Dissociation pathways of a single dimethyl disulfide on Cu(111): reaction induced by simultaneous excitation of two vibrational modes.

We present a novel reaction mechanism for a single adsorbed molecule that proceeds via simultaneous excitation of two different vibrational modes excited by inelastic tunneling electrons from a scanning tunneling microscope. Specifically, we analyze the dissociation of a single dimethyl disulfide (DMDS, (CH3S)2) molecule on Cu(111) by using a versatile theoretical method, which permits us to simulate reaction rates as a function of sample bias voltage. The reaction is induced by the excitation of C-H stretch and S-S stretch modes by a two-electron process at low positive bias voltages. However, at increased voltages, the dissociation becomes a single-electron process that excites a combination mode of these stretches, where excitation of the C-H stretch is the energy source and excitation of the S-S stretch mode enhances the anharmonic coupling rate. A much smaller dissociation yield (few orders of magnitude) at negative bias voltages is understood in terms of the projected density of states of a single DMDS on Cu(111), which reflects resonant excitation through the molecular orbitals.

Adsorption-induced switching of magnetic anisotropy in a single iron(II) phthalocyanine molecule on an oxidized Cu(110) surface.

We examined the zero-field splitting of an iron(II) phthalocyanine (FePc) attached to clean and oxidized Cu(110) surfaces and the dependence on an applied magnetic field by inelastic electron tunneling spectroscopy with STM. The symmetry of the ligand field surrounding the Fe atom is lowered on the oxidized surface, switching the magnetic anisotropy from the easy plane of the bulk to the easy axis. The zero-field splitting was not observed for FePc on a clean Cu(110) surface, and the spin state converts from triplet to singlet due to the strong coupling of Fe d states with the Cu substrate, as is also confirmed by photoelectron spectroscopy. These findings demonstrate the importance of coupling at the molecule-substrate interface for manipulating the magnetic properties of adsorbates.

Role of molecular orbitals near the fermi level in the excitation of vibrational modes of a single molecule at a scanning tunneling microscope junction.

Inelastically tunneled electrons from the tip of a scanning tunneling microscope were used to induce S-S bond dissociation of a (CH(3)S)(2) and lateral hopping of a CH(3)S on Cu(111) at 4.7 K. Both experimental results and theoretical calculations confirm that the excitation mechanism of the vibrationally induced chemistry reflects the projected density of states of molecular orbitals that appear near the Fermi level as a result of the rehybridization of the orbitals between the adsorbed molecules and the substrate metal atoms.

Scanning tunneling microscope imaging of (CH3S)2 on Cu(111).

Scanning tunneling microscope (STM) images of isolated molecules of dimethyl disulfide, (CH(3)S)(2), adsorbed on the Cu(111) surface were successfully obtained at a sample temperature of 4.7 K. A (CH(3)S)(2) molecule appears as an elliptic protrusion in the STM images. From density functional theory calculation, it was suggested that the bright part in the protrusion corresponds to the molecular orbital which is widely spread around H atoms in each CH(3) group in the (CH(3)S)(2) molecule. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Cu(111), which are given by the combination of three equivalent adsorption sites and two conformational isomers for each adsorption site.