Lifetime of the excited state in vivo. II. Bacteriochlorophyll in photosynthetic bacteria at room temperature.

Lifetime of the excited state (tau) of bacteriochlorophyll (BChl) in photosynthetic bacteria, measured with a mode-locked argon laser (oscillating at 488 nm; mode locked at 56 MHz) as light source, ranged from 0.3 to 2.5 nsec. These tau values are reported with a precision of +/-0.1 nsec. The value of tau at high exciting light intensity (I) was two to three times that at low intensity. For young cultures of green bacterium Chloropseudomonas ethylicum, tau ranged from 0.5 (low I) to 1.0 nsec (high I); for those of the purple bacterium Rhodospirillum rubrum, from 0.4 (low I) to 1.0 nsec (high I); and for those of the BChl b-containing Rhodopseudomonas viridis, from 1.0 (low I) to 2.5 nsec (high I). These data provide information regarding the efficiencies of the photochemical process in these bacteria. Quantum yield (ø) of BChl fluorescence, calculated from ø = tau/tau(0) (where tau(0) is the intrinsic lifetime of fluorescence), ranges from 2-6% at low intensities to 6-14% at high intensities.