The (2-0) R(0) and R(1) transition frequencies of HD determined to a 10-10 relative accuracy by Doppler spectroscopy at 80 K.

The Doppler broadened R(0) and R(1) lines of the (2-0) vibrational band of HD have been measured at liquid nitrogen temperature and at pressures of 2 Pa, with a comb referenced continuous-wave cavity ring-down spectrometer set-up. Transition frequencies of 214905335185 kHz and 217105181898 kHz were derived from 33 and 83 recordings, with corresponding root mean squared deviation of 53 and 33 kHz for the R(0) and R(1) transition, respectively. This is the first sub-MHz frequency determination of the R(0) transition frequency and represents a three order of magnitude accuracy improvement compared to literature. The R(1) transition frequency is in very good agreement with previous determinations in saturation regime reported with similar accuracy. To achieve such accuracy, the transition frequency of the (101)-(000) 211-312 line of H216O interfering with the R(0) line had to be precisely determined and is reported with a standard error of 100 Hz at 214904329826.49(10) kHz (relative uncertainty of 5 × 10-13). These measurement sets provide stringent reference values for validating future advances in the theoretical description of the hydrogen (and water) molecule.

Comb coherence-transfer and cavity ring-down saturation spectroscopy around 1.65 µm: kHz-accurate frequencies of transitions in the 2ν3 band of 12CH4.

Comb Coherence Transfer (CCT) uses a feed-forward frequency correction to transfer the optical phase of a frequency comb to the beam of a free-running diode laser. This allows the amplification of a selected comb tooth by 50 dB while adding agile and accurate frequency tuning. In the present work, SI-traceable frequency calibration and comb tooth narrowing down to 20 kHz is additionally provided by comb frequency locking to an ultrastable optical frequency reference distributed from Paris to Grenoble through the RENATER optical fiber network [Lisdat et al., Nat. Commun., 2016, 7, 12443]. We apply this CCT broadly tunable source for saturated cavity ring-down spectroscopy of ro-vibrational R0 to R10 multiplets in the 2ν3 band of 12CH4 (from 6015 to 6115 cm-1). Indeed, efficient cavity injection with large intra-cavity power build-up induces saturation of the ro-vibrational transitions at low pressure and Doppler-free Lamb dips are observed with high signal/noise. kHz-accurate transition frequencies are derived improving by three orders of magnitude previous values from spectra in the Doppler regime, which are strongly affected by line blending. While previous saturation spectroscopy investigations addressed specific 2ν3 multiplets (R6 or R9), the CCT approach allowed for a rapid coverage of the entire R0-R10 series. Measured transition frequencies are compared with experimental and theoretical line lists available in the literature.

Electric quadrupole transitions in carbon dioxide.

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.

Detection of electric-quadrupole transitions in water vapour near 5.4 and 2.5 µm.

Nowadays, the spectroscopic databases used for the modeling of Earth and planetary atmospheres provide only electric-dipole transitions for polyatomic molecules (H2O, CO2, N2O, CH4, O3…). Very recently, electric-quadrupole transitions have been detected in the high sensitivity cavity ring down spectrum (CRDS) of water vapour near 1.3 µm [A. Campargue et al., Phys. Rev. Res., 2020, 2, 023091, DOI: 10.1103/PhysRevResearch.2.023091]. This discovery paved the way to systematic searches of quadrupole transitions in water vapor and other polyatomic molecules. In the present work, on the basis of high accuracy ab initio predictions, H216O quadrupole lines are detected for the first time in the 5.4 µm and 2.5 µm regions where they are predicted to have their largest intensities (up to 10-26 cm per molecule). A total of twelve quadrupole lines are identified in two high sensitivity Fourier transform spectra recorded with a 1064 m path length. Ten lines in the 4030-4150 cm-1 region are assigned to the ν3 band while the lines near 1820 and 1926 cm-1 belong to the ν2 band. The derived line intensities which are largely above the dipole intensity cut-off of the standard spectroscopic databases, agree nicely with the theoretical predictions. We thus conclude that the calculated line list of quadrupole transitions, validated by the present measurements, should be incorporated in the spectroscopic databases.

Ab initio variational predictions for understanding highly congested spectra: rovibrational assignment of 108 new methane sub-bands in the icosad range (6280-7800 cm(-1)).

A detailed study of methane spectra in the highly congested icosad range of 6280-7800 cm(-1) has been performed using global variational calculations derived from accurate ab initio potential energy and dipole moment surfaces. About 13,000 (12)CH4 lines of the WKLMC line lists recorded at 80 and 296 K using very sensitive laser techniques (DAS, CRDS) have been rovibrationally assigned from first principles predictions. Overall, a total of 7436 upper energy levels were determined. Among the 20 bands and the 134 sub-levels contained in the icosad system, 20 and 108 have been identified for the first time, respectively. The assigned transitions represent 98% of the sum of the experimental intensity at 80 K. This work demonstrates for the first time how accurate first principles global calculations allow assigning complicated spectra of a molecule with more than 4 atoms.

Communication: Saturated CO2 absorption near 1.6 µm for kilohertz-accuracy transition frequencies.

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.

Cavity ring down spectroscopy with 5 × 10(-13) cm-1 sensitivity.

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 a(1)Δ(g)(0)-X (3)Σ(g) (-)(1) hot band of (16)O(2) near 1.58 µm and (ii) first detection of an electric quadrupole transition in the second overtone band of nitrogen ((14)N(2)) near 1.44 µm.

The absorption spectrum of D2: ultrasensitive cavity ring down spectroscopy of the (2-0) band near 1.7 µm and accurate ab initio line list up to 24,000 cm(-1).

Eleven very weak electric quadrupole transitions Q(2), Q(1), S(0)-S(8) of the first overtone band of D(2) 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 α(min) ≈ 3 × 10(-11) cm(-1). By averaging a high number of spectra, the noise level was lowered to α(min) ≈ 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 D(2) 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.

The absorption spectrum of H2: CRDS measurements of the (2-0) band, review of the literature data and accurate ab initio line list up to 35000 cm(-1).

Five very weak transitions-O(2), O(3), O(4), O(5) and Q(5)-of the first overtone band of H(2) are measured by very high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) between 6900 and 7920 cm(-1). The noise equivalent absorption of the recordings is on the order of α(min)≈ 5 × 10(-11) cm(-1) allowing for the detection of the O(5) transition with an intensity of 1.1 × 10(-30) cm per molecule, the smallest intensity value measured so far for an H(2) absorption line. A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift of the O(2) and O(3) lines was accurately determined from a series of recordings with pressure ranging between 10 and 700 Torr. From an exhaustive review of the literature data, the list of H(2) absorption lines detected so far has been constructed. It includes a total of 39 transitions ranging from the S(0) pure rotational line near 354 cm(-1) up to the S(1) transition of the (5-0) band near 18,908 cm(-1). These experimental values are compared to a highly accurate theoretical line list constructed for pure H(2) at 296 K (0-35,000 cm(-1), intensity cut off of 1 × 10(-34) cm per molecule). The energy levels and transition moments were computed from high level quantum mechanics calculations. The overall agreement between the theoretical and experimental values is found to be very good for the line positions. Some deviations for the intensities of the high overtone bands (V > 2) are discussed in relation with possible pressure effects affecting the retrieved intensity values. We conclude that the hydrogen molecule is probably a unique case in rovibrational spectroscopy for which first principles theory can provide accurate spectroscopic parameters at the level of the performances of the state of the art experimental techniques.

Accurate determination of low state rotational quantum numbers (J < 4) from planar-jet and liquid nitrogen cell absorption spectra of methane near 1.4 micron.

An improved procedure for accurate determination of empirical lower state rotational quantum numbers from molecular absorption spectra is demonstrated for methane. We combine the high resolution absorption spectra in the 7070-7300 cm(-1) frequency range obtained in liquid nitrogen cooled cryogenic cell (T = 81 K) and in supersonic planar jet expansion (T(R) = 25 K). Empirical lower state energies of 59 transitions are determined from the ratio of the absolute absorption line strengths at 25 and 81 K. The procedure relies on the realistic description of rotational state populations in the supersonic jet expansion where non-equilibrium nuclear spin isomer distributions are generated due to the rapid cooling. The accuracy of the experimental determination of the lower state energies with J < or = 3 is found to considerably improve the results of the same approach applied to spectra at 296 and 81 K. The 59 transitions with determined lower J values provide a good starting point for the theoretical interpretation of the highly congested icosad region of methane. In particular, the centres of nine vibrational bands are estimated from the transitions with J = 0 lower state rotational quantum number.

The near-infrared (1.30-1.70 microm) absorption spectrum of methane down to 77 K.

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.30 and 1.70 microm (5850-7700 cm(-1)) using a newly developed cryogenic cell and a series of DFB diode lasers. The investigated spectral range includes part of the tetradecad and the full icosad regions for which only very partial theoretical analysis are available. The analysis of the low temperature spectrum will benefit from the reduction of the rotational congestion and from the narrowing by a factor of 2 of the Doppler linewidth allowing the resolution of a number of multiplets. Moreover, the energy value and rotational assignment of the angular momentum J of the lower state of a given transition can be obtained from the temperature variation of its line intensity. This procedure is illustrated in selected spectral regions by a continuous monitoring of the spectrum during the cell cool-down to 77 K, the temperature value being calculated at each instant from the measured Doppler linewidth. A short movie showing the considerable change of a spectrum during cool-down is attached as Supplementary Material. The method applied to a 30 cm(-1) section of the tetradecad spectrum around 6110 cm(-1) has allowed an unambiguous determination of the J values of part of the observed transitions.

The absorption spectrum of water near 750 nm by CW-CRDS: contribution to the search of water dimer absorption.

The absorption spectrum of natural water vapour around 750 nm has been recorded with a typical sensitivity of 3 x 10(-10) cm(-1) using a cw cavity ring down spectroscopy set up based on a Ti:sapphire laser. The 13 312.4-13 377.7 cm(-1) spectral interval was chosen as it corresponds to the region where water dimer absorption was recently measured (K. Pfeisticker et al., Science, 2003, 300, 2078-2080). The line parameters (wavenumber and intensity) of a total of 286 lines of water vapor were measured by a one by one fit of the lines to a Voigt profile. For the main water isotopologue, 276 lines were measured with line intensities as weak as 5 x 10(-29) cm molecule(-1)i.e. about 50 times smaller than the weakest H(2)16O line intensities included in the 2004 edition of the HITRAN database. On the basis of the predictions of Schwenke and Partridge, all but 16 lines could be assigned to different isotopologues of water (H(2)16O, H(2)18O, and HD16O) present in natural abundance in the sample. A total of 272 energy levels of H(2)16O were determined and rovibrationally assigned to 18 upper vibrational states. Half of them had not been reported previously. The importance of the additional absorbance resulting from the observation of many new weak lines is discussed in relation to the detection of water dimer absorption and compared to the absorbance predicted by Schwenke and Partridge. The quality of the line parameters of water monomer is shown to be of crucial importance to identify the absorbance of the water dimer in the considered region.

Spectroscopy and Intensity Measurements of the 3nu(1)+3nu(3) Tetrad of (12)CO(2) and (13)CO(2).

Three of the four components of the 3nu(1)+3nu(3) tetrad of (12)C(16)O(2) and (13)C(16)O(2), labeled 30031, 30032, and 30033 in HITRAN notation, have been observed by intracavity laser absorption spectroscopy in the 10 450- to 11 000-cm(-1) region. The rotational analysis has yielded the rovibrational parameters of the vibrational states. The experimental values are found to be in very good agreement with the rovibrational energies recently predicted from variational calculations and reduced effective Hamiltonians. The absolute band intensity of these extremely weak transitions have been measured. The study of the relative intensities within the 3nu(1)+3nu(3) tetrad suggests that part of the oscillator strength is carried by the (22(0)3) state. Copyright 2001 Academic Press.