An algorithm for calculating a speed-dependent Lorentzian profile

We present an algorithm for calculating speed-dependent Lorentzian profiles. Specifically, we show that the sum of two simple Lorentzians, subject to certain constraints, accurately reproduces a Lorentzian profile with a speed-dependent broadening. Further, the algorithm may be used in more complicated lineshapes in which the speed-dependent Lorentzian is embedded. The benefits resulting from this construction are simplicity of form, the ability to use the form in further symbolic manipulations, and the use of preexisting fast computer codes for the evaluation of the function. Such practical considerations make it possible to use speed-dependent lineshapes in applications such as atmospheric modeling, which demand accurate but computationally efficient absorption curves. In addition, several biases or problems, that occur when speed-dependent experimental profiles are fitted with a simple Lorentzian, are discussed. (C) 1999 Academic Press.