Atomic absorption with ultracold atoms

Recent advances in laser-atom cooling techniques and diode-laser technology now allow one to conduct an idealised atomic absorption experiment comprising a sample of ultracold, quasi-stationary absorbing atoms and a source of near-monochromatic resonant light. Under such conditions, the atomic absorption coefficient at line centre is independent of the oscillator strength of the atomic resonance line. This offers the prospect of ‘oscillator-strength-free’ atomic absorption spectroscopy in which the absorption signal is equally large for both strong and weak (closed) transitions of the same wavelength and in which absolute atomic absorption could be performed without knowledge of the oscillator strength. Moreover, the resolution and sensitivity for a given atom density are greatly enhanced, typically by approximately three orders of magnitude (and even more for weak transitions), compared with conventional flame or graphite-furnace atomic absorption. We describe an atomic absorption experiment based on samples of ultracold, laser-cooled caesium atoms and a narrow-bandwidth diode laser source that approximates the idealised conditions for oscillator-strength-free atomic absorption. The absorption measurements are used to determine the number density and temperature (approx. 6 μK) of the sample of ultracold atoms. Some of the technical obstacles that would have to be overcome before samples of ultracold atoms and diode laser sources could be used in analytical atomic absorption spectroscopy are discussed.     

Isotope selective element analysis by diode laser atomic absorption spectrometry

The analytical figures of merit of isotope selective diode laser atomic absorption spectrometry (DLAAS) in low-pressure graphite furnaces are given for lithium and rubidium. While⁶Li and⁷Li were measured by Doppler-limited as well as by Doppler-free absorption spectroscopy of the 670.79 nm resonance line, Doppler-free saturation spectroscopy was applied for analysis of the⁸⁵Rb and⁸⁷Rb D2 resonance line at 780.03 nm. Three different modulation techniques were applied and compared: (i) intensity modulation, (ii) wavelength modulation, and (iii) a combination of intensity and wavelength modulation.     

Laser atomic absorption spectrometry of excited Hg in a discharge applying sum frequency mixing of two diode lasers (preliminary results)

Wavelength modulation diode laser atomic absorption spectrometry is applied to the detection of atomic mercury. Transitions from metastable energy levels highly populated in a radio-frequency discharge are induced with laser diodes by use of nonlinear techniques. The wavelength of one strong transition at 365.119 nm with a high oscillator strength is obtained by sum frequency generation of two diode lasers. The cold vapor technique is used to transfer ionic into atomic mercury. The mercury in the vapor phase is transported by an argon stream into the discharge tube. From the time-dependent absorption signals detection limits of 100 ng/L are achieved at this state of research.     

Laser atomic absorption spectrometry of excited Hg in a discharge applying sum frequency mixing of two diode lasers (preliminary results)

Wavelength modulation diode laser atomic absorption spectrometry is applied to the detection of atomic mercury. Transitions from metastable energy levels highly populated in a radio-frequency discharge are induced with laser diodes by use of nonlinear techniques. The wavelength of one strong transition at 365.119 nm with a high oscillator strength is obtained by sum frequency generation of two diode lasers. The cold vapor technique is used to transfer ionic into atomic mercury. The mercury in the vapor phase is transported by an argon stream into the discharge tube. From the time-dependent absorption signals detection limits of 100 ng/L are achieved at this state of research.     

Application of the 2f-wavelength modulation technique for the measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy

The measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy was investigated using a low pressure atomizer and 2f-wavelength modulation (WM) detection. The measurements were supported by computer simulations. Compared with a direct absorption measurement, the relative absorption sensitivity for ⁶Li is considerably reduced when 2f-WM is performed in the center of the ⁶Li D1 fine structure component, but it will be enhanced, when the center of modulation is tuned to the maximum in the red wing of the 2f-WM line profile. The results of calculated 2f-WM line strengths were used to deconvolute overlapping lines and enabled the measurement of ⁷Li/⁶Li isotope ratios as large as approximately 2000. The Li content of a sample with a strong sodium chloride matrix was determined by isotope dilution.     

Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-atomic absorption spectrometry

A diode laser was used for the selective detection of ²³⁵U and ²³⁸U in a laser-induced plasma ignited by a Nd:YAG laser beam focused onto uranium oxide samples. The diode laser was sequentially tuned to the absorption lines of both isotopes (682.6736 nm for ²³⁵U, and 682.6913 nm for ²³⁸U). The absorption was measured on a pulse-to-pulse basis; the transient absorption peak was used as an analytical signal. Three samples were used with the relative abundance of the minor isotope ²³⁵U of 0.204%, 0.407% and 0.714%. Optimal conditions for the detection of the minor isotope were obtained at a distance of ∼3 mm from the sample surface, an argon pressure of ∼3 kPa and for 7.5 mJ pulse energy of the Nd:YAG laser. Absorption in the wing of the broadened line of the ²³⁸U isotope was found to be the main source of background for the measurement of the absorption of the minor isotope. The limit of detection of the minor isotope, evaluated on the basis of the 3σ criteria was estimated to be 100 μg g⁻¹. At the optimal conditions for the detection of the minor isotope optical thick conditions in the line centre of the main isotope were observed. Therefore, the isotope ratio measurements were performed by rationing the intensity of the net absorption signal measured in the line centre of the minor isotope and the absorption signal measured in the wing of the main isotope. This strategy was checked by determination of the isotope ratios for the two samples with depleted ²³⁵U concentration using the sample with the natural isotope composition (0.714%) as a standard. The accuracy and precision for this measurement strategy was evaluated to approximately 10%.     

Narrow and broad band diode laser absorption spectrometry—concepts,limitations and applications

Implementation concepts as well as the fundamental aspects concerning the analytical capability of diode laser spectrometry with respect to narrow and broad band absorption are discussed. The applicability is illustrated by means of the element-selective analysis of flames or plasmas and the molecular analysis of liquids or turbid media. While in narrow band absorption one diode laser and different modulation techniques can be applied to obtain very low detection limits, two diode lasers and a double-beam scheme should be used when the absorption bands are very broad. The two-laser, double-beam method is demonstrated by means of absorption measurements in a turbid medium and by concentration analyses of liquid samples applying the surface plasmon resonances technique.     

Element selective detection of molecular species applying chromatographic techniques and diode laser atomic absorption spectrometry

Tunable diode laser atomic absorption spectroscopy (DLAAS) combined with separation techniques and atomization in plasmas and flames is presented as a powerful method for analysis of molecular species. The analytical figures of merit of the technique are demonstrated by the measurement of Cr(VI) and Mn compounds, as well as molecular species including halogen atoms, hydrogen, carbon and sulfur.     

Methyl Iodide Photodissociation at 193 nm: The I((2)P(1/2)) Quantum Yield

Methyl iodide photolysis at 193 nm has been studied through probing the I((2)P(1/2)-(2)P(3/2)) transition in the atomic iodine photofragment using diode laser spectroscopy. The I((2)P(1/2)) quantum yield has been determined through two different diode laser techniques and then compared. Frequency-modulated diode laser based absorption spectroscopy was used to extract nascent Doppler lineshapes from which an I((2)P(1/2)) quantum yield of unity is inferred. However when diode laser gain/absorption measurements were made, an I((2)P(1/2)) quantum yield of 0.68 ± 0.04 was found. The reason for this discrepancy is shown to lie in the diode laser gain/absorption method. Molecular iodine is found to be formed during the experiment via atomic iodine recombination and then in turn dissociates to produce both I((2)P(1/2)) and I((2)P(3/2)), thus distorting the returned quantum yield. This conclusion is supported both by the reduction of the I((2)P(1/2)) quantum yield with number of photolysis laser shots when measured using this technique and by the presence of fluoresence which is shown to have excited-state lifetimes and quenching rates that are consistent with those previously measured for the D and D' states of molecular iodine.     

Tunable UV generation at 283 nm by frequency doubling and sum frequency mixing of two semiconductor lasers for the detection of Pb

High resolution atomic absorption measurements of lead at 283 nm in a vapor cell were performed by frequency doubling an 850 nm laser diode to obtain 425 nm light, followed by sum frequency generation of the harmonic radiation with a second 850 nm laser diode.     

Quantitative element analysis by isotope dilution in diode laser graphite furnace atomic absorption spectrometry

The paper reviews the application of the isotope dilution technique in optical atomic absorption spectrometry by use of a low-pressure graphite tube furnace as atomizer and diode lasers as radiation sources. The principles and the methodology to obtain accurate quantitative results despite of the occurrence of interferences are presented. The successful application of different Doppler-limited and Doppler-free spectrometric techniques is also presented. The perspectives but also the limitations of this new method are discussed.     

Real-time monitoring of Yb vapor density using an extended cavity violet diode laser

Based on laser absorption spectroscopy (LAS), we developed a vapor density monitor for controlling the vaporization rate of Yb using a tunable diode laser. The laser source consisted of an extended cavity violet diode laser which has an emission wavelength of 398.8 nm coincident with the Yb absorption transition line, 6s(2) 1S(0)-6s6p 1P(1). The light emitted from the diode laser was transmitted across an atomic vapor column generated by heating the Yb metal, while the laser frequency was scanned across the atomic transition line. By comparing the amount of incident light to the amount of light transmitted after the light passed through the vapor column, the vapor density was determined using the Beer's law. From the experimental results, we demonstrated that the diode-laser-based LAS operated successfully for the real-time monitoring of the Yb vapor density.     

Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry

A diode laser is used for the selective excitation of ²³⁵U and ²³⁸U in a laser-induced plasma applying Nd:YAG laser pulses to UO₂ samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ (880 MW cm⁻²). Three samples with 0.204, 0.407 and 0.714% ²³⁵U were measured. For example, for the UO₂ pellet with the natural uranium isotopic composition (99.281% ²³⁸U and 0.714% ²³⁵U), the accuracy and precision were 7% and 5% (460 shots), respectively, limited by the continuum emission background from the laser-induced plasma.     

Plasmas for lab-on-the-chip applications

Two small-size plasmas used as detectors of halogenated hydrocarbons and suitable for miniaturized instrumentation are discussed. A reduced pressure dielectric barrier discharge was integrated in a diode laser atomic absorption spectrometer and the already reported chlorine detection limits of 5 ppm (v/v) can be improved with one order of magnitude by spatially resolved measurements. A microstructured electrode discharge at atmospheric pressure was coupled with a miniaturized Échelle spectrometer and detection limits were found to be 20 ppb for chlorine as well as for fluorine.     

Photo-acoustic measurements of gas and aerosol absorption with diode lasers

The results of designing multipurpose high-sensitive photo-acoustic (PA) detectors and their application to high-resolution diode laser spectroscopy of molecular gases, gas analysis, and aerosol absorption measurements are summarized in this paper. The hardware and software of the diode laser spectrometer with a Helmholtz resonant PA detector providing an absorption sensitivity limit of better than 10(-7)Wm(-1)Hz(-1/2) are described. A procedure is proposed for an experiment involving the measurements of the rotational structure of hot vibrational bands of molecules. The results of the application of the nonresonant PA cell with temporal resolution of signals to measurements of weak nonresonant absorption of gases and soot aerosols are presented, and the possibility of creating a broad-band PA laser diode aerosol-meter is discussed.     

Isotope ratio analysis on micron-sized particles in complex matrices by Laser Ablation-Absorption Ratio Spectrometry

Laser ablation has been combined with dual tunable diode laser absorption spectrometry to measure ¹⁵²Gd:¹⁶⁰Gd isotope ratios in micron-size particles. The diode lasers are tuned to specific isotopes in two different atomic transitions at 405.9 nm (¹⁵²Gd) and 413.4 nm (¹⁶⁰Gd) and directed collinearly through the laser ablation plume, separated on a diffraction grating, and detected with photodiodes to monitor transient absorption signals on a shot-by-shot basis. The method has been characterized first using Gd metal foil and then with particles of GdCl₃·xH₂0 as binary and ternary mixtures with ¹⁵²Gd:¹⁶⁰Gd isotope ratios ranging from 0.01 to 0.43. These particulate mixtures have been diluted with Columbia River sediment powder (SRM 4350B) to simulate environmental samples and we show the method is capable of detecting a few highly-enriched particles in the presence of a >100-fold excess of low-enrichment particles, even when the Gd-bearing particles are a minor component (0.08%) in the SRM powder and widely dispersed (1178 particles detected in 800,000 ablation laser shots). The implications for monitoring ²³⁵U:²³⁸U enrichment ratios in airborne particle samples, as related to the nuclear industry, are discussed.     

Comparison of electrothermal atomization diode laser Zeeman- and wavelength-modulated atomic absorption and coherent forward scattering spectrometry

Atomic absorption and coherent forward scattering spectrometry by using a near-infrared diode laser with and without Zeeman and wavelength modulation were carried out with graphite furnace electrothermal atomization. Analytical curves and limits of detection were compared. The magnetic field was modulated with 50 Hz, and the wavelength of the diode laser with 10 kHz. Coherent forward scattering was measured with crossed and slightly uncrossed polarizers. The results show that the detection limits of atomic absorption spectrometry are roughly the same as those of coherent forward scattering spectrometry with crossed polarizers. According to the theory with bright flicker noise limited laser sources the detection limits and linear ranges obtained with coherent forward scattering spectrometry with slightly uncrossed polarizers are significantly better than those obtained with crossed polarizers and with atomic absorption spectrometry. This is due to the fact that employing approaches of polarization spectroscopy reduce laser intensity fluctuations to their signal carried fractions.     

Laser atomic absorption spectroscopy applying semiconductor diode lasers

The application of tunable single mode semiconductor diode lasers in atomic absorption spectroscopy is discussed in general. The use of several diode lasers, periodical modulation of the laser powers and Fourier analysis of the absorption signals allow background-corrected multi-element atomic absorption spectroscopy with extended dynamic range and internal standardization. This is demonstrated by the simultaneous determination of rubidium and barium in aqueous solutions with a commercial graphite tube atomizer.     

A discussion about the significance of Absorbance and sample optical thickness in conventional absorption spectrometry and wavelength-modulated laser absorption spectrometry

One of the most frequently used concepts in atomic absorption spectrometry (AAS) is that of Absorbance, an easily measurable quantity that is linear with the concentration of the analyte over a large range of concentrations whenever Beer's law is valid. Laser-based modulation techniques, however, in particular wavelength modulated diode laser absorption spectrometry (WMAS), do not measure exactly the same physical quantities as conventional AAS techniques do, since they do not rely upon separate measurements of the intensities in the presence and in the absence of the sample, but rather (a certain fraction of) the difference between the signals ‘on’ and ‘off’ resonance. Hence, Absorbance is not as natural for the modulated laser-based absorption techniques as it is for the conventional AAS techniques. The entity called sample optical thickness (SOT) appears to be a natural entity as well as a natural unit (denoted SOT units) for the quantification of signals in WMAS. The present paper discusses the concept of measurable quantities and their units in WMAS in some detail and compares them (theoretical and practical considerations) with those of conventional AAS. In particular, it makes a distinction between the ‘observed’ sample optical thickness and the ‘true’ sample optical thickness and shows how these two entities are related to the Absorbance entity. Finally, this work also introduces a dimensionless sensitivity of the WMAS technique, and shows this quantity to be equal to the nth Fourier coefficient of the wavelength modulated, peak-normalized line shape function.     

Diode laser emission linewidth determination: application to H2O line profile studies in the 5 and 1.4 microm regions

In order to study absorption line profiles using the stabilized diode laser spectrometer of Laboratoire de Physique Moléculaire et Applications (LPMA), a reliable determination of the emission line shape of different diodes laser is needed. In the near infrared region (1.39 and 1.66 microm) we used Distributed Feed Back diode lasers which operate around room temperature and in the middle infrared (5 and 8 microm) we used lead salt diode lasers cooled in a helium closed cycle cryostat or in a liquid nitrogen dewar. Some results obtained in H2O line profile studies in the 1.39 and 5 microm regions are presented as examples demonstrating how absorption line profile measurements can lead to erroneous values of the spectroscopic parameters when the contribution of the diode laser emission line width is neglected.