Photon echo and reverse transient grating measurements of the loss of electronic coherence for molecular iodine are presented. Systematic measurements of the coherence decay rate were made as a function of buffer gas. From the dependence of decay rate on numerical density, we calculated experimental cross sections of decoherence. These values range from 135 Angstrom(2) for helium to 1170 Angstrom(2) for I-2. We find Lennard-Jones parameters for the long-range interactions responsible for decoherence which can be modeled by dispersion forces. (C) 2003 American Institute of Physics.