Higher-order fluorescence from antenna bacteriochlorophyll

A study was made of the fluorescence emitted by the bacteriochlorophyll of purple bacteria at wavelengths shorter than that of the exciting light. The excitation was in the form of laser pulses of $\thicksim25$ nsec duration and $\lambda=694$ and $868$ nm wavelengths. The maximum of this fluorescence corresponded to twice the energy of the $S_1$ level of the bacteriochlorphyll. Estimates were obtained of the fluorescence quantum yields of various samples and a study was made of the dependence of the fluorescence intensity on the excitation energy. Various mechanisms of the formation of the $S_n$ state (including two-photon absorption, $S_1-S_1$ fusion, etc.) were considered.