title: Vibrational excitations in molecular layers probed by ballistic electron microscopy.
abstract: We demonstrate the information on molecular vibrational modes via the second derivative (d(2)I(B)/dV(2)) of the ballistic electron emission spectroscopy (BEES) current. The proposed method does not create huge fields as in the case of conventional derivative spectroscopy and maintains a zero bias across the device. BEES studies carried out on three different types of large polycyclic aromatic hydrocarbon (PAH) molecular layers show that the d(2)I(B)/dV(2)  spectra consist of uniformly spaced peaks corresponding to vibronic excitations. The peak spacing is found to be identical for molecules within the same PAH family though the BEES onset voltage varies for different molecules. In addition, injection into a particular orbital appears to correspond to a specific vibrational mode as the manifestation of the symmetry principle.
authors.affiliation: Institute of Materials Research and Engineering, 3 Research Link, 117602, Singapore.
authors.forename: Rasanayagam Sivasayan
authors.initials: RS
authors.lastname: Kajen
authors.forename: Natarajan
authors.initials: N
authors.lastname: Chandrasekhar
authors.forename: Xinliang
authors.initials: X
authors.lastname: Feng
authors.forename: Klaus
authors.initials: K
authors.lastname: Müllen
authors.forename: Haibin
authors.initials: H
authors.lastname: Su