The modulated photoconductivity was measured in single crystal YB66, e
ssentially consisting of B-12 icosahedral clusters like many other bor
on-rich borides. A model, in which localized states participate both i
n photocarrier generation and trapping well explains the phase shift b
etween excitation and photocurrent. For the localized states involved,
an ionization energy of 0.19 eV is estimated. (C) 1997 Academic Press
.