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Very recently, a spectroscopic determination of the Boltzmann constant, kB, has been performed at the Second University of Naples by means of a rather sophisticated implementation of Doppler Broadening Thermometry (DBT)1. Performedon a 18O-enriched water sample, at a wavelength of 1.39 µm, the experiment has provided a value for kB with a combined uncertainty of 24 parts over 106, which is the best result obtained so far, by using an optical method.In the spectral analysis procedure, the partially correlated speed-dependent hard-collision (pC-SDHC) model was adopted. The uncertainty budget has clearly revealed that the major contributions come from the statistical uncertainty (type A) and from the uncertainty associated to the line-shape model (type B)2. In the present work, we present the first results of a theoretical and numerical work aimed at reducing these uncertainty components. It is well known that molecular line shapes exhibit clear deviations from the time honoured Voigt profile. Even in the case of a well isolated spectral line, under the influence of binary collisions, in the Doppler regime, the shape can be quite complicated by the joint occurrence of velocity-change collisions and speed-dependent effects. The partially correlated speed-dependent Keilson-Storer profile (pC-SDKS) has been recently proposed as a very realistic model, capable of reproducing very accurately the absorption spectra for self-colliding water molecules, in the near infrared3. Unfortunately, the model is so complex that it cannot be implemented into a fitting routine for the analysis of experimental spectra. Therefore, we have developed a MATLAB code to simulatea variety of H218O spectrain thermodynamic conditions identical to the one of our DBT experiment, using the pC-SDKS model. The numerical calculations to determine such a profile have a very large computational cost, resulting from a very sophisticated iterative procedure. Hence, the numerically simulated spectra (with the addition of random noise) have been used to test the validity of simplified line shape models, such as the speed-dependent Galatry (SDG) profile and pC-SDHC model. In particular, we have used the global fitting procedure that is described in Amodio et al4. Such a procedure is very effective in reducing the uncertainty resulting from statistical correlation among free parameters. Therefore, the analysis of large amounts of simulated spectra has allowed us to study the influence of the choice of the model and quantify the achievable precision and accuracy levels, at the present value of the signal-to-noise ratio.

We present the outcomes of a specific study on the quadratic approximation in the partially-Correlated Speed-Dependent Hard-Collision profile (pC-SDHC), which is currently the recommended profile to replace the Voigt convolution for the shape of isolated high-resolution rotational-vibrational transitions, when perturbed by neutral gas-phase molecules. It includes the main effects occurring in the line formation, in particular the Dicke narrowing and the speed-dependent effects, as well as the possible correlation between them. We tested the quadratic (q-) and hypergeometric (hg-) versions for the speed dependence of the pC-SDHC on high quality H218O absorption spectra, in coincidence with three vibration-rotation transitions of the ν1 + ν3 band, at 1.39µm, looking for possible differences in the retrieved parameters. The absorption spectra were observed in the Doppler regime, with unprecedented spectral fidelity, by using a dual-laser absorption spectrometer, recently developed for the aim of a spectroscopic determination of the Boltzmann constant. The investigated transitions were: 22,1 → 22,0 , 44,1 → 44,0 and 30,3 → 20,2. The pC-SDHC profile is found to be quite robust, regardless of the choice of the particular speed dependence, provided that the velocity-changing collision frequency is considered as a free parameter. In particular, the pressure broadening and shifting parameters, retrieved by using the quadratic and hypergeometric versions, were found to be fully consistent. Similarly, the integrated absorbance was found to be completely unaffected by the choice of the speed-dependence, in the whole pressure range that we have explored. It should be said, however, that the velocity-changing collision frequency resulted to be physically meaningful only for the hg-version, but not for the q-version. Therefore, in the quadratic approximation, the collision frequency must be considered just as an indispensable parameter to be included in the fitting procedure for the aims of a successful fit.

We describe an implementation of Doppler broadening thermometry based upon a room-temperature continuous-wave quantum cascade laser at 9.07 mu m, coherently phase locked to a thulium optical frequency comb centered at 2 mu m. The thermodynamic temperature is retrieved from the analysis of multiple-line absorption spectra of the NH3 nu(2) band, adopting semiclassical line-shape models and taking into full account the speed dependence of collisional broadening. We demonstrate that a precision of approximately five parts over 10(5) can be reached as a result of the analysis of only 90 absorption spectra, as acquired as a function of the gas pressure.

Doppler-free saturated-absorption Lamb dips are observed for weak vibration-rotation transitions of C2H2 between 7167 and 7217 cm−1, using a frequency-comb assisted cavity ring-down spectrometer based on the use of a pair of phase-locked diode lasers. We measured the absolute center frequency of sixteen lines belonging to the 2 ν 3 + ν 5 1 band, targeting ortho and para states of the molecule. Line pairs of the P and Q branches were selected so as to form a ‘V’-scheme, sharing the lower energy level. Such a choice made it possible to determine the rotational energy separations of the excited vibrational state for J-values from 11 to 20. Line-center frequencies are determined with an overall uncertainty between 3 and 13 kHz. This is over three orders of magnitude more accurate than previous experimental studies in the spectral region around the wavelength of 1.4 μm. The retrieved energy separations provide a stringent test of the so-called MARVEL method recently applied to acetylene.

We realized a UV laser spectrometer at 253.7 nm for Doppler broadening thermometry on the 1 S 0 − 3 P 1 intercombination line in mercury vapors. Our setup is based on the two-stage duplication of a 1014.8 nm diode laser in a fiber-coupled periodically poled lithium niobate waveguide crystal and a beta-barium borate crystal in enhancement cavity, and we exploit injection locking of a 507.4 nm diode laser to boost the available optical power after the first duplication. Our setup addresses spectroscopic features that allow the thermodynamic temperature determination of the atomic sample from the absorption profile with 10 − 6 accuracy. The realized UV laser source has 1 × 10 − 4 relative intensity stability, Gaussian shape, and over 10 GHz mode-hop-free tunable range. These features are crucial for the practical realization of the kelvin in the new International System of Units through a spectroscopic technique.

International audience; We review progress, since publication of the book "Collisional effects on molecular spectra Laboratory experiments and models, consequences for applications" (Elsevier, Amsterdam, 2008), on measuring, modeling and predicting the influence of pressure (ie of intermolecular collisions) on the spectra of gas molecules. We first introduce recently developed experimental techniques of high accuracy and sensitivity. We then complement the aforementioned book by presenting the theoretical approaches, results and data proposed (mostly) in the last decade on the topics of isolated line shapes, line-broadening and-shifting, line-mixing, the far wings and associated continua, and collision-induced absorption. Examples of recently demonstrated consequences of the progress in the description of spectral shapes for some practical applications (metrology, probing of gas media, climate predictions) are then given. Remaining issues and directions for future research are finally discussed.

2015 - 2016, This thesis deals with the development of frequency-stabilized laser absorption spectrometers in the near-infrared to perform precision measurements on molecular spectra of atmospheric interest, such as acetylene and carbon dioxide. Two novel schemes have been implemented: the former is based on a pair of phase-locked diode lasers, one of them being a reference oscillator at 1.4 micron; the latter makes use of a self-referenced optical frequency comb synthesizer. In the second apparatus, a diode laser at a wavelength of 2 micron is frequency locked to the nearest tooth of the comb. In both cases, an absolute frequency scale is produced underneath any absorption spectrum. Line intensity factors and line widths have been determined with unprecedented accuracy. For carbon dioxide, our measurements give an important experimental test of ab-initio calculations, which have been recently performed at University College London by Jonathan Tennyson and coworkers. [edited by author], XXIX n.s.

A step forward in Doppler-broadening thermometry is demonstrated using a comb-assisted cavity-ring-down spectroscopic approach applied to an isolated near-infrared line of carbon dioxide at thermodynamic equilibrium. Specifically, the line-shape of the ${P}_{\mathrm{e}}(12)$ line of the $(30012)\ensuremath{\leftarrow}(00001)$ band of $\mathrm{C}{\mathrm{O}}_{2}$ at 1.578 \textmu{}m is accurately measured and its Doppler width extracted from a refined multispectrum fitting procedure accounting for the speed dependence of the relaxation rates, which were found to play a role even at the very low pressures explored, from 1 to 7 Pa. The thermodynamic gas temperature is retrieved with relative uncertainties of $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ (type A) and $11\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ (type B), which ranks the system at the first place among optical methods. Thanks to a measurement time of only $\ensuremath{\approx}5\phantom{\rule{0.16em}{0ex}}\mathrm{h}$, the technique represents a promising pathway toward the optical determination of the thermodynamic temperature with a global uncertainty at the ${10}^{\ensuremath{-}6}$ level.

We report on absolute measurements of saturated-absorption line-center frequencies of room-temperature trifluoromethane using a quantum cascade laser at 8.6 μm and the frequency modulation spectroscopy method. Absolute calibration of the laser frequency is obtained by direct comparison with a mid-infrared optical frequency comb synthesizer referenced to a radio-frequency Rb standard. Several sub-Doppler transitions falling in the υsub5/subvibrational band are investigated at around 1158.9 cmsup-1/supwith a fractional frequency precision of 8.6·10sup-12/supat 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10sup-11/supis mainly limited by the reproducibility of the frequency measurements.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 m. Lasergas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the nu1 + 2nu2 + nu3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

Previous research effort towards the determination of the Boltzmann constant has significantly improved the supporting theory and the experimental practice of several primary thermometry methods based on the measurement of a thermodynamic property of a macroscopic system at the temperature of the triple point of water. Presently, experiments are under way to demonstrate their accuracy in the determination of the thermodynamic temperature T over an extended range spanning the interval between a few kelvin and the copper freezing point (1358 K). We discuss how these activities will improve the link between thermodynamic temperature and the temperature as measured using the International Temperature Scale of 1990 (ITS-90) and report some preliminary results obtained by dielectric constant gas thermometry and acoustic gas thermometry. We also provide information on the status of other primary methods, such as Doppler broadening thermometry, Johnson noise thermometry and refractive index gas thermometry. Finally, we briefly consider the implications of these advancements for the dissemination of calibrated temperature standards.

A method of performing comb-assisted spectroscopy in the 2- and 4-mu m wavelength regions is deeply discussed and applied to the spectroscopic characterization of absorption lines of a CO2 gas sample. The method relies on the use of a sum-frequency-generation scheme to relate the frequency of the probing laser to a frequency comb provided by an Er:fiber femtosecond oscillator. Controlled scans of the comb repetition frequency allow highly accurate and reproducible acquisition of the absorption spectra, which are used for subsequent retrieval of the underlying spectroscopic parameters.

This article reports on the construction details of an isothermal cell, referenced to the triple point of water (TPW), together with characterization of its temperature uniformity and stability. The traceability of the temperature measurements is also defined and reported. The cell has an inner chamber of 15 mm diameter, and it is 150 mm long. Its temperature is actively controlled and maintained stable within 0.1 mK, for an unlimited time. The temperature gradient is limited to less than one millikelvin over the length of the cell which is kept in a horizontal position. This accurate temperature control is achieved by means of a series of three vacuum chambers, one inside the other. A special heater, reflectors, standard platinum resistance thermometers, several feedthroughs, an auxiliary thermostat, specific electronics, and dedicated software are used for the active control. The device represents a mixture of cryogenic and contact thermometry techniques, and it has been designed, assembled, and characterized at the Istituto Nazionale di Ricerca Metrologica. This temperature-stabilized cell is a part of a more complex experimental setup, based on near-infrared precision laser spectroscopy, devoted to the experimental determination of the Boltzmann constant.

Small molecules exhibit characteristic ro-vibrational transitions in the near- and mid-infrared spectral regions, which are strongly influenced by isotopic substitution. This gift of nature has made it possible to use laser spectroscopy for the accurate analysis of the isotopic composition of gaseous samples. Nowadays, laser spectroscopy is clearly recognized as a valid alternative to isotope ratio mass spectrometry. Laser-based instruments are leaving the research laboratory stage and are being used by a growing number of isotope researchers for significant advances in their own field of research. In this review article, we discuss the current status and new frontiers of research on high-sensitivity and high-precision laser spectroscopy for isotope ratio analyses. Although many of our comments will be generally applicable to laser isotope ratio analyses in molecules of environmental importance, this paper concerns itself primarily with water and carbon dioxide, two molecules that were studied extensively in our respective laboratories. A complete coverage of the field is practically not feasible in the space constraints of this issue, and in any case doomed to fail, considering the large body of work that has appeared ever since the review by Kerstel in 2004 (Handbook of Stable Isotope Analytical Techniques, Chapt. 34, pp. 759–787).

Laser spectroscopy in the linear regime of radiation–matter interaction is a powerful tool for measuring thermodynamic quantities in a gas at thermodynamic equilibrium. In particular, the Doppler effect can be considered a gift of nature, linking the thermal energy to an optical frequency, namely the line centre frequency of an atomic or molecular spectral line. This is the basis of a relatively new method of primary gas thermometry, known as Doppler broadening thermometry (DBT). This paper reports on the efforts that have been carried out, in the last decade, worldwide, to the end of making DBT competitive with more consolidated and accurate methodologies, such as acoustic gas thermometry and dielectric constant gas thermometry. The main requirements for low-uncertainty DBT, of both theoretical and technical nature, will be discussed, with a special focus on those related to the line shape model and to the frequency scale. A deep comparison among the different molecules that have been selected in successful DBT implementations is also reported. Finally, for the first time, to the best of my knowledge, the influence of refractive index effects is discussed.

Frequency fluctuations of an optical frequency standard at 1.39 µm have been measured by means of a highly-sensitive optical frequency discriminator based on the fringe-side transmission of a high finesse optical resonator. Built on a Zerodur spacer, the optical resonator exhibits a finesse of 5500 and a cavity-mode width of about 120 kHz. The optical frequency standard consists of an extended-cavity diode laser that is tightly stabilized against the center of a sub-Doppler H(2) (18)O line, this latter being detected by means of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. The emission linewidth has been carefully determined from the frequency-noise power spectral density by using a rather simple approximation, known as β-line approach, as well as the exact method based on the autocorrelation function of the laser light field. It turns out that the linewidth of the optical frequency standard amounts to about 7 kHz (full width at half maximum) for an observation time of 1 ms. Compared to the free-running laser, the measured width corresponds to a line narrowing by a factor of ~220.

We developed a high-precision spectroscopic system at 8.6 mu m based on direct heterodyne detection and phaselocking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The similar to 30 dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of similar to 500 kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of similar to 21 kHz, limited by the measured linewidth of the mid-IR comb. The system was used to perform absolute measurement of line-center frequencies for the rotational components of the nu(2) vibrational band of N2O, with a relative precision of 3 x 10(-10). (C) 2015 Optical Society of America

A compact and portable diode-laser spectrometer is developed for high-precision measurements of the (CO2)-C-13 /(CO2)-C-12 isotope ratio. Ro-vibrational transitions belonging to different combination bands, around 2.008 mum, are detected for the two isotopomers, using wavelength-modulation spectroscopy with first-harmonic detection. The determination of the isotope ratio is performed through a comparison between the spectra, simultaneously acquired in two different multiple-reflection cells designed to contain the sample and reference gases. The reproducibility of our method is extensively investigated over different time scales. In particular, the short-term precision of the isotope ratio determination is found to be 0.03%. We discuss the presence of possible systematic shifts in the delta-value and optimise the operating conditions to ensure accuracy within the precision level. A possible use for in-situ measurements in volcanic gases is also discussed.

We demonstrate the feasibility of the accurate and simultaneous measurement of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water vapor by means of tunable diode laser spectroscopy. The absorptions are due to the nu(1) + nu(3) combination band, observed using a room temperature, distributed feedback (DFB) diode laser at 1.39 mum. The precision of the instrument is approximately 3, 1, and 0.5% for the H-2, O-17, and O-18 isotope ratios, respectively, and is at present limited by residual optical feedback to the laser. The signal-to-noise, however, is superior to that obtained in a similar experiment using a color center laser at 2.7 mum. Replacing the current laser with a better unit, we are confident that a precision well below 1parts per thousand is attainable for all three isotope ratios. The diode laser apparatus is ideally suited for applications demanding a reliable, cheap, and/or portable instrument, such as the biomedical doubly labeled water method and atmospheric sensing. (C) 2002 Elsevier Science B.V. All rights reserved.

We describe an implementation of Doppler broadening thermometry based upon a room-temperature continuous-wave quantum cascade laser at 9.07 $\ensuremath{\mu}$m, coherently phase locked to a thulium optical frequency comb centered at 2 $\ensuremath{\mu}$m. The thermodynamic temperature is retrieved from the analysis of multiple-line absorption spectra of the NH${}_{3}$ ${\ensuremath{\nu}}_{2}$ band, adopting semiclassical line-shape models and taking into full account the speed dependence of collisional broadening. We demonstrate that a precision of approximately five parts over 10${}^{5}$ can be reached as a result of the analysis of only 90 absorption spectra, as acquired as a function of the gas pressure.

Complementary tests of the partially correlated speed-dependent Keilson-Storer (pCSDKS) model for the shape of isolated transition of pure water vapor[N. H. Ngo et al. , J. Chem. Phys.136, 154310 (Year: 2012)]10.1063/1.4704675 are made using new measurements. The latter have been recorded using a high sensitivity cavity ring down spectrometer, for seven self-broadened H2O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H2 18O spectra of [M. D. De Vizia et al. , Phys. Rev. A83, 052506 (Year: 2011)]10.1103/PhysRevA.83.052506 in the 1.38 μm region, measured at 273.15 K and for pressures from 0.3 to 3.75 Torr have also been used for comparison with the model. Recall that the pCSDKS model takes into account the collision-induced velocity changes, the speed dependences of the broadening and shifting coefficients as well as the partial correlation between velocity and rotational-state changes. All parameters of the model have been fixed at values previously determined, except for a scaling factor applied to the input speed-dependent line broadening. Comparisons between predictions and experiments have been made by looking at the results obtained when fitting the calculated and measured spectra by Voigt profiles. The good agreement obtained for all considered lines, at different temperature and pressure conditions, confirms the consistency and the robustness of the model. Limiting cases of the model have been then derived, showing the influence of different contributions to the line shape.

The outcomes of a specific study on line shape deviations from the usual Voigt profile, in the near-IR spectrum of water, will be discussed. Particularly, a great attention is paid to the role of speed-dependence of collisional broadening. From the experimental point of view, high spectral fidelity has been reached by using the technique of offset-frequency locking between two extended-cavity diode lasers. In such a system, the master laser is frequency stabilized against a narrow sub-Doppler molecular line, while the slave laser is actively controlled so that its emission frequency maintains a given offset with respect to the frequency of the master. This method allows one to perform highly accurate and reproducible frequency scans of the slave laser around a given vibration-rotation line. I will present recent findings on the speed dependence of collision parameters, influencing the shape of H218O absorption lines. Finally, I will discuss the implications for the spectroscopic determination of the Boltzmann constant.

This work presents a very simple yet effective way to obtain direct referencing of a quantum-cascade-laser at 4.3 μm to a near-IR frequency-comb. Precise tuning of the comb repetition-rate allows the quantum-cascade-laser to be scanned across absorption lines of a CO2 gaseous sample and line profiles to be acquired with extreme reproducibility and accuracy. By averaging over 50 acquisitions, line-centre frequencies are retrieved with an uncertainty of 30 kHz in a linear interaction regime. The extension of this methodology to other lines and molecules, by the use of widely tunable extended-cavity quantum-cascade-lasers, paves the way to a wide availability of high-quality and traceable spectroscopic data in the most crucial region for molecular detection and interrogation.

We describe a continuous-wave diode laser spectrometer for water-vapour precision spectroscopy at 1.38 μm. The spectrometer is based upon the use of a simple scheme for offset-frequency locking of a pair of extended-cavity diode lasers that allows to achieve unprecedented accuracy and reproducibility levels in measuring molecular absorption. When locked to the master laser with an offset frequency of 1.5 GHz, the slave laser exhibits residual frequency fluctuations of 1 kHz over a time interval of 25 minutes, for a 1-s integration time. The slave laser could be continuously tuned up to 3 GHz, the scan showing relative deviations from linearity below the 10{-6} level. Simultaneously, a capture range of the order of 1 GHz was obtained. Quantitative spectroscopy was also demonstrated by accurately determining relevant spectroscopic parameters for the 22,1→22,0line of the H2(18)O v1+v3 band at 1384.6008 nm.

We used polarization spectroscopy to perform a high-resolution investigation of atomic oxygen in the visible and near-infrared spectral region by means of dye and semiconductor diode lasers. We measured the fine structure of the highly excited 4d5D level and resolved the hyperfine structure of the metastable 3s5S2 level of 17O. Isotope shift values are also reported for several transitions of the three stable oxygen isotopes (16O, 17O, 18O). Polarization lineshapes are discussed and compared with those obtained in saturation techniques.

We report on a new optical implementation of primary gas thermometry based on laser absorption spectrometry in the near infrared. The method consists in retrieving the Doppler broadening from highly accurate observations of the line shape of the R(12) $\nu_{1} + 2 \nu_{2}^{\phantom{1}0} + \nu_{3}$ transition in CO$_{2}$ gas at thermodynamic equilibrium. Doppler width measurements as a function of gas temperature, ranging between the triple point of water and the gallium melting point, allowed for a spectroscopic determination of the Boltzmann constant with a relative accuracy of $\sim1.6\times10^{-4}$., Comment: Submitted to Physical Review Letters

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 mu m, using a CW quantum cascade laser phase-locked to a mid-infrared optical frequency comb. This latter is generated by a nonlinear down-conversion process starting from a dual-branch Er: fiber laser and is stabilized against a GPS-disciplined rubidium clock. By tuning the comb repetition frequency, several transitions falling in the nu(5) vibrational band are recorded with a frequency resolution of 20 kHz. Due to the very dense spectra, a special multiple-line fitting code, involving a Voigt profile, is developed for data analysis. The combination of the adopted experimental approach and survey procedure leads to fractional accuracy levels in the determination of line center frequencies, down to 2 x 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided. (C) 2015 AIP Publishing LLC.

International audience; We report on the first application of extended–wavelength DFB diode lasers to Cavity–Enhanced Absorption Spectroscopy in-situ trace measurements on geothermal gases. The emission from the most active fumarole at the Solfatara volcano near Naples (Italy) was probed for the presence of CO and CH4. After passing through a gas dryer and cooler, the volcanic gas flow (98% CO2) was analysed in real time for the concentration of these species, whose relatively strong absorption lines could be monitored simultaneously by a single Distributed Feed–Back (DFB) GaSb–based diode laser emitting around 2.33 μm (4300 cm−1) at room temperature. The concentrations were found to be about 3 ppm and 75 ppm, respectively, while actual detection limits for these molecules are around 1 ppb. We discuss the possibility of detecting other species of interest for volcanic emission monitoring

Near-infrared laser spectroscopy is used to measure the 13C/12C isotope abundance ratio in gas phase carbon dioxide. The spectrometer, developed expressly for field applications, is based on a 2 μm distributed feedback diode laser in combination with sensitive wavelength modulation detection. It is characterized by a simplified optical layout, in which a single detector and associated electronics are used to probe absorptions of a pair of 13CO2 and 12CO2 lines, simultaneously in a sample, as well as a reference gas. For a careful investigation of the achievable precision and accuracy levels, we carried out a variety of laboratory tests on CO2 samples with different isotopic compositions, calibrated with respect to the international standard material by means of isotope ratio mass spectrometry. The 1-σ accuracy of the 13CO2/12CO2 determinations, reported in the so-called δ notation, is about 0.5‰ (including both statistical and systematic errors), for δ-values in the range from -30‰ to +20‰. We show that the major source of systematic errors is a consequence of the non-linearity of the Lambert–Beer absorption law, and can be corrected for to a very high degree of accuracy.

A novel diode laser spectrometer was developed using dual-wavelength multiplexing, ensuring ideal conditions for high-precision and simultaneous measurements of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water. A 1.4-mum diode laser probed a (HOH)-O-16/(HOH)-H-2 line pair near 7198 cm(-1), while a similar laser observed (HOH)-O-16, (HOH)-O-17, and (HOH)-O-18 rovibrational lines around 7183 cm(-1), or a (HOH)-O-16/(HOH)-O-18 line pair near 7200 cm(-1). The 1-sigma standard deviation is 0.2parts per thousand for O-18/O-16, and 0.5parts per thousand for the H-2/H-1 and O-17/O-16 isotope ratios. Preliminary experiments with repeated injections of a natural abundance sample point to an accuracy of about 1parts per thousand for all three isotope ratios in natural samples. (C) 2003 Optical Society of America.

A dual laser absorption spectrometer operating at 1.39 μm was employed to acquire high-quality absorption spectra in coincidence with the 44,1 → 44,0 line of the H2 (18)O ν1 + ν3 band, at the temperature of the triple point of water. A rather sophisticated global fitting procedure was developed and used to simultaneously fit spectra across the explored pressure range, roughly between 1 and 4 Torr. Adopting the speed-dependent Galatry profile with a hypergeometric dependence of the collisional width and shift on the absorbers' velocity, we could take into account narrowing effects ascribed to collision-induced velocity changes and speed-dependence of relaxation rates. The global approach allowed us to implement and exploit some relevant physical constraints at the aim of reducing significantly statistical correlation issues among free parameters. Consequently, we demonstrated the possibility of determining pressure broadening and shifting parameters with high precision and accuracy, at the level of one part over 10(3). Furthermore, even being in the Doppler regime, it was possible to determine the effective frequency of velocity-changing collisions for self-colliding H2 (18)O molecules.

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d1D2, 5p4d3F2, 5s4d3D1, 5s6s3S1, 5s5p3P1 and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.

The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 microm has been long-term stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal. Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz.

An overview of novel laser techniques suitable for volcanic monitoring, based on different kinds of infrared laser sources, is presented. Their main advantages and drawbacks are discussed focusing on the achievable sensitivity and precision levels in analysis of gaseous species. Some of the most recent experimental results obtained in laboratory development as well as in field tests of home-built laser spectrometers are reported. New perspectives in optical devices aimed at geochemical and geophysical applications are also considered.