Abstract: A method and system are provided for determining temperature of a low density gas seeded with molecules of iodine. The Cordes bands of the iodine molecules are excited with light energy of a known wavelength (e.g., between 175-210 nanometers) to generate fluorescent emission. At least a portion of the emission's light intensity is collected and used to generate an emission spectrograph. The spectrograph's vibrational energy level peaks are fitted to a vibrational Boltzmann distribution. Specifically, one of the vibrational energy level peaks associated with molecule population in a ground vibrational energy level at a wavelength greater than the known wavelength of the light energy is selected. Next, a plurality of ratios are generated that define a unique linear relationship for a temperature of the seeded environment.