We present evidence for a biexciton state In a molecular aggregate. Fe
mtosecond pump-probe spectroscopy of the H aggregate of an epitaxially
grown trivalent metal-halogen-phthalocyanine gives distinct photoindu
ced absorptions to a continuum of two-exciton states and to a bound bi
exciton state. No signature of the biexciton is found in the J aggrega
te of a very similar trivalent metal-halogen-phthalocyanine. The stron
g exciton-exciton interaction in the H-aggregate system has its origin
in the unique cofacial stacking of the molecular components in this m
aterial. [S0163-1829(98)51608-6].