Room temperature detection of the (H2)2 dimer.

The hydrogen dimer, (H2)2, is among the most weakly bound van der Waals complexes and a prototype species for first principles ab initio studies. The detection of the (H2)2 infrared absorption spectrum was reported more than sixty years ago at a temperature of 20 K. Due to the sharp decrease of the (H2)2 abundance with temperature, detection at room temperature was generally considered hardly achievable. Here we report the first room temperature detection of partly resolved rotational structures of (H2)2 by cavity ring down spectroscopy at sub-atmospheric pressures, in the region of the first overtone band of H2 near 1.2 µm. The quantitative analysis of the absorption features observed around ten allowed or forbidden transition frequencies of the monomer provides insight on the structure of this elusive species and a benchmark for future theoretical studies.

First detection and absolute transition frequencies in the (3-0) band of D2.

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.

Accurate absolute frequency measurement of the S(2) transition in the fundamental band of H2 near 2.03 µm.

A series of spectra of the quadrupolar electric S(2) transition of H2 in the 1-0 band near 4917 cm-1 has been recorded at seven pressure values between 2 and 100 Torr. The comb-referenced cavity ring down spectroscopy (CR-CRDS) technique was used for the recording of this very weak transition. The accuracy of the spectrum frequency axis is achieved by linking the CRDS setup to an optical frequency comb referenced to a GPS-referenced 10 MHz rubidium clock. Applying a multi-spectrum fit procedure to the seven averaged spectra with a quadratic speed dependence Nelkin-Ghatak profile, the transition frequency is determined (ν0 = 147 408 142 357 kHz) with an uncertainty of 150 kHz (∼1 × 10-9 in relative). This represents the smallest uncertainty achieved so far for a transition in the fundamental band of H2. The experimental frequency reported in this work is 1.53 MHz higher than the best-to-date theoretical value. This difference represents 1.5 times the 1σ-uncertainty (about 1 MHz) of the calculated frequency. The measurements also allow for the determination of the absolute intensity value of the S(2) line which shows an agreement with the ab initio value at the per mil level. In addition, the cross section of the collision induced absorption (CIA) underlying the S(2) line is accurately retrieved from the quadratic pressure dependence of the baseline level of the recorded spectra.

The high-accuracy spectroscopy of H2 rovibrational transitions in the (2-0) band near 1.2 µm.

Accurate transition frequencies of six lines of the (2-0) vibrational band of H2 are reported near 1.2 µm, namely Q1-Q4, S0, and S1. These weak electric-quadrupole transitions were measured at room temperature by comb-referenced cavity ring-down spectroscopy. Accurate transition frequencies were determined by applying a multi-spectrum fit procedure with various profile models including speed-dependent collisional broadening and shifting phenomena. Although none of the considered profiles allows reproducing the shape of the strongest lines at the noise level, the zero-pressure line centers are found mostly independent of the used profile. The obtained values are the first H2 (2-0) transition frequencies referenced to an absolute frequency standard. As a result, a 1σ-accuracy better than 100 kHz was achieved for the Q1, S0, and S1 transition frequencies, improving by three orders of magnitude the accuracy of previous measurements. For the six measured transitions, the most recent calculated frequencies were found to be systematically underestimated by about 2.51 MHz, about twice their claimed uncertainties. The energy separation between J = 2 and J = 0 rotational levels of the vibrational ground state was derived from Q2 and S0 transition frequencies and found within the 110 kHz uncertainty of its theoretical value. The same level of agreement was achieved for the energy separation between the J = 3 and J = 1 rotational levels obtained by the difference of Q3 and S1 transition frequencies. The ab initio values of the intensity of the six transitions were validated within a few thousandths.

12CO2 transition frequencies with kHz-accuracy by saturation spectroscopy in the 1.99-2.09 µm region.

Saturation spectroscopy has been used to determine the absolute frequencies of 107 ro-vibrational transitions of the two strongest 12CO2 bands of the 2 µm region. The considered 20012-00001 and 20013-00001 bands are of importance for the CO2 monitoring in our atmosphere. Lamb dips were measured using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator or to an ultra-stable optical frequency. The comb-coherence transfer (CCT) technique was applied to obtain a RF tunable narrow-line comb-disciplined laser source using an external cavity diode laser and a simple electro-optic modulator. This setup allows obtaining transition frequency measurements with kHz-level accuracy. The resulting accurate values of the energy levels of the 20012 and 20013 vibrational states are reproduced with a (1σ)-rms of about 1 kHz using the standard polynomial model. The two upper vibrational states appear thus to be highly isolated except for a local perturbation of the 20012 state leading to an energy shift of 15 kHz at J = 43. A recommended list of 145 transition frequencies with kHz accuracy is obtained providing secondary frequency standards across the 1.99-2.09 µm range. The reported frequencies will be valuable to constrain the zero-pressure frequencies of the considered transitions in 12CO2 retrieval from atmospheric spectra.