UNOCCUPIED ORBITAL MEDIATED TUNNELING - RESONANCE-LIKE STRUCTURES IN THE TUNNELING SPECTRA OF POLYACENES

Tunneling spectra obtained from [Al-AlOx-pA-Pb] (where pA = anthracene , tetracene, pentacene, perylene, and coronene) tunnel diodes are repo rted in the bias region extending from -2.3 to +2.3 V. Large intense f eatures (delta sigma/sigma similar to 0.1) are seen in the spectrum wh en Pb is biased positively but are absent in the other bias. These fea tures are not scan direction dependent and do not correlate with inela stic electronic transitions. The positions of the band maxima vary lin early with the Huckel LUMO energy and can be fit using a value of beta = 67 kcal/mol. These novel bands are attributed to elastic tunneling mediated by the unoccupied pi orbitals of the polyacenes. While this type of tunneling has been previously postulated for the case of coppe r phthalocyanine, we believe that the present study is the first unequ ivocal demonstration of resonance-like elastic tunneling via unoccupie d molecular orbitals. The implications of these results to molecular i maging in scanning tunneling microscopy are also discussed.