Multispectrum analysis of 12CH4 from 4100 to 4635cm−1: II. Air-broadening coefficients (widths and shifts)

Abstract

Room temperature Lorentz air-broadened halfwidth and pressure-induced air-shift coefficients were measured for 1011 transitions in the octad region of methane between 4100 and 4635 cm À1 . These measurements were made by analyzing 10 laboratory absorption spectra recorded at 0.011 cm À1 resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. The spectra were obtained using two absorption cells with path lengths of 2.05 and 150 cm. The total sample pressures ranged from 99 to 400 torr with CH 4 volume mixing ratios of 0.01 in dry air. The spectral line parameters were retrieved using a multispectrum nonlinear least squares technique. Transitions belonging to five bands of the octad polyad were observed, namely m 2 + 2m 4 , m 1 + m 4 , m 3 + m 4 , 2m 2 + m 4 , and m 2 + m 3 . The numbers of measurements by bands are: 33 for m 2 + 2m 4 , 180 for m 1 + m 4 , 635 for m 3 + m 4 , 33 for 2m 2 + m 4 , and 130 for m 2 + m 3 . Transitions with rotational quantum number J up to 16 are included in the analysis. The measured width and shift coefficients vary according to the tetrahedral symmetry species and the rotational quantum numbers of the transitions. The retrieved parameters from this study are compared with prior results, in part to estimate absolute accuracy and determine the extent of vibrational dependence of widths and shifts.

Line broadening coefficient calculations for methane perturbed by nitrogen

“…This octad spectral region consists mainly in two vibrational bands. The ν 1 + ν 4 band is around 4223 cm −1 and the ν 3 +ν 4 band is around 4319 cm −1 [34]. The latter has a greater vibrational degeneracy, which is reflected by the trends in the line broadening values on Fig.…”

“…The experimental values for the line positions and line broadening coefficients were taken from Refs. [34,42,47,52,80]. For the ν 4 band and the octad region, the perturber was in fact air [34,42], for which nitrogen contributes to about 80%.…”

“…The 1013 measurements range from 4101.8 cm −1 to 4627.8 cm −1 with angular momenta up to J ′′ = 16 [34]. 750 lines were calculated, which was thought to be sufficient for a qualitative discussion of the results.…”

“…The subject of infrared line shapes studies for methane is extensively documented from an experimental point of view. Some of the most recent works dealt with measurements in the first four polyads of methane (dyad near 1300 cm −1 , pentad near 3000 cm −1 , octad near 4300 cm −1 and tetradecad near 6000 cm −1 ) [31][32][33][34][35][36][37][38][39][40][41][42][43]. We note that the temperature dependencies of line shape parameters and/or line mixing parameters could be determined in some of these studies.…”

Water vapour total columns from SCIAMACHY spectra in the 2.36 μm window

“…Use of new CH 4 spectroscopic data (broadening coefficients) by Predoi-Cross et al (2006) does not lead to a significant change in the results (cf. Gloudemans et al, 2008).…”