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
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.
•We analyze Cassini VIMS spectra of Titan using improved methane spectroscopic data.•We constrain the aerosol single scattering albedo and derive the surface albedo over specific ...terrains.•Differences in surface albedos can be explained by different amounts of tholins and water ice particles.•The surface albedo of the Huygens landing site at 2μm is inconsistent with water ice being a major compound.•No reversal of the North-to-South asymmetry in the stratospheric haze is seen till mid-2010.
We present an analysis of Titan data acquired by the Cassini Visual and Infrared Mapping Spectrometer (VIMS), making use of recent improvements in methane spectroscopic parameters in the region 1.3–5.2μm. We first analyzed VIMS spectra covering a 8×10-km2 area near the Huygens landing site in order to constrain the single scattering albedo (ω0) of the aerosols over all of the VIMS spectral range. Our aerosol model agrees with that derived from Huygens Probe Descent Imager/Spectral Radiometer (DISR) in situ measurements below 1.6μm. At longer wavelengths, ω0 steadily decreases from 0.92 at 1.6μm to about 0.70 at 2.5μm and abruptly drops to about 0.50 near 2.6μm, a spectral variation that differs from that of Khare et al.’s (Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W. 1984. Icarus 60, 127–137) laboratory tholins. Our analysis shows that the far wings of the strong methane bands on both sides of the transparency windows provide a significant source of opacity in these windows, and that their unknown sub-Lorentzian behavior limits our ability to determine precisely the surface albedos. Below 1.6μm, the retrieved surface albedos agree with those derived from Huygens/DISR. The VIMS spectrum at 2.0μm indicates a surface albedo of 0.11±0.01, larger than derived in previous studies, and inconsistent with the signature of water ice. A series of VIMS data taken from 2004 to 2010 between 40°S and 40°N were then analyzed to monitor the latitudinal and temporal evolution of the atmospheric aerosol content. In the 2004–2008 period, the haze extinction is larger at Northern mid-latitudes by ∼20% with respect to the Huygens site, whereas Southern mid-latitudes are depleted by ∼15–20%. In 2009–2010, a progressive decline of the haze content in the Northern hemisphere is observed but no reversal of the North-to-South asymmetry is seen till mid-2010. Finally, data from five regions in Tui Regio and Fensal that show markedly different spectral behaviors and morphologies were analyzed to investigate the wavelength dependence of their surface albedo. The difference between bright and dark regions can be explained by different contents of small-sized tholins at the surface, brighter regions being more tholin-rich than dark regions, including the Huygens landing site. On the other hand, the albedo spectrum of the so-called blue regions, either dark or bright, can be explained by an excess of water ice particles, compared with the Huygens landing site. The spectrum of a 5-μm bright region in Tui Regio indicates a large excess of small-sized tholins relative to the Huygens site, but does not point to any particular surface composition.
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