Search

Quantitative UV laser absorption spectroscopy is implemented to measure the strength of the (6s^{2})^{1}S_{0}→(6s6p)^{3}P_{1} intercombination transition of Hg atoms near the wavelength of 253.7 nm and to retrieve the lifetime of the excited state. Tunable, single-mode, coherent radiation in the deep UV is produced through a double-stage second-harmonic generation process, starting from the output radiation of an external-cavity diode laser at 1014.8 nm. This latter is referenced to an optical frequency comb synthesizer. The measurement strategy exploits the relationship between the strength of an optical transition and the spontaneous emission Einstein coefficient. The complete uncertainty budget leads to an overall relative uncertainty of 0.6%, with a statistical contribution of 0.28 ns, which is the lowest ever obtained for the (6s6p)^{3}P_{1} lifetime. The new value, amounting to 125.12±0.79ns, compares well with most of the past determinations within two times the quoted uncertainties. Despite this, it does not agree with the weighted average of the 12 prior values, thus suggesting that a critical reassessment of the uncertainty of some of the past measurements is probably necessary.

We present a mid-IR high-precision spectrometer capable of performing accurate Doppler-free measurements with absolute calibration of the optical axis and high signal-to-noise ratio. The system is based on a widely tunable mid-IR offset-free frequency comb and a Quantum-Cascade-Laser (QCL). The QCL emission frequency is offset locked to one of the comb teeth to provide absolute-frequency calibration, spectral-narrowing, and accurate fine frequency tuning. Both the comb repetition frequency and QCL-comb offset frequency can be modulated to provide, respectively, slow- and fast-frequency-calibrated scanning capabilities. The characterisation of the spectrometer is demonstrated by recording sub-Doppler saturated absorption features of the CHF3 molecule at around 8.6 μm with a maximum signal-to-noise ratio of ∼7 × 103 in 10 s integration time, frequency-resolution of 160 kHz, and accuracy of less than 10 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.

Doppler-free saturated-absorption Lamb dips are observed for weak vibration-rotation transitions of C _2 H _2 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 ${\nu }_{3}+{\nu }_{5}^{1}$ 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 μ m. The retrieved energy separations provide a stringent test of the so-called MARVEL method recently applied to acetylene.

We report on precision spectroscopy of the 6s^{2} ^{1}S_{0}→6s6p ^{3}P_{1} intercombination line of mercury in the deep ultraviolet, by means of a frequency-comb referenced, wavelength-modulated, saturated absorption technique. This method allowed us to perform sub-Doppler investigations with an absolute frequency axis at 254 nm, while ensuring a relatively high signal-to-noise ratio. The absolute line center frequencies of the ^{200}Hg and ^{202}Hg bosonic isotopes were measured with a global uncertainty of 8 and 15 kHz (namely, 6.8×10^{-12} and 1.3×10^{-11}, in relative terms), respectively, the statistical and systematic components being significantly reduced as compared to past determinations. This remarkable result was achieved also thanks to an in-depth study of the ac Stark effect. Furthermore, we found the most accurate ^{200}Hg-^{202}Hg isotope shift ever obtained before, namely, 5 295 57 0±15_{stat}±8_{syst}  kHz.

Lamb dips of twenty lines in the P, Q, and R branches of the ν 1 + ν 3 + ν 4 1 vibrational band of 12 C 2 H 2 , in the spectral window of 7125-7230 cm -1 , have been measured using an upgraded comb-calibrated frequency-stabilized cavity ring-down spectrometer, designed for extensive sub-Doppler measurements. Due to the large number of carefully executed Lamb-dip experiments, and to the extrapolation of absolute frequencies to zero pressure in each case, the combined average uncertainty of the measured line-center positions is 15 kHz (5 × 10 -7 cm -1 ) with a 2-σ confidence level. Selection of the twenty lines was based on the theory of spectroscopic networks (SN), ensuring that a large number of transitions, measured previously by precision-spectroscopy investigations, could be connected to the para and ortho principal components of the SN of 12 C 2 H 2 . The assembled SN contains 331 highly precise transitions, 119 and 121 of which are in the ortho and para principal components, respectively, while the rest remain in floating components. The para - and ortho - 12 C 2 H 2 energy-level lists, determined during the present study, contain 82 and 80 entries, respectively, with an accuracy similar to that of the lines. Based on the newly assembled lists of para - and ortho - 12 C 2 H 2 empirical energy levels, a line list, called TenkHz, has been generated. The TenkHz line list contains 282 entries in the spectral range of 5898.97-7258.87 cm -1 ; thus far, only 149 of them have been measured directly via precision spectroscopy. The TenkHz line list includes 35 intense lines that are missing in the HITRAN2020 database.

Modulation transfer spectroscopy is used to demonstrate absolute frequency stabilization of an 8.6-µm-wavelength quantum cascade laser against a sub-Doppler absorption of the C H F 3 molecule. The obtained spectral emission properties are thoroughly characterized through a self-referenced optical frequency comb, stabilized against either a GPS-disciplined Rb clock or a 1.54-µm Er-fiber laser locked to a high-finesse ultra-low-expansion optical cavity. Fractional long-term stability and accuracy at a level of 4×10 -12 (at 100 s) and 3×10 -10 , respectively, are demonstrated, along with an emission linewidth as narrow as 10 kHz for observation times of 0.1 s.

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.

The International Committee for Weights and Measures (CIPM), at its meeting in October 2017, followed the recommendation of the Consultative Committee for Units (CCU) on the redefinition of the kilogram, ampere, kelvin and mole. For the redefinition of the kelvin, the Boltzmann constant will be fixed with the numerical value 1.380 649 × 10 -23 J K -1 . The relative standard uncertainty to be transferred to the thermodynamic temperature value of the triple point of water will be 3.7 × 10 -7 , corresponding to an uncertainty in temperature of 0.10 mK, sufficiently low for all practical purposes. With the redefinition of the kelvin, the broad research activities of the temperature community on the determination of the Boltzmann constant have been very successfully completed. In the following, a review of the determinations of the Boltzmann constant k , important for the new definition of the kelvin and performed in the last decade, is given.

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 μm. Laser-gas 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 CO 2 samples at different gas pressures, in coincidence with three lines of the R-branch of the ν 1 + 2ν 2 + ν 3 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.

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 υ 5 vibrational band are investigated at around 1158.9  cm -1 with a fractional frequency precision of 8.6·10 -12 at 1-s integration time, limited by the Rb-clock stability. The demonstrated frequency uncertainty of 6.6·10 -11 is mainly limited by the reproducibility of the frequency measurements.

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., (© 2016 The Authors.)

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., (© 2016 The Author(s).)

We report a high-precision spectroscopic study of room-temperature trifluoromethane around 8.6 μm, 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 υ5 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 × 10(-10). Line intensity factors, pressure broadening, and shifting parameters are also provided.

We developed a high-precision spectroscopic system at 8.6 μm based on direct heterodyne detection and phase-locking of a room-temperature quantum-cascade-laser against an harmonic, 250-MHz mid-IR frequency comb obtained by difference-frequency generation. The ∼30  dB signal-to-noise ratio of the detected beat-note together with the achieved closed-loop locking bandwidth of ∼500  kHz allows for a residual integrated phase noise of 0.78 rad (1 Hz-5 MHz), for an ultimate resolution of ∼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 ν2 vibrational band of N2O, with a relative precision of 3×10(-10).

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 report on a narrow-linewidth distributed-feedback quantum cascade laser at 8.6 μm that is optical-feedback locked to a high-finesse V-shaped cavity. The spectral purity of the quantum cascade laser is fully characterized using a high-sensitivity optical frequency discriminator, leading to a 1 ms linewidth of less than 4 kHz and a minimum laser frequency noise spectral density as low as 0.01 Hz2/Hz for Fourier frequencies larger than 100 kHz. The cumulative standard deviation of the laser intensity is better than 0.1% over an integration bandwidth from 2 Hz to 100 MHz.

We implemented an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) at 1.39 μm. The emission frequency of an extended-cavity diode laser was actively stabilized against the center of the 4(4,1)→4(4,0) transition of the H(2)(18)O ν1+ν3 band, under optical saturation conditions. The nonlinear regime of laser-gas interaction was reached by using an optical cavity with a finesse of about 8700. By filling it with an 18O-enriched water sample at a pressure of a few Pa, the Lamb dip could be observed with a full width at half-maximum of about 2 MHz. Absolute frequency stabilization was obtained by locking the cavity resonance to the center of the sub-Doppler signal, which was provided by the NICE-OHMS technique under the dispersion regime of operation. An Allan deviation analysis demonstrated a relative frequency stability of ∼5×10(-13) for an integration time of 1 s. For longer integration times, the flicker frequency noise floor set the stability at the level of 4×10(-14).

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 report on a new implementation of Doppler broadening thermometry based on precision absorption spectroscopy by means of a pair of offset-frequency locked extended-cavity diode lasers at 1.39  μm. The method consists in the highly accurate observation of the shape of the 4(4,1)→4(4,0) line of the H2(18)O ν1+ν3 band, in a water vapor sample at thermodynamic equilibrium. A sophisticated and extremely refined spectral analysis procedure is adopted for the retrieval of the Doppler width as a function of the gas pressure, taking into account the Dicke narrowing effect, the speed dependence of relaxation rates, and the physical correlation between velocity-changing and dephasing collisions. A spectroscopic determination of the Boltzmann constant with a combined (type A and type B) uncertainty of 24 parts over 10(6) is reported. This is the best result obtained so far by means of an optical method. Our determination is in agreement with the recommended CODATA value.

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 (2012)] are made using new measurements. The latter have been recorded using a high sensitivity cavity ring down spectrometer, for seven self-broadened H(2)O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H(2) (18)O spectra of [M. D. De Vizia et al., Phys. Rev. A 83, 052506 (2011)] 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.

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1 μm to a Tm-fiber laser frequency comb centered at 2 μm, with an integrated residual phase error of 0.9 rad (30 mHz to 1.5 MHz). This resulted in a QCL linewidth reduction from 525 to 25 kHz at 1 ms observation time, limited by the linewidth of the free-running frequency comb.

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.