Analytical expressions for the expansion coefficients in the series of exponents s(g) are derived in the case of an individual line with the Lorentzian, Doppler, and Voigt contours. The method of estimation of the absorption function at small pressures is suggested based on the asymptotic value of the corresponding integral, written with the use of a series of exponents for the individual line and for the arbitrary number of lines. It is shown numerically that asymptotic estimations may be used in a wide range of pressures and are simple in applications. Qualitative evaluations of their areas of applicability are given. The availability is given of bending points on the curve s(g) in places corresponding to the line maxima, and their possible influence on the calculation accuracy at small pressures is noted.
series of exponents, low pressures, asymptotic estimates of transmission