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Previously unobserved nitrous oxide transitions around 2.5 μm are measured by intracavity laser absorption spectroscopy (ICLAS) analyzed by time-resolved Fourier transform (TRFT) spectrometer. With an accuracy of the order of 10−3 cm−1, measured positions of 1637 assigned weak transitions are provided. They belong to 42 vibrational transitions, among which 33 are observed for the first time. These data are believed to be useful in particular to monitoring atmosphere purposes.
In this note, intracavity laser absorption spectroscopy (ICLAS) coupled (1) with time-resolved Fourier transform (TRFT) spectrometer is applied to the measurement of previously unobserved N2O transitions around 2.5 μm. High resolution absorption spectra of N2O in natural isotopic abundance are recorded around 4000 cm−1 with kilometric absorption path lengths. Their analysis reveals the observed weak transitions belong to 42 vibrational transitions, among which 33 are observed for the first time. These data are believed to be useful in particular for atmospheric applications.
The laser, installed in a vacuum chamber, is made of Cr2+: ZnSe amplifying medium inserted in X-fold cavity. It is optically pumped with an Er3+ fiber doped laser emitting at 1.6 μm. The pumping beam is chopped by an acousto-optic modulator. The laser build-up is recorded with a TRFTS equipped with two InSb detectors cooled at 77 K. More details on the experimental setup may be found in the instrumentally oriented paper (2) and in (3) where new data measurements and analysis for 23 C2H2 molecular bands located around 4000 cm−1 are also reported.
Three N2O time-resolved spectra (numbered 508, 509, 510) have been recorded (4) with natural sample pressures respectively equal to 3.49, 70.62, 70.58 hPa (2.62, 53.0, and 52.9 Torr). The gas was inserted in the vacuum chamber. This provides, with the elimination of the parasitic atmospheric absorption, the advantage of a laser cavity filling ratio practically equal to unity. All TRFT spectra of the laser emission were recorded with 64 time samples, 1.6 μs time resolution and 32 co-additions. Unapodized spectral resolutions and recording times were respectively 0.037, 0.037, 0.007 cm−1 and 5, 5, and 25 minutes.
Figure 1 displays the general temporal behavior of the spectrum nb. 510 with restricted spectral resolution. The laser line is narrowing with increasing generation times reaching at most 120 μs. With 7.1 km absorption path length the spectrum covers 150 cm−1, approximately from 3900 to 4050 cm−1. With 33.5 km absorption path length the spectral coverage is restricted to 60 cm−1, from 3955 to 4005 cm−1. As in Ref.(3), it has been checked that no deviation to the linear evolution versus the generation time of the peak absorbance is observed. No attempt was made to tune the Cr2+: ZnSe laser.
Wavenumber scale of the spectra have been calibrated against lines from residual water vapor in the laser chamber, using (5). Line positions reported here were measured in the 3 temporal samples of spectrum 510. Their sequence number is 6, 16, 26 corresponding to the equivalent absorption paths 7, 12, 17 km. Their respective wavenumber scales were first checked to be consistent within an average of 4 10−4 cm−1. Full width at half maximum of the N O profiles in spectrum 510 (pressure: 70.58 hPa) is of the order of 20 10−3 cm−1, revealing as expected contribution of collisional broadening. The accuracy of the line position measurements is of the order of 10−3 cm−1. Only well-resolved and unsaturated lines were taken into account. Line sequences detected with a Loomis-Wood program (6), were left unprocessed and are not reported in this note, when at least 15 lines were not fitting the selection criteria. Altogether, 42 bands are observed. Among them, 32 belong to 14N216O, 4 to 14N15N16O, 4 to 15N14N16O, and 2 to 14N218O. The 14N216O bands consist of 1 Σ-Σ, 11 Σ-Π, 2 Σ-Φ, 2 Π–Φ, 4 Π–Π, 8 Π–Δ and 4 Δ–Φ transitions. All the 14N15N16O, 15N14N16O, and 14N218O bands are Σ-Π transitions. Practically all the observed lines are transitions between already known energy levels. Only the 13312 14N216O energy level is observed for the first time. In order to appreciate the validity of our measurements, least-squares fits of individual bands were performed using the polynomial expression:
The 42 observed bands are summarized with their assignments and calculated band centers in Table 1. A full resolution small part of the temporal sample n° 6 already shown in Figure 1 is given on Figure 2 with spectral assignments of the resolved lines. In the Journal supplementary material, 3 additional tables are given. Table 2 reports for 14N216O the effective parameters obtained from our calculations and their corresponding values taken in (7) and (8). Table 3 reports similar results for the isotopologues 14N15N16O, 15N14N16O, and 14N218O. Table 4 is aimed to be a convenient tool. It provides in increasing order 1637 measured line positions, with their corresponding “observed – calculated” values and their isotopologue and rovibrational assignments.
The work has been supported by the French-Austrian exchange program Amadeus and the FWF project P17973.