Temperature-dependence exponents for CH3I-N2 line-broadening coefficients

Abstract

Temperature-dependence exponents of the traditional power law relating line-broadening coefficients at experimental and reference temperatures are deduced for CH3I-N2 lines in the ν6 band from theoretical estimates in the temperature interval 200–400 K recommended for HITRAN. Perfectly linear fits in log-log coordinates indicate that this simple law is sufficient and more advanced models such as the double-power law are not applicable. Calculations are performed by a semi-empirical method employing room-temperature-adjusted model parameters which remain valid for other temperatures. Results are provided for all six sub-branches of the considered band and cover a large range of quantum numbers (0 ≤ J ≤ 70, K ≤ 20) requested typically by spectroscopic databases. In the absence of measurements at not-room temperatures, these data can be extremely useful for atmospheric applications involving methyl iodide, in particular for the terrestrial atmosphere. Given the negligible vibrational dependence of nitrogen-broadened CH3I line widths, the values computed for the ν6 fundamental can be safely used for other parallel and perpendicular bands.