analysis, so that PAS reveals itself as a complementary method to attenuated total reflectance spectroscopy. Samples with inhomogeneous profiles, e.g. laminated polymer films, can often be prepared as microtome slices perpendicular to the layered structure. Using infrared microscopy it is now possible to investigate different regions of the cross-section easily. The size of the regions that can be studied in this way may be as small as a few microns.

Photoacoustic investigation of iodine‐doped polystyrene

Abstract: The kinetics of iodine doping of atactic polystyrene films is investigated using photoacoustic spectroscopy. The changes in the photoacoustically measured physical properties, such as nonradiative relaxation time and thermal diffusivity, are present as a function of the doping time. The results show strong evidence that an order‐disorder transition is taking place as a function of the doping time. The suggested order‐disorder transition is also evident in the dielectric constant measurements of the doped films.

Method and apparatus for the detection of a gas using photoacoustic spectroscopy

Abstract: In a process for the detection of a first gas in a gas mixture comprising a second gas, the absorption spectre of which interferes with the absorption spectre of the first gas, a photoacoustic measurement is carried out in the presence of a third gas which in combination with the first or the second gas exhibits kinetic cooling. During measurement the gas mixture is influenced by pulsating laser light having a constant repetition frequency where the frequency of the laser light is varied gradually. The measurement comprises at least one detection of the phase of the photoacoustic signal as a function of the laser light frequency. The invention further relates to an apparatus for carrying out the invention.

Phase-resolved photoacoustic spectroscopy and EPR investigation of MnO2- and CoO-doped soda-lime glasses.

Abstract: A discussion on the use of the phase shifts of the photoacoustic signal of different constituents of a composite sample for resolving their individual spectra is presented. It is experimentally shown that as long as we are interested in a qualitative analysis the method is simple and fast. For a quantitative analysis in which the nonradiative relaxation time and the characteristic diffusion time within the optical-absorption depth are sought, the method presents some limitations. This is demonstrated using MnO~and CoO-doped soda-lime —silica glass samples.

Simulation of photoacoustic IR spectra of multilayer structures

Abstract: In photoacoustic spectroscopy a sample in a closed gas cell is heated by periodically modulated light. When thermal waves generated by the warm spots inside the sample reach the surface they heat up the adjacent gas. This causes periodic pressure variations which are detected by a microphone. We present a matrix method which enables us to calculate the surface temperature of a multilayer structure with any number of homogeneous lamellae of any optical and thermal properties. This method is based on the multiple reflections and interferences of thermal waves inside the system of lamellae. Photoacoustic spectra simulated by this method reproduce the measured spectra. This is demonstrated for a mylar (polyethyleneterephthalate, PETP) foil coated with a thin antimony layer.

Concept, design, and use of the photoacoustic heat pipe cell

Abstract: A resonant photoacoustic cell suitable for studies of liquid samples having low vapor pressures has been developed and tested. The cell, the working of which is based on that of the heat pipe, is of a simple, compact design; its operational temperature range is limited only by the choice of working fluid and the material used to construct the cell. The feasibility of this novel‐type cell has been demonstrated by obtaining the absorption spectrum of geraniol C10H18O at 403 K in the spectral region covered by the CO2 laser emission.

Thermal Lens Measurements in Liquids on a Submicrosecond Time Scale

Abstract: The use of the thermal lens method is shown to be quite suitable for kinetic studies of quenching on a submicrosecond time scale. The lower limit of time resolution that can be achieved is determined by the acoustic transit time, τa, in the medium. A thermal lens signal with a 100-ns time constant due to the quenched triplet state of benzophenone is readily measured. The thermal lens method is superior to the photoacoustic (PA) method in the breadth of the accessible time range, and in the significantly fewer measurements required to obtain accurate data, including no requirement for a reference sample; it is also less sensitive to geometrical and laser power requirements than is the PA method.

Infrared Photoacoustic Spectroscopy of Carbon Black Filled Rubber: Concentration Limits for Samples and Background

Abstract: Infrared spectroscopy studies of the cure chemistry, state of cure, and surface bloom on rubber materials have always been limited by the presence of carbon black in samples. One of the modern methods for recording infrared spectra of solid samples is photoacoustic detection Fourier transform spectroscopy. It has been demonstrated in the past that surface-segregated species can be identified with this technique, but the results are complicated by the presence of carbon black, which limits the optical depth. As for the study of bulk chemistry, photoacoustic detection does not require that the sample be infrared transparent, and the method can be used with samples containing as much as a 15 wt % carbon. At loadings higher than 30 wt %, the material becomes a total absorber and can be used to record an instrument background spectrum.

Laser Photoacoustic Spectroscopy

Abstract: Fig. 7 — Principle of the photoacoustic effect. Photoacoustic spectroscopy (PAS), also known as optoacoustic spectro­ scopy, was pioneered by Alexander Graham Bell more than a century ago. The advent of the laser strongly stimu­ lated research and applications of this technique and the interest is such that international topical meetings on photo­ acoustic and photothermal phenomena are now held biennially. The photoacoustic (PA) effect is es­ sentially an energy-conversion process. When a sample (solid, liquid or gas) is irradiated by a laser beam or some other radiation source, part of the absorbed energy is converted into translation energy of the molecules by radiation­ less transitions. It is this de-excitation channel which is responsible for heat production within the sample although secondary reactions may also play a role (see Fig. 1). If either the incident radiation or the absorption by the sam­ ple is modulated, the periodic heating finally results in a pressure modulation. The PA signal thus originates from the thermal response of the sample to the absorbed energy detected as pressure fluctuations acoustically. Owing to ther­ mal diffusion processes the magnitude of this signal depends not only on the thermal and optical characteristics of the sample but also on the nature of the coupling mechanism between sample and detector. Consequently the cou­ pling medium plays an important role and one can differentiate between the light-to-heat conversion and the heatto-sound conversion efficiencies.

High-sensitivity, interference-free, Stark-tuned CO2 laser photoacoustic sensing of urban ammonia

Abstract: Low‐concentration (few ppbv), interference‐free, on‐line photoacoustic detection of ambient ammonia (NH3) is reported by Stark tuning the Q(J=5, K=5, M=5) NH3 absorption line into resonance with the CO2 laser. Measurements were made over a range of total pressure between 600 and 50 mbar.

The intensity of the 105–000 transition of HCN

Abstract: We have remeasured the intensity of the anomalously strong 105–000 band in HCN using long‐path length absorption and a cw dye laser. Our initial measurement of this intensity using calibrated intracavity photoacoustic spectroscopy was challenged by a low‐resolution photoacoustic spectrum which gave a ratio of intensity of the 105–000 to the 006–000 band that was close to the ratio of 1.5 predicted by theory. Our present result of 16.7 cm/mol, is 20% larger than our earlier result, yielding an intensity ratio of the 105–000 to the 006–000 of 7, which increases the disagreement with theory.

Photoacoustic detection of rapid-scan Fourier transform infrared spectra from low-surface-area solid samples

Abstract: We have demonstrated that the PAS cell volume, and the state of the background material as well as the interferometer mirror velocity and the cell gas composition, must be controlled when one is recording spectra of solid samples. An optically thick, totally absorbing material with a volume matching that of the sample is needed in order to properly normalize spectra of solid samples. With the proper detection bandwidth, mirror speeds of up to 0.181 cm/s can be used with helium as a transfer medium, for the cell specified. The improvement attained at high frequency by using helium can be as much as 10-fold over that obtained with air. The Helmholtz design of the cell produces a resonance at 1.4 kHz with air and 2.65 kHz with helium. These resonances are also affected by the volume and composition of the gas in the sample chamber. Thus, it is essential to select an appropriate background sample for the purpose of normalizing spectra.

A photoacoustic study of LiAl5O8 doped with several percent of iron(III)

Abstract: The behavior of the non-radiative relaxation time τ and the characteristic diffusion time τ β of excited state of Fe 3+ in LiAl 5 O 8 are investigated using photoacoustic spectroscopy. The results are interpreted in terms of the frequency dependence predicted by the theory of Rosencwaig and Gersho.

Photopyroelectric Spectroscopy (P2ES) of a-Si:H Thin Semiconducting Films on Quartz

Abstract: Photopyroelectric Spectroscopy has proven to be a sensitive qualitative [1] and quantitative [2] technique for thin film spectroscopic applications. An important feature of this back-surface detection technique, not shared with the more conventional front-surface photothermal detection methods (Photothermal Deflection Spectroscopy, PDS; and Photoacoustic Spectroscopy, PAS) is its ability to measure directly and separately two independent spectrally-varying parameters: the optical absorption coefficient [3] and the nonradiative quantum efficiency. PDS of thin semiconducting films of amorphous hydrogenated Si [4] readily yields information about the product of the optical absorption coefficient, α(λ), and the nonradiative quantum efficiency, η(λ). The standard assumption is, however, that η(λ) is not a sensitive function of the exciting photon energy. This assumption is generally wrong, and nonradiative quantum efficiencies have been found photoacoustically to vary by one order of magnitude [5] across the optical gap in Ge doped As 2 Se 3 chalcogenide glasses. PAS yields amorphous thin film spectra similar to PDS [6]. The working assumption has been that PA spectra are essentially accurate above the optical gap, as η(λ) is expected to be independent of photon energy. Kitamura et al. [5] were able to derive extended η(λ) spectra of (As 2 Se 3 )100-x Ge x glasses upon combining PA spectra with optical absorption coefficient information obtained in an independent spectrophotometric experiment using ordinary polished bulk samples. These authors, however, were not able to guarantee that the glasses and the bulk samples had the same (or even nearly similar) α(λ) spectra.

Photoacoustic EXAFS of Solid Phase

Abstract: Fine structures observed in X-ray photoacoustic spectra of various solids were compared with those in the absorption spectra. For a precise comparison, the photoacoustic spectra were corrected for energy-depending X-ray absorption of gas existing in the optical path. Finally corrected photoacoustic spectra showed a leveled or decreasing trend with increasing photon energy. Good correspondence between the X-ray photoacoustic spectra and the absorption spectra further confirmed that the fine structures in the photoacoustic spectra are identical with the extended X-ray absorption fine structures (EXAFS).

Fourier-Transform Infrared Photoacoustic Spectroscopy of Synthetic Polymers

Abstract: Although not yet widely used in analytical laboratories, the tremendous development of low-cost FTIR spectrometers should allow photoacoustic spectroscopy to become a commonly used technique for polymer analysis. The ability to determine depth profiles in a broader range than ATR makes FTIR-PAS a particularly attractive method to follow the effect of chemical treatments or the penetration of coatings in polymeric materials. The possibility to obtain a spectrum from as-received samples is also of great interest to the analytical chemist. The only limitation of this technique is the need to use a great number of scans to enhance the signal-to-noise ration. However, continued improvements in PA cell design should increase the overall sensitivity, leading to shorter data-collection times.

Determination Of Fluorescent Quantum Yields Using Pulsed-Laser Photoacoustic Calorimetry

Abstract: Pulsed-laser, time-resolved photoacoustic calorimetry is a technique which provides information on nonradiative channels of deactivation of molecular excited states. It is thus complementary to fluorescence techniques, and can provide information on fluorescent quantum yields. This paper describes pulsed photoacoustic calorimetry, how the data can be deconvolved to give dynamic and enthalpic information, and how this information can be used to determine fluorescent quantum yields.

Time-resolved laser optogalvanic spectroscopy of iodine in a radio frequency discharge

Abstract: Pulsed laser optogalvanic (LOG) spectra of iodine vapor in a ∼32 MHz rf discharge were excited at 14 900–17 100 cm−1. Two distinct, time‐resolved components were observed: a fast component, synchronous with the laser pulse, width ∼1 μs, followed by a slow component, width ∼100 μs, delayed relative to the laser pulse. The fast component exhibits atomic transitions of I(I) and I(II). The slow component reproduces the B←X photoacoustic (PA) spectrum of molecular I2. The signal delay of the slow component accords with the velocity of acoustic waves in iodine vapor. The rf electrode region is the ‘‘sensitive’’ region where the acoustic wave generates the slow LOG signal. Two mechanisms of signal generation and propagation are involved. The fast signal originates in a two‐step laser photoionization of plasma‐excited atoms, the first‐step being resonant, and/or in changes of the atomic collisional ionization rates. These processes occur on time scales shorter than the laser pulse and generate an ‘‘instantaneou...

Laser-Based Photoacoustic Densitometer for Two-Dimensional Scanning of Thin-Layer Chromatographic Plates:

Abstract: Laser-based photoacoustic densitometry was applied to two-dimensional analysis in thin-layer chromatography. An acousto-optic device was used to provide both intensity modulation and a rastering scan. The spatial resolution is 25 × 60 spots for an area of 25 × 50 mm on the thin-layer chromatographic plate. Three detection modes, normal modulation, resonant modulation, and unmodulated rapid scan modes were compared. Detection limits in the most sensitive mode and in the fastest mode are 350 pg with scan time of 153 s and 6.9 ng with scan time of 1.5 s, respectively.

Photoacoustic technique for the measurement of absorption line profiles.

Abstract: A spectrophone utilizing a resonant cylindrical cavity and operated by driving the first azimuthal mode of the cavity has been developed for the study of weak absorption lines of gases at pressures from 100 to 1300 Torr. Presented are the acoustic resonant amplification factor as a function of pressure, and a description of the noise sources inherent in this spectrophone. An example is given of the optical frequency resolution resulting when this spectrophone is used in conjunction with a tunable ring dye laser as a high resolution spectrometer.