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Three electric quadrupole transitions in the second overtone band of D2 are newly measured by comb-referenced cavity ring down spectroscopy around 1.18 µm. These extremely weak transitions (line intensities smaller than 10−29 cm/molecule) are the first to be detected in the (3–0) band of D2. The spectra of the O(3), O(2), and Q(2) lines near 8321, 8446, and 8607 cm−1, respectively, are recorded at room temperature for pressure values ranging between 100 and 600 Torr. Accurate transition frequencies and line intensities of the three D2 transitions are determined from a line fitting procedure using beyond-Voigt profiles, including strong Dicke narrowing. Considering statistical fit errors and possible biases due to the interference with water lines (which are six orders of magnitude stronger than the studied D2 lines), total uncertainties on the frequencies extrapolated at zero pressure are estimated below 14 MHz (∼4.7 × 10−4 cm−1). The derived experimental frequencies and intensities are compared to ab initio values. An overall agreement is achieved, confirming the positional accuracy of the most advanced theoretical calculations. [ABSTRACT FROM AUTHOR]

Recent advances in high sensitivity spectroscopy have made it possible, in combination with accurate theoretical predictions, to observe, for the first time, very weak electric quadrupole transitions in a polar polyatomic molecule of water. Here, we present accurate theoretical predictions of the complete quadrupole rovibrational spectrum of a non-polar molecule CO2, important in atmospheric and astrophysical applications. Our predictions are validated by recent cavity enhanced absorption spectroscopy measurements and are used to assign few weak features in the recent ExoMars Atmospheric Chemistry Suite mid-infrared spectroscopic observations of the Martian atmosphere. Predicted quadrupole transitions appear in some of the mid-infrared CO2 and water vapor transparency regions, making them important for detection and characterization of the minor absorbers in water- and CO2-rich environments, such as those present in the atmospheres of Earth, Venus, and Mars. [ABSTRACT FROM AUTHOR]

Three electric quadrupole transitions in the second overtone band of D2 are newly measured by comb-referenced cavity ring down spectroscopy around 1.18 µm. These extremely weak transitions (line intensities smaller than 10−29 cm/molecule) are the first to be detected in the (3–0) band of D2. The spectra of the O(3), O(2), and Q(2) lines near 8321, 8446, and 8607 cm−1, respectively, are recorded at room temperature for pressure values ranging between 100 and 600 Torr. Accurate transition frequencies and line intensities of the three D2 transitions are determined from a line fitting procedure using beyond-Voigt profiles, including strong Dicke narrowing. Considering statistical fit errors and possible biases due to the interference with water lines (which are six orders of magnitude stronger than the studied D2 lines), total uncertainties on the frequencies extrapolated at zero pressure are estimated below 14 MHz (∼4.7 × 10−4 cm−1). The derived experimental frequencies and intensities are compared to ab initio values. An overall agreement is achieved, confirming the positional accuracy of the most advanced theoretical calculations. [ABSTRACT FROM AUTHOR]

In spite of its importance for the evaluation of the Earth radiative budget, thus for climate change, very few measurements of the water vapor continuum are available in the near infrared atmospheric windows especially at temperature conditions relevant for our atmosphere. In addition, as a result of the difficulty to measure weak broadband absorption signals, the few available measurements show large disagreements. We report here accurate measurements of the water vapor self-continuum absorption in the 2.1 μm window by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) for two spectral points located at the low energy edge and at the center of the 2.1 μm transparency window, at 4302 and 4723 cm-1, respectively. Self-continuum cross sections, CS, were retrieved with a few % relative uncertainty, from the quadratic dependence of the spectrum base line level measured as a function of water vapor pressure, between 0 and 16 Torr. At 296 K, the CS value at 4302 cm-1 is found 40% higher than predicted by the MT_CKD V2.5 model, while at 4723 cm-1, our value is 5 times larger than the MT_CKD value. On the other hand, these OF-CEAS CS values are significantly smaller than recent measurements by Fourier transform spectroscopy at room temperature. The temperature dependence of the self-continuum cross sections was also investigated for temperatures between 296 K and 323 K (23-50 °C). The derived temperature variation is found to be similar to that derived from previous Fourier transform spectrometer (FTS) measurements performed at higher temperatures, between 350 K and 472 K. The whole set of measurements spanning the 296-472 K temperature range follows a simple exponential law in 1/T with a slope close to the dissociation energy of the water dimer, D0 ≈ 1100 cm-1. [ABSTRACT FROM AUTHOR]

Doppler-free saturated-absorption Lamb dips were measured on weak rovibrational lines of 12C16O2 between 6189 and 6215 cm-1 at sub-Pa pressures using optical feedback frequency stabilized cavity ring-down spectroscopy. By referencing the laser source to an optical frequency comb, transition frequencies for ten lines of the 30013 ← 00001 band P-branch and two lines of the 31113 ← 01101 hot band R-branch were determined with an accuracy of a few parts in 1011. Involving rotational quantum numbers up to 42, the data were used for improving the upper level spectroscopic constants. These results provide a highly accurate reference frequency grid over the spectral interval from 1599 to 1616 nm. [ABSTRACT FROM AUTHOR]

We show how to effectively introduce a proper description of the velocity-changing collisions into the model of isolated molecular transition for the case of self- and Ar-perturbed H² .We demonstrate that the billiard-ball (BB) approximation of the H²-H² and H²-Ar potentials gives an accurate description of the velocity-changing collisions. The BB model results are compared with ab initio classical molecular dynamics simulations. It is shown that the BB model correctly reproduces not only the principal properties such as frequencies of velocity-changing collisions or collision kernels, but also other characteristics of H² and H²-Ar gas kinetics like rate of speed-changing collisions. Finally, we present line-shape measurement of the Q(1) line of the first overtone band of self-perturbed H² . We quantify the systematic errors of line-shape analysis caused by the use of oversimplified description of velocity-changing collisions. These conclusions will have significant impact on recent rapidly developing ultra-accurate metrology based on Doppler-limited spectroscopic measurements such as Doppler-width thermometry, atmosphere monitoring, Boltzmann constant determination, or transition position and intensity determination for fundamental studies. [ABSTRACT FROM AUTHOR]

The ultimate sensitivity performances obtained with a continuous wave-cavity ring down spectroscopy setup in the near infrared are investigated. At fixed frequency, the noise of the photodetector is found to be the main limitation and the best limit of detection (about 10-11 cm-1) is reached after a 10 s averaging. We show that long term baseline fluctuations can be efficiently averaged over several days allowing us to reach a detection limit as low as 5 × 10-13 cm-1. The achieved sensitivity is illustrated on narrow spectral intervals where the weakest lines detected so far by absorption spectroscopy are observed: (i) ultra-weak transitions of the a1Δg(0)-X 3Σg-(1) hot band of 16O2 near 1.58 μm and (ii) first detection of an electric quadrupole transition in the second overtone band of nitrogen (14N2) near 1.44 μm. [ABSTRACT FROM AUTHOR]

Eleven very weak electric quadrupole transitions Q(2), Q(1), S(0)-S(8) of the first overtone band of D2 have been measured by very high sensitivity CW-cavity ring down spectroscopy (CRDS) between 5850 and 6720 cm-1. The noise equivalent absorption of the recordings is on the order of αmin ≈ 3 × 10-11 cm-1. By averaging a high number of spectra, the noise level was lowered to αmin ≈ 4 × 10-12 cm-1 in order to detect the S(8) transition which is among the weakest transitions ever detected in laboratory experiments (line intensity on the order of 1.8 × 10-31 cm/molecule at 296 K). A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift and position at zero pressure limit were determined from recordings with pressures ranging between 10 and 750 Torr. A highly accurate theoretical line list was constructed for pure D2 at 296 K. The intensity threshold was fixed to a value of 1 × 10-34 cm/molecule at 296 K. The obtained line list is provided as supplementary material. It extends up to 24 000 cm-1 and includes 201 transitions belonging to ten v-0 cold bands (v = 0-9) and three v-1 hot bands (v = 1-3). The energy levels include the relativistic and quantum electrodynamic corrections as well as the effects of the finite nuclear mass. The quadrupole transition moments are calculated using highly accurate adiabatic wave functions. The CRDS line positions and intensities of the first overtone band are compared to the corresponding calculated values and to previous measurements of the S(0)-S(3) lines. The agreement between the CRDS and theoretical results is found within the claimed experimental uncertainties (on the order of 1 × 10-3 cm-1 and 2% for the positions and intensities, respectively) while the previous S(0)-S(3) measurements showed important deviations for the line intensities. [ABSTRACT FROM AUTHOR]

Using intracavity laser absorption spectroscopy combined with a supersonic slit jet, we have observed and analyzed seven vibronic cold bands of NO[SUB2] between 10 800 and 11 680 cm[SUP-1]. The vibronic energies, relative intensities, A, B, and C rotational constants, and the spin-rotation constants have been determined. The rotational constants play a crucial role in the vibronic assignments. The seven observed states are vibronically mixed. However, three of them have a dominant à [SUP2]B[SUB2] electronic character while the four others have a dominant &Xtilde; [SUP2]A[SUB1] character. The vibrational assignments and energies of the three à [SUP2]B[SUB2]) levels are :(1, 0, 0) at 10 999.42 cm[SUP-1], (0,2,) at 11 210.50 cm[SUP-1], and (0, 0, 2) at 11 283.15 cm[SUP-1]. The frequencies of the symmetric stretch, (ω[SUB1] ≃ 1265 cm[SUP-1]), and bending, (ω[SUB2] ≃ 738 cm[SUP-1]) modes derived from the (1, 0, 0) and (0, 2, 0) levels are in agreement with ab initio calculations. In contrast, the frequency of the antisymmetric stretch, ω[SUB3] ≃ 775 cm[SUP-1], derived from the observed (0, 0, 2) level, agrees only with the ab initio value obtained by Kaldor while other ab initio values ranging from 390 to 1750 cm[SUP-1] were reported. The four levels with a dominant &Xtilde;[SUP2] A[SUB1] character have been tentatively vibrationally assigned. The present experimental data allows for a first experimental determination of the two Ã[SUP2] B[SUB2] stretch vibrational frequencies which are necessary to model the &Xtilde; [SUP2]A[SUB1] - à [SUP2]B[SUB2] vibronic interactions, i.e., the conical intersection between &Xtilde; [SUP2]A[SUB1] and à {SUP2]B[SUB2] potential energy surfaces. [ABSTRACT FROM AUTHOR]

Examines the rotationally resolved overtone transitions of the acetylenic stretching of propyne using Fourier transform spectroscopy, various intracavity laser absorption spectrometers and cavity ring down spectroscopy. Mixing coefficient of the two interacting states; Comparative study of the fine structure in all investigated overtone bands.

Studies the vibrational energy pattern in acetylene. Details of the cluster model used in the study; Consideration of an extended data set on the vibrational energy levels in [sup 12]C[sub 2]H[sub 2]; Combination of investigations of experimental and theoretical results.

The near infrared and visible absorption spectrum of nitrous oxide 14N216O has been recorded by Fourier transform absorption spectroscopy, between 6500 and 11 000 cm-1, and by Intracavity Laser Absorption Spectroscopy, between 11 700 and 15 000 cm-1. Nineteen new bands are observed and, altogether, 34 cold and 10 hot bands are rotationally analyzed. The related upper term values, vibrational assignments, and principal rotational constants, as well as the relative band intensities are quantitatively discussed in terms of the formation of vibrational clusters, on the basis of the effective Hamiltonian developed by J. L. Teffo, V. I. Perevalov and O. M. Lyulin [J. Mol. Spectrosc. 168, 390 (1994)]. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The third stretching overtone transition of jet cooled methane has been recorded between 11 170 and 11 315 cm-1 by intracavity laser absorption spectroscopy associated with a supersonic slit jet expansion. By varying the methane percentage seeded in the carrier gas, we have studied the evolution of the spectrum vs the rotational temperature. The rotational cooling drastically reduces the congestion of the spectrum and only a few lines are observed in the spectrum corresponding to the lowest rotational temperature. In the 11 245–11 315 cm-1 range, the rotational structure of two superimposed bands is clearly observed. The lowest rotational levels of each band are assigned and the corresponding band origins, rotational constants and Coriolis parameters are obtained. The two bands are assigned to the two components of a Fermi dyad corresponding to the interaction of the 3ν1+ν3 (F2) stretching level carrying the oscillator strength with a near resonant vibrational state of the background. The high mixing of the vibrational wave functions is reflected by the close values of the rotational constants obtained for the two bands. In the lower energy range of the spectrum (11 170–11 245 cm-1), two weaker vibrational bands are tentatively assigned. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Here we report the preliminary analysis of the high resolution spectra of 70GeH4 (6000) and (7000) stretch overtone bands obtained by intracavity laser absorption spectroscopy (ICLAS). The observed P(J+1), Q(J), and R(J-1) transitions agree well with the combination difference relations, indicating that the Coriolis coupling in the degenerate states is negligible. This is an important feature for a spherical top molecule near the local mode limit. The vibration–rotational energy levels of upper states as well as those of some perturber states are obtained. The improved local mode parameters of GeH4 are also derived. [ABSTRACT FROM AUTHOR]

Absorption spectra in the 14 900–18 700 cm-1 range have been recorded for gaseous CHD3 at high resolution using the intracavity laser absorption spectroscopy (ICLAS) technique. The observed transitions correspond to the N=6 and 7 overtones of the C–H chromophore. Five bands were identified and rotational constants determined for four of them. The comparison of our experimental data with the calculated band origins and rotational constants obtained by Lewerenz and Quack is discussed. Absolute band intensities are given for seven bands corresponding to the N=5, 6, and 7 polyads. [ABSTRACT FROM AUTHOR]

The absorption spectrum of CHD3 has been recorded for the N=5 polyad near 13 500 cm-1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm-1. The rotational structure of the four observed bands has been resolved and the rotational constants given. The behavior of these constants versus the quantum number of the C–H stretching and bending modes are also discussed. [ABSTRACT FROM AUTHOR]

The absorption spectra of CHF3 and CHD3 have been recorded between 12 900 and 14 300 cm-1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm-1. The rotational structure of the four observed bands of CHD3 has been resolved while only the PQR contours of the five rotational bands of CHF3 were observed at this resolution. The observed vibrational levels of CHF3 are assigned as the vs =5 CH chromophore band system of the recent model developed by Quack et al. A simple anharmonic development of the energies vs vs and vb is sufficient to predict the energies of the observed bands of CHD3. [ABSTRACT FROM AUTHOR]

Examines the absorption spectroscopy of silylene near the 640 nanometer. Generation of the silylene radical in a flowing mixture of silane and argon; Factors attributing the perturbations in the spectrum; Role of silylene for the deposition of amorphous silicon films.

The interpretation of third harmonic generation, as obtained by crossing and focusing a pulsed laser beam on the axis of a continuous supersonic jet of nitrogen, is discussed and treated quantitatively. The geometry of the laser modes, as well as the inhomogeneity of the free jet zone of silence surrounded by the shock wave structure and the background gas, are taken into account. In this way, the variation of the phase matching factor is calculated in terms of the axial distance from the nozzle to the laser focus. This makes it possible to obtain the free jet characteristics from careful measurements of two-photon resonant, or nonresonant, frequency conversion rates. Thus the variations along the jet axis of the number density and the rotational temperature (decreasing down to 6 K), are obtained in good agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

At least for its most probable mechanisms, the reaction O(1D)+H2(X 1∑+g) →OH(A 2∑+)+H(1 2S) entirely takes place, from reagents to products, on electronically excited-state channels. Due to its endoergicity and to probable energy barriers (E*≊2.8 eV) encountered on these channels, the reaction can be produced in a crossed beam experiment only if the hydrogen molecule is excited on high vibrational levels (v≥5 or v≥7, according to the presence or the absence of energy barriers). This is obtained here by low-energy electron bombardment of the H2 molecular beam. The reaction is observed through the spontaneous fluorescence of the product OH. The measured spectrum corresponding to the 0–0 band of the A 2∑+→X 2Π transition is compared to simulated spectra, as obtained with a simple statistical model and with fitted rotational distribution for OH(A 2∑+,v=0): a broad rotational distribution and a strongly nonstatistical effect on the spin doublet populations are observed. [ABSTRACT FROM AUTHOR]

In the theory, the two-photon resonant third-order nonlinear susceptibility of a molecular gas is calculated. The variations of this susceptibility, with the rotational quantum numbers of the resonant states, are given in the case of two singlet states of different symmetries, Σ and Π. In the experiment, third harmonic generation of XUV light (94.5–97.5 nm) is obtained with and without two-photon resonance. The gaseous nonlinear medium is formed by CO as used either in a cell, or in a continuous supersonic free jet zone of silence. The intensity of the third harmonic wave generated is well related to the laser power, phase-matching conditions, two-photon resonances, and rotational temperature, through the nonlinear susceptibility, as described theoretically. [ABSTRACT FROM AUTHOR]

Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H216O near 7180 cm−1, using optical feedback frequency stabilized cavity ring-down spectroscopy. The saturation of the considered lines is so high that at the early stage of the ring down, the cavity loss rate remains unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies are determined down to 100 Hz precision and kHz accuracy. The developed setup allows resolving highly K-type blended doublets separated by about 10 MHz (to be compared to a HWHM Doppler width on the order of 300 MHz). A comparison with the most recent spectroscopic databases is discussed. The determined K-type splittings are found to be very well predicted by the most recent variational calculations. [ABSTRACT FROM AUTHOR]

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 H2O lines in the 1.6 μm region at room temperature and for pressures from 0.5 to 15 Torr. Furthermore, the H218O 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. [ABSTRACT FROM AUTHOR]

Fourier transform spectroscopy and intracavity laser absorption spectroscopy are used to record the absorption spectrum of formic acid at high spectral resolution, in the ranges of the 2ν[sub OH] (6968.258 cm[sup -1]) and 4ν[sub OH] (13 284.075 cm[sup -1]) vibrational bands of the trans-rotamer, respectively. Numerous perturbations combined with a large line density limit the extent to which the vibration-rotation analysis is performed. Some 689 lines are assigned in the first overtone band and related vibration-rotation constants are determined. Only the band origin and upper state principal A-rotational constant are determined for the n=4 overtone band, because of much higher spectral density. Interpolation, helped by literature data, provides all missing principal rotational constants in the n[sub OH] series, for n=1 to 4. All major vibration-rotation parameters appear to evolve very smoothly along the series. This trend is fully supported by ab initio calculations performed at the MP2/cc-pVTZ level of theory and based on an effective one-dimensional model describing the OH local mode stretching vibration. Results of these calculations are detailed in terms of the mean nuclear structure in the excitation series, up to n=4. The related calculated dipole moment surface allows the experimental a:b subband intensity ratio in the series to be interpreted in terms of a decrease in the HOC angle, from &Thgr;approx. 90° (n=1) to 44°(n=4). The potential occurrence of a proton exchange mechanism between the two oxygen nuclei, close to a tautomerism mechanism, is discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

On the basis of new results obtained by intracavity laser absorption spectroscopy, a number of HDO lines have been identified in the E-PAPS electronic archive of H[sub 2]O absorption lines provided in the paper entitled "The near infrared, visible and near ultraviolet overtone spectrum of water" [J. Chem. Phys. 111, 2444 (1999)]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The rotationally resolved absorption spectrum of the O[sub 2] dimer involving the [O[sub 2]([sup 1]Δ[sub g])[sub v=0]][sub 2]←[O[sub 2]([sup 3]Σ[sub g][sup -])[sub v=0]][sub 2] transition has been recorded near 632.6 nm by continuous wave Cavity Ring Down Spectroscopy in a supersonic slit jet expansion of pure O[sub 2]. A quadratic dependence of the absorption in the jet versus the stagnation pressure is observed. A rotational temperature of 12 K is derived from the (O[sub 2])[sub 2] rotational analysis. The high spectral resolution of the CW-CRDS measurements limited by the residual Doppler broadening in the jet and the low rotational temperature allow the first rotational analysis in this open-shell complex. The same spectrum was also recorded by Intracavity Laser Absorption Spectroscopy and the comparison of the performances of the two methods is discussed. Among more than 600 lines measured between 15 800 and 15 860 cm[sup -1] from the CW-CRDS spectrum, 40 were assigned to the [sup R]P[sub 0], [sup R]Q[sub 0], and [sup R]R[sub 0] branches of two subbands associated with B[sub 1][sup -]←A[sub 1][sup +] and A[sub 1][sup +]←B[sub 1][sup -] transitions between the ground and excited rovibrational levels, labeled following the G[sub 16] permutation inversion representation. Forty five lines were assigned to [sup P]P[sub 2], [sup P]Q[sub 2], and [sup P]R[sub 2] branches of two subbands associated with B[sub 1][sup -]←A[sub 1][sup +] and A[sub 1][sup +]←B[sub 1][sup -] transitions. The subbands centered at 15 808.401(49) [A[sub 1][sup +]←B[sub 1][sup -]] and 15 813.134(37) cm[sup -1] [B[sub 1][sup -]←A[sub 1][sup +]] for those arising from K=0, and at 15 812.656(20) [A[sub 1][sup +]←B[sub 1][sup -]] and 15 818.277(35) [B[sub 1][sup -]←A[sub 1][sup +]] when arising from K=2, are analyzed considering (O[sub 2])[sub 2] as a slightly asymmetric prolate top. The rotational analysis of the two K=0 subbands le... [ABSTRACT FROM AUTHOR]

Three electric quadrupole transitions in the second overtone band of D2 are newly measured by comb-referenced cavity ring down spectroscopy around 1.18 µm. These extremely weak transitions (line intensities smaller than 10-29 cm/molecule) are the first to be detected in the (3-0) band of D2. The spectra of the O(3), O(2), and Q(2) lines near 8321, 8446, and 8607 cm-1, respectively, are recorded at room temperature for pressure values ranging between 100 and 600 Torr. Accurate transition frequencies and line intensities of the three D2 transitions are determined from a line fitting procedure using beyond-Voigt profiles, including strong Dicke narrowing. Considering statistical fit errors and possible biases due to the interference with water lines (which are six orders of magnitude stronger than the studied D2 lines), total uncertainties on the frequencies extrapolated at zero pressure are estimated below 14 MHz (∼4.7 × 10-4 cm-1). The derived experimental frequencies and intensities are compared to ab initio values. An overall agreement is achieved, confirming the positional accuracy of the most advanced theoretical calculations., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H 2 16 , using optical feedback frequency stabilized cavity ring-down spectroscopy. The saturation of the considered lines is so high that at the early stage of the ring down, the cavity loss rate remains unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies are determined down to 100 Hz precision and kHz accuracy. The developed setup allows resolving highly K-type blended doublets separated by about 10 MHz (to be compared to a HWHM Doppler width on the order of 300 MHz). A comparison with the most recent spectroscopic databases is discussed. The determined K-type splittings are found to be very well predicted by the most recent variational calculations.-1 , using optical feedback frequency stabilized cavity ring-down spectroscopy. The saturation of the considered lines is so high that at the early stage of the ring down, the cavity loss rate remains unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies are determined down to 100 Hz precision and kHz accuracy. The developed setup allows resolving highly K-type blended doublets separated by about 10 MHz (to be compared to a HWHM Doppler width on the order of 300 MHz). A comparison with the most recent spectroscopic databases is discussed. The determined K-type splittings are found to be very well predicted by the most recent variational calculations.

Doppler-free saturated-absorption Lamb dips were measured on weak rovibrational lines of (12)C(16)O2 between 6189 and 6215 cm(-1) at sub-Pa pressures using optical feedback frequency stabilized cavity ring-down spectroscopy. By referencing the laser source to an optical frequency comb, transition frequencies for ten lines of the 30013←00001 band P-branch and two lines of the 31113←01101 hot band R-branch were determined with an accuracy of a few parts in 10(11). Involving rotational quantum numbers up to 42, the data were used for improving the upper level spectroscopic constants. These results provide a highly accurate reference frequency grid over the spectral interval from 1599 to 1616 nm.