The infrared bands ν2 and ν5 of CH3Br with Coriolis interaction

The infrared bands ν₂ and ν₅ of CH₃Br have been measured at a resolution of 0.015 cm^?1. The lines due to the isotopic species CH₃⁷⁹Br and CH₃⁸¹Br were resolved and altogether about 3000 lines were assigned. The bands were analyzed simultaneously by taking into account the xy-Coriolis resonance between the states v₂ = 1 and v₅ = 1. A local perturbation observed in ν₅ could be explained in terms of a (1,?2) resonance between v₂ = 1 and v₅ = 1. Some perturbation allowed transitions could be assigned and they give an equation between A₀ and D₀^K.     

Submillimeter spectrum of methyl bromide (CH3Br)

Methyl bromide is a ubiquitous component of the atmosphere, but has yet to be remotely detected in the upper atmosphere. Due to the strong ozone depletion capability of the activated bromine species, the total atmospheric bromine load needs to be carefully monitored. Combined analysis of precise measurements and cataloging of the rotational spectrum of methyl bromide may enable its concentration to be monitored with future remote sensing instrumentation. In an effort to extend and improve previous work for this molecule, the spectrum of CH₃Br has been measured at JPL. Using an isotopically enriched ¹³CH₃Br (90%) sample, spectra have been recorded from 750 to 1200 GHz. Quantum number assignments cover the CH₃⁷⁹Br, CH₃⁸¹Br, ¹³CH₃⁷⁹Br and ¹³CH₃⁸¹Br isotopologues with J < 66 and K < 17 for the ground and ν₃ vibrational states. The dataset for the ¹²C isotopologues is more precise than previous THz measurements resulting in reductions of rotational and distortion parameter uncertainties by factors of 2-15. Parameters of the ν₃ state of the ¹²C isotopologues are improved by 2-105. The spectra of the ¹³C isotopologues are the first reported beyond J = 2.     

Determination of line strengths for selected transitions in the v2 band relative to the v1 and v5 bands of H2CO

The spectral line strengths in the v₂ band of H₂CO (segments spanning 1720-) have been determined relative to two sets of spectral line groups in the v₁ and v₅ band, using tunable diode laser spectroscopy. Simultaneous detection using a dual-diode instrument with a absorption cell was employed to assure identical H₂CO column density for the two spectral regions. The results in the selected regions of this study are in good agreement with the line positions and the relative intensities specified in an unpublished complete line listing for the v₂ band prepared by Linda Brown (see full text for reference). Based upon measurements of individual groups of spectral lines in the P, Q and R branches, the absolute band strength has been determined to be .     

The stretching fundamental bands ν1, ν2 and ν4 of HSiD3

The ν₁(A₁), Si-H stretching, ν₂(A₁) and ν₄(E), Si-D stretchings, fundamental bands of HSiD₃ have been recorded at an effective resolution of ca. 0.003 cm⁻¹ between 2080 and 2280 cm⁻¹ and between 1480 and 1720 cm⁻¹, respectively. Ro-vibrational transitions of the H²⁸SiD₃ isotopologue have been assigned in the two spectral ranges, about 700 belonging to ν₁, with J′ up to 25 and K up to 21, and about 1600 to the ν₂/ν₄ dyad, with J′ up to 24 and K′ up to 19. The spectra of all the bands evidence the existence of several perturbations. The transitions of ν₁ have been analyzed either neglecting or including in the model A₁/E Coriolis-type interactions with nearby dark states. The υ₂ = 1 and υ₄ = 1 states have been fitted simultaneously taking into account several ro-vibrational interactions between them and, in addition, with the υ₅ = 2, l = 0 component, and with few other close dark states. The standard deviation of the fit for both ν₁ and the ν₂/ν₄ dyad is, however, more than one order of magnitude larger than the estimated experimental precision and is independent on the adopted model.     

High resolution analysis of the ν12 and ν17 fundamental bands of acrolein, CH2CHCHO, in the 600 cm region

Synchrotron radiation from the new Canadian Light Source facility has been used to obtain a high resolution (0.0012 cm⁻¹) absorption spectrum of acrolein vapor in the 550-660 cm⁻¹ region. Almost 2000 transitions have been included in a detailed analysis of the ν₁₂ (∼564 cm⁻¹) and ν₁₇ (∼593 cm⁻¹) fundamental bands which yielded precise values for the band origins, rotational and centrifugal distortion parameters. The analysis included the a- and b-type Coriolis interactions connecting ν₁₂ and ν₁₇, as well as an a-type Coriolis interaction between ν₁₇ and a “dark” perturbing state, identified as 4ν₁₈. We believe that this is the first high resolution infrared study of acrolein.     

The ν5 and ν3 Raman bands of CH2D2

The ν₅ and ν₃ Raman bands of CH₂D₂ have been recorded with a resolution of 0.35 cm^?1. The ν₃ state is well known from infrared studies. Three hundred twenty-nine transitions of the ν₅ band were analyzed, assuming an unperturbed upper state, giving a standard deviation on the fit of the upper-state energies of 0.037 cm^?1, The constants A, B, C, Δ_J, Δ_JK, and Δ_K differed significantly from the ground-state values, and ν₅ was determined as 1331.41 ± 0.05 cm^?1. This work represents the first complete analysis of the fine structure of a rotation-vibrational Raman band for an asymmetric rotor. The ν₅ state could not be analyzed in infrared so this investigation, once more, demonstrates the usefulness of the Raman method.     

Line mixing calculation in the ν6Q-branches of N2-broadened CH3Br at low temperatures

In an early study [H. Tran, D. Jacquemart, J.Y. Mandin, N. Lacome, JQSRT 109 (2008) 119-131], line mixing effects of the ν₆ band of methyl bromide were observed and modeled at room temperature. In the present work, line mixing effects have been considered at low temperatures using state-to-state collisional rates which were modeled by a fitting law based on the energy gap and a few fitting parameters. To validate the model, several spectra of methyl bromide perturbed by nitrogen have been recorded at various temperatures (205-299 K) and pressures (230-825 hPa). Comparisons between measured spectra and calculations using both direct calculation from relaxation operator and Rosenkranz profile have been performed showing improvement compared to the usual Lorentz profile. Note that the temperature dependence of the spectroscopic parameters has been taken into account using results of previous studies.     

Theoretical calculation of CH3Br/N2-broadening coefficients at various temperatures

The N₂-broadening coefficients of CH₃⁷⁹Br and CH₃⁸¹Br have been calculated for transitions belonging to the ^PP-, ^PQ-, ^PR-, ^RP-, ^RQ- and ^RR-branches of the ν₆ band near 10 μm, using a semiclassical impact theory. The intermolecular potential used, includes in addition to the overwhelming electrostatic interactions, induction and dispersion energy contributions, which are significant only for low J transitions with K approaching or equal to J. Comparisons have been performed with the extensive set of previous measurements at room temperature [4]. The theoretical results are in satisfactory agreement with the experimental data, and the J and K dependences are reasonably well reproduced. From calculations at 200, 230, 260 and 296 K, the temperature exponent of the collisional broadenings derived from a simple power law has been determined for each transition of each sub-branch and compared with a recent experimental evaluation [5]. Finally the N₂-broadening coefficients calculated at 296 K and their temperature exponents are given as supplementary materials of this paper for atmospheric applications and databases.     

High-resolution FTIR spectrum and analysis of the ν2+ν4 combination band of SF6

The spectroscopic knowledge of sulfur hexafluoride, which is necessary for a correct remote sensing and monitoring of this species in the Earth’s atmosphere, is still very partial. In particular, the hot bands in the strongly absorbing ν₃ region (near 948 cm⁻¹) have not been analyzed yet. Their study implies the analysis of many vibrational levels and thus the spectroscopy of various fundamental, harmonic, and combination bands. The present work is a new contribution to this topic, concerning the ν₂+ν₄ combination band. The FTIR spectrum of this region has been recorded at room temperature with a resolution of 0.002 cm ⁻¹. The data have been analyzed thanks to the HTDS software (http://www.u-bourgogne.fr/LPUB/shTDS.html) developed in Dijon for XY₆ octahedral molecules. Seven hundred and fifty-nine lines could be assigned up to J=112, and the standard deviation is 0.0022 cm⁻¹. The distance between the two vibrational sublevels with respective symmetry F_1u and F_2u is 0.348 cm⁻¹.     

High-resolution FTIR measurement of the 2ν8 band of methylene fluoride (CH2F2)

A high-resolution (0.003 cm⁻¹) infrared absorption spectrum of the first overtone of the fundamental mode ν₈ of methylene fluoride (CH₂F₂) has been measured on a Bruker IFS 120-HR Fourier transform infrared spectrometer. More than 2000 ro-vibration transitions in the range of 2770-2900 cm⁻¹ with J ? 45 and K_a ? 20 have been assigned in this B-type band centered at 2838.5 cm⁻¹. Precise value for the band origin (2838.579799 cm⁻¹) and centrifugal distortion constants up to third order (Φ_JK, Φ_KJ, and Φ_K) have been obtained by fitting a total of 1474 unblended ro-vibration transitions (J ? 45 and K_a ? 13) of the 2ν₈ band with a standard deviation of 0.00029 cm⁻¹ using a Watson’s A-reduced Hamiltonian in the I^r representation. Signature of perturbations with nearby states has been seen.     

N2O emission in the 4.5 μM region: high excitation of the bending mode transitions v1v2v3→v1v2(v3−1) with (2v1+v2)=5

Eight emission spectra of pure N₂O and N₂O + N₂ + He mixtures excited by a radio frequency discharge were recorded by Fourier Transform Spectroscopy at a resolution of 0.005 and 0.004 cm⁻¹ in the 4.5 μm region. Results (wavenumbers, band centers, and spectroscopic constants) concerning nine new vibrational transitions which have not been observed before, and which occur between highly excited levels of the bending mode are reported. The derived spectroscopic parameters allow us to reproduce the experimental wavenumbers with an RMS error lower than 4.5 × 10⁻⁴ cm⁻¹.     

High resolution infrared spectra of the ν1-ν4 bands of BiH3, and ab initio calculations of the spectroscopic parameters

The infrared spectrum of short-lived BiH₃ has been studied by Fourier transform technique. The BiH stretching bands ν₁/ν₃ at 1733.2546/1734.4671 cm⁻¹ and the bending fundamentals ν₂/ν₄ at 726.6992/751.2385 cm⁻¹ have been measured with a resolution of 5.5 and 6.6 × 10⁻³ cm⁻¹, respectively. The spectra were analyzed using different reductions of the rovibrational Hamiltonian accounting for the numerous resonance interactions in particular within the strongly Coriolis-coupled bending dyad. About 1150 and 980 transitions belonging to the ν₁/ν₃ and ν₂/ν₄ bands were fitted with an rms deviation of 0.62 and 0.53 × 10⁻³ cm⁻¹, respectively. High-level ab initio calculations at the coupled cluster CCSD(T) level with an energy-consistent small-core pseudopotential and large basis sets were carried out to determine the equilibrium structure, the anharmonic force field, and the associated spectroscopic constants of BiH₃. The theoretical results are in good agreement with the available experimental data.     

High-resolution study of some doubly excited vibrational states of PH2D: the ν1+ν2, ν2+ν5, ν2+ν3, and ν2+ν6 bands

The absorption bands ν₁+ν₂, ν₂+ν₃, and ν₂+ν₆ of PH₂D have been recorded for the first time using a high-resolution Bruker 120 HR interferometer, and rotationally analyzed. Some transitions belonging to the very weak band ν₂+ν₅ and enhanced in intensity by strong interactions with the ν₁+ν₂ band were also assigned. Sets of parameters obtained from the fit reproduce experimental line position of the bands ν₁+ν₂ and ν₂+ν₃ with about the experimental accuracy. The residuals of the ro-vibrational energies of the ν₂+ν₆ band are about 10 times larger. Reasons for the poorer reproduction of the latter data are given.     

Determination of A0 of CH3Br from its Raman spectrum

A value of A₀ = 5.1800 ± 0.0010 cm^?1 for CH₃Br has been determined from an analysis of the ν₄ Raman band, based both on a direct fit of Q-branch frequencies and on ground state combination differences. The constants ν₄, (Aζ)₄, η₄₄, and A_e = 5.2442 ± 0.0015 cm^?1 were also determined. The equilibrium distance of the H atoms from the figure axis is calculated as 0.32077 ± 0.00005 Å. All the fundamental Raman bands of CH₃Br were observed, and experimental results for the ν₁, ν₂ and ν₅ bands are included.     

Study of the ν2 + ν3 and 2ν6 infrared bands of CH379Br and CH381Br

The rotational analysis of the ν₂ + ν₃ band, centered around 1912 cm^?1, and of both components 2ν₆^±2 and 2ν₆⁰, centered about 1912 and 1904 cm^?1, respectively, has been carried out from a Fourier transform spectrum having a resolution limit of 0.005 cm^?1. A standard deviation of about 0.001 cm^?1 was obtained for about 750 lines of the unperturbed 2ν₆^±2 component for both isotopic species. The ν₂ + ν₃ band, stronger than 2ν₆^±2, is perturbed by two resonances: a Coriolis resonance with the very weak ν₃ + ν₅ band, no line of which has been observed, and an anharmonic resonance with 2ν₆⁰, only four K subbands of which have been observed. For both isotopic species, a standard deviation of about 0.002 cm^?1 has been obtained for about 750 lines of ν₂ + ν₃ and 2ν₆⁰.     

The Infrared Spectrum of C2D2: The Bending States up to v4+v5=2

The vibration-rotation spectrum of ¹³C₂D₂ has been recorded in the infrared region between 420 and 1100 cm⁻¹ with an effective resolution ranging from 0.004 to 0.006 cm⁻¹, and in the millimeter-wave region between 68 and 518 GHz. A total of about 1400 rovibrational transitions (66 of which have been measured in the millimeter-wave region) have been assigned to 8 bands with 15 l-vibrational components involving the bending states up to v_t=v₄+v₅=2. The ground state and nine vibrationally excited states have been characterized. All the measured transitions have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the usual vibration and rotation l-type resonances, together with the Darling-Dennison coupling between the v₄=2 and v₅=2 bending states. The derived spectroscopic parameters reproduce the transition wavenumbers with a standard deviation of the fit of the order of the experimental uncertainty.     

The Coriolis interaction between the fundamentals ν2 and ν5 of CD3Br

The bands ν₂ and ν₅ of CD₃Br have been measured at a resolution of 0.010 cm^?1. They were analyzed simultaneously by taking into account the xy-Coriolis interaction. More than 1600 transitions were assigned for each isotopic species CD₃⁷⁹Br and CD₃⁸¹Br. The Coriolis coupling term proved to be ζ_2,5^y = 0.559. The band centers are (in cm^?1) ν₂: 991.401 (CD₃⁷⁹Br), 991.390 (CD₃⁸¹Br); ν₅: 1055.474 (CD₃⁷⁹Br), 1055.471 (CD₃⁸¹Br).     

The far-infrared spectrum of acrolein, CH2CHCHO: The ν18 fundamental and (ν17 + ν18) − ν18 hot bands

The ν₁₈ fundamental band (∼158 cm⁻¹) of acrolein is studied at high resolution (0.0015 cm⁻¹) using synchrotron radiation from the Canadian Light Source facility and a Bruker IFS 125HR Fourier transform spectrometer. By fitting this band, together with some pure rotational transitions, molecular parameters are obtained to accurately determine the energies of the ν₁₈ = 1 state levels for values of (J, K_a) up to at least (45, 24). These parameters should be useful for future high resolution studies of acrolein hot bands. This is demonstrated here by a detailed analysis of the (ν₁₇ + ν₁₈) − ν₁₈ hot band at ∼589 cm⁻¹. The upper state (ν₁₇ + ν₁₈) of this band is found to be perturbed by Coriolis interactions analogous to those affecting the ν₁₇ state.     

Contribution to the study of the 2ν5 (A1) rovibrational band of CH3Br near 2860 cm−1

This band was studied on a Fourier Transform spectrum (resolving power of the apparatus: 0.005 cm^?1). For each isotopic species CH₃⁷⁹Br and CH₃⁸¹Br, about 800 lines were assigned. The band is well explained by a x-y type Coriolis interaction with ν₂ + ν₅. Several local resonances occur for low K values as well as a doubling of the K = 3 levels. Their study provides interesting information on neighboring bands, especially 3ν₆(E), ν₂ + ν₃ + ν₆, and 2ν₅(E).     

The Infrared Spectrum of CCH2: The Bending States up to v4+v5=4

The vibration-rotation spectra of ¹³C monosubstituted acetylene, ¹²C¹³CH₂, have been recorded in the region between 450 and 3200 cm⁻¹ with an effective resolution ranging from 0.004 to 0.006 cm⁻¹. A total of about 5300 rovibrational transitions have been assigned to 53 bands involving the bending states up to v_t=v₄+v₅=4, allowing the characterization of the ground state and of 30 vibrationally excited states. All the bands involving states up to v_t=3 have been analyzed simultaneously by adopting a model Hamiltonian which takes into account the vibration and rotation l-type resonances. The derived spectroscopic parameters reproduce the transition wavenumbers with a RMS value of the order of the experimental uncertainty. Using the same model larger discrepancies between observed and calculated values have been obtained for transitions involving states with v_t=4. These could be satisfactorily reproduced by only adopting, in addition to the previously determined parameters which were constrained in the analysis, a set of effective constants for each vibrational manifold.