Laser-induced Fluorescence Spectra Research Articles

CH3O Substitution Effect Revisited in the Vibrationally Resolved Laser-Induced Fluorescence Spectra of Methoxycyclohexoxy Radicals.

The oxy-substituted alkoxy radicals have attracted wide attention due to the increasing application of oxygenated volatile organic compounds as fuel additives and solvents. Direct detection of these intermediate radicals is desired for measuring the reaction rate and investigating the oxidation mechanism of organic compounds in the atmosphere. A charge-transfer excited state induced by CH3O substitution was identified in our previous study of 3-methoxy-1-propoxy radical [Xue, J. Phys. Chem. Chem. Phys. 2021, 23, 2586]. As the C-C bonds of chain alkoxy radicals can freely rotate, further studies are needed to understand the mechanism of this long-range charge-transfer effect. In this work, vibrational-resolved laser-induced fluorescence (LIF) spectra of 3- and 4-methoxycyclohexoxy radicals were obtained under jet-cooled conditions. A large red-shift of ∼454 cm-1 of the origin band was observed when the CH3O substituent moved from the δ site to the γ site of the cyclohexoxy radical. The LIF spectra are assigned to 3-cis (e, e) and 4-trans (e, e) conformers, respectively, with the assistance of structural optimization and electron excitation studies conducted at the CAM-B3LYP/6-311++G(d,p) level of theory. Natural transition orbital analysis reveals that the intramolecular charge transfer from the C-O-C p orbital to the radical O p orbital in 3-methoxycyclohexoxy has a strong effect on the radical CO σ → O p excitation and hence results in a spectral change. On the other hand, the spectral effect of CH3O substitution almost vanishes at δ carbon. The results propose a through-bond interaction between CH3O and radical CO groups.

Three-dimensional flow velocity determination using laser-induced fluorescence method with asymmetric optical vortex beams

Laser-induced fluorescence (LIF) Doppler spectroscopy using an optical vortex beam with an asymmetric intensity distribution, referred to as aOVLIF, is proposed as a new method to measure plasma flow velocity. LIF spectra were calculated numerically using typical laboratory low-temperature plasma parameters, and it was revealed that an ion flow across the beam produces a frequency shift of the spectra. This method also has the capability of temperature measurements. The propagation effects of asymmetric optical vortex beams are discussed assuming an actual experiment, and it is found that the sensitivity to the transverse flow velocity is approximately unchanged. The aOVLIF method, which exploits the inhomogeneous phase structure of optical vortices, can be applied to the determination of three-dimensional velocity vectors and promises to enhance the usefulness of conventional LIF spectroscopy using plane waves.

Optical spectroscopy of the previously unobserved palladium monosulfide (PdS) radical.

The optical spectra of the palladium monosulfide (PdS) molecule in gas phase have been investigated for the first time through laser-induced fluorescence (LIF) and single-vibronic-level (SVL) emission spectroscopies. The I3Σ- - X3Σ- transition system containing 16 vibronic bands was identified in the LIF spectra, covering the energy range of 22 030-23 400cm-1. The spectra with rotational resolution allowed for the determination of the molecular constants in both the ground X and excited I electronic states, involving the spin-orbit splitting, rotational constant, vibrational frequency, and isotope shift. Isotopically resolved SVL emission spectra permitted the observation of the spin-orbit splitting, vibrational frequency, and vibrational isotope shift of the X3Σ-0+,1 and A3Π2,1,0-,0+ states as they transitioned from the excited I state to the vibrational levels of the X and A states. Ab initio calculations presented plenty of the Λ-S and Ω states of PdS below 28 000cm-1 and provided strong support for the assignments of the experimental observation.

High-resolution laser spectroscopy of the [formula omitted][formula omitted] transition in YbOD

Ytterbium monohydroxide (YbOH) and its isotopologues are good candidate molecules for use in the search for physics beyond the standard model. We present a rotational analysis of the Ã2Π1/2−X̃2Σ+ transition in 174YbOD, the deuterated analogue of 174YbOH, based on high-resolution laser-induced fluorescence spectra. The band origin T0= 17993.81014(33) cm−1 was shifted to the red of the 174YbOH origin (T0= 17998.5875(2) cm−1). The ground state rotational parameter is B = 0.2205648(72) cm−1, which compares favourably to the value of 0.2203 cm−1 estimated by isotopically scaling the 174YbOH value of B = 0.245116257(1) cm−1.

Femtosecond laser-induced fluorescence for rapid monitoring of cardiac troponin 1 as a cardiovascular disease biomarker

Medical diagnosis usually requires blood analysis of various biomarkers which are essential for disease detection and health status monitoring. Cardiac troponin 1 (cTn1) is a protein member of the cardiac troponin complex used for the diagnosis of several pathologies associated with cardiomyocyte necrosis. Laser-induced fluorescence (LIF) is a technique with high sensitivity and specificity, and it is one of the most significant developments used as an analytical tool for qualitative and quantitative analysis. The current study investigated the potential application of femtosecond LIF as a novel detection technique for rapid monitoring of cTn1 in clinical analysis. In the present study, the cTn1 (8 ng/ml) was excited over wavelengths ranging from 350 to 400 nm, and the LIF spectra were recorded. The results demonstrated that the maximum fluorescence intensity was observed at an excitation wavelength of 350 nm, with an emitted fluorescence peak centeredat 494 nm. At an excitation wavelength of 350 nm, different concentrations of cTn1 have been investigated and LIF spectra were obtained. The results revealed that the fluorescence peak intensity is concentration-dependent and increases linearly with increasing cTn1 concentration. These findings show that femtosecond LIF presents a unique, highly selective, precise, and direct approach to monitoring cTn1.

Prediction of brown tide algae using improved Gramian angular field and deep learning based on laser-induced fluorescence spectrum

The frequent occurrence of algal blooms has seriously affected the marine environment and human production activities. Therefore, it is crucial to monitor the phytoplankton concentration in water bodies. In this study, a prediction method for brown tide algae using improved Gramian angular field (IGAF) and deep learning based on the laser-induced fluorescence spectrum was proposed. The method combined one-dimensional (1D) fluorescence spectrum with IGAF for image coding. The internal normalizing approach of the original Gramian angle field algorithm was upgraded from local to global, which can increase the difference between samples with various concentrations. Then, we established a novel technique that fully takes into account the Gramian angular difference field and Gramian angular summation field features, allowing it to control the main and sub-diagonal features and successfully convert 1D sequences into images by adding various weight factors. Using depthwise separable convolutional neural network to extract image features helps reduce model training parameters, paired with long short-term memory network to rapidly predict the concentration of brown tide. To confirm the actual performance of the given approach, ablation and contrast experiments were carried out, and the results showed that the method’s regression accuracy, R 2 was 97.8%, with the lowest mean square error and mean absolute error. This study investigated the transformation of 1D spectra into images using IGAF, which not only explored the application of the fluorescence spectrum image coding method for algal regression but also enabled the introduction of the potent benefits of deep learning image processing into the field of spectral analysis.

Black Plastic Waste Classification by Laser-Induced Fluorescence Technique Combined with Machine Learning Approaches

Sensor-based sorting devices commonly used in plastic recycling plants, mainly working in the near infrared range (NIR), are unable to identify black plastics, due to their low spectral reflectance. The aim of this work was to investigate the potentialities offered by laser-induced fluorescence (LIF) technique (spectral range 270–750 nm) for the identification of black polymers inside a plastic waste stream, thus allowing the possibility to build efficient sorting strategies to be applied in recycling plants. Representative samples of black plastics collected among the most utilized in household packaging were selected, constituted by four different types of polymers, i.e., expanded polystyrene (EPS), polystyrene (PS), polypropylene (PP) and high-density polyethylene (HDPE). The acquired LIF spectra were processed using multivariate approaches in order to optimize polymer classification. The developed hierarchical—partial least square-discriminant analysis (Hi-PLS-DA) classification model, showed excellent performances, confirmed by the values of sensitivity and specificity values in prediction, being equal to 1. The correctness of classification obtained by LIF was confirmed by the application of Fourier Transform Infrared spectroscopy (FTIR) on the same samples. The achieved results demonstrated the potential of LIF technique combined with a machine learning approach as sorting/quality control tool of black polymers in recycling plants.Graphical

Detection Experiment of Six Vitamins by Laser-Induced Fluorescence Spectra

Detection Experiment of Six Vitamins by Laser-Induced Fluorescence Spectra

Simple, Fast Nitric Oxide Planar Laser-Induced Fluorescence Model for Computational Fluid Dynamics Applications

The current work demonstrates the efficacy of a simple and fast model for nitric oxide (NO) laser-induced fluorescence (LIF). The model is validated with experimental LIF spectra computed from captured images of fluorescence from a gas cell filled with a known amount of NO. These data are obtained for several segments of the NO fluorescence spectrum. The selected segments encompass LIF lines with rotational quantum numbers appropriate for low-to-moderate temperature flows. The spectra obtained using the current model are also compared to those obtained from other theoretical models, namely, LIFBASE and LINUS. The current model is based on a previously developed version of a two-level fluorescence model and is developed specifically for applications in computational fluid dynamics (CFD) where rapid on-the-fly evaluations are required due to the potentially large number of grid cells for which LIF is being computed. Good agreement between the experimental and theoretical results provides confidence in current LIF modeling for use with CFD. An example application of the current model in a Mach 6.5 hypersonic mixing flowfield is also presented.

Electronic Spectroscopy of Jet-Cooled NdO

Chemi-ionization reactions of the type M + O → MO+ + e- (M = Nd or Sm) are currently being investigated as a method to artificially increase the electron density in the ionosphere for control of micro- and radio wave propagation. Experiments involving the release of atomic Nd into the upper atmosphere have resulted in the production of a cloud that, on excitation by solar radiation, emits green light. It has been assumed that NdO was the carrier of this emission, but the existing spectroscopic data needed for this attribution is lacking. While the electronic spectrum of NdO has been well-characterized at wavelengths greater than 590 nm, relatively little spectroscopic data exist for emission wavelengths in the blue-green spectral range. In this study, spectra for jet-cooled NdO were recorded over the range 15,500-21,000 cm-1. Rotationally resolved laser induced fluorescence and vibronically resolved dispersed laser-induced fluorescence spectra were recorded, and nine new electronically excited states were identified. The data indicate that the electronic spectrum of NdO has relatively few allowed transitions in the green spectral range, casting doubt on the assignment of the Nd high-altitude release cloud green emission to NdO.

Laser-induced fluorescence spectroscopy for kinetic temperature measurement of xenon neutrals and ions in the discharge chamber of a radiofrequency ion source

Application of single-photon absorption laser-induced fluorescence (LIF) spectroscopy for non-intrusive measurement of neutral xenon and singly charged xenon ion kinetic temperatures in the discharge chamber of a gridded radiofrequency ion source is demonstrated. A LIF spectrum analysis approach including hyperfine structure reconstruction and inverse filtering (Fourier deconvolution) is outlined. Special focus is set on optimization of post-deconvolution filtering as well as retracing of deconvolution result imperfection due to hyperfine structure parameter uncertainty, incorrect natural linewidth, and saturation of the LIF signal. The corresponding contributions to the kinetic temperature estimation error are quantified via simulation of spectral lineshapes. Deconvolution of almost unsaturated LIF spectra recorded in the center of the ion source discharge chamber reveals that the neutral xenon and xenon ion kinetic temperatures range between approximately 500 and 700 K and, respectively, 700 and 1000 K depending on the radiofrequency power supplied to the discharge.

Simultaneous measurements of NH2 and major species and temperature with a novel excitation scheme in ammonia combustion at atmospheric pressure

This work reports the first 1D, simultaneous, laser-induced fluorescence (LIF) measurements of the amidogen (NH2) radical concentration and Raman/Rayleigh measurements of major species and temperature in laminar counterflow non-premixed and premixed NH3/H2/N2–air flames. The instrument presents a novel excitation scheme for NH2, which add no complexity to the existing Raman/Rayleigh instrument. The NH2 LIF is excited by three Nd:YAG lasers, temporally stretched to 500 ns using a pulse stretcher, resulting in total energy of ∼ 1 J at 532 nm. LIF spectra collected in NH2 produced from photolysis of NH3, in an oxygen-free environment, match those obtained in the counterflow flames. The NH2-LIF signal is sampled in the 620–630 nm, together with the Raman signal, using the same collection hardware. The weak absorption cross-section at 532 nm is compensated by the high laser energy available, and the long pulse allows repeated excitation and emission from the same molecule, leading to the partial saturation region. A simplified model which neglects quenching, and contributions from other species, but includes Raman crosstalk from H2, N2, and NH3 is proposed and calibrated by comparing measurements and 1D Chemkin simulation of a counterflow flame. Finally, this simplified model was validated in more than 23 flames with varying fuel compositions, strain rates, and non-premixed and premixed flames to assess accuracy and precision. The overall agreement between the measured and simulated NH2 using the same mechanism is within 5%, and the single-shot detection limit is below 100 ppm. NO2-LIF is one of the main interferences to NH2-LIF, and its effects can be negligible on the fuel side, but care only need to be taken on the air side with T < 800 K, in which the maximum interference ∼33 ppm was observed. The accuracy of chemical kinetic models, with an uncertainty of ∼ 20%, limits the accuracy of the proposed NH2 measurements. The technique is best suited to investigate the turbulence-chemistry interaction in turbulent NH3 flames combined with major species and temperature from Raman/Rayleigh scattering, and it can provide useful validation data for the numerical model when the same chemical model is used.

High-resolution isotope-shift spectroscopy of Cd i

We present absolute frequency measurements of the ${}^{1}\text{P}_1 \leftarrow {}^{1}\text{S}_0$ ($229$nm) and ${}^{3}\text{P}_1 \leftarrow {}^{1}\text{S}_0$ ($326$nm) transitions for all naturally occurring isotopes of cadmium. The isotope shifts and hyperfine intervals of the fermionic isotopes are determined with an accuracy of 3.3MHz. We find that quantum interference in the laser-induced fluorescence spectra of the ${}^{1}\text{P}_1 \leftarrow {}^{1}\text{S}_0$ transition causes an error of up to 29(5)MHz in determining the hyperfine splitting, when not accounted for with an appropriate model. Using a King-plot analysis, we extract the field- and mass-shift parameters and determine nuclear charge radius differences for the fermions. The lifetime of the $^1\text{P}_1$ state is determined to be 1.60(5)ns by measuring the natural linewidth of the ${}^{1}\text{P}_1 \leftarrow {}^{1}\text{S}_0$ transition. These results resolve significant discrepancies among previous measurements.

Research on transformer fault diagnosis based on an IWHO optimized MS1DCNN algorithm and LIF spectrum.

Accurate diagnosis of transformer faults can effectively improve the enduring reliability of power grid operation. Aiming at overcoming the problems of long time consumption and low diagnostic rate in the past diagnosis methods, this article designs a laser-induced fluorescence (LIF) detection system, which can be combined with a multi-scale one-dimensional convolution neural network (MS1DCNN) to diagnose transformer fault categories. The structural parameters of MS1DCNN are optimized using the improved wild horse optimizer (IWHO). Electrical fault oil, thermal fault oil, normal oil and locally damped oil are used as raw materials for the experiment. First, the LIF spectral data of the four kinds of oil samples are obtained, and the spectral data obtained are pretreated by standard normal variate (SNV) and multiple scattering correction (MSC), and the dimensions are reduced by linear discriminant analysis (LDA) and kernel principal component analysis (KPCA). Then the dimensionality reduced data are imported into the MS1DCNN algorithm for learning, and the parameters of MS1DCNN are optimized using the IWHO algorithm. Finally, the experiment shows that the efficiency and precision of LIF technology for raw data extraction are higher than for traditional methods; in comparison with the same type of algorithm, MSC has a better preprocessing effect, KPCA has a better dimensionality reduction effect, MS1DCNN has a better prediction effect, and IWHO has a better optimization effect. Compared to them, the MSC-KPCA-IWHO-MS1DCNN model has the best diagnostic ability, with a mean square error (MSE) of 4.9037 × 10-4, mean absolute error (MAE) of 0.0179, and goodness of fit (R2) of 0.9996. Transformer fault intelligent diagnosis is necessary for the sustained and stable operation of power networks.

Iron(Ⅲ)-modified resin YZS60 combined with laser-induced fluorescence spectra for the detection of phosphorus after solid-phase extraction in aqueous solutions

Phosphorus elements were extracted from aqueous solutions by iron-modified chelate resin YZS60. The characterization of the iron-modified chelate resin by using scanning electron microscopy, energy disperse spectroscopy, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy showed that iron(Ⅲ) was successfully grafted onto the iminodiacetic acid functional group of chelating resin. The resin chelated with phosphorus emitted strong fluorescence after excited by very low power semiconductor laser, which was used for the detection of phosphorus in aqueous solutions, resulting in improving the sensitivity significantly for the detection of phosphorus in aqueous solutions. The fluorescence intensities of the chelates were proportional to the phosphorus concentrations with highly linear correlation coefficients R2 = 0.991 and the limit of detection of 0.018 mg/L was achieved. This work provides a simple, low-cost, and non-destructive method for the extraction and detection of phosphorus in aqueous solutions.

Laser induced fluorescence spectroscopy of the jet cooled SiNSi radical: Rotational analysis of the [formula omitted][formula omitted] – [formula omitted][formula omitted] transition

We have generated SiNSi in supersonic free jet expansions, and have measured laser induced fluorescence (LIF) spectra in the ultra-violet (UV) region. On the basis of rotational analyses, four vibronic bands in the LIF spectra have been attributed to those of the Ẽ2Σu+ – X̃2Πg electronic transition of SiNSi. The two bands at 32918 and 33059 cm−1 have been assigned to the two spin–orbit components of the 000 band. The other two bands at 34173 and 34314 cm−1 have been attributed to two subbands of a vibronic band, although no conclusive vibrational assignment have been obtained yet. All of the rotational line widths, ∼ 0.10 cm−1, of the four bands are significantly wider than that of the instrumental resolution, ∼ 0.03 cm−1. The widths have been interpreted as coupling of the rotational levels of the Ẽ2Σu+ state with quasi-continuum background levels consisting of low-lying electronic states. For the upper vibronic levels of the 34173 and 34314 cm−1 subbands, a very local perturbation has been found only at the f rotational level, J′=16.5.

A Weighted-LSM Method to Improve Classification and Concentration Evaluation from Laser-Induced Fluorescence Spectra.

The detection of biological agents using optical systems is an open field of research. Currently, different spectroscopic techniques allow to detect and classify chemical agents while a fast and accurate technique able to identify biological agents is still under investigation. Some optical techniques, such as Laser-Induced Breakdown Spectroscopy (LIBS) or Laser-Induced Fluorescence (LIF), are already used as classification methods. However, the presence of background, spectrum similarities and other confounders make these techniques not very specific. This work shows a new method to achieve better performances in terms of classification and concentration evaluations. The method is based on the Weighted Least Square Minimization method. In fact, by using ad hoc weights, the LSM looks at specific features of the spectra, resulting in higher accuracy. In order to make a systematic analysis, numerical tests have been conducted. With these tests, the authors were able to highlight the various advantages and drawbacks of the new methodology proposed. Then, the method was applied to some LIF measurements to investigate the applicability of the method to preliminary experimental cases. The results show that, by using this new weighted LSM, it is possible to achieve better classification and concentration evaluation performances. Finally, the possible application of the new method is discussed.

Electronic Spectroscopy of SmO in the 645 to 670 nm Range.

The associative ionization reaction Sm + O → SmO+ + e- is being investigated as an electron source that could transiently modify high-altitude electron densities via Sm vapor release. Electronic spectra have been obtained from tests where sounding rockets released Sm vapor, but the interpretation of these results has been hampered by the limited laboratory spectral data available for both SmO and SmO+. The present study extends the spectroscopic characterization of SmO in the 645-670 nm range, where the field data show the most prominent molecular emission features. Rotationally resolved excitation spectra, dispersed laser-induced fluorescence spectra, and fluorescence decay lifetimes are reported. The results are consistent with the assignment of a subset of the red-region bands to configurational transitions of the form Sm2+(4f56s)O2- ↔ Sm2+(4f55d)O2-. Analysis of the excited state hyperfine structure supports this configurational description.

Intensities of KCs [formula omitted] band system up to dissociation threshold: an interplay between spin-orbit, hyperfine and rovibronic coupling effects

The relative intensity distribution in the rotationally resolved laser-induced fluorescence spectra belonging to the E(4)1Σ+→(a3Σ1+,X1Σ+) band systems of the KCs molecule was analyzed. The experimental intensities in doublet P,R progressions assigned to spin-allowed E→X and spin-forbidden E→a transitions up to their common ground dissociation limit were described in the framework of a coupled-channels (CC) deperturbation model applied for the interacting X1Σ+ and a3Σ+ states. The CC intensity simulation was based solely on fixed electronic structure parameters as functions of the internuclear distance R, namely: accurate empirical potential energy curves for all three states, ab initio estimates for matrix elements A(R) of the hyperfine structure (HFS), and transition dipole moments dEX(R) and dEa(R). A comparison between the measured intensities and their theoretical counterparts demonstrates a strong competition between different intramolecular interactions. A weak spin-orbit coupling of the upper E(4)1Σ+ state with the remote 3Π states is responsible for appearance of the E→a vibrational bands for the intermediate va-values. In turn, the HFS coupling between X1Σ+ and a3Σ+ states leads to peculiarities in E→(X,a) intensities, which are pronounced for high vX/a-values in the vicinity of K(42S)+Cs(62S) dissociation threshold. Both adiabatic ro-vibrational and non-adiabatic electronic-rotational interactions explain the abrupt deviation of some observed P/R intensity ratios from the expected Hönl-London factors.

Fine and hyperfine interactions of PbF studied by laser-induced fluorescence spectroscopy.

The fine and hyperfine interactions in PbF have been studied using the laser-induced fluorescence (LIF) spectroscopy method. Cold PbF molecular beam was produced by laser-ablating a Pb rod under jet-cooled conditions, followed by the reaction with SF6. The LIF excitation spectrum of the (0, 0) band in the B2Σ+-X2Π1/2 system of the 208PbF, 207PbF, and 206PbF isotopologues has been recorded with rotational, fine structure, and hyperfine-structure resolution. Transitions in the LIF spectrum were assigned and combined with the previous X2Π3/2-X2Π1/2 emission spectrum in the near-infrared region [Ziebarth et al., J. Mol. Spectrosc. 191, 108-116 (1998)] and the X2Π1/2 state pure rotational spectrum of PbF [Mawhorter et al., Phys. Rev. A 84, 022508 (2011)] in a global fit to derive the rotational, spin-orbit, spin-rotation, and hyperfine interaction parameters of the ground (X2Π1/2) and the excited (B2Σ+) electronic states. Molecular constants determined in the present work are compared with previously reported values. Particularly, the significance of the hyperfine parameters, A⊥ and A‖, of 207Pb is discussed.