High Sensitivity Cavity Ring Down Spectroscopy Research Articles

Experiment on Recording Ozone Absorption Transitions to 3A2 Triplet Electronic State by High-Sensitivity Cavity Ring-Down Spectroscopy in the Range 9350–10 000 cm−1

Experiment on Recording Ozone Absorption Transitions to 3A2 Triplet Electronic State by High-Sensitivity Cavity Ring-Down Spectroscopy in the Range 9350–10 000 cm−1

高灵敏腔衰荡光谱技术及其应用研究

高灵敏腔衰荡光谱技术及其应用研究

Detection and assignment of ozone bands near 95% of the dissociation threshold: Ultrasensitive experiments for probing potential energy function and vibrational dynamics

Ozone formation and depletion play a key role in various physical and chemical atmospheric processes which remain to be understood in more detail. The modeling of such phenomena based on ozone dynamics calculations requires a detailed knowledge of quantum states, transition probabilities, and ozone potential energy surface (PES) near the dissociation threshold ${D}_{0}$. Knowledge of highly excited rovibrational states with energies approaching ${D}_{0}$ is necessary for a reliable interpretation of satellite measurements of the ozone absorption or emission in the upper atmosphere in nonlocal thermodynamic equilibrium conditions. In addition, these states provide an ideal probe of PESs computed by ab initio methods but their detection by absorption spectroscopy is particularly challenging due to the sharp decrease of the intensity with the energy of the corresponding vibrational combination and overtone bands. In this work, two very high-energy vibrational bands are detected by high sensitivity cavity ring-down spectroscopy providing a noise equivalent absorption ${\ensuremath{\alpha}}_{\mathrm{min}}$ on the order of a few ${10}^{\ensuremath{-}11}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. The corresponding cold bands, assigned as ${\ensuremath{\nu}}_{1}+6{\ensuremath{\nu}}_{2}+3{\ensuremath{\nu}}_{3}$ and $6{\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{2}+{\ensuremath{\nu}}_{3}$ centered at 7969 and $7993\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, respectively, are about nine orders of magnitude less intense than the well-known ${\ensuremath{\nu}}_{3}$ fundamental band near 10 \textmu{}m. The spectrum analysis allowed for the experimental determination of 240 rovibrational energy levels of the upper vibrational states. The corresponding rotational patterns, located between 93.1% and 96.7% of the dissociation threshold, are the most excited ones measured so far by absorption spectroscopy. The values of the band centers and rotational constants derived from the spectrum analysis provide further confirmation of the absence of an activation barrier on the minimum-energy path towards the dissociation threshold [Tyuterev et al., Phys. Rev. Lett. 113, 143002 (2014)]. The vibrational dynamics is discussed in the context of the radiative transition probabilities and related Einstein coefficients for the observed bands. Another important question in the context of the nuclear dynamics concerns the interaction among the three identical potential wells of the ozone molecule in the ground electronic state, which appears to be due to the Jahn-Teller effect. Recent theoretical results accounting for the three potential wells in a full symmetry approach predicted that vibration states should exhibit deviations from the conventional one-potential-well ab initio calculations, near the dissociation threshold. In the present work, we observe a better agreement of the experimentally determined energy value with the three-wells prediction for the (1,6,3) vibrational state involving a large simultaneous excitation of both bending (${\ensuremath{\upsilon}}_{2}=6$) and asymmetric stretch (${\ensuremath{\upsilon}}_{3}=3$) vibrations pointing towards a nearby well. This could be tentatively interpreted as a manifestation of the three-wells effect.

New transitions and energy levels of water vapor by high sensitivity CRDS near 1.73 and 1.54 µm

This contribution is part of a long term project aiming at improving the water absorption spectroscopy by high sensitivity cavity ring down spectroscopy (CRDS) in the near infrared.Two new sources of CRDS spectra are considered:(i) The room temperature absorption spectrum of water vapor in natural isotopic abundance is recorded near 1.73 µm. A series of recordings was performed from 5693 to 5991 cm−1 with a pressure value of about 6 Torr. The noise equivalent absorption (αmin) of the spectra is better than 10−10 cm−1. A total of 1453 lines were assigned to 1573 transitions of four water isotopologues (H216O, H217O, H218O and HD16O). Their intensities span more than five orders of magnitude from 3.0 × 10−30 to 4.7 × 10−25 cm/molecule at 296 K. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge.Two hundred fifty-one lines (assigned to 280 transitions) are observed for the first time and twelve energy levels are newly determined. The comparison of the obtained line parameters with those of the HITRAN database is discussed. Forty-six line positions are observed to significantly differ from their HITRAN values (δν = │νHITRAN – νCRDS│ > 0.02 cm−1). The derived set of energy levels is compared to those recommended by an IUPAC task group.(ii) The room temperature CRDS spectrum of water vapor highly enriched in 17O was recorded near 1.54 µm (6223–6672 cm−1) at a pressure of 12 Torr. Compared to a previous study, the higher pressure of the recordings allowed for extending the observations. Overall, twenty-six new levels were determined for both H217O and HD17O.All these observations together with other recent measurements will allow for an extension and an update of our empirical database in the 5693– 8340 cm−1 region.

High-sensitivity CRDS absorption spectrum of 17O enriched carbon dioxide near 1.74 µm

The room temperature absorption spectrum of 17O enriched carbon dioxide is investigated by high sensitivity cavity ring down spectroscopy (CRDS) between 5695 and 5850 cm−1. About 1100 lines are measured and assigned to 26 bands of the six 17O containing carbon dioxide isotopologues: 16O12C17O, 16O13C17O, 17O12C18O, 17O13C18O, 12C17O2 and 13C17O2. The set of observed lines are rovibrationnally assigned on the basis of the predictions of the effective Hamiltonian (EH) model. The observed bands belong to the ΔP = 8 and 9 series of transitions where P = 2 V1 + V2 + 3 V3 is the polyad number (Vi being the vibrational quantum numbers). They consist in 12 previously unobserved bands and 14 already-known bands, for which additional high J lines are assigned. Bands of the 12C17O2 and 13C17O2 isotopologues (two for each) are observed for the first time in this spectral interval. The spectroscopic parameters of the upper vibrational levels are determined from a band-by-band fit of the line positions (typical rms deviations are less than 0.001 cm−1). The comparison to the most recent carbon dioxide spectroscopic databases is presented. The reported data will be used to improve the modeling of the line positions of 17O containing carbon dioxide isotopologues.

Time-resolved measurements of NO2 concentration in pulsed discharges by high-sensitivity cavity ring-down spectroscopy

To describe the complex kinetics of formation and destruction mechanism of nitrogen dioxide (NO2), there is an increasing demand for real-time and in situ analysis of NO2 in the discharge region. Pulsed cavity ring-down spectroscopy (CRDS) provides an excellent diagnostic approach. In the present paper, CRDS has been applied in situ for time evolution measurement of NO2 concentration which is rarely investigated in gas discharges. In pulsed direct current discharge of NO2/Ar mixture at a pressure of 500 Pa, a peak voltage of −1300 V and a frequency of 30 Hz, for higher initial NO2 concentration (3.05 × 1014 cm−3, 8.88 × 1013 cm−3), the NO2 concentration sharply decreases at the beginning of the discharge afterglow and then becomes almost constant, and the pace of decline increases with pulse duration; however, for lower initial NO2 concentration of 1.69 × 1013 cm−3, the NO2 concentration also decreases at the beginning of the discharge afterglow for 200 ns and 1 μs pulse durations, while it slightly increases and then declines for 2 μs pulse duration. Thus, the removal of low-level NO2 could not be promoted by a higher mean energy input.

The CO2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (I) Rovibrational lines

The absorption of carbon dioxide is very weak near 2.3µm which makes this transparency window of particular interest for the study of Venus’ lower atmosphere. As a consequence of the weakness of the transitions located in this region, previous experimental data are very scarce and spectroscopic databases provide calculated line lists which should be tested and validated by experiment. In this work, we use the Cavity Ring Down Spectroscopy (CRDS) technique for a high sensitivity characterization of the CO2 absorption spectrum in two spectral intervals of the 2.3µm window: 4248–4257 and 4295–4380cm−1 which were accessed using a Distributed Feed Back (DFB) diode laser and a Vertical External Cavity Surface Emitting Laser (VECSEL) as light sources, respectively. The achieved sensitivity (noise equivalent absorption, αmin, on the order of 5×10−10cm−1) allowed detecting numerous new transitions with intensity values down to 5×10−30cm/molecule.The rovibrational assignments were performed by comparison with available theoretical line lists in particular those obtained at IAO Tomsk using the global effective operator approach. Hot bands of the main isotopologue and 16O12C18O bands were found to be missing in the HITRAN database while they contribute importantly to the absorption in the region.Additional CRDS spectra of a CO2 sample highly enriched in 18O were recorded in order to improve the spectroscopy of this isotopologue. As a result about 700 lines of 16O12C18O, 16O12C17O, 17O12C18O, 12C18O2 and 13C18O2 were newly measured.The status of the different databases (HITRAN, CDSD, variational calculations) in the important 2.3µm transparency window is discussed. Possible improvements to correct evidenced deficiencies are suggested.

Analyses of FT spectra of C2V ozone isotopologues in the 4500–5700 cm−1 region: 16O18O16O, 18O16O18O and 18O3

This work continues the systematic investigation of high resolution infrared spectra of 18O enriched ozone isotopologues in the 4500–6300cm−1 spectral range. So far, the 16O18O16O and 18O16O18O spectra have been recorded and analysed using Fourier Transform Spectrometer (FTS) below 5000cm−1 and using high sensitivity Cavity Ring-Down Spectroscopy above 6000cm−1. In order to fill the gap between 5000 and 6000cm−1, new infrared spectra were recorded with the Reims FTS. The analyses of the 2ν1+ν2+3ν3 band of 16O18O16O and of the 5ν3, ν1+2ν2+3ν3 and ν1+5ν3 bands of 18O16O18O are reported. New ν1+2ν2+3ν3 and ν1+5ν3 bands of 18O3 were also observed and analysed. Observed line positions were fitted using effective Hamiltonian models accounting for the dark state perturbations. The derived band centres and rotational constants are in a good agreement with new theoretical calculations from the molecular potential function.

Absolute measurements of electron impact excitation cross-sections of atoms using cavity ringdown spectroscopy

A new method of using cavity ringdown spectroscopy (CRDS) technique to measure apparent electron impact excitation cross-sections (EIECS) for different energy levels of an atom is described. The method involves measurements of absolute population densities of the energy levels in concern using the high sensitivity CRDS, leading to an analytical expression of EIECS as a function of the population densities. Determination of EIECS in this method does not require an external calibration. The proposed method is validated by determining EIECS of the metastable levels of Hg (6s6p 3P0) and Ne (2p53s 3P2), and the results are in good agreement with the cross-sections reported in the literature. The population densities in both cases were extracted from the published data. This method exhibits promising potential for absolute measurements of EIECS of metastable energy levels.

High-sensitivity CRDS absorption spectroscopy of acetylene between 5851 and 6341 cm−1

The absorption spectrum of acetylene has been recorded at room temperature (297 K) using high-sensitivity cavity ring-down spectroscopy (αmin ∼ 5×10−11 cm−1) in the 5851 and 6341 cm−1 interval corresponding to a region of very weak absorption. A list of about 10,700 absorption features with estimated absolute line intensities was constructed. The smallest intensities are of the order of 5×10−29 cm molecule−1. The line list includes about 2500 absorption lines of ethylene present at the ppm level in the acetylene sample and identified on the basis of a high-resolution Fourier transform spectrum specifically recorded. A total of more than 2700 lines of 12C2H2 were rovibrationally assigned in comparison with accurate predictions provided by a global effective operator model. Overall, the present effort adds about 2260 new assignments to the set of about 500 assigned transitions available in the literature. The new assignments correspond to 45 new bands and 17 already-known bands, for which additional J lines were assigned. Spectroscopic parameters were derived for the upper vibrational levels from a band by band fit of the line positions (typical root mean square deviation values are of the order of 0.001 cm−1). A few of the analysed bands were found to be affected by rovibrational perturbations, which are discussed. The new data will be valuable to refine the parameters of the global effective Hamiltonian and dipole moments of 12C2H2.

Two schemes for trace detection using cavity ringdown spectroscopy

We describe and compare two schemes of high-sensitivity cavity ringdown spectroscopy (CRDS), both functioning with telecom diode lasers. The first (cw-CRDS) gives high spectral resolution, which is useful for low-pressure trace detection or for laboratory spectroscopy applications. We present a compact prototype partly based on fiber technology. The second scheme exploits optical feedback (of-CRDS) and results in a much simpler setup, more appropriate for realizing low-cost trace-detection devices.

Density of N2 (X 1Σg+; v = 18) molecules in a dc glowdischarge measured by cavity ringdown spectroscopy at 227 nm; validitydomain of the technique

The high sensitivity cavity ringdown spectroscopy (CRDS)absorption technique is used to measure the absolute density of the groundstate nitrogen molecules in the v'' = 18 vibrational level in a dc glowdischarge. The experimental conditions are 2.3 Torr nitrogen pressure,100 mA current and a discharge tube of 3.6 cm diameter and 80 cm length.The excimer-pumped dye laser is tuned on theLyman-Birge-Hopfield, N2 (a 1Πg; 8-X 1Σg; 18) band at227 nm and absorption rates on spectrally-resolved rotational lines areobtained. The gas temperature deduced from the rotational distribution is500 K. We measure a total density of (9±3.5)×1019 molecules m-3 in the v'' = 18 vibrational level, equivalent to0.2% of the total nitrogen density. So high density suggests that thev'' = 18 level should be located in the Treanor plateau of the vibrationaldistribution function. We also analyse the validity domain of the CRDSmeasurements, regarding the optical saturation due to the high intensity ofthe laser radiation inside the CRDS cavity and depletion of the absorbingspecies. If, for our measurements, the linear absorption conditions arefulfilled, we show that very often the optical saturation becomes dominant,inducing a non-single exponential decay of the ringdown signal and leadingto an underestimated absorber density.