Frequency-stabilized Cavity Ring-down Spectrometer Research Articles

Measurement and calculation of CO (7-0) overtone line intensities.

Intensities of 14 lines in the sixth overtone (7-0) band of carbon monoxide (12C16O) are measured in the visible range between 14300 and 14500 cm-1 using a frequency-stabilized cavity ring-down spectrometer. This is the first observation of such a high and weak overtone spectrum of the CO molecule. A theoretical model is constructed and tested based on the use of a high accuracy abinitio dipole moment curve and a semi-empirical potential energy curve. Accurate studies of high overtone transitions provide a challenge to both experiment and theory as the lines are very weak: below 2 × 10-29cmmolecule-1 at 296K. Agreement between theory and experiment within the experimental uncertainty of a few percent is obtained. However, this agreement is only achieved after issues with the stability of the Davidson correction to the multi-reference configuration interaction calculations are addressed.

On the 12C2H2 near-infrared spectrum: absolute transition frequencies and an improved spectroscopic network at the kHz accuracy level.

Lamb dips of twenty lines in the P, Q, and R branches of the ν1 + ν3 + ν41 vibrational band of 12C2H2, 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 12C2H2. 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-12C2H2 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-12C2H2 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.

Line mixing in the oxygen B band head.

We present the results of direct measurements of the line mixing parameters for two pairs of overlapping transitions at the band head of the oxygen B band. Measurements were performed with the frequency-stabilized cavity ring-down spectrometer assisted by an optical frequency comb. The recorded spectra were analyzed with line profiles comprising speed dependence, Dicke narrowing, and line mixing. Incorporation of the line mixing into the model eliminated previous discrepancies for pressure shift and their speed dependence coefficients. First-order line mixing was determined directly from the line shape fitting at relatively low pressure (0.04 atm) together with other line shape parameters and compared with that calculated by Sung et al. [J. Quant. Spectrosc. Radiat. Transfer 235, 232-243 (2019)].

Ultrahigh finesse cavity-enhanced spectroscopy for accurate tests of quantum electrodynamics for molecules.

We report the most accurate, to the best of our knowledge, measurement of the position of the weak quadrupole S(2) 2-0 line in $ {{\rm D}_2} $D2. The spectra were collected with a frequency-stabilized cavity ringdown spectrometer (FS-CRDS) with an ultrahigh finesse optical cavity ($ {\cal F} = 637 000 $F=637000) and operating in the frequency-agile, rapid scanning spectroscopy (FARS) mode. Despite working in the Doppler-limited regime, we reached 40 kHz of statistical uncertainty and 161 kHz of absolute accuracy, achieving the highest accuracy for homonuclear isotopologues of molecular hydrogen. The accuracy of our measurement corresponds to the fifth significant digit of the leading term in quantum electrodynamics (QED) correction. We observe $ 2.3\sigma $2.3σ discrepancy with the recent theoretical value.

Accurate deuterium spectroscopy for fundamental studies

We present an accurate measurement of the weak quadrupole S(2) 2-0 line in self-perturbed D2 and theoretical ab initio calculations of both collisional line-shape effects and energy of this rovibrational transition. The spectra were collected at the 247–984 Torr pressure range with a frequency-stabilized cavity ring-down spectrometer linked to an optical frequency comb (OFC) referenced to a primary time standard. Our line-shape modeling employed quantum calculations of molecular scattering (the pressure broadening and shift and their speed dependencies were calculated, while the complex frequency of optical velocity-changing collisions was fitted to experimental spectra). The velocity-changing collisions are handled with the hard-sphere collisional kernel. The experimental and theoretical pressure broadening and shift are consistent within 5% and 27%, respectively (the discrepancy for shift is 8% when referred not to the speed averaged value, which is close to zero, but to the range of variability of the speed-dependent shift). We use our high pressure measurement to determine the energy, ν0, of the S(2) 2-0 transition. The ab initio line-shape calculations allowed us to mitigate the expected collisional systematics reaching the 410 kHz accuracy of ν0. We report theoretical determination of ν0 taking into account relativistic and QED corrections up to α5. Our estimation of the accuracy of the theoretical ν0 is 1.3 MHz. We observe 3.4σ discrepancy between experimental and theoretical ν0.

Response of an optical cavity to phase-controlled incomplete power switching of nearly resonant incident light.

The response of an optical cavity to incomplete extinction of nearly resonant incident light was experimentally examined. Measurements were performed using a Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer (CRDS) that allowed the laser frequency detuning from the cavity resonance center to be controlled at Hz-level resolution. It is shown that an insufficient laser light extinction ratio combined with a phase shift and frequency detuning may lead to non-exponential cavity pumping and decay signals. The experimental results can be explained with a simple analytical model. The non-exponential decay can lead to a systematic shift as high as 0.5% in the ring-down time constants, dependent on the laser frequency detuning from the cavity mode center and on the extinction ratio. This can lead to appreciable systematic errors in the absorption coefficients determined with the CRDS technique.

Absolute frequency determination of molecular transition in the Doppler regime at kHz level of accuracy

We measured absolute frequency of the unperturbed P7 P7 O2 B-band transition ν0= 434783.5084857(82) GHz and the collisional self-shift coefficient δ=−9.381(62)×10−21 GHz/(molecule/cm3). With Doppler-limited spectroscopy we achieved the relative standard uncertainty of 2×10−11 on line position, typical for Doppler-free techniques. Shapes of the spectral line were measured with a Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer referenced to an 88Sr optical atomic clock via an optical frequency comb.

Line positions, pressure broadening and shift coefficients for the second overtone transitions of carbon monoxide in argon

Absolute positions and spectral line-shape parameters of carbon monoxide 0→3 band, P branch transitions are reported. The speed-dependent effects and the influence of velocity-changing collisions were taken into account in the fitted line-shape models. For the first time the values of pressure shift coefficients of CO in argon for this band were determined. The measurements were made with the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer, with the frequency axis linked through an optical frequency comb to the UTC(AOS) frequency reference based on a hydrogen maser. Achieved uncertainties of line positions are between 70kHz and 420kHz.

A new approach to spectral line shapes of the weak oxygen transitions for atmospheric applications

We propose to construct a new database of O2 molecular spectral lines for atmospheric application, consistent with recent IUPAC recommendation [Tennyson et al. Pure Appl Chem 2014;86:1931] going beyond Voigt profile by incorporation of the speed dependence of collisional broadening and shifting. For this purpose we collected the laboratory data for the self-perturbed oxygen B-band transitions. Line shapes were measured at low pressures by the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer linked to the optical frequency comb. Data were analyzed by means of the quadratic speed-dependent Voigt profile. The absolute transition frequencies are determined with accuracy even as good as 150kHz. Line intensities, pressure width and shift coefficients and the speed-dependent parameters are determined with subpercent accuracy.

CRDS investigation of line shapes of the nitrogen-broadened oxygen B-band transition

SynopsisWe present precise line-shape measurements of the nitrogen-broadened P9 P9 oxygen B-band transition occurring near 689 nm. Data were obtained using the optical frequency comb-assisted Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer. Several line-shape models describing physical effects such as Dicke narrowing, the speed dependence of collisional broadening and shifting, and the correlation between velocity- and phase-changing collisions were used in the analysis. The multispectrum fitting technique is used to minimize correlation between line-shape parameters. Observed line narrowing is mostly determined by the speed dependence of the collisional broadening.

Speed-dependent effects and Dicke narrowing in nitrogen-broadened oxygen

We present the line-shape analysis of the nitrogen-broadened P9 P9 oxygen B-band transition measured by the optical frequency comb-assisted Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer. Perturbation by both oxygen and nitrogen molecules is taken into account simultaneously in the line-shape analysis. Several line-shape models describing physical effects such as Dicke narrowing, the speed dependence of collisional broadening and shifting, and the correlation between velocity- and phase-changing collisions were used in the analysis. The comparison between the hypergeometric and quadratic approximations of the speed-dependent effects is presented. Observed line narrowing is mostly determined by the speed dependence of the collisional broadening.

Spectral line shapes and frequencies of the molecular oxygen B-band R-branch transitions

We present the line-shape parameters for the first 11 lines of the oxygen B-band R-branch self-broadened transitions measured at low pressures by the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer (PDH-locked FS-CRDS) linked to the optical frequency comb. The collisional self-broadening, shifting and narrowing parameters were determined together with the quadratic speed-dependence as well as phase- and velocity-changing correlations parameters. The absolute frequencies of the transitions with combined standard uncertainties below 150kHz are reported. Dependence of line parameters on choice of the line-shape model is discussed.

Spectral line-shapes of oxygen B-band transitions measured with cavity ring-down spectroscopy

Results of line-shape measurements of self- and N2-broadened P9 P9 transition of the oxygen B band are presented. Spectra were acquired using the optical frequency comb- assisted Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer (PDH- locked FS-CRDS). In the line-shape analysis the line narrowing described by Dicke narrowing or/and the speed dependence of collisional broadening were taken into account. The multispectrum fitting technique was used to minimize numerical correlations between line-shape parameters. Collisional broadening and shifting coefficients are reported with sub-percent uncertainties. Influence of the spectral line-shape model used in data analysis on determined line intensities and collisional broadening is discussed.

Spectral line shapes of self-broadened P-branch transitions of oxygen B band

We used the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer assisted by the optical frequency comb for systematic line-shape study of self-broadened P-branch transitions of the O216B band [b1Σg+(v=1)←X3Σg−(v=0)]. In the line-shape analysis we take into account the line-narrowing effects described by Dicke narrowing or the speed dependence of collisional broadening. The relation between the parameters describing Dicke narrowing with the use of the soft- and hard-collision models is discussed and verified experimentally in the low pressure regime using the multispectrum fitting technique. We report line positions with uncertainties of about 170kHz, the collisional broadening coefficients with 0.45% uncertainties, and line intensities with 0.5% uncertainties. We compare these results to data available in the literature.

Low pressure line-shape study of self-broadened CO transitions in the (3←0) band

We present high-sensitivity and high-spectral-resolution line-shape measurements of self-broadened CO (3←0) and (4←1) bands transitions measured using the frequency-stabilized cavity ring-down spectrometer. Ten of twelve investigated lines were measured for the first time. We have shown that the narrowing of the spectral lines should be taken into account to correctly determine their collisional broadening parameters. In our conditions line narrowing can be equally well described by Dicke narrowing or the speed dependence of collisional broadening. We compare line intensities measured with subpercent relative uncertainties and line positions to data available in the HITRAN spectroscopic database.

Transition frequencies of oxygen B-band lines measured with optical frequency comb assisted cavity ring-down spectroscopy

Transition frequencies of weak B-band lines of O2 were measured with low uncertainties. These results were compared to data available in the literature. In order to achieve reported accuracy we took into account in the data analysis several subtle line-shape effects, such as speed dependence of collisional broadening and Dicke narrowing. We found that the speed-dependent Nelkin-Ghatak profile can properly describe experimental line shapes in the investigated pressure range of up to 22 Torr. Our experimental setup consists of the Pound-Drever-Hall locked frequency-stabilized cavity ring-down spectrometer. This spectrometer provides high-resolution and very high signal-to-noise ratio of measured spectra. It was recently linked to the optical frequency comb working in the visible region of spectrum. This upgrade allows for accurate determination of the absolute frequency axis of the CRD spectrum by direct measurement of the probe laser frequency.

Cavity ring-down spectroscopy of the oxygen B-band with absolute frequency reference to the optical frequency comb

Absolute positions of several oxygen B-band lines were measured with the Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectrometer. The frequency axis of spectra was linked to the optical frequency comb. Achieved uncertainties of line positions are between 0.9 and 2.9 MHz. Self-pressure shifts coefficients are also reported.

High-accuracy transition frequencies for the O 2A-band

Frequencies for 32 P-branch transitions of the O 2 A-band ( b 1 Σ g + ← X 3 Σ u - ) were measured between 12 975 and 13 115 cm −1 using the frequency-stabilized cavity ring-down spectrometer located at National Institute of Standards and Technology, Gaithersburg, MD. Absolute line positions were calibrated using the hyperfine components of 39K D 1 and D 2 atomic transitions yielding uncertainties <1 MHz (∼3 × 10 −5 cm −1). Spectroscopic parameters for the b 1 Σ g + ( v = 0 ) state were calculated and compared with previously determined values. These new transition frequencies should serve as a convenient secondary calibration standard in the 760 nm region.

Experimental intensity and lineshape parameters of the oxygen A-band using frequency-stabilized cavity ring-down spectroscopy

Line intensities, self- and air-broadened linewidths, pressure-induced shifts, and collisional narrowing coefficients were measured from 2 ⩽ J′ ⩽ 32 in the P branch of the O 2 A-band (12 975–13110 cm −1) utilizing Galatry line profiles. Spectra were recorded using the frequency-stabilized cavity ring-down spectrometer located at NIST, Gaithersburg, MD with a spectral resolution <0.0001 cm −1 and noise-equivalent absorption coefficient of 6 × 10 −8 m −1 Hz −1/2. Line intensities, obtained from calibrated gas samples for 2 ⩽ J′ ⩽ 32, are ∼1% lower than the values in current spectroscopic databases. At higher J (18 ⩽ J′ ⩽ 32), the measured air- and self- broadened half widths are up to 20% lower than the extrapolated values given in HITRAN 2004, while corresponding half-widths for 2 ⩽ J′ ⩽ 15 are in better agreement. Available self-broadened half widths are fitted to empirical expressions with an rms of 0.8%. We discuss the implications of our results for accurate remote sensing of surface pressure and photon path length distributions.

Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the ν1+ν3 water vapor combination band near ˜ν=7181.156 cm-1, we compare spectroscopic and thermodynamic determinations of trace water vapor in N2, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2×10-10 cm-1 Hz-1/2, which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7×10-9 for an absorption spectrum of 10 minutes duration.