Pulsed Laser Photoacoustic Instrument Operating under Visible and Ultraviolet Range

A pulsed laser photoacoustic (PA) instrument operating under visible and ultraviolet range was reported, which used a YAG pumped dye laser (width 8 ns, repetition rate 10 Hz) as the light source. Photoacoustic spectra of C2H2 overtone excitation and A1Au←X1Σ+g electronic transition were obtained using the instrument and the dependence of PA signal intensity on the sample pressure and the laser energy was measured. The application of pulsed PA to electronic transition was less reported before. Because of the simplification of its theoretical treatment and experimental operation, the pulsed PA instrument can replace resonant (or non-resonant) moderate cw PA instrument in almost all the cases except the ultra high-resolution spectroscopy. The difference between pulsed and cw moderate PA was also discussed in the text.     

Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications

An ammonia traces analyser based on photoacoustic spectroscopy is described. The system uses a CO(2) laser and a properly designed resonant photoacoustic cell to achieve ammonia detection at sub-parts-per-billion (ppb) level. The instrument features unattended automatic on-line monitoring of ammonia with a detection limit of 0.1 ppb. Interferences from atmospheric CO(2) and H(2)O are efficiently suppressed by a careful selection of the laser wavelength and a compensation of the water vapour signal made with a high-precision hygrometer. The cell design enables continuous measurement at high flow rates (up to 5 l/min), which guarantees a fast response time of the system for the monitoring of ammonia, a sticky polar molecule that adheres to most surfaces. Various examples of applications of the instrument in the semiconductor industry and for atmospheric pollution monitoring are presented. They demonstrate the excellent performances of the system and its suitability for these applications.     

溶液中三价稀土Ho3+、Nd3+的脉冲激光光声光谱研究及其测定

采用自制的液体光声传感器,研究了在620～665nm波长范围不同溶剂中稀土Ho³⁺的光谱精细结构;探讨了入射激光脉冲能量、测试温度以及各种与水混溶的有机溶剂对光声信号强度的影响。在乙腈水溶液中测定稀土Ho³⁺、Nd³⁺检测限分别为5×10^-8mol/L和1.0×10^-7mol/L,相应的吸光度为1.5×10^-7和6.3×10^-7。     

Accurate Measurement of Raman Depolarization Ratio in Gaseous CO2

The Raman depolarization ratios of gaseous CO₂ in the spectral range of 1240-1430 cm^-1 are determined with a sensitive photoacoustic Raman spectroscopy, and more accurate data compared to the literature results are presented. The precision of the obtained depolarization ratio is achieved by measuring and fitting the dependence of the PARS signal intensity on the cross angle between the polarizations of two incident laser beams.     

Fe3+ concentration dependence of photoacoustic absorption spectroscopy on ZnGa2O4 ceramic powders

In this paper we present the photoacoustic spectroscopy of ZnGa2O4 with 5 and 10% of Fe3+. The ZnGa2O4 is a normal spinel, with Zn2+ ions in tetrahedral sites and Ga3+ ions in octahedral sites. It is expected for these hosts that Fe3+ ions occupy Ga3+ sites. Therefore, the optical properties would be associated to an octahedral symmetry. The energy levels were identified and the crystal field and electronic interaction parameters calculated with Tanabe-Sugano matrices for d5 configuration. We have also studied the dependence of the photoacoustic signal amplitude on the excitation light modulation frequency. Our results indicate that thermal expansion is the predominant phenomenon for the photoacoustic signal generation.     

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10¹² molecules cm⁻³ with a signal-to-noise ratio (SNR) of 1.     

CO2-laser photoacoustic detection of heavy water vapour. Basic principles

A CO₂-laser cw system and a photoacoustic detection apparatus, based on a nonresonant gas cell, was constructed and built. Coincidences of D₂O absorption with several laser lines were observed. The strongest absorption was observed with the line at 9.26 m, which was used in the experiments. Samples of deuterated water in the range from 10 to 100% deuterium were used to examine the behaviour of the photoacoustic signal under conditions of varying deuterium content and total vapour pressure. A principal calibration procedure with respect to this is proposed. The behaviour of the system on buffer gas introduction was examined with hydrogen and dry air. Self-buffering of water vapour is also discussed on the basis of the results.     

Application of ion-exchanger phase photoacoustic spectroscopy to flow analysis of trace amounts of iron in water

Ion-exchanger phase photoacoustic spectroscopy has been applied to the flow analysis of trace amounts of iron in water. The reaction product of iron(II) with 4,7-diphenyl-1,10-phenanthroline-disulphonate, introduced into a carrier solution stream in the flow system, was concentrated on a small amount of QAE-Sephadex gel settled in a fused silica glass tube (1.5 mm i.d.) of a photoacoustic cell. The photoacoustic signal produced by pulsed laser irradiation of the gel beads at 532 nm was detected by a cylindrical piezoelectric transducer which was attached outside the glass tube. When 3.7 ml of a sample solution was introduced into the flow system, the sensitivity of this method was 590 times higher than that of the corresponding solution photoacoustic spectrometry and the detection limit of iron was 0.33 ng/ml. Because the coloured complex was desorbed from the cell with a desorbing agent solution, the present method could afford repeated analyses of natural water samples containing iron at ng/ml levels without any preconcentration procedures.     

Photoacoustic detection of circular dichroism

An analysis is presented for an experimental technique involving the measurement of circular dichroism using polarization-modulated photoacoustic spectroscopy. The technique is referred to as photoacoustic circular dichroism (PACD). In the PACD experiment, a photoacoustic signal is induced by using polarization-modulated excitation light which is alternately left-and right-circularly polarized. Expression appropriate for analyzing the PACD experimental observables (signal strength and phase) in terms of sample circular dichroism are developed within the general framework of the Rosencwaig and Gersho model for the photoacoustic effect in solids and liquids. Calculations based on these expressions are reported and the applicability of PACD for measuring the circular dichronic properties of optically opaque samples is discussed.     

Optimisation of photoacoustic resonant cells with commercial microphones for diode laser gas detection

The theoretical and experimental study of the differential Helmholtz resonant (DHR) cell sensitivity under variation of the total gas pressure is made for various commercial microphones. Near-infrared lasers (room-temperature diode lasers) were used to measure the response of DHR cell versus pressure of the absorbing gas and frequency of the laser radiation modulation. Several molecular absorbers like H2O, CH4, mixed with molecular buffer gases were used to investigate the behavior of the photoacoustic (PA) signal characteristics with a DHR cell. The experimental data are compared with the results of computer simulation. The minimal detectable concentrations of gases were determined for the DHR cell for each commercial microphone.     

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10¹² molecules cm⁻³ with a signal-to-noise ratio (SNR) of 1.     

Photoacoustic study of CO2-laser coincidences with absorption of some organic solvent vapours

Detection of specific molecules in the atmosphere is motivated by the need for monitoring the emission of industrial pollutants. CO₂-laser photoacoustic (PA) spectroscopy was used here to obtain coincidence spectra of the vapours of three organic solvents: trichloroethylene (TCE), benzene, and dioxan. Spectra of coincidences were obtained as a function of total pressure, adding dry air to the vapour of the solvents up to 1000 mbar.A narrow cavity photoacoustic cell with window acoustic buffers was built for improved sensitivity, and is reported here.The vapour molecules studied do have some significant absorption in the CO₂-laser region, or close to it, but that fact does not necessarily lead to usable coincidences with the narrow laser lines. The results reveal several prominent coincidences that can be used for practical purposes, especially in the case of trichloroethylene.     

Multi-gas sensing based on photoacoustic spectroscopy using tunable laser diodes

Multi-hydrogenated compounds detection based on photoacoustic (PA) spectroscopy is reported. Three near-infrared semiconductor lasers are used with a resonant PA cell operated in its first longitudinal mode to monitor methane, water vapour and hydrogen chloride in the parts per million range. The design of our cell results from simulations performed in order to optimise its performances. Influence of the buffer gas on the PA signal has also been analysed, both theoretically and experimentally. A reduction of the PA signal of almost one order of magnitude has been observed between N(2) and He, which demonstrates the importance of the buffer gas in PA spectroscopy. Finally, detection limits of 0.5 ppm of CH(4) and 3 ppm of HCl has been achieved experimentally in nitrogen and an H(2)O sensitivity of 0.2 ppm has been estimated.     

Quantum cascade laser-based photoacoustic spectroscopy of volatile chemicals: application to hexamethyldisilazane

The use of photoacoustic spectroscopy and mid-infrared quantum-cascade lasers (QCLs) for the detection of hexamethyldisilazane (HMDS) is reported. A detection limit of 200 parts in 10(9) is found using a Fabry-Perot QCL operated at 8.4 microm in pulsed mode and a photoacoustic cell equipped with four electret microphones. The laser multimode spectrum matches the range of the N-H bending absorption band of HMDS. Further improvements to reach lower detection limits are discussed.     

Effects of exothermic chemical reaction on the photoacoustic effect from particulate suspensions

Irradiation of chemically reactive particulate suspensions by high power, pulsed laser radiation initiates reactions at the sites of the particles so that besides the absorbed optical energy, chemical energy is liberated. In addition to the release of chemical energy, chemical reaction can result in gas production both of which result in enhancement in the amplitude of the photoacoustic effect. Here we report photoacoustic and transient grating experiments with colloidal C in mixtures of H(2)O(2) with H(2)O. The inclusion of H(2)O(2) in an aqueous C suspension changes the normally endothermic reaction of C with H(2)O into the highly exothermic reaction of C with H(2)O(2) leading to both an enhanced photoacoustic effect and an increase in light emission from the suspension. As well, laser-initiated exothermic reactions in suspensions of C with CH(3)NO(2) and particulate Hg(CNO)(2) in H(2)O are shown to result in greatly enhanced photoacoustic signal amplitudes.     

Measurement of A Two Layer Sample Using Pseudo Pulse Excited Photoacoustic Spectrometry

Abstract

To obtain surface layer thickness easily, a simple pseudo pulse excited photoacoustic spectrometric method was proposed. An argon ion laser was chopped mechanically to generate a pseudo pulse (pulse width; 6.6 ms, duty 1.67%), which was then led to a sample enclosed in photoacoustic cell. Two layer samples made of polymethyl methacrylate (PMMA) were used as model samples. The photoacoustic signal waveform observed showed a maximum from the negative edge of the pseudo pulse of the laser. The delay of the signal increased concomitant with the sample surface thickness. The delay time of the signal was calculated by a cross-correlation method. A linear relationship was obtained between the delay time of the photoacoustic signal from the input pseudo pulse and the surface transparent layer thickness in the range of 0–90 mm. The regression line between the film thickness x (cm) and the delay time was expressed with the thermal diffusivity of the film k, as follows; Δτ (s) = 1.16 × 10^?1 κ^?½ x + 0.006. Using this result, the method proposed was successfully applied to the measurement of the thickness for laminated polyethylene film on papers. The method proposed is simple and easy to perform without any modification of usual photoacoustic instrumentation.     

Photoacoustic spectrometry for trace gas analysis and leak detection using different cell geometries

A photoacoustic (PA) spectrometer with high selectivity and sensitivity has been developed for trace gas analysis and for the detection of gas leak at part per trillion by volume (pptV) level. This PA system comprises of a resonant photoacoustic cell, a pulsed line tunable CO₂ laser as an excitation source and a sensitive electret microphone as a photoacoustic detector with an option to trigger the safety alarm system for early warning of gas leaks. In this work, three resonant PA cells with various geometries have been developed at our laboratory for the detection of photoacoustic signal using pulsed laser system and their comparative performance have been studied. As a special application of this PA system, the detection of sulfur hexa fluoride (SF₆) gas using these three cells has been carried out for optimizing the sensitivity. Besides this, our PA system can very well be applied for pollution monitoring and detection of hazardous gases in a noisy environment.     

Dynamics of L-tryptophan in aqueous solution by simultaneous laser induced fluorescence (LIF) and photoacoustic spectroscopy (PAS)

An experimental system for measuring simultaneously photoacoustic (PA) and fluorescence signals is described. The simultaneous measurement of laser induced fluorescence and photoacoustic signals provide a suitable method for the study of different quenching phenomena occurring in fluorescent systems. In this paper we report tryptophan solvation dynamics in water using fluorescence and photoacoustic spectra recorded simultaneously by photoacoustic and fluorescence signals as functions of concentration, indicate that quantum yield is maximum at low concentrations. Also, the energy lost in the fluorescence path of tryptophan, due to different quenching phenomena like self quenching, Resonance energy transfer (RET), solvation relaxation, etc. is clearly seen from the photoacoustic signal intensity which increases as the fluorescence intensity decreases.     

Effect of laser pulse energies in laser induced breakdown spectroscopy in double-pulse configuration

In this paper, the effect of laser pulse energy on double-pulse laser induced breakdown spectroscopy signal is studied. In particular, the energy of the first pulse has been changed, while the second pulse energy is held fixed. A systematic study of the laser induced breakdown spectroscopy signal dependence on the interpulse delay is performed, and the results are compared with the ones obtained with a single laser pulse of energy corresponding to the sum of the two pulses. At the same time, the crater formed at the target surface is studied by video-confocal microscopy, and the variation in crater dimensions is correlated to the enhancement of the laser induced breakdown spectroscopy signal. The results obtained are consistent with the interpretation of the double-pulse laser induced breakdown spectroscopy signal enhancement in terms of the changes in ambient gas pressure produced by the shock wave induced by the first laser pulse.     

Ion-exchanger phase photoacoustic spectrometry for trace analysis of metal ions

Laser-induced photoacoustic spectroscopy has been applied to the determination of chromate, uranyl, and Cr(VI) and Fe(II) ions as coloured complexes that were sorbed on ion-exchanger beads. A simple photoacoustic cell has been constructed; it consisted of a fused silica glass tube and an attached cylindrical piezoelectric transducer. A pulsed laser fight at 532 or 355 nm irradiated the ion-exchanger beads that had been collected in the glass tube of the cell. Since a sample material was effectively concentrated on the ion exchanger, the sensitivity was much higher than that in the corresponding photoacoustic spectrometry in a solution state. The lowest detection limit was 0.21 ng/ml Fe for Fe(II)-DPPS (4,7-diphenyl-1,10-phenanthrolinedisulphonate) complex sorbed on QAE-Sephadex gel. Analytical characteristics of the present method are described.