First high-resolution analysis of the 4ν13 band of nitrogen dioxide near 1.5 μm

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Abstract

The high-resolution absorption spectrum of the 4ν13 band of the 14N16O2 molecule was recorded by CW-Cavity Ring Down Spectroscopy between 6575 and 6700 cm−1. The assignments involve energy levels of the (4,0,1) vibrational state with rotational quantum numbers up to Ka=8 and N=48. A large majority of the spin-rotation energy levels were reproduced within their experimental uncertainty using a theoretical model which takes explicitly into account the Coriolis interactions between the spin-rotational levels of the (4,0,1) vibrational state and those of the (4,2,0) and of (0,9,0) dark states, the anharmonic interactions between the (4,2,0) and (0,9,0) states together with the electron spin-rotation resonances within the (4,0,1), (4,2,0) and (0,9,0) states. Precise vibrational energies, rotational, spin-rotational, and coupling constants were determined for the {(4,2,0), (0,9,0), (4,0,1)} triad of interacting states. Using these parameters and the value of the transition dipole-moment operator determined from a fit of a selection of experimental line intensities, the synthetic spectrum of the 4ν13 band was generated and is provided as Supplementary Material.

Introduction

Nitrogen dioxide (14N16O2) is an important atmospheric trace species involved in the photochemistry of the stratosphere and in the pollution of the troposphere. Accurate measurements of NO2 concentration in the atmosphere are now commonly performed in the 6.2 and 3.4 μm regions which correspond to the ν3 and ν13 bands, respectively, by means of infrared remote sensing methods [1], [2]. For this reason numerous detailed spectroscopic studies of the 14N16O2 absorption bands were performed from the microwave up to the 1.7 μm region [3], [4], [5], [6], [7], [8], [9], [10], [11]. The results of these studies lead to lists of NO2 line positions and intensities [12] which are now implemented in spectroscopic databases such as HITRAN [13] and GEISA [14].

14N16O2 is an asymmetric rotor exhibiting in the infrared region a spectrum with a doublet structure due to the electron spin–rotation interaction, and a hyperfine structure in microwave and far infrared spectral regions. Also depending on the spectral range of interest, one has to consider rovibrational interactions in order to account for the measured line positions and intensities. Indeed strong C-type Coriolis resonances are coupling the spin-rotational levels of the (v1,v2,v3) and (v1,v2±2,v3±1) vibrational states [6], [8], [9], [10], [11], [15]. In addition, for the first triad of interacting states {(1,0,0), (0,2,0), (0,0,1)}, a weaker C-type Coriolis resonance connects the (1,0,0) and (0,0,1) energy levels [6].

It is also important to understand the complex absorption behavior of the NO2 species at high energies. Using the laser induced dispersed fluorescence (LIDFS) technique, the complete set of the 191 lowest vibrational levels of the X2A1 ground electronic state, up to 10,000 cm–1 was measured [16]. However, at wavelengths smaller than 2.5 μm only some NO2 infrared bands were the subject of detailed rotational analysis, like the 2ν13, 3ν3 [11], 3ν13 [17] and ν1+3ν3 [15] bands located at 4179.938, 4754.209, 5437.54 and 5984.705 cm−1, respectively. Indeed in this frequency range, the absorption bands become extremely weak, except for series of A-type bands corresponding to (v1,v2,v3)−(0,0,0) vibrational transitions with, v3=odd for symmetry reasons, high vibrational excitation of the stretching modes (v1+v3=high), and no excitation of the bending mode (v2=0). One has to underline that during the investigation of the ν1+3ν3 band [15], additional high order vibration–rotation interactions involving vibrational states differing by a large number of vibrational quanta were identified in addition to the resonances classically observed in the infrared region. It is interesting to search for such high order resonances by investigating NO2 bands at higher frequency.

The present work is devoted to the analysis of the 4ν13 band near 6677 cm−1. This band is the highest combination band of nitrogen dioxide in the X2A1 ground state reported so far at high-resolution. The spectrum was recorded in Grenoble by CW-Cavity Ring Down Spectroscopy (CW-CRDS). The analysis was performed by using a Hamiltonian matrix which explicitly accounts for the electron spin-rotation resonances, Coriolis-type resonances and anharmonic interactions.

Section snippets

Experimental details

The high-sensitivity absorption spectrum of nitrogen dioxide was recorded in the 6575–6700 cm−1 region. The fibered distributed feedback (DFB) laser CW-CRDS spectrometer used for these recordings has been described in Refs. [18], [19], [20]. Each DFB laser diode has a typical tuning range of about 35 cm−1 by temperature tuning from −5 to 60 °C. Five DFB laser diodes were sufficient to cover the 6575–6700 cm−1 region of interest. The stainless steel ring down cell (l=1.42 m, Φ=10 mm) is fitted by a

Assignment

Nitrogen dioxide is an asymmetric rotor which is rather close to the prolate symmetric top limit, due to the value of its rotational constants (A∼8.0023, B∼0.4337, C∼0.4104 cm−1 in the ground vibrational state [5]). According to symmetry properties, the 4ν13 band is an A-type band with only ΔKa=even transitions. However since NO2 is a near prolate symmetric top, only Ka=0 transitions are observable.

In principle 14N16O2 exhibits a spectrum with doublet structure due to the electron

Conclusion

The high-resolution Cavity Ring Down laser spectrum of nitrogen dioxide was recorded near 1.5 μm which allowed the first extensive analysis of the 4ν13 band of 14N16O2. This band is the highest combination band of nitrogen dioxide in the X2A1 ground state, analyzed so far at high-resolution. A Hamiltonian matrix which accounts for Coriolis, electron spin–rotation and anharmonic interactions was constructed for the determination of the experimental spin–rotation energy levels of the {(4,2,0),

Acknowledgment

A.P. is grateful to the INSU (Institut national des sciences de l’Univers) of the CNRS for financial support.

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