G. Poussigue

The ground state of methane 12CH4 through the forbidden lines of the ν3 band

Abstract High resolution spectra of the ν3 band of methane, 12CH4, were recorded by using a “third generation vacuum Fourier interferometer”; a large pressure range (from 0.009 to 10 Torr) with a sample path fixed at eight meters was used, enabling observation of transitions with intensity ratios as low as 1 10 000 . More than 350 forbidden transitions of the ν3 band, including about 125 transitions of the Q+ branch, were unambiguously identified. Of the 277 transitions retained for computations, one-hundred have 11 ≤ J ≤ 16. From combination difference relations using pairs of transitions having the same upper state energy level (forbidden-allowed and forbidden-forbidden pairs were used), 276 independent differences between ground state energy levels could be determined with uncertainties of about 0.001 cm−1. These data yielded the following values for the ground state structure constants of 12CH4 along with their standard deviations (in cm−1): β o hc =5.2410356±0.0000096 , γ o hc =(−1±0.00074) 10 −4 , π o hc =(5.78±0.18) 10 −9 , ϵ o hc =(−1.4485±0.0023) 10 −6 , ϱ o hc =(1.768±0.126) 10 −10 , ξ o hc =(−1.602±0.067) 10 −11 , Thus, for the first time, the scalar constant π0 has been evaluated and ir values have been obtained for the two tetrahedral constants ϱ0 and ξ0; furthermore, these values are in very good agreement with the ones recently determined from radiofrequency data, i.e., in cm−1: ϵ o hc =(−1.45061±0.00014) 10 −6 , ϱ o hc =(1.7634±0.0068) 10 −10 , ξ o hc =(−1.5432±0.0040) 10 −11 From these values, the 276 differences can be reproduced with an overall rms deviation equal to 0.0009 cm−1. Finally, the ground state energies of 12CH4 have been calculated for J ≤ 16.

Rotational analysis of vibrational polyads in tetrahedral molecules: Simultaneous analysis of the pentad energy levels of 12CH4

Abstract A theoretical model for the rovibrational analysis of clustered vibrational states for tetrahedral molecules is presented. The results of a comprehensive study of the five bands ν1, ν3, 2ν2, ν2 + ν4, and 2ν4 of 12CH4 are reported. The Hamiltonian has been developed to the third order in the Amat-Nielsen classification. Twenty-two parameters related to the ground state and the dyad ν 2 ν 4 have been transferred unchanged and 52 new parameters have been adjusted to fit experimental data, mainly interferometric ir data in the region 2250–3250 cm−1. The analysis has been realized through J′ = 18 involving 1919 observed upper-state energies. The overall standard deviation of the fit is 0.022 cm−1. A prediction of upper-state energies for the five bands has been carried out through J′ = 20. The model reproduces qualitatively and quantitatively the rotational fine structure of the upper levels in the full range of J′ values including level crossings (J′ ≥ 12), for the first time. The problem of higher excited vibrational states in methane is briefly discussed.

Rotational analysis of vibrational polyads in tetrahedral molecules: line parameters of the infrared spectrum of 12CH4 in the range 2250-3260 cm−1 ― theory versus experiment

Abstract Experimental and theoretical line parameters of the infrared spectrum of 12 CH 4 in the range 2250–3260 cm −1 covering the pentad ν 1 , ν 3 , 2 ν 2 , ν 2 + ν 4 , and 2 ν 4 are reported. The individual line strengths are reproduced with a relative precision of 12% comparable to the experimental accuracy. In all, 6499 transitions have been calculated in the spectral region 2250–3260 cm −1 . Their intensities range from 2.5 to 213 000 × 10 −24 cm/molecule. Virtually all the absorptions of 12 CH 4 in this range are satisfactorily reproduced.

The pentad rotation-vibrational states of 12CD4: Wavenumber analysis of infrared and Raman spectra of the ν1, ν3, 2ν2, ν2 + ν4, and 2ν4 bands

Abstract The so-called pentad of 12 CD 4 consists of the vibrational states v 1 = 1(symmetry A 1 ), v 3 = 1( F 2 ), v 2 = 2( A 1 + E ), v 2 = v 4 = 1( F 1 + F 2 ), and v 4 = 2( A 1 + E + F 2 ). All states are located in the 1950 to 2250-cm −1 region and all are strongly interacting. In the present work we have assigned more than 5000 infrared rotation-vibrational transitions and 163 isotropic Raman transitions from the vibrational ground state to the pentad. We have used infrared and Raman spectra of a resolution better than 0.01 cm −1 . From the experimental wavenumbers 2567 pentad rotation-vibrational energy levels with J ≦ 20 have been determined. These levels are reported in the paper. The levels have been used for refinements of the spectroscopic constants of two physically different effective Hamiltonians for the pentad states. For all levels with J ≦ 12 an unweighted standard deviation of 0.004 cm −1 is obtained for both Hamiltonians, whereas the standard deviation increases more or less rapidly with J above 12 due to the imperfections of the Hamiltonians. The values of the spectroscopic constants of both Hamiltonians (85 and 106, respectively) are reported and the effects of the approximations are discussed.

Forbidden lines of the ν3 band of 13CH4: Ground-state constants

Abstract The ν3 fundamental vibration-rotation band of carbon-13 enriched methane (13CH4) was recorded using a high-resolution vacuum infrared grating spectrograph. Forbidden transitions of this band are reported for the first time. Of the nearly 900 transitions identified, 560 are forbidden transitions and 347 of the forbidden transitions have 11 ≤ J ≤ 18. Pairs of forbidden and allowed transitions having the same upper-state energy levels were used to calculate 550 independent differences between ground-state term values. From these data, a least-squares analysis was used to calculate the following values for ground-state structure constants and their standard deviations (in cm−1): β O hc = 5.240820 ± 0.000056 , λ O hc =−(1.0856 ± 0.0015) × 10 −4 , ϵ O hc = −(1.4174 ± 0.0034) × 10 −4 , η hc = −(1.73 ± 0.37) × 10 −11 . The 550 values for the ground-state combination differences retained for analysis can be reproduced with an overall standard deviation of 0.0155 using the stated values for the structure constants. The note added in proof refines the above constants by including the newly observed microwave data.

Absorption of monodeuteromethane 12CH3D at 4.5 μM, analysis of the overtone band 2ν6

Abstract The overtone band 2 ν 6 of 12 CH 3 D is analyzed in the range 2088–2433 cm −1 . The parallel and perpendicular components, centered at 2316.266 and 2323.297 cm −1 , are strongly interacting, giving rise to a number of “forbidden” transitions and large A 1 A 2 splittings. Six hundred twelve transitions including J ′ values up to 13 are assigned; the vibration-rotation constants for the upper state v 6 = 2 are derived from these data, allowing the reproduction of the experimental wavenumbers with a rms equal to 0.007 cm −1 . Some intensity measurements are used to estimate the overall band strength of 2 ν 6 .

Absorption of carbon tetrafluoride at 16 μm. Analysis of the ν4 band

Abstract The absorption spectrum of 12 CF 4 was recorded between 595 and 670 cm −1 with a resolution of 0.010 cm −1 . About 900 line clusters belonging to the ν 4 band were assigned, ranging from P (70) to R (70). A linear least-squares fit on 867 of these data led to six significant spectroscopic constants, allowing to reproduce the experimental wavenumbers with an overall standard deviation of 0.003 cm −1 .

Analysis of the absorption spectrum of 12CD4 between 2180 and 2320 cm−1. Comparative study of ν3 and 2ν3

Abstract The high-resolution spectrum of the ν3 band of 12CD4 has been recorded and analyzed. Corresponding to 367 allowed transitions of this band, 218 lines have been identified between 2180 and 2320 cm−1. Twelve significant spectral constants have been determined in such a way as to reproduce the spectrum with a standard deviation equal to 7 × 10−3 cm−1. A comparative analysis between the present results on ν3 and those obtained by K. Fox [J. Mol. Spectrosc. 9, 381 (1962)] for 2ν3 showed that the interaction between the two sub-levels E and F2 of the v3 = 2 state produces a significant effect of the second order and enabled us to determine the interval between these sublevels, i.e., 20 T33 ∼ 30 cm−1.

The pentad of 12CD4: Line parameter analysis of the IR spectrum in the range 1775–2350 cm−1

Abstract In a previous paper (J.-E. Lolck, G. Poussigue, E. Pascaud, and G. Guelachvili, J. Mol. Spectrosc. 111, 235–274 (1985)), we presented the first comprehensive assignment and wavenumber analysis of high-resolution (nearly Doppler limited) rotation-vibrational spectra of the interacting upper states of the ν 1 , ν 3 , 2 ν 2 , ν 2 + ν 4 , and 2 ν 4 bands of the 12 CD 4 pentad. The present paper continues this work by describing the determination and quality estimation of experimental infrared line strengths from the Fourier transform spectra. These line strengths are interpreted in terms of a theoretical model which contains, as parameters, the dipole moment derivatives and the main Hermann-Wallis coefficients of the infrared-allowed bands: ν 3 ( F 2 ), ν 2 + ν 4 ( F 2 ), and 2 ν 4 ( F 2 ). This model also explains the appearance of infrared transitions to upper states, forbidden in infrared in an isolated state approach, through the mixing of states caused by the intervibrational interactions. The intensity analysis leads to the determination of all six parameters in the model and to a reproduction of the experimental intensities with a precision comparable to the experimental accuracy of 10 to 15%.