Phosphorescence has been detected from the dimeric primary electron donor in Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers. For Rb. sphaeroides the emission maximum occurs at 1318 + 2 nm at 20 K and is nearly independent of temperature. The emission linewidth (full width at half maximum) is 240 + 20 cm-! at 20 K and increases to 580… (More)