I am doing line shape studies of Methane broadened by Air and Methane in 4100 to 4300 wave number ranges. The pressure induced shift coefficients always show negative values with respect to J values. Could you please give me an explanation why it shows negative effects? How the values are influenced by atmospheric conditions and a wide range of pressures and temperatures. Need to know the proper explanation and the basis of negative effects for shift parameters.
We used 14 high-resolution spectra recorded at JPL, California with 0.005 cm-1 resolution among which 8 were pure and 6 mixed with air methane spectra. The atmospheric conditions for recording spectra varying in between 298 -148 K and 4.5 - 385 Torr. Is it possible to make a comparison between off-diagonal relaxation matrix elements and first-order calculations? How much percentage difference in intensity between measured and calculated is acceptable. Mention that we used initial line parameters from HITRAN-2012 and fitted data with a multispectrum nonlinear least square fitting software called Labfit.
The pressure shifts can be both negative and positive, however majority of transitions of atmospheric gases have negative pressure shifts in the NIR region. Moreover the further in the vibrational ladder you go the larger those negative shifts are. In fact for many transitions in HITRAN we use an estimate which is a linear function of wavenumber. It has to do with interaction potential of target molecule and the perturber.
I am not sure what you mean by "How much percentage difference in intensity between measured and calculated is acceptable?" Acceptable for what? Also did you try using HITRAN2016? it is available at www.hitran.org
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