Measurement of boron isotopic ratio with non-gated molecular spectroscopy of femtosecond laser-produced plasma

Laser-induced breakdown spectroscopy can be augmented by concurrent spectroscopic measurements of molecules and molecular ions resulting from recombination of laser-ablated plasmas with the ablated sample and surrounding environment. Molecular emissions can exhibit significantly greater isotopic shifts than atomic emissions, making this approach attractive for isotopic characterization of nuclear materials. We describe a measurement of boron isotopic ratios in four boron-containing samples utilizing the emission from boron monoxide radicals formed in the expanding laser-induced ablation plume. Femtosecond laser excitation is used and the emission in the 532–540 nm region of interest is studied, where a linear superposition of characteristic spectra for two boron isotopes has been applied for reconstruction of the boron isotopic ratio. It is also demonstrated that the use of non-gated measurements of emission spectra suffices for accurate isotopic characterization using this method. The application of multivariate regression to deconvolve the isotopic ratio from the measured emission spectra is discussed in detail, including the limitations and subtleties of this approach.     

Laser Ablation Molecular Isotopic Spectrometry

A new method of performing optical isotopic analysis of condensed samples in ambient air and at ambient pressure has been developed: Laser Ablation Molecular Isotopic Spectrometry (LAMIS). The technique uses radiative transitions from molecular species either directly vaporized from a sample or formed by associative mechanisms of atoms or ions in a laser ablation plume. This method is an advanced modification of a known atomic emission technique called laser-induced breakdown spectroscopy (LIBS). The new method — LAMIS — can determine not only chemical composition but also isotopic ratios of elements in the sample. Isotopic measurements are enabled by significantly larger isotopic shifts found in molecular spectra relative to atomic spectra. Analysis can be performed from a distance and in real time. No sample preparation or pre-treatment is required. Detection of the isotopes of hydrogen, boron, carbon, and oxygen are discussed to illustrate the technique.     

Laser Ablation Molecular Isotopic Spectrometry: Parameter influence on boron isotope measurements

Laser Ablation Molecular Isotopic Spectrometry (LAMIS) was recently reported for optical isotopic analysis of condensed samples in ambient air and at ambient pressure. LAMIS utilizes molecular emissions which exhibit larger isotopic spectral shits than in atomic transitions. For boron monoxide ¹⁰BO and ¹¹BO, the isotopic shifts extend from 114 cm⁻¹ (0.74 nm) to 145–238 cm⁻¹ (5–8 nm) at the B²Σ⁺ (v = 0) → X²Σ⁺ (v = 2) and A²Π_i (v = 0) → X²Σ⁺ (v = 3) transitions, respectively. These molecular isotopic shifts are over two orders of magnitude larger than the maximum isotopic shift of approximately 0.6 cm⁻¹ in atomic boron. This paper describes how boron isotope abundance can be quantitatively determined using LAMIS and how atomic, ionic, and molecular optical emission develops in a plasma emanating from laser ablation of solid samples with various boron isotopic composition. We demonstrate that requirements for spectral resolution of the measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric analysis of spectra. Sensitivity can be improved by using a second slightly delayed laser pulse arriving into an expanding plume created by the first ablation pulse.     

Laser ablation molecular isotopic spectrometry of water for 1D2/1H1 ratio analysis

Laser Ablation Molecular Isotopic Spectrometry (LAMIS) has been investigated for optical isotopic analysis of the deuterium to protium ratio in enriched water samples in ambient air at atmospheric pressure. Multivariate PLSR (Partial Least Squares Regression) based calibrations were carried out and validated using multiple statistical parameters. Comparisons of results are reported using two spectrometers having two orders of magnitude difference in spectral resolution. The accuracy and precision of isotopic analysis depends on the spectral resolution and the inherent isotope shift of the elements. The requirements for spectral resolution of the measurement system can be significantly relaxed when the isotopic abundance ratio is determined using chemometric processing of the spectra. Large isotopic shifts in the individual rotational branches of OH/OD molecular emission spectra were measured. Optimized temporal conditions for LAMIS measurements were established. Several sub-regions of spectra were used for PLSR calibration and the results demonstrate that both the emission intensity and degree of spectral differentiation affect the quality of the PLSR calibration. LAMIS results also were compared with traditional LIBS results obtained using PLSR and a spectral deconvolution method, demonstrating the advantages of LAMIS over LIBS with respect to isotopic composition determination.     

Laser Ablation Molecular Isotopic Spectrometry: Strontium and its isotopes

The experimental details are reported of Laser Ablation Molecular Isotopic Spectrometry (LAMIS) and its application for performing optical isotopic analysis of solid strontium-containing samples in ambient atmospheric air at normal pressure. The LAMIS detection method is described for strontium isotopes from samples of various chemical and isotopic compositions. The results demonstrate spectrally resolved measurements of the three individual ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr isotopes that are quantified using multivariate calibration of spectra. The observed isotopic shifts are consistent with those calculated theoretically. The measured spectra of diatomic oxide and halides of strontium generated in laser ablation plasmas demonstrate the isotopic resolution and capability of LAMIS. In particular, emission spectra of SrO and SrF molecular radicals provided clean and well resolved spectral signatures for the naturally occurring strontium isotopes. A possibility is discussed of using LAMIS of strontium isotopes for radiogenic age determination.     

Underwater sediment analyses by laser induced breakdown spectroscopy and calibration procedure for fluctuating plasma parameters

Laser Induced Breakdown Spectroscopy (LIBS) was applied on sediments directly under water. The aim of the research was to develop a method for measuring the sediment elemental composition, including minor elements, which could be implemented in-situ. The plasma was generated by a double-pulse, Q-Switched Nd:YAG laser operated at 1064 nm. For signal detection, both ICCD and non-gated, compact detectors were used. The major difficulties in underwater sediment analyses are related to the natural and laser induced surface roughness, and to the sample softness. The latter is responsible for the formation of particle clouds above the surface, which scatter both the laser and plasma radiation, and often results in breakdown formation above the analyzed surface. In such cases, a broad sonoluminescence emission from water, formed during the gas bubble collapse was sometimes registered. Under optimized experimental conditions, even by using a non-gated detector and single shot acquisition, it was possible to detect several minor sediment constituents, such as titanium, barium, manganese and others. A crude estimation of the Limit of Detection (LODs) for these elements was performed by underwater measurements on certified soils/sediments. Due to strong shot-to-shot fluctuations in the plasma temperature, well correlated calibration curves, aimed for quantitative analyses, could only be obtained after applying an appropriate data processing procedure. The latter selects automatically only the spectra characterized by similar plasma parameters, which are related to their continuum spectral distribution. Application of such a procedure improves the measurement accuracy also in other surroundings and on samples different from the ones analyzed here.     

UF6 enrichment measurements using TDLS techniques

The objective of this work was investigation of possibility of tunable diode laser spectroscopy (TDLS) technique application for gaseous uranium hexafluoride (UF6) isotope measurement. Spectra of uranium hexafluoride gas mixture were investigated using two different Fourier Transform Spectrometers Vector 22 and Bruker 66v. Observed spectral features were identified and model spectra of different gas mixture components were developed. Optimal spectral range for measurements was determined near maximum of UF6 combination band nu1+nu3. Laboratory prototype of multi-channel instrument under consideration based on tunable diode lasers was built and algorithms were developed to measure gaseous UF6 isotopic ratios. Diode laser used operated at the wavelengths near lambda=7.68 microm. It was placed in a liquid nitrogen cooled cryostat. Three instrument channels were used for laser frequency calibration and spectra recording. Instrument was tested in measurements of real UF6 gas mixtures. Measurement accuracy was analyzed and error sources were identified. The root-mean-square random error in the 235U isotopic content is characterized by a spread of about 0.27% for quick measurements (at times less than 1 min) and 1% for periods of more than an hour. It was estimated that the measurement accuracy could be improved by at least an order of magnitude by minimizing the error sources.     

A compact and portable laser-induced breakdown spectroscopy instrument for single and double pulse applications

We present LIBS experimental results that demonstrate the use of a newly developed, compact, versatile pulsed laser source in material analysis related to art and archaeological applications in view of research aiming at the development of portable LIBS instrumentation. LIBS qualitative analysis measurements were performed on various samples and objects, and the spectra were recorded in gated and non-gated modes. The latter is important because of advantages arising from size and cost reduction when using simple, compact spectrograph-CCD detection systems over the standard ICCD-based configurations.     

Spectral power distribution and quantum yields of Sm3+-doped heavy metal tellurite glass under the pumping of blue lighting emitting diode

Quantum yields for multichannel transition emissions have been determined in Sm3+-doped heavy metal tellurite glass under the pumping of blue lighting emitting diode for the first time. To achieve this goal, the necessary fluorescence spectra were measured and calibrated in an integrating sphere, which was connected to a CCD detector with a 400 microm-core optical fiber. The spectral power distribution of the sample under the blue LED pumping was derived from the measured spectra firstly, and then the quantum yields for the visible emissions of Sm3+ were calculated based on the distribution and the total quantum yields in visible region is 7.55%. For accurate measurements, integrating sphere method is proved to be a reliable and reproducible way to characterize luminescence and laser materials.     

Pulse chirp effects in ultrafast laser-induced breakdown spectroscopy

When compared to many other sensitive methods for material detection, such as inductively coupled mass spectroscopy and thermal ionization mass spectroscopy, laser-induced breakdown spectroscopy (LIBS) typically exhibits a lower signal-to-noise ratio (SNR), resulting in higher detection limits. Increasing the SNR of LIBS would improve the ability to characterize the sample composition with increased accuracy and speed and reduce the amount of material needed to perform analysis. We have been investigating the effect of simple ultrashort laser pulse shaping on the SNR of LIBS. Our goal is to control the dynamics of the ionization and recombination processes in the laser-produced plasma to favorably affect the SNR associated with the line emission from the plasma. Pulse shaping is performed using an acousto-optic programmable dispersive filter. An adaptive learning algorithm is being developed to automate the pulse shape optimization process for maximization of LIBS SNR in nuclear security-relevant material characterization scenarios. We report a 27 % increase of the SNR for non-gated LIBS measurements of uranium by utilizing simple pulse shaping limited exclusively to excess quadratic spectral phase of the laser pulse.     

Spectrum shift fitting technique for atomic emission spectrometry

The use of the principle of additivity for the atomic emission spectra registered by a multi-element solid-state detector (SSD) requires to take into account the possible non-controlled spectrum shift. The method of solution for the problem, based on the construction of parabolic model of signal change on pixels under the spectrum shift with respect to detector, is suggested. The algorithm of the model construction depends neither on the spectral line shape nor on the spectrometer apparatus function. The method can be applied with equal success both for the short spectral regions (10–20 pixels) and for the long regions (10 000 and more pixels). The values of relative spectrum shifts can lie in a continuous range from 0.05 to 100 and more pixels. The advantages of this method are shown on the examples of subtraction of spectra in the process of registration on diffraction spectrograph PGS-2 (Carl Zeiss, Jena) with the help of a linear solid state detector (MAES-10, VMK-Optoelectronika, Russia, Novosibirsk).     

光电子成像方法研究Xe时间分辨多光子电离过程

利用自行研制的离子成像检测器研究了Xe的飞秒时间分辨双色多光子电离过程.Xe的408nm多光子电离对比实验结果表明,该离子成像检测器与相应的进口产品具有相近的光电子能量分辨率.在272nm飞秒激光作用下,3光子电离产生能量为1.57和0.26eV的光电子,分别对应于Xe+的两个自旋态;在408nm飞秒激光作用下,还观察到第一级阈上电离产生的光电子.在双色飞秒时间分辨实验中,随着两束光相对时间的改变,光电子能谱出现了一系列的变化;随着两束光时间重合程度的增加,由双色多光子电离(3+1'或4'+1)产生的光电子信号逐渐加强;在第二束光的作用下,由第一束单色光产生的光电子出现能量红移,第二束光同时也导致中间态布居数减少.这种光电子能谱的红移现象反映了原子体系中激光场诱导有质动力势的时间分辨动态调制过程.     

Determination of phosphorus, sulfur and the halogens using high-temperature molecular absorption spectrometry in flames and furnaces—A review

The literature about the investigation of molecular spectra of phosphorus, sulfur and the halogens in flames and furnaces, and the use of these spectra for the determination of these non-metals has been reviewed. Most of the investigations were carried out using conventional atomic absorption spectrometers, and there were in essence two different approaches. In the first one, dual-channel spectrometers with a hydrogen or deuterium lamp were used, applying the two-line method for background correction; in the second one, a line source was used that emitted an atomic line, which overlapped with the molecular spectrum. The first approach had the advantage that any spectral interval could be accessed, but it was susceptible to spectral interference; the second one had the advantage that the conventional background correction systems could be used to minimize spectral interferences, but had the problem that an atomic line had to be found, which was overlapping sufficiently well with the maximum of the molecular absorption spectrum. More recently a variety of molecular absorption spectra were investigated using a low-resolution polychromator with a CCD array detector, but no attempt was made to use this approach for quantitative determination of non-metals. The recent introduction and commercial availability of high-resolution continuum source atomic absorption spectrometers is offering completely new possibilities for molecular absorption spectrometry and its use for the determination of non-metals. The use of a high-intensity continuum source together with a high-resolution spectrometer and a CCD array detector makes possible selecting the optimum wavelength for the determination and to exclude most spectral interferences.     

Anlysenstörungen durch strukturierten Untergrund in der Atomabsorptionsspektrometrie

In atomic absorption spectrometry the determination of the spectral background is very difficult. The problem of the background measurement is namely to determine the spectral background under the very narrow resonance line, which cannot be resolved by the monochromator. The normal method of background measurement with a source of continuous radiation is correct only in that case when the background is a spectral continuum, e.g., a dissociation continuum of molecules. The background measurement may be faulty, if the background is due to line-rich electronic excitation spectra of molecules. The actual background is totally dependent upon whether or not a rotational line of the molecular spectrum coincides with the, atomic absorption line. This cannot be deduced from measurements obtained from those atomic absorption instruments normally used. Therefore in order to avoid systematic errors in atomic absorption analysis a systematic study of interfering molecular spectra with high resolution instruments is the only way to solve this problem.     

LIBS using dual- and ultra-short laser pulses

Pre-ablation dual-pulse LIBS enhancement data for copper, brass and steel using ns laser excitation are reported. Although large enhancements are observed for all samples, the magnitude of the enhancement is matrix dependent. Whereas all of the dual-pulse studies used ns laser excitation we see interesting effects when using ps and fs laser excitation for single-pulse LIBS. LIBS spectra of copper using 1.3 ps and 140 fs laser pulses show much lower background signals compared to ns pulse excitation. Also, the atomic emission decays much more rapidly with time. Because of relatively low backgrounds when using ps and fs pulses, non-gated detection of LIBS is shown to be very effective. The plasma dissipates quickly enough using ps and fs laser pulses, that high pulse rates, up to 1,000 Hz, are effective for increasing the LIBS signal, for a given measurement time. Finally, a simple near-collinear dual-pulse fiber-optic LIBS probe is shown to be useful for enhanced LIBS measurements.     

Semi-empirical and vibrational studies of flavone and some deuterated analogues

The infrared solid state, Raman solid state and tetrachloride solution spectra of flavone have been obtained. Assignments of most of the vibrational data have been performed by comparison between the spectra of flavone and three isotopic species, deuterated on the A, B and C rings, respectively. The vibrational frequencies for all the investigated compounds have been calculated from the conformational analysis of flavone using the semi-empirical AM1 method and compared with experimental values. The correlation is more or less satisfactory; however, for some vibrational modes, the calculated isotopic shifts agree better with experiment than do the frequencies themselves. Specific vibrational modes which retain a benzene ring mono-substituted and ortho-distributed character have been recognized in the spectra, according to literature data, isotopic frequency shifts and graphic representation of the atomic displacements.     

Frequency shifts due to the interference of resolved peaks in magnitude-mode Fourier-transform ion cyclotron resonance mass spectra

We have obtained relationships for frequency shifts resulting from the interference of spectral components for the magnitude mode Fourier transform. The approximation of a weak perturbation of well resolved peaks has been used. Both the low- and high-pressure limits for Fourier-transform ion cyclotron resonance (FTICR) operation have been considered. We have found that the shifts can be either negative or positive, depending on the initial phase and/or the choice of the time-domain interval. The magnitude of shifts generally does not exceed the peak width. In the approximation of small perturbations the shifts produced by multiple peaks are additive. We have compared theoretical results with experimental shifts for isotopic clusters of multiply charged insulin. Up to 1 ppm frequency variations were experimentally observed for the insulin 5+ charge state, consistent with theoretical estimates. The peak interference is of particular significance in the case of bio-molecular mass spectra having a large number of peaks and covering a considerable dynamic range (i.e., relative abundance). We conclude that the common mass measurement procedure based on the location of the magnitude mode maxima of well resolved peaks can result in systematic mass measurement errors. The relationships obtained provide corrections for the frequency shifts and thus improve the mass measurement accuracy.     

Investigating ion-surface collisions with a niobium superconducting tunnel junction detector in a time-of-flight mass spectrometer

The performance of an energy sensitive, niobium superconducting tunnel junction (STJ) detector is investigated by measuring the pulse height produced by impacting molecular and atomic ions at different kinetic energies. Ions are produced by laser desorption and matrix-assisted laser desorption in a time-of-flight mass spectrometer. Our results show that the STJ detector pulse height decreases for increasing molecular ion mass, passes through a minimum at around 2000 Da, and then increases with increasing mass of molecular ions above 2000 Da. The detector does not show a decline in sensitivity for high mass ions as is observed with microchannel plate ion detectors. These detector plus height measurements are discussed in terms of several physical mechanisms involved in an ion-surface collision.     

Intrinsic self-calibration of non-destructive gamma spectrometric measurements determination of U,Pu and241Am isotopic ratios

The intrinsic consistency and discrete character of high resolution gamma spectrometric measurements are used in order to make self-calibration of these measurements. Both energy and overall relative efficiency (including all geometrical self-attenuation and attenuation factors as well as the detector efficiency) can be performed using the same measured spectrum. The method is particularly useful for non-destructive gamma spectrometric measurements of isotopic ratios on different nuclear material samples. As an illustration, a complete isotopic analysis of U/Pu mixed oxide rods is given. The accuracy of the isotopic measurements depends strongly on the accuracy of the available nuclear data and comparison with destructive analysis when possible facilitates the process of obtaining more accurate nuclear data for the isotopes involved. The attainable accuracy is estimated to be better than 0.5% for²³⁹Pu and²⁴¹Pu measurements.     

Time-resolved ultraviolet laser-induced breakdown spectroscopy for organic material analysis

Ultraviolet pulses (266 nm) delivered by a quadrupled Nd:YAG laser were used to analyze organic samples with laser-induced breakdown spectroscopy (LIBS). We present characteristics of the spectra obtained from organic samples with special attentions on the emissions of organic elements, O and N, and molecular bonds CN. The choice of these atomic or molecular species is justified on one hand, by the importance of these species to specify organic or biological materials; and on the other hand by the possible interferences with ambient air when laser ablation takes place in the atmosphere. Time-resolved LIBS was used to determine the time-evolution of line intensity emitted from these species. We demonstrate different kinetic behaviors corresponding to different origins of emitters: native atomic or molecular species directly vaporized from the sample or those generated through dissociation or recombination due to interaction between laser-induced plasma and air molecules. Our results show the ability of time-resolved UV-LIBS for detection and identification of native atomic or molecular species from an organic sample.