A cryogenic scanning tunneling microscope (STM) has been used to perform atomic scale imaging and spectroscopy measurements of individual fullerene nanotube peapods. We show that the encapsulation of the C_(60) molecules does not modify the structure of the single-wall carbon nanotube (SWNT) cage yet strongly affects SWNTs' electronic properties. Spatially resolved measurements show that chains of C_(60) molecules induce periodic modifications of the unoccupied density of states in semiconducting SWNTs. These changes are caused by the coupling of the lowest unoccupied t_(1u) orbitals of the C_(60) molecules to the unoccupied SWNTs' one-dimensional electronic states.
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