Knowledge Bank

Absolute strengths and wavenumbers of 1500 vibration-rotation lines of natural methane have been measured at 297 K in the spectral region $4433-4719cm-1$ from high resolution spectra recorded on the F. T. spectrometer at Kitt Peak. Accuracy of the strengths can be estimated to 3% for clean lines. Most of these lines belong to the $\nu z+\nu 3$ combination band. The upper state levels being largely perturbed, the analysis of strengths on the basis of an isolated band model -the only one presently available- requires a second order development of the dipole moment including 8 parameters. The strengths of a severe selection of 229 single lines are reproduced with a relative standard deviation of 5.4 % whereas reduced developments with only 3 parameters (first order) or 1 parameter (zero order) lead to 16% and 133% respectively.The obtained parameters are used to predict the associated $\nu 3-\nu z$ difference band.