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The High-Resolution Infrared Spectrum of 34 S 16 O 2 up to 4000 cm-1 J.-M. Flaud, W.J. Lafferty, R.L. Sams, and El Hadji Abib Ngom

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Introduction The spectrum of sulfur dioxide is obviously of interest as an atmospheric pollutant monitoring tool especially after a volcanic eruption. It is also present in the Venus atmosphere and has recently been discovered in the atmosphere of Io. The spectrum of the normal isotopic species, 32 S 16 O 2 has been very well studied. However, that of the 34 SO 2 species which is 4% abundant has only been studied in natural samples. In this work we have recorded the spectra of a number of bands of a sample enriched to 95.3% in 34 SO 2 and retrieved much improved spectroscopy constants of this isotopic species.

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The fundamental bands: ν 1 (8.7μm),ν 2 (19.5μm), ν 3 (7.4μm) The combination bands: ν 2 + ν 3 (5.4μm),ν 1 + ν 3 (4.0μm) 2ν 3 (3.7μm) and 2ν 1 + ν 3 (2.8μm). The hot bands: 2ν 2 ‑ ν 2, 3ν 2 ‑ 2ν 2, ν 1 + ν 2 ‑ ν 2, ν 2 + ν 3 ‑ ν 2, ν 1 + ν 2 + ν 3 ‑ ν 2 Bands of 34 S 16 O 2 Accurate rotational levels for the: (000), (010), (020), (030), (100), (001), (110), (011), (101), (002), (111), (201) vibrational states.

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EXPERIMENTAL DETAILS Sulfur dioxide sample enriched in 34 SO 2 (95.3 %) Bruker IFS 120 HR (PNNL) evacuated to about 4 Pa (0.030 Torr). Potassium bromide beamsplitter. Detectors: Liquid helium silicon bolometer from 410 cm-1 to 660 cm-1 Mercury cadmium telluride from 1000 cm-1 to 2000 cm-1 Indium antimonide detector from 2200 cm cm-1. Pressure measured with three MKS Baratron manometers (1, 10 or 1000 Torr full scale) (stated uncertainty of 0.05 % of full scale). Cells: cm cell temperature regulated to 24.36C +/- 0.1 C White type long path cell at room temperature of about 21.5+/- 1.0C

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LIST OF SPECTRA Spectral region (cm -1 ) Resolution (cm -1 ) Number of Scans Pressure Millibar (Torr) Optical path length (cm) T(C)Calibration (1.205) N 2 O [7] (0.500) N 2 O [7] (0.2164) N 2 O [7] ( ) N 2 O [7] (.0471) N 2 O [7] (0.220) CO 2 [8] (0.604) CO 2 [8] (1,017) HDO [9] (1.013) CO 2 [8] (1.570) H 2 O [10] (0.679) CO 2 [8] (0.1645) CO 2 [8]

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Initial line assignments performed: either by following line series and using combination differences to verify the J and Ka assignments, or by using predictions based on the corresponding spectra analyzed for the main isotopic species. ANALYSIS Then the various ground state combination difference obtained in this work were used together with all existing microwave, sub millimeter and terahertz rotational frequencies to obtain an improved set of ground state rotational constants. Finally these GS constants were used to calculate the upper state energy levels of the assigned bands..

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HAMILTONIAN MODELS (020)(100)(001) (020) HWHW Herm conj (100) FHWHW Herm conj (001)C HWHW A-type Watson Hamiltonian written in the I r representation Except for F = h F J xy 2 C C = i*J y J 2 + ( J J + 3 ) J ± = J x -/+ iJ y

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LINE POSITIONS:RESULTS(1) Vibrational State(010)(100)(001)(020)(110)(011) Number of levels J MAX K max ≤δ< %82.7%69.0%66.3% ≤δ< %12.2%24.7%24.9% ≤δ< %5.1%10.3%8.8% Std. Deviation (10 -3 cm -1 )

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Vibrational state(030)(101)(002)(111)(201) Number of levels J max K max ≤ δ < ≤ δ < ≤ δ < % 20.6% 7.2% 59.1% 25.6% 15,3% 36.5% 28.3% 35.2% 49.4% 30.9% 19.7% 33.7% 22.0% 44.3% Std. Devi, (10 -3 cm ‑ 1) LINE POSITIONS:RESULTS(2)

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Obs. and calc. spectra of the ν 1 band of 34 S 16 O 2 showing the distinctive b-type contour of the band.

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LINE INTENSITIES Nb of fitted intensities: 359 Statistical analysis: 0% <δI/I < 3% 66.6 % of the lines 3% < δI/I < 6% 23.4 % of the lines 6% < δI/I < 12% 10.0 % of the lines Weighted Standard Deviation: 0.94D SO 2 34 SO 2 32 SO 2 34 SO 2 S(ν 1 )0.3513(8)0.3452(17)S(8.7μm) (90)0.3767(15) S(ν 3 )2.721(4)0.2673(13)S(7.3μm)2.9657(43)2.959(10) Total band intensities ( cm -1 /(molecule cm -2 ) at 296K

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The Q-branch of the Ka = 9-10 subband of the b-type ν 2 band of 34 S 16 O 2

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Small portion of the R-branch of the a-type ν 3 band of 34 S 16 O 2

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Obs. and calc. transitions of the a-type hot band. The transitions of the ν 1 + ν 3 band are off-scale. The lower-state rotational quantum numbers are given.

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SO 2 EQUILIBRIUM STRUCTURE

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34 S 16 O 2 Rotation-vibration interaction constants ABC XeXe (210) (800) (110) α1α (170) (730)41.186(100) α2α (390) (710)15.832(120) α3α (240) (610) (850) γ (710) γ (810) (730) γ (810)0.1877(580) (610) γ (210) (200) (240) γ (820) (730) γ (830) γ (360)

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Corrections from centrifugal distortion terms

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Corrections from electron-rotation interaction effects I Fit ΔI elec ΔI cd I IAIA (31) a,b (60) c IBIB (77) (150) c ICIC (76) (140) c Moments of inertia of 34 S 16 O 2 in amu Ǻ 2 These moments of inertia have been least squares fit together with those previously obtained[1] for 32 S 16 O 2 leading to the equilibrium structural parameters: r e (S=O) = (40) Ǻ and < e (O=S=O)= (24) o. [1] J.-M. Flaud and W.J. Lafferty, J. M. S. 16 (1993)

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CONCLUSION Much improved spectroscopic constants of the 34 S isotopic species. Accurate line intensities ~3% at best Very precise equilibrium structure

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