Absorption spectra were measured by means of an optical multichannel analyzer in Rhodopseudomonas sphaeroides R-26 reaction centers (RCs) modified by treatment with NaBH(4) at various times (>/=1 ps) after the onset of a short excitation flash at 880 nm. Most of these RCs (75-95%) have only one "monomeric" bacteriochlorophyll-800 (B(1)) molecule and are as active as the original RCs. The duration of the excitation and measuring pulses was approximately 33 ps. If the center of the excitation pulse preceded the center of the measuring pulse by 36-40 ps, the formation of a state P(E) (early state), which is converted to the state P(F) (P(+) bacteriopheophytin(-)) in 4 +/- 1 ps (1/e time), was observed. Also the kinetics and the spectrum of the stimulated emission (reflecting the kinetics and the emission spectrum of the excited state P()) were determined. The difference spectrum of the state P(E) approximately equals the sum of the spectra of the states P() ( approximately 65%) and (1)[P(+)B(1) (-)] ( approximately 35%). This indicates that B(1) (-) is an intermediate in the electron transfer from P(*) to bacteriopheophytin, H(1), transferring this electron with a rate constant of (4 x 0.35 ps)(-1) = 7 x 10(11) s(-1).