M. Dang-Nhu

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 .

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.

The ground-state rotational constants of silane

From thirty-nine combination difference equations we have determined three significant ground-vibronic state constants of silane: β 0/hc=2·85941 cm-1, γ 0/hc=-3·82×10-5 cm-1 and e 0/hc=-7·97×10-7 cm-1 or in Hechts notation B 0=2·85941 cm-1, D s=3·82×10-5 cm-1 and D t=2·436×10-6 cm-1.

A Doppler-limited study of the infrared spectrum of allene from 2965 to 3114 cm−1

Abstract A difference-frequency laser spectrometer has been used to measure the infrared absorption spectrum of the ν 5 and ν 8 bands of allene (C 3 H 4 ). In addition, the A 1 A 2 - E and B 1 B 2 - E components of the ν 8 + ν 11 − ν 11 hot band have been measured and analyzed. For the ν 8 and ν 11 states both the 〈 k, l k ± 2, l ± 2 〉 and 〈 k, l k ± 2, l ∓ 2 〉 splittings have been observed. For the ν 8 + ν 11 state the A 1 A 2 and B 1 B 2 interactions have been observed. Many weak perturbations have been found in the ν 5 and ν 8 bands. Recognition that a perturbation in the ν 5 band shares intensity between two Q Q 4 subband features has led to a reassignment of the K > 3 subbands of ν 5 . The intensities of lines in both ν 5 and ν 8 have been measured and the least perturbed transitions have been fit to obtain intensity parameters for both bands.

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 .

Spectral intensities of the 4-μm ν1 + ν3 combination band of SO2

Abstract The transition intensities of the 4-μm bands of SO 2 are measured with high precision using a tunable laser difference-frequency spectrometer. The band strength calculated from the ν 1 + ν 3 combination band data at 295.2K is S v 0 = 10.54 ± 0.04 cm −2 atm −1 . Here the uncertainty (three standard deviations) quoted is for the relative precision only; the absolute accuracy, which depends on the sample pressure calibration, is ∼1%. F -factors and “hot” band results are also obtained.

Spectral intensities in the v 3-band of 12CH3 35Cl at 13 μm

A tunable diode laser spectrometer was used to perform measurements of absolute intensities for 98 lines in the v 3-fundamental of methyl chloride. From these data, the vibrational band strength S 0 v was calculated at 296K and for hundred percent of the 12CH3 35Cl-species to be S 0 v = 90·7 ± 1·3cm-2 atm-1 with the uncertainty covering three times the standard deviation. Furthermore the α1-coefficient involved in the Herman-Wallis factor F = (1 + α1 m + …)2 was significantly derived, α1 = -0·00120 ± 0·00018. These results can be used for further applications such as sensing and determination of atmospheric methyl chloride via its absorption in the 13 μm spectral region.

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.

Absolute line intensities of the ν9 band of propyne at 15.5 μm

Abstract A tunable diode laser spectrometer was used to perform measurements of absolute line intensities for 20 transitions in the ν 9 -fundamental of propyne centered at 15.5 μm. From these data, the vibrational band-strength S v o was significantly determined to be S v o = 196.8 ± 6.0 cm −2 atm −1 at 297 K. These results will be used for further applications of astrophysical interest such as a refined contribution for modelling Titans atmosphere.