G. Tarrago

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.

Analysis of phosphine absorption in the region 9–10 μm and high-resolution line-by-line simulation of the ν2 and ν4 bands

Abstract Phosphine absorption is analyzed in the range 818–1340 cm −1 from laboratory spectra recorded at Denver University with a resolution of 0.05 cm −1 . A refined treatment of the Coriolis interaction between ν 2 and ν 4 is introduced, allowing interpretation of the rotational structure of the two bands up to high J ′ values, including the large observed A 1 A 2 splittings. A fit, based on 1187 experimental transitions, leads to a set of spectroscopic constants applicable to a high-resolution, line-by-line simulation of the two bands ν 2 and ν 4 . The overall standard deviation of the fit is 0.017 cm −1 . About 70 transitions belonging to the “hot” band 2 ν 2 - ν 2 are also identified, consistent with a band center near 980.4 cm −1 .

Absorption of 12CH3D at 6–10 μm: Triad ν3, ν5, ν6

The absorption of 12CH3D at 6–10 μm was recorded under vacuum with a resolution of 0.0054 cm−1. For the first time the ν5 band was assigned extensively, allowing the analysis of ν3, ν5, and ν6 together within a “triad model,” including a rigorous treatment of all the Coriolis couplings involved. The assignments in the spectral range were anlarged from the original 1000 to the present 3500. The newly assigned lines essentially concerned the ν5 band and a great number of perturbation-allowed transitions (about 30% of all observations). Ground state energy parameters were refined from 2641 combination differences involving J up to 19 and |ΔK| up to 6; the standard deviation of the fit was 0.0003 cm−1. A set of 35 triad upper state energy parameters was derived, reproducing all observations with a standard deviation of 0.0076 cm−1. The three dipole moment derivatives involved in the intensity calculations were estimated theoretically, by taking advantage of the recent accurate determination of the band strengths S3 and S4 of 12CH4. Finally, about 6000 triad transitions predicted with linestrengths at least equal to 4 × 10−25 cm·molecule−1 were tabulated with assignments, wavenumbers, linestrengths, and lower and upper energy levels.

Spectrum of phosphine at 4 to 5 μm: Analysis of ν1 and ν3 bands

Abstract The absorption spectrum of phosphine has been investigated in the region 2087–2482 cm−1. About 1200 transitions belonging to the bands ν1 and ν3 were assigned. A strong Coriolis interaction between these bands gives rise to many “forbidden” transitions and large A1A2 splittings. The simultaneous analysis of the two bands enabled us to get a set of vibration-rotation constants for the vibrational states v1 = 1 and v3 = 1, and to obtain a value for the ratio S 1 S 3 between the band strengths of ν1 and ν3.

Analysis of the rotational spectrum of C3v molecules by using factorization and diagonalization of the energy matrix: Application to CH3C15N

Abstract A method is given for the analysis of the rotational spectrum in the ground and excited states of C 3 v molecules; it consists in a direct diagonalization of the energy matrix including all elements whose contribution can become significant for the analysis up to the sixth order of approximation. The method of factoring the energy matrix into four submatrices A 1 , A 2 , E , E , according to the symmetry species of the full point group C 3 v , is given. The programm enables the calculation of the rotational frequencies and also carries out by a least-squares method the fitting of the molecular constants for vibrational states v = 0 (ground state) and v E = 1, 2, 3, and 4, separately or simultaneously over several of these states. The analysis of the rotational spectrum of CH 3 C 15 N in the v 8 = 0, 1, 2 states is given as an example.

Ground state rotational energies of C3v quasi-spherical top molecules: Applications to 16OPF3 and PH3

Abstract Rotational spectra of the quasi-spherical symmetric molecules, 16 OPF 3 and PH 3 , are analyzed with all the data currently available. In both cases the current formulation that uses rotational contact transformations to completely diagonalize the ground state energy matrix appears to be inadequate, due to the smallness of the quantity |; A 0 − B 0 |. This quantity represents only 5% of A 0 or B 0 in the case of OPF 3 and 11% in the case of PH 3 . Thus, both analyses require the resonance terms in Δ K = ±3 and/or Δ K = ±6 to be taken into account through diagonalization of the energy matrix. The ground state parameters as well as the ground state energies are given for both molecules.

Absolute absorption intensities in the triad ν3, ν5, ν6 of 12CH3D at 6–10 μm

Abstract The absolute absorption intensities of 80 lines belonging to ν3, ν5, and ν6 of 12CH3D were measured in the range 1062–1480 cm−1 using a dual beam tunable diode laser spectrometer. The data were analyzed within a triad model using the energy and intensity formulation recently set up for investigating all three bands present at 6–10 μm. The fit led to the determination of the three dipole moment derivatives δμ δq s (s = 3, 5, 6) involved in the triad. Three first-order Herman-Wallis type coefficients were also required to account for the observations. All linestrengths were reproduced with an overall standard deviation of 3.4%, to be compared to an experimental uncertainty of 3.0%. The values derived for the band strengths of ν3, ν5, and ν6 are 40.1, 13.4, and 53.2 cm−2 atm−1, respectively, at 296 K. Comparison is made with literature data.

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.

High-resolution Fourier transform spectra of H3SiD and monoisotopic H370GeD in the region 600 to 1100 cm−1: Analysis of the ν3, ν5, and ν6 fundamentals

Abstract Fourier Transform infrared spectra of H 3 SiD and monoisotopic H 3 70 GeD have been recorded with a resolution of 0.005 cm −1 in the 600- to 1100-cm −1 region covering the fundamentals ν 3 , ν 5 , and ν 6 . About 3000 normal and perturbation-allowed transitions have been measured and assigned for each molecule. Ground state constants have been determined up to sextic coefficients. Strong interactions were found to occur between ν 3 , ν 5 , and ν 6 which required a simultaneous rovibrational analysis. A model was chosen which accounted for all the Coriolis-type couplings among the three excited vibrational states by a diagonalization method. The experimental data were fitted to 32 and 31 excited state parameters for H 3 SiD ( σ = 7.4 × 10 −3 cm −1 ) and H 3 70 GeD ( σ = 6.4 × 10 −3 cm −1 ), respectively. The vibrational fundamentals ν 0 ( H 3 SiD H 3 70 GeD ) were determined: ν 3 = 912.997(1) 821.598(1) , ν 5 = 950.576(1) 898.946(1) , ν 6 = 784.324(1) 709.930(1) cm −1 . Absolute intensities of lines belonging to ν 3 , ν 5 , and ν 6 have been estimated on the basis of calculated values for the dipole moment derivatives ∂μ ∂q s (s = 3, 5, 6) .

Triad νn(A1), νt(E), νt′(E) in C3v molecules: Energy and intensity formulation (computer programs)

Abstract The lower frequency IR absorption of mono- and trideuterated hydrides X H 3 D and X D 3 H ( X = C, Si, Ge, Sn) is due to the three bending modes ν n ( A 1 ), ν t ( E ), v t ′ ( E ), forming a triad with rather strong Coriolis couplings. Formulation and computer programs suitable for analyzing such a triad are presented here. Wavenumbers and absolute line strengths are calculated according to the strict selection rules on J and symmetry species, without any restriction on K , so that the perturbation allowed transitions are accounted for in a straightforward manner. Programs have been developed allowing us either to fit the energy parameters in the ground state (via GS differences) and the triad upper states v n = 1, v t = 1, v t ′ = 1, or to predict frequencies and intensities for the whole absorption covered by the triad ν n , ν t , ν t ′ . The application to the analysis of ν 3 , ν 5 , ν 6 of 12 CH 3 D is presented elsewhere.