Laser-induced Fluorescence Spectra Research Articles

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.

Prolonged Use of Insecticide Dimethoate Inhibits Growth and Photosynthetic Activity of Wheat Seedlings: A Study by Laser-Induced Chlorophyll Fluorescence Spectroscopy.

This paper is an extension of the work published in Journal of Fluorescence (2011) 21: 785-791. In the previous work, we studied the effect of dimethoate (50, 100 and 200ppm) on growth and photosynthetic activity of wheat seedlings after 10days of dimethoate treatment. In the present study, new measurement conditions (dimethoate concentration: 25ppm, treatment period: 20days and 30days) were used in addition to those used in the past work. Various plant growth parameters, photosynthetic pigment content, laser-induced chlorophyll fluorescence (LICF) spectra and fluorescence induction kinetics (FIK) curves were recorded after 10, 20 and 30days of dimethoate treatments. LICF spectra were recorded in the region of 650-780nm using violet diode laser (405nm). FIK curves were recorded at 685nm using red diode laser (635nm). Fluorescence intensity ratio (FIR) of two fluorescence peaks around 685 and 730nm, and variable chlorophyll fluorescence decrease ratio (Rfd) were determined from LICF spectra and FIK curves respectively. Curve-fitted parameters of LICF spectra were used for determination of FIR (F685/F730). The effect of treatment of the insecticide dimethoate on growth and photosynthetic activity of wheat seedlings was examined by using these parametersas well as the past work. In 10-days treatment, 25 and 50ppm dimethoate showed stimulatory effect with better stimulation being observed at 25ppm. All studied concentrations higher than 50ppm exhibited inhibitory effect on wheat seedlings. In case of dimethoate treatment studied for longer durations (more than 10days), all concentrations showed inhibitory effect. Lower doses which showed some positive response for short time duration become toxic with the extension of treatment periods. Thus, this study clearly confirms the toxic effect of dimethoate on wheat plants.

LIF spectroscopy of epithelial tissues: Assay of structural changeover due to the cancer progression.

Most of the human cancers occur in epithelial tissues containing basic cells with different shapes, and not only do the spectral properties of the tissue pigments alter due to cancer, but the cellular architecture also change. However, in optical diagnosis of the cancerous tissues, attention has been paid to the spectral changeover of native chromophores as bio-markers. Here, we have attempted to assay the structural alterations of the epithelial tissues during the cancer progression utilizing Laser induced fluorescence (LIF) spectroscopy as a fast, sensitive and easy-to-use method. In this regard, angular dependence of the LIF spectral features of the healthy and cancerous epithelial tissues (soaked in Rhodamine 6G solution) from three different human organs i. e. uterus, colon and kidney with distinct microstructures have been examined. In general, both wavelength and intensity at the peak of the LIF spectra depend on the tissue orientation and the angle of detection respect to the laser beam direction. Those optical parameters also demonstrate distinctive alterations in different tissues that is explicated based on the morphological alteration of the epithelial cells in each carcinoma type provided by pathology data.

Spectroscopic identification and characterization of the aluminum methylene (AlCH2) free radical.

The AlCH2 free radical has been spectroscopically identified for the first time. This highly reactive species was produced in an electric discharge jet using trimethylaluminum vapor in high pressure argon as the precursor. The laser-induced fluorescence spectrum of the B̃2A2-X̃2B1 band system in the 513-483nm region was recorded, and the 0-0 bands of AlCH2 and AlCD2 were studied at high resolution. The fine structure splittings were found to be due primarily to the Fermi contact interaction in the excited state rather than the usual spin-rotation coupling. Rotational analysis gave the molecular constants of the combining states, and the geometries were obtained as r″A1-C=1.9591A◦,r″C-H=1.1061A◦,θHCH ″=110.41◦ and r'A1-C=1.9431A◦,r'C-H=1.0911A◦,θHCH '=115.41◦. The bond lengths correspond to an aluminum-carbon single bond in both states.

Water quality assessment in mosquito breeding habitats based on dissolved organic matter and chlorophyll measurements by laser-induced fluorescence spectroscopy.

Rapid urbanization and its associated pollution can affect water quality in mosquito breeding habitats and, as a result, the ecology and control of mosquito vectors. To understand the effects of pollution on mosquito vectors, an accurate assessment of water quality in breeding habitats is needed. Presently, water quality assessment of mosquito breeding habitats is usually based on the measurement of individual physicochemical parameters. However, several parameters are sometimes difficult to interpret or may not give a clear picture of the overall water quality of the breeding habitats, especially when the pollutants are in complex mixtures. This study employed the use of Laser-Induced Fluorescence (LIF) spectroscopy to assess water quality in breeding habitats of Anopheles, Aedes, and Culex mosquitoes in urban areas in Cape Coast, Ghana. The LIF spectra, using a 445-nm diode laser, were measured from field-collected water samples in the laboratory. The LIF spectra showed the presence of dissolved organic matter (DOM) and chlorophyll in the breeding habitats. The DOM and chlorophyll fluorescence signals were normalised by the Raman vibrational signals to determine water quality in each habitat. The overall water quality was better in Aedes breeding habitats than in Anopheles and Culex breeding habitats. The poor water quality in Anopheles and Culex breeding habitats was due to the presence of high fulvic acid and chlorophyll content, which often reflect pollutants from anthropogenic sources. Anopheles and Aedes habitats were made up of mainly An. coluzzii and Ae. aegypti respectively while Culex species were identified to genus level. The results add up to the growing concern about the breeding of Anopheles in polluted habitats. The study demonstrated for the first time the ability of LIF spectroscopy to assess water quality in mosquito breeding habitats.

Burst-mode 1-methylnaphthalene laser-induced fluorescence: extended calibration and measurement of temperature and fuel partial density in a rapid compression machine

In this work, tracer-based laser-induced fluorescence (LIF) with the tracer 1-methylnaphthalene is utilized to study temperature and fuel courses in a rapid compression machine (RCM) under high temperature and pressure conditions. A burst-mode Nd:YAG laser at 266 nm is applied for excitation of tracer fluorescence at a frame rate of 7.5 kHz. A high-speed intensified CMOS camera equipped with an image doubler is used for 2-color LIF (2c-LIF) thermometry. With known local temperature, the fuel partial density can be determined using the signal of the channel covering the complete LIF spectrum. Both temperature and fuel partial density are determined during the compression and expansion strokes in nitrogen and air atmospheres. For this purpose, first-time 1-MN LIF calibration measurements in air atmosphere were performed for cylinder pressures up to 2.8 MPa. This significantly extends the calibration data base generated in current calibration cells. Although the LIF signal dropped significantly due to oxygen quenching, first promising measurements of temperature and fuel partial density were conducted in the RCM at relevant equivalence ratios. The influence of the RCM driving gas pressure on the temperature course is shown for cylinder pressures up to 7.4 MPa in nitrogen atmosphere. Although the temperature and concentration fields are very homogeneous at early points in time during compression, inhomogeneities in terms of millimeter-sized hot and cold gas regions were resolved especially near top dead center (TDC) using the present approach. These structures were also visible in the fuel partial density field. These inhomogeneities are due to the heat transfer between the hot gas and the cool walls and are probably also induced by the piston movement. Especially at TDC, the minimum gas temperature is about 300 K lower than the peak temperature in the wall region of the cylinder head. These cool region temperatures are much lower than in piston engines and other RCMs reported in the literature at comparable conditions, which may due to the special design of the present layout of the machine.

Olive oil classification with Laser-induced fluorescence (LIF) spectra using 1-dimensional convolutional neural network and dual convolution structure model

Laser-induced fluorescence (LIF) spectroscopy is widely used for the analysis and classification of olive oil. This paper proposes the classification of LIF data using a specific 1-dimensional convolutional neural network (1D-CNN) model, which does not require pre-processing steps such as normalisation or denoising and can be flexibly applied to massive data. However, by adding a dual convolution structure (Dual-conv) to the model, the features of the 1-dimensional spectra are more scattered within one convolution-pooling process; thus, the classification effects are improved. The models were validated through an olive oil classification experiment which contained a total of 72,000 sets of LIF spectra data, and the classification accuracy rate reached ∼99.69%. Additionally, a common classification approach, the support vector machine (SVM), was utilised for the comparison of the results. The results show that the neural networks perform better than the SVM. The Dual-conv model structure has a faster convergence speed and higher evaluation parameters than those of the 1D-CNN in the same period of iterations, without increasing the data dimension.

Barely fluorescent molecules. II. Twin-discharge jet laser-induced fluorescence spectroscopy of HSnBr and DSnBr.

HSnBr and DSnBr have been detected for the first time by a combination of laser-induced fluorescence (LIF), fluorescence hole-burning, and wavelength resolved emission spectroscopies. The transient molecules were produced in a twin-discharge jet using separate precursor streams of SnH4/SnD4 and HBr/DBr, both diluted in high pressure argon. The Ã1A″-X̃1A' spectrum of HSnBr only consists of the 00 0 and 20 1 cold bands that show clearly resolved subband structure with fluorescence lifetimes varying from 526 to 162ns. The DSnBr LIF spectrum exhibits four bands (00 0, 20 1, 20 2, and 10 1) whose fluorescence lifetimes decrease from 525ns (00) to 175ns (11). Single vibronic level emission spectra have provided extensive information on the ground state vibrations, including all the anharmonicities and the harmonic frequencies. Fluorescence hole-burning experiments have shown that a few higher HSnBr nonfluorescent levels are very short-lived but still detectable. The ab initio studies of Tarroni and Clouthier [J. Chem. Phys. 156, 064304 (2022)] show that these molecules dissociate into SnBr + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the LIF spectra. HSnBr is a barely fluorescent molecule in the sense that only vibrational levels less than or equal to 317cm-1 in the excited state emit detectable photons down to the ground state.

Barely fluorescent molecules. I. Twin-discharge jet laser-induced fluorescence spectroscopy of HSnCl and DSnCl.

The divalent tin transient molecules HSnCl and DSnCl have been detected for the first time by laser-induced fluorescence (LIF) spectroscopy. HSnCl/DSnCl were produced in a twin-discharge jet using separate precursor streams of SnH4/SnD4 and the discharge products from HCl/DCl, both diluted in high pressure argon. The Ã1A″-X̃1A' spectrum of HSnCl consists of a single vibronic 00 0 band with a very short fluorescence lifetime (∼30ns). In contrast, the LIF spectrum of DSnCl exhibits three bands (00 0,20 1,and20 2), whose fluorescence lifetimes decrease from 393ns (00) to less than 10ns (22). Single vibronic level emission spectra have been recorded, providing information on all three vibrational modes in the ground state. Previous detailed ab initio studies indicate that these molecules dissociate into SnCl + H on the excited state potential surface and this is the cause of the short fluorescence lifetimes and breaking off of the fluorescence. It is fortunate that the HSnCl excited state zero-point level is still fluorescent or it would not be detectable by LIF spectroscopy.

Utilizing laser spectrochemical analytical methods for assessing the ripening progress of tomato

To meet market demands and minimize losses, the tomato crop (Solanum Lycopersicum L.) requires a simple, rapid, and cost-effective method to distinguish between different maturity stages with high accuracy. This study aimed at evaluating two spectrochemical analytical techniques, namely laser-induced fluorescence (LIF) and laser-induced breakdown spectroscopy (LIBS), to discriminate three different maturity stages of tomato fruit (‘Green/Breaker’; ‘Turning/Pink’; and ‘Light-red/Red’). The simple linear regression confirmed the obtained LIF results with chlorophyll content (mg/100 g), hue angle (h°), and firmness (kg/cm2) of the different maturity stages (measured by conventional methods). Furthermore, the findings showed that the peak intensities of LIF spectra decreased with the chlorophyll content depletion during ripening. Moreover, the data exposed a reasonably good association between LIF spectra and chlorophyll content with a regression coefficient of 0.85. On the other hand, firmness and skin hue have shown an excellent predictor for the spectra with a high regression coefficient of 0.94. For LIBS spectra of each maturity stage, the ratios of Ca’s ionic-to-atomic spectral lines intensities have followed the same trend as conventionally measured firmness. The results demonstrated that LIF and LIBS are accurate, easy, and fast techniques used to define tomatoes’ different ripening stages. Both methods are useable in situ without any prior laboratory work.

Remote laser fluorescence method for oil leak detection at an excitation wavelength of 266 nm

Subject of study. The possibility of implementing a remote laser fluorescence method to detect oil leaks on the Earth’s surface at a fluorescence excitation wavelength of 266 nm was analyzed and substantiated. Method. Laser-induced fluorescence spectra of clean oil, oil spilled on different types of soil at different time intervals after leakage, and natural and anthropogenic terrain elements were investigated. The experimental setup for remote measurements of laser-induced fluorescence spectra of oil pollution on the Earth’s surface using a laser source with a wavelength of 266 nm, pulse energy of 0.3 mJ, and a single spectral channel of a detector with a central wavelength of 510 nm and a bandwidth of 84 nm is described. The detection possibility of oil pollution against a background of natural vegetation (shrubs and grass) and soil without oil pollution were investigated remotely at a distance of 75 m using emission with a fluorescence excitation wavelength of 266 nm. Main results. Laser-induced fluorescence spectra of commercial oil from the Ryazan oil refinery and oil from Almetyevsk oil field were obtained in addition to the fluorescence spectra of vegetation, soil, and asphalt. Analysis of the data in laser-induced fluorescence spectra indicates that the maximum of the oil spectra is in the range 450–550 nm. Laser-induced fluorescence from vegetation is the largest interference signal in the detection of oil leaks on the Earth’s surface. Remote detection of oil pollution against the background of vegetation and other landscape elements was experimentally demonstrated both immediately after pollution and two days later. Practical significance. The possibility of remote monitoring of oil pollution on the Earth’s surface was experimentally demonstrated at a fluorescence excitation wavelength of 266 nm at distances close to safe aircraft flying altitudes.

Laser induced fluorescence spectra of the [formula omitted][formula omitted] – [formula omitted][formula omitted] transition of jet cooled [formula omitted]NO3 and [formula omitted]NO3 I: [formula omitted] progressions in the ground [formula omitted][formula omitted] state

14NO3 and 15NO3 were generated in a supersonic free jet, and their laser excitation spectra and dispersed laser induced fluorescence (LIF) spectra of the B∼2E′ – X∼2A2′ transitions from the vibrationless B∼2E′ state were observed. Because of the cooling of the gas in the supersonic jet, the observed spectra are with resolution an order of magnitude higher than those previously reported and allowed us to resolve, for example, the splitting between the a1′ and e′ components of the 2ν4 state. The spectra of 14NO3 and 15NO3 are similar but small isotope shift in the latter allowed us to separate the ν1 band from the 3ν4 (a1′) band which were overlapped in the former.The high resolution dispersed LIF spectra revealed panoramic views of the vibrational level structures in the X∼2A2′ ground electronic states of the two isotopomers. Although the accuracies of the measurement of energy levels are much less than those obtained from the direct high resolution infrared spectroscopy, the obtained patterns are more complete because totally symmetric (a1′) vibrations such as ν1,2ν2,2ν1,ν1 + 2ν2 etc. are also observed. Such clear and complete spectra have revealed remarkable ν4 progressions with detailed vibrational structures in the ground X∼2A2′ state. Near complete progressions of nν4 overtone bands, nν4 with n = 1 ∼ 5, ν1 + nν4 bands with n = 1 ∼ 4, some 2ν2 + nν4 and ν3 + nν4 have been assigned leaving few lines unassigned. There is no evidence of anomaly in the degenerate ν4 bending progression of the B∼2E′ – X∼2A2′ electronic transition of NO3.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging.

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state‐dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser‐induced fluorescence (LIF) spectra of samples comprising both the mass‐selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non‐emissive VOPc⋅ forms a high‐spin diradical cation (VOPc+..) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR‐emitting electro‐optic devices.

Radiative force from optical cycling on magnesium monofluoride

We demonstrate radiative force deflection by using an optical cycling of the $X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{1/2}^{+}\ensuremath{-}A{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Pi}}}_{1/2}$ electronic transition in diatomic molecule magnesium monofluoride (MgF). For the (0,0) and (0,1) bands of the electronic transition, the Franck-Condon factors---obtained by the dispersed laser-induced fluorescence spectrum---are ${f}_{00}=0.972$ and ${f}_{01}=0.028$, which suggests that the vibrational branching is quasiclosed. Furthermore, the dark Zeeman sublevels are destabilized by applying an external magnetic field. The cycling fluorescence is clearly observed to reveal that the molecular beam is deflected by 4.9 mm via scattering $\ensuremath{\sim}200$ photons per molecule. This work is encouraging toward laser cooling and magneto-optical trapping of MgF molecules.

Observation and modeling of bound-free transitions to the X1Σ+ and a3Σ+ states of KCs

The oscillation continuum in laser-induced fluorescence spectra of bound-free c3Σ+ → a3Σ+ and (4)1Σ+ → X1Σ+ transitions of the KCs molecule was recorded by a Fourier-transform spectrometer and modeled under the adiabatic approximation. The required interatomic potentials for ground a3Σ+ and X1Σ+ states were reconstructed in an analytical Chebyshev-polynomial-expansion form in the framework of the regularization direct-potential-fit procedure based on the simultaneous consideration of experimental line positions from Ferber et al. [Phys. Rev. A 80, 062501 (2009)] and the present ab initio calculation of short-range repulsive potential data. It was proved that the repulsive part over the dissociation limit of the derived a3Σ+ potential reproduces the experiment better than the potentials reported in the literature. It is also shown that all empirical and semi-empirical potentials available for the X1Σ+ state reproduce the bound-free (4)1Σ+ → X1Σ+ spectrum with equal quality in the range of observations.