Pulsed Photoacoustics Research Articles

Trace gases analysis in pulsed photoacoustics based on swarm intelligence optimization

Trace gases analysis in pulsed photoacoustics based on swarm intelligence optimization

Exact solution for a photoacoustic wave from a finite-length cylindrical source.

In wide-field pulsed photoacoustics, a nearly instantaneous source of electromagnetic energy is applied uniformly to an absorbing medium to create an acoustic wave. In this work, an exact solution is derived for the photoacoustic wave originating from a finite-length solid cylindrical source in terms of known analytic functions involving elliptic integrals of canonical form. The solution is compared with the output of a finite-element simulation.

Computationally intelligent pulsed photoacoustics

In this paper, the application of computational intelligence in pulsed photoacoustics is discussed. Feedforward multilayer perception networks are applied for real-time simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases. Networks are trained and tested with theoretical data adjusted for a given experimental set-up. Genetic optimization has been used for calculation of the same parameters, fitting the photoacoustic signals with a different number of generations. Observed benefits from the application of computational intelligence in pulsed photoacoustics and advantages over previously developed methods are discussed, such as real-time operation, high precision and the possibility of finding solutions in a wide range of parameters, similar to in experimental conditions. In addition, the applicability for practical uses, such as the real-time in situ measurements of atmospheric pollutants, along with possible further developments of obtained results, is argued.

Spatial laser beam determination by pulsed photoacoustics: detection radius/signal wavelength approximation

Utilizing a mathematical algorithm developed for photoacoustic tomography, it is possible to deduce the laser beam spatial profile within pulsed photoacoustic measurements. This method is based on the temporal shape of the photoacoustic signal analysis. The exact solution for the laser beam spatial profile obtained with this method involves summation of a series and may take much time to compute. Sometimes the simplification of this solution can be done if the detection radius is much larger than the photoacoustic signal wavelength. This simplification is the so-called detection radius/signal wavelength approximation. Our numerical results obtained with this approximation are presented here. These results are compared with the one obtained using a different method and an exact solution. Experimental conditions for photoacoustic signal generation used in our numerical analysis correspond to the multiphoton absorption in SF6–Ar mixtures. Together with the laser beam spatial profile, the molecular vibrational to relaxational time of the excited molecules is calculated simultaneously. Application of neural computation and genetic optimization in pulsed photoacoustics is also discussed.

Neural Networks-Based Real-Time Determination of the Laser Beam Spatial Profile and Vibrational-to-Translational Relaxation Time Within Pulsed Photoacoustics

This paper concerns with the possibilities of computational intelligence application for simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases by pulsed photoacoustics. Results regarding the application of neural computing through the use of feed-forward multilayer perception networks are presented. Feed-forward multilayer perception networks are trained in an offline batch training regime to estimate simultaneously, and in real-time, the laser beam spatial profile (profile shape class) and the vibrational-to-translational relaxation time from given (theoretical) photoacoustic signals. The proposed method significantly shortens the time required for the simultaneous determination of the laser beam spatial profile and relaxation time and has the advantage of accurately calculating the aforementioned quantities.

In Situ Characterization of Laser Ablation by Pulsed Photoacoustics: The Case of Organic Nanocrystal Synthesis

Here, a new methodology based on the pulsed photoacoustic (PA) technique for real-time monitoring of the ablation process used to synthesize organic nanocrystals is described. The methodology is implemented by ablating microcrystals grown from an organic chromophore with nonlinear optical properties. It was determined that the PA signal from the ablation process increases in amplitude and is time-shifted as the ablation process advances. Comparing the PA signals generated at different ablation times under different laser fluences with the nanocrystal characterization obtained through light scattering, optical microscopy, and AFM, it was demonstrated that the pulsed PA technique can be useful for monitoring the process and determining the threshold of ablation.

Photosystem trap energies and spectrally-dependent energy-storage efficiencies in the Chl d-utilizing cyanobacterium, Acaryochloris marina

Acaryochloris marina is the only species known to utilize chlorophyll (Chl) d as a principal photopigment. The peak absorption wavelength of Chl d is redshifted ≈40nm in vivo relative to Chl a, enabling this cyanobacterium to perform oxygenic phototrophy in niche environments enhanced in far-red light. We present measurements of the in vivo energy-storage (E-S) efficiency of photosynthesis in A. marina, obtained using pulsed photoacoustics (PA) over a 90-nm range of excitation wavelengths in the red and far-red. Together with modeling results, these measurements provide the first direct observation of the trap energies of PSI and PSII, and also the photosystem-specific contributions to the total E-S efficiency. We find the maximum observed efficiency in A. marina (40±1% at 735nm) is higher than in the Chl a cyanobacterium Synechococcus leopoliensis (35±1% at 690nm). The efficiency at peak absorption wavelength is also higher in A. marina (36±1% at 710nm vs. 31±1% at 670nm). In both species, the trap efficiencies are ≈40% (PSI) and ≈30% (PSII). The PSI trap in A. marina is found to lie at 740±5nm, in agreement with the value inferred from spectroscopic methods. The best fit of the model to the PA data identifies the PSII trap at 723±3nm, supporting the view that the primary electron-donor is Chl d, probably at the accessory (ChlD1) site. A decrease in efficiency beyond the trap wavelength, consistent with uphill energy transfer, is clearly observed and fit by the model. These results demonstrate that the E-S efficiency in A. marina is not thermodynamically limited, suggesting that oxygenic photosynthesis is viable in even redder light environments.

Computational intelligence based simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time by pulsed photoacoustics

This paper is concerned with the possibilities of computational intelligence application for simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases by pulsed photoacoustics. Results regarding the application of neural computing and genetic optimization are presented through the use of feed forward multilayer perception networks and real-coded genetic algorithms. Feed forward multilayer perception networks are trained in an offline batch training regime to estimate simultaneously, and in real-time, laser beam spatial profile R(r) (profile shape class) and vibrational-to-translational relaxation time ?V?T from a given (theoretical) photoacoustic signals ?p(r,t). The proposed method significantly shortens the time required for the simultaneous determination of the laser beam spatial profile and relaxation time and has the advantage of accurately calculating the aforementioned quantities. Real coded genetic algorithms are used to calculate ?V?T by fitting the ?p(r,t) with the theoretical one. The previously developed methods determine the laser beam profile and relaxation time with sufficient precision, but the methods based on the application of artificial intelligence are more suitable for practical applications, such as the real-time in-situ measurements of atmospheric pollutants.

Numerical modeling of photoacoustic imaging of brain tumors.

Photoacoustic imaging has shown great promise for medical imaging, where optical energy absorption by blood haemoglobin is used as the contrast mechanism. A numerical method has been developed for the in silico assessment of the photoacoustic image reconstruction of the brain. Image segmentation techniques were used to prepare a digital phantom from MR images. Then, light transport through brain tissue was modeled using the finite element approach. The resulting acoustic pressure was then estimated by pulsed photoacoustics considerations. The forward acoustic wave propagation was modeled by linearized coupled first order wave equations and solved by the acoustic k-space method. Since skull bone is an elastic solid and strongly attenuates ultrasound (due to scattering and absorption), a k-space method was developed for elastic media. To model scattering effects, a new approach was introduced based on propagation in random media. In addition, absorption effects were incorporated using a power law. Finally, the acoustic pressure was reconstructed using the k-space time reversal technique. The simulations were run in three dimensions to produce the photoacoustic tomogram of a brain tumor. The results show that relying on optical energy absorption by blood hemoglobin as the contrast mechanism, as is the current practice, leads to poor image quality.

Laser beam spatial profile determination by pulsed photoacoustics: exact solution

The pulsed photoacoustic spectroscopy technique is one of the oldest but respectable techniques which offer some unique features that are relevant to trace gas monitoring. Despite obvious advantages there are some drawbacks that have so far alienated this spectroscopy technique from routine air monitoring. One of them is the additional instrumentation necessity to fulfill the requirement for an accurate knowledge of the spatial and temporal profiles of the excitation light source. Here we present one method for a laser beam spatial profile determination and simultaneous vibrational–translational relaxation time calculation. It is based on the temporal shape analysis of the photoacoustic signals. Symmetric laser beam spatial profiles are obtained with a high level of accuracy. Even though this method does not work in real time, it gives a good basis for future beam profile investigations. Experimental signals used in our analysis are obtained after infrared multiphoton absorption in the SF6–Ar mixture.

Determination of the laser beam spatial profile by pulsed photoacoustics

Pulsed photoacoustics should fulfil some of the requirements to be a powerful and precise trace gas detection technique. Beside the high sensitivity and selectivity, large dynamic range and good temporal resolution, pulsed photoacoustics has the potential to measure numerous parameters of the laser beam spatial and temporal characteristics with one or no additional instruments. The different numerical methods can be distinguished for this purpose with respect to the measuring procedure and result analysis. In the following article the numerical method for the laser beam spatial profile determination will be presented. Simultaneously this method allows calculation of the molecular vibrational to translational relaxation time in different gas mixtures.

Multifractality in the copolymerization of Bis‐GMA/TEGDMA by pulsed photoacoustics

AbstractFractal development during polymerization of a dimethacrylate system (Bis‐GMA/TEGDMA) has been studied through the analysis of time series obtained sequentially by pulsed photoacoustics. The photoacoustic signals as obtained in situ during photopolymerization of Bis‐GMA/TEDGMA were analyzed by rescaled range analysis, dispersion analysis, and detrended fluctuation analysis methods. The analysis reveals the presence of more than one scaling parameter and, therefore, belongs to a complex process known as multifractal. The evolution of fractality during photopolymerization was compared under equivalent conditions with the chemical conversion (of dimethacrylate double bonds) as a function of time, as monitor by infrared spectroscopy. This kinetic study was also used to correlate the fractal nature of the dimethacrylate reaction system with the condensed‐state transitions emerging during the photochemical reaction. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Theory and analysis of frequency-domain photoacoustic tomography

A new frequency-domain approach to photoacoustic tomography has recently been proposed, promising to overcome some of the shortcomings associated with the pulsed photoacoustic approach. This approach offers many of the benefits of pulsed photoacoustics but requires a different set of equations for modeling of the forward and inverse problems due to the longer time scales involved in the optical input signal. The theory of photoacoustic tomography with an optical input that is not necessarily a short pulse is considered in this paper. The full optical, thermal, and acoustic governing equations are derived. A transfer function approach is taken for the solution and analysis of this problem. The results and implications are compared with those of pulsed photoacoustics and traditional ultrasonic diffraction tomography. A Fourier diffraction theorem is also presented, which could be used as a basis for the development of tomographic imaging algorithms.

Iterative method for determination of the laser beam profile and τV-T

Measuring the vibrational-to-translational relaxation time ?V-T in gases is one of the first applications of the photoacoustic effect. The spatial profile of the laser beam is crucial in these measurements because the multiphoton excitation is investigated. The multiphoton absorption is a non-linear process. Because of this, the top hat profile is preferable. It allows one to deal with nonlinearity in a simple manner. In order to reveal the real laser beam profile, we have slightly changed the theoretical profiles in such a manner that the best matching is obtained between theoretical and experimental photoacoustic signals. Still, there was a question: Is it possible to deduce the laser beam profile directly from the photoacoustic signal, thus avoiding manual changing of the laser beam profile? According to this paper, it is possible. The appropriate method has been found in another photoacoustics application: photoacoustic tomography. Thus, the method for the simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time is presented in this paper. It employs pulsed photoacoustics and an algorithm developed for photoacoustic tomography.

Background subtraction in pulsed photoacoustics through neural-network processing

We report on the application of neural-network processing to pulsed photoacoustics for improving the detection limit by subtracting the window-heating-associated background. This technique was applied to the measurement of ethylene traces excited by a TEA (transverse electrical discharge in gas at atmospheric pressure) CO(2) laser. The signal contains a term that shows absorption saturation, characteristic of the absorbing gas, and another, generated by window heating, linearly dependent on laser energy. At low concentrations, normalization to laser energy is not possible owing to the different absorption mechanisms. To overcome this problem we relied on a neural-network filter, trained with experimentally obtained patterns, that subtracts the background and returns the sample concentration. This way, we reduced the detection limit to 20% of the previous limit obtained by reading the main resonance peak amplitude.

Simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time by pulsed photoacoustics

Measuring the vibrational-to-translational relaxation time τV-T in gases is one of the applications of pulsed photoacoustic spectroscopy. Unfortunately, the spatial profile of the laser beam can significantly influence the accuracy of these measurements. Namely, minor changes in the spatial profile of the laser beam cause significant errors in the τV-T measuring. We present a method for simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time. It is based on the temporal shape of the photoacoustic pulse and utilizes a mathematical algorithm developed for photoacoustic tomography.

The A-FXto FA/BStep inSynechocystis6803 Photosystem I Is Entropy Driven

We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P(700) to F(A/B) iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P(700) --> A(1)(-)F(X) (<10 ns) and (2) A(1)(-)F(X) --> F(A/B)(-) (20-200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (-0.8 +/- 0.1 eV) and large volume change (-23 +/- 2 A(3)), which are assigned to the P(700) --> A(1)(-)F(X) step, and (2) a component with approximately 200 ns lifetime, which has a positive enthalpy (+0.4 +/- 0.2 eV) and a small volume change (-3 +/- 2 A(3)) that are attributed to the A(1)(-)F(X) --> F(A/B)(-) step. For the fast reaction using the redox potentials of A(1)F(X) (-0.67 V) and P(700) (+0.45 V) and the energy of P(700) (1.77 eV), the free energy change for the P(700) --> A(1)(-)F(X) step is -0.63 eV, and thus the entropy change (TDeltaS, T = 25 degrees C) is -0.2 +/- 0.3 eV. For the slow reaction, A(1)(-)F(X) --> F(A/B)(-), taking the free energy of -0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283-310], the entropy change (TDeltaS) is positive, +0.54 +/- 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A(-)F(X) to F(A/B)(-) step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.

System of pulsed photoacoustics based on a TEA CO2laser applied to SF6-N2mixtures

A transversal single-mode TEA CO 2 laser, whose cavity design is a diffraction coupled resonator, tuned at the 10P(16) line, is used for photoacoustic detection of SF 6 traces in N 2 . The acoustic cavity, resonant at a longitudinal mode, is designed on the basis of an electric analog model of a one-dimensional resonator. The acoustic signal from SF 6 -N 2 and the signal from a laser energy detector enter simultaneously into a PC through its sound port. The photoacoustic signal, proportional to the average number of absorbed photons per molecule, is found to increase with the laser energy as a power of 0.5. The constant of the setup, previously measured from pulsed photoacoustics on NO 2 -N 2 mixtures excited with the second harmonic of a Q-switched Nd:YAG laser, allows to quantitatively determine the SF 6 absorption cross section as a function of the CO 2 laser fluence. The calibration of this system is performed down to 150 ppbV of SF 6 in N 2 at atmospheric pressure for different values of laser energy.

Photoacoustic analysis indicates that chloroplast movement does not alter liquid-phase CO2 diffusion in leaves of Alocasia brisbanensis.

Light-mediated chloroplast movements are common in plants. When leaves of Alocasia brisbanensis (F.M. Bailey) Domin are exposed to dim light, mesophyll chloroplasts spread along the periclinal walls normal to the light, maximizing absorbance. Under high light, the chloroplasts move to anticlinal walls. It has been proposed that movement to the high-light position shortens the diffusion path for CO(2) from the intercellular air spaces to the chloroplasts, thus reducing CO(2) limitation of photosynthesis. To test this hypothesis, we used pulsed photoacoustics to measure oxygen diffusion times as a proxy for CO(2) diffusion in leaf cells. We found no evidence that chloroplast movement to the high-light position enhanced gas diffusion. Times for oxygen diffusion were not shorter in leaves pretreated with white light, which induced chloroplast movement to the high-light position, compared with leaves pretreated with 500 to 700 nm light, which did not induce movement. From the oxygen diffusion time and the diffusion distance from chloroplasts to the intercellular gas space, we calculated an oxygen permeability of 2.25 x 10(-)(6) cm(2) s(-)(1) for leaf cells at 20 degrees C. When leaf temperature was varied from 5 degrees C to 40 degrees C, the permeability for oxygen increased between 5 degrees C and 20 degrees C but changed little between 20 degrees C and 40 degrees C, indicating changes in viscosity or other physical parameters of leaf cells above 20 degrees C. Resistance for CO(2) estimated from oxygen permeability was in good agreement with published values, validating photoacoustics as another way of assessing internal resistances to CO(2) diffusion.

The Enthalpy and Entropy of Reaction for Formation of P+QA- from Excited Reaction Centers of Rhodobacter sphaeroides

The enthalpy and volume changes for the charge-transfer reaction between excited donor and ionized donor and acceptor in bacterial reaction centers were determined using pulsed photoacoustics. Excitation in the lowest absorption band of the centers at 860 nm minimized the thermal signal caused by degradation of excess energy. Knowing the free energy of this reaction, −0.86 eV, the determination of the enthalpy, −0.44 eV, fixes the entropy at 25 °C as about one-half (TΔS = +0.42 eV) of the free energy for the normal ubiquinone-10 containing centers. This is a larger contribution than anticipated from previous estimates of the enthalpy. The unexpected sign of the entropy is assigned to the release of counterions from the reaction center surfaces when the charge transfer cancels the dominant opposite charges at the interfaces. The enthalpy and entropy of six reaction centers containing exchanged quinones did not correlate with their free energies. The volume contractions ranged from −28 to −42 A3 and roughly...