The conventional photographic process(1-3) involves several steps: the phot
ogeneration of electron-hole pairs in crystals of a silver halide; the redu
ction of silver cations to atoms by some fraction of these electrons; the s
ubsequent build up of atoms to give clusters (the 'latent image'); and the
complete reduction by a developer of crystallites having more than a critic
al number of silver atoms per cluster. The effective quantum yield, Phi(eff
), of photoinduced electron-hole pairs produced per photon absorbed is less
than the theoretical limit (Phi(theory) = 1), because of the fast recombin
ation of some fraction of the pairs(1-6). Here we describe an approach for
enhancing the yield of useful photogenerated electrons, in which the silver
halide is doped with formate ions, HCO2-. The dopant ions act as hole scav
engers, thus enhancing the escape of electrons from pair recombination. Mor
eover, the resulting CO2.- radical can itself transfer an electron to anoth
er silver cation, so raising the theoretical yield to two silver atoms per
photon absorbed. This photoinduced bielectronic transfer mechanism is stric
tly proportional to the light quanta absorbed-the dopant ions do not induce
spontaneous reduction of silver cations in the dark-and appears to be clos
e to the theoretical limit of efficiency. The efficiency is constant at all
illumination levels and applies to both dye-sensitized and unsensitized cr
ystals. We suggest that this approach is a promising route for improving th
e performance of photographic emulsions(7).