Solutions of cerium(III)/(IV)
and formic acid in 0.4 M sulfuric acid have been photolysed
under 254 nm and 365 nm light. Marked differences in the reaction kinetics and
quantum yields are observed at the two different wavelengths. At 365 nm, the
reactions leading to cerium(IV) reduction are caused almost exclusively by the
SO4- radical. The ratio of rate constants, k(SO4-
+ CeIII)/ k(SO4- + HCOOH), is 116 � 11 and the
quantum yield of sulfate radicals, ф(SO4-), is
0.023 � 0.002. At 254 nm, the reactions leading to cerium(IV) reduction are
caused mainly by the OH radical, but approximately 35% of the oxidizing
radicals formed in the primary photochemical reaction are SO4-.
Cerium(III) species, excited at 254 nm, transfer energy to cerium(IV) and this
results in an additional yield of OH and SO4- radicals.
Fluorescence measurements confirmed the efficiency of the energy transfer
reaction. The ratio of rate constants, k(OH+CeIII)/k(OH+HCOOH), is
2.22 � 0.18 and ф(CeIV*)
and ф(CelIII*) giving oxidizing radicals are 0.116 � 0.010 and
0.0083 � 0.0008 respectively. Thus about 5 times more total oxidizing radicals
are produced from excited cerium(IV) species at 254 nm than at 365 nm.
Early Archean origin of Photosystem II