Accurate transition frequencies of six lines of the (2-0) vibrational band of H
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 H
(2-0) transition frequencies referenced to an absolute frequency standard. As a result, a 1
-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
= 2 and
= 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
= 3 and
= 1 rotational levels obtained by the difference of Q3 and S1 transition frequencies. The
values of the intensity of the six transitions were validated within a few thousandths.
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.
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.
Saturation spectroscopy has been used to determine the absolute frequencies of 107 ro-vibrational transitions of the two strongest
CO
bands of the 2 μm region. The considered 20012-00001 and ...20013-00001 bands are of importance for the CO
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 (1
)-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
= 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
CO
retrieval from atmospheric spectra.
A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a ...high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ~20 000 (ringdown time ~20 *ms) and allows recording spectra of up to 200 cavity modes (2 cm) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ~5X10 cm for 1 s averaging over scans, with a dynamic range of four orders of magnitude.
The water vapour self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy at room temperature in the 1.6 mu m window. The real time pressure dependence of the continuum ...was investigated during pressure cycles up to 12 Torr for fifteen selected wavenumber values. The continuum absorption coefficient measured between 5875 and 6450 cm-1 shows a minimum value around 6300 cm-1 and ranges between 110-9 and 810-9 cm-1 for 8 Torr of water vapour. The continuum level is observed to deviate significantly from the expected quadratic dependence versus the pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively.
A self-referenced frequency comb has been combined with a cavity ring down (CRD) spectrometer to achieve a sub-MHz accuracy on the derived positions of the absorption lines. The frequency emitted by ...the distributed feedback (DFB) laser diode used in the spectrometer was obtained from the frequency of its beat note with the closest mode of the frequency comb. This delivers excellent frequency accuracy over a broad spectral region with sensitivity (noise equivalent absorption) of 1x10 super(-11) cm super(-1) Hz super(-1/2). This setup is used to measure the absorption spectrum of CO over a wide range corresponding to the 3-0 band (6172.5-6418.0 cm super(-1)). Accurate values of line centers are measured for a total of 184 lines of four CO isotopologues, namely super(12)C super(16)O, super(13)C super(16)O, super(12)C super(18)O and super(12)C super(17)O present in "natural" abundances in our sample. The measurements include the first extensive study of the 3-0 band of super(12)C super(18)O and super(12)C super(17)O, of the 4-1 hot band of super(12)C super(16)O and the detection of new high-J transitions of the 3-0 band of super(12)C super(16)O up to J=34. The line centers were corrected for the self-pressure shift and used to derive the upper state spectroscopic parameters. The obtained standard deviation of about 300 kHz and 500 kHz for the 3-0 band of super(12)C super(16)O and of the minor isotopologues, respectively, is a good estimate of the average accuracy of the reported line centers. The resulting 3-0 line list of super(12)C super(16)O provided as Supplementary material includes 69 reference line positions with a 300 kHz accuracy for the 6183-6418 cm super(-1) region.
The very weak water vapor self-continuum has been investigated by high-sensitivity cavity ring-down spectroscopy in the 1.6µm window at five temperatures between 302K and 340K. The absorption cross ...sections, Cs(ν, T), were retrieved for 10 selected wave numbers from a fit of the absorption coefficients measured in real time during pressure ramps, after subtraction of the contributions of the local water monomer lines and of water adsorbed on the cell mirrors. The values measured between 5875 and 6665cm-1 range between 1.5×10-25 and 2×10-24cm2 mol-1atm-1 with a minimum around 6300cm-1. At 302K, an agreement within 50% is observed over the whole window with the cross sections provided by the MT_CKD V2.5 model. Nevertheless, while our measurements show that the Cs(ν, T) decrease from 302K to 340K is no more than 50% for all our selected wave numbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the center of the window, the agreement being better on the edges of the window. The obtained results are discussed in relation with theoretical modeling of the water vapor self-continuum as far wings of monomer lines or water dimer absorption. For potential atmospheric applications, cross sections are provided at each temperature with a sampling step of 10cm-1 for the entire 5850-6700cm-1 range. Key Points Water self-continuum absorption cross sections are measured by CRDS The temperature dependence of the cross sections is studied
The room temperature self- and foreign-continua of water vapor have been measured near 4250 cm
−1
with a newly developed high sensitivity cavity ring down spectrometer (CRDS). The typical sensitivity ...of the recordings is
α
min
6 × 10
−10
cm
−1
which is two orders of magnitude better than previous Fourier transform spectroscopy (FTS) measurements in the spectral region. The investigated spectral interval is located in the low energy range of the important 2.1 μm atmospheric transparency window. Self-continuum cross-sections,
C
S
, were retrieved from the quadratic dependence of the spectrum base line level measured for different water vapor pressures between 0 and 15 Torr, after subtraction of the local water monomer lines contribution calculated using HITRAN2012 line parameters. The
C
S
values were determined with 5% accuracy for four spectral points between 4249.2 and 4257.3 cm
−1
. Their values of about 3.2 × 10
−23
cm
2
molecule
−1
atm
−1
are found 20% higher than predicted by the MT_CKD V2.5 model but two times weaker than reported in the literature using FTS. The foreign-continuum was evaluated by injecting various amounts of synthetic air in the CRDS cell while keeping the initial water vapor partial pressure constant. The foreign-continuum cross-section,
C
F
, was retrieved from a linear fit of the spectrum base line level
versus
the air pressure. The obtained
C
F
values are larger by a factor of 4.5 compared to the MT_CKD values and smaller by a factor of 1.7 compared to previous FTS values. As a result, for an atmosphere at room temperature with 60% relative humidity, the foreign-continuum contribution to the water continuum near 4250 cm
−1
is found to be on the same order as the self-continuum contribution.
Water vapor self and foreign continuum absorption cross sections are measured with unprecedented accuracy around 2.35 μm using high sensitivity CRDS.
Since the discovery of anomalies in ozone isotope enrichment, several fundamental issues in the dynamics linked to the shape of the potential energy surface in the transition state region have been ...raised. The role of the reeflike structure on the minimum energy path is an intricate question previously discussed in the context of chemical experiments. In this Letter, we bring strong arguments in favor of the absence of a submerged barrier from ultrasensitive laser spectroscopy experiments combined with accurate predictions of highly excited vibrations up to nearly 95% of the dissociation threshold.
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