Supplementary Information for "High-Resolution Laser Spectroscopy: The A2PI-X2Sigma Transition" The files on this disk are in an ASCII text format and can be read by most text editors, such as "Editor". Each file is a back calculation on an individual band using the parameters given in Tables 3 and 4 of the manuscript for the ground and excited states, respectively. To best describe the naming system, we'll use 4YbCl01.txt as an example. The "4" refers to the 174Yb35Cl isotopomer (2 is used for the corresponding 172Yb isotope) and the "01" refers to the 0-1 band. Each file begins with the parameters used to calculate the line positions and then lists all observed line positions for that band as well as values for obs-calc. The branches are listed seperately by the rotational numbering J. Todd C. Melville Dalhousie University, 1999.