Methyl Bromide : Spectroscopic line parameters in the 7- and 10-μm region D. Jacquemart 1, N. Lacome 1, F. Kwabia-Tchana 1, I. Kleiner 2 1 Laboratoire de Dynamique, Interactions et Réactivité; Université Pierre et Marie Curie- Paris6, CNRS, UMR 7075, France 2 Laboratoire Inter-Universitaire des Systèmes Atmosphériques; Universités Paris 12 et Paris 7, CNRS, UMR 7583, FRANCE

→ Atmospheric trace gas (≈ 10 pptv) of both natural and anthropogenic origins (oceanic emission, biomass burning, leaded gasoline, agricultural pesticide …) → Deadly toxic gas for human and animal life when exposed to high concentration → Major contributor to stratospheric bromine which participates to ozone destruction Methyl Bromide (CH 3 Br)

→ Previous works concern mainly line positions analysis (see Graner JMS 1981;90: ) → Two recent works on line positions and intensities in the 7-μm spectral region Spectroscopic line parameters in literature (Kwabia Tchana et al. JMS 2004;228: ; Kwabia Tchana et al. JMS 2006;235:132-43) → No spectroscopic data is available in atmospheric database such as HITRAN or GEISA → No work on broadening coefficients Presented in the second part of this talk

CH 3 Br in our atmosphere → Not yet detected in atmospheric spectra → Compare to CH 3 Cl, the quantity in our atmosphere is 10 time less → Compare to Cl, Br radicals are 10 time more efficient for the ozone destruction Complete line lists are necessary to detect CH 3 Br

CH 3 Br in the 10 μm spectra region

Experimental conditions for spectra recorded around 10 μm → Rapid scan interferometer Bruker IFS 120 HR (LADIR, Paris) (Δmax = 450 cm; FWHM =1.1  10  3 cm  1 ) Absorbing sample Natural CH 3 Br50.54 % of CH 3 79 Br % of CH 3 81 Br Stated purity99.50 % Experimental conditions S/N ratio  100 _________________________________________________________________ _ # CH 3 Br pressure N 2 pressure Temperature Absorption path (mbar) (mbar) (K) (cm) ________________________________________________________________ __ ________________________________________________________________

Preliminary work → Phase correction for each spectrum (Mertz method) → Determination of an average effective iris radius → Wavenumber calibration using NH 3 transitions and HITRAN2004 wavenumbers as etalon = 1.789(40)×10 –6 scattering (1SD) of 0.04×10 –3 cm –1 at 1000 cm –1

Line parameters measurement for transitions having J and K ranging from 0 to 55 and from 0 to 9 → Use of a multispectrum fitting procedure (Eur Phys J D 2001;14:55-69.) Position, intensity and broadening coefficients of a same line are constrained to be the same during the simultaneous fit of the six spectra. Use of a Voigt profile. For broadening coefficients we assumed that: → 1200 transitions fitted between 880 and 1050 cm  1 of both CH 3 79 Br and CH 3 81 Br

Two models have been used to analyze measured line intensities → Treatment using the eigenvectors as a linear combination of the zero order basis wavefunction (ℓ-type interactions) (Tarrago G, Delaveau M. Triad v n (A 1 ), v t (E), v t’ (E) in C 3v Molecules: Energy and Intensity Formulation (Computer Programs). J Mol Spectrosc 1986;119: ) → Classical Herman Wallis treatment with |v,ℓ,J,K> as eigenvectors (Watson JKG. Quadratic Herman-Wallis Factors for Symmetric- and Asymmetric- Top Molecules. J Mol Spectrosc 1992;153: )

= 0.2 ± 3.7 % = –0.01 ± 3.84 % ;  R 0  2 = 2.688(6)10  3 Debye 2 A K = 5.3(2)10  3 d 6 (2) =1.41(4)×10 -4 d 6 2 = (8)10  3 Debye 2 → weak ℓ-type interactions for the v 6 level A J = 0

Strong K-dependence No significant J -dependence

Ratio of the two calculations for measured transitions (1200) P Q(1) R Q(1) R Q(2) P P(1) R R(1) P Q(2)

Ratio of the two calculations for extrapolated transitions (18000 transitions) → No line intensity cutoff, but J max =60 and K max =30 P Q(1) R Q(1) R Q(2) P P(1) R R(1) P Q(2)

P Q(1) branch R Q(1) branch Comparison with measurements

Empirical model has been used to compute measured self and N 2 widths → For C 3v molecules the J-and K-dependences of the widths have already been observed for: NH 3 (Nemtchinov V, Sung, K, Varanasi P. Measurements of line intensities and half-widths in the 10-μm bands of 14 NH 3. JQSRT 2004;83: ) CH 3 D ( Predoi-Cross A, Hambrook K, Brawley-Tremblay S, Bouanich JP, Malathy Devi V, Smith MAH. Measurements and theoretical calculations of N 2 -broadening and N 2 -shifting coefficients in the ν 2 band of CH 3 D. J Mol Spectrosc 2006;235;35-53.) Analysis of the measured self and N 2 widths

Empirical model used to compute measured self and N 2 widths → For transitions having same value of J inf

Empirical model used to compute measured self widths → Fit of the two coefficients a J 0 and a J 2

Empirical model used to compute measured N 2 widths → Fit of the two coefficients a J 0 and a J 2

Comparison between measured and calculated self-widths

Comparison between measured and calculated N 2 -widths

Conclusion for the 10 μm region based on the fit of 1200 measurements For positions: The average discrepancy obs-calc is equal to (0.001 ± 0.114)×10 -3 cm -1, The accuracy is estimated to be better than 0.2×10 -3 cm -1. For intensities: The average discrepancy obs-calc is equal to 0.2 ± 3.8 %, The rotational dependence is reproduced with accuracy around 5 %. For widths: The average discrepancy %self is equal to 0.8 ± 6.4 %, The average discrepancy % N 2 is equal to –0.3 ± 3.3 %. The J and K dependence of the measurement is reproduced with accuracy better than 10 % for the self-broadening coefficients, and around 5 % for the N 2 -broadening coefficients. → List of these parameters will be proposed to atmospheric databases

CH 3 Br in the 7 μm spectra region

Experimental conditions for spectra recorded around 7 μm → Rapid scan interferometer Bruker IFS 120 HR (LADIR, Paris) (Δmax = 450 cm; FWHM =1.1  10  3 cm  1 ) Experimental conditions Absorbing sample Natural CH 3 Br % of CH 3 79 Br and % of CH 3 81 Br Stated purity % Experimental conditions (SNR  100) ____________________________________________ # CH 3 Br pressure Temperature Absorption path (mbar) (K) (cm) ____________________________________________ ____________________________________________

ν 2 parallel band ν 5 perpendicular band

Determination of line intensities using a single spectrum fitting procedure → Around 320 transitions have been measured

→ Treatment using the eigenvectors as a linear combination of the zero order basis wavefunction (ℓ-type interactions) (Tarrago G, Delaveau M. Triad v n (A 1 ), v t (E), v t’ (E) in C 3v Molecules: Energy and Intensity Formulation (Computer Programs). J Mol Spectrosc 1986;119: ) Analysis of line intensities

3.0 % 2.6 % % rms % 3.0 % % rms # lines 5 (b) # lines 2 (b) 3 % 2 %10% ≤ δ < 17% 3 % 2 %7% ≤ δ < 10% 22 % 16 %4% ≤ δ < 7% 72 % of the lines 80 %0 ≤ δ (a) < 4% CH 3 81 Br CH 3 79 BrStatistics – 0.111(10)– (92) × (79) (72) (18) (16) CH 3 81 Br CH 3 79 BrDipole Moment Derivatives Dipole Moment Derivatives (in Debye) for the 2 and 5 Bands of CH 3 79 Br and CH 3 81 Br (320 lines studied) (a) δ = |calc – obs| / obs in %. (b) Number of transitions included in the least squares fits.

Synthetic spectrum

Conclusion for the 7 μm region For positions: (based on the fit of 7500 line positions) The average discrepancy obs-calc is equal to (0.002 ± 0.784)×10 -3 cm -1, The accuracy is estimated to be better than 1×10 -3 cm -1. For intensities: (based on the fit of 300 line intensities) The average discrepancy obs-calc is equal to 0.04 ± 3.9 %, The rotational dependence is reproduced with accuracy around 5 %. For widths: New spectra have been recorded with CH 3 Br and N 2, and will be analyzed to observe or not a vibrational dependence for broadening coefficients. At the present time we suggested that widths obtained in the 10 μm region could be applied to the 7 μm region. → List at 7 μm will be proposed to atmospheric databases

Methyl Bromide will probably be the next new molecule in HITRAN The number 40