Lateral Hopping of CO on Ag(110) by Multiple Overtone Excitation

A novel type of action spectrum representing multiple overtone excitations of the $v(M\text{−}\mathrm{C})$ mode was observed for lateral hopping of a CO molecule on Ag(110) induced by inelastically tunneled electrons from the tip of a scanning tunneling microscope. The yield of CO hopping shows sharp increases at $261\pm 4\mathrm{mV}$, corresponding to the C-O internal stretching mode, and at $61\pm 2$, $90\pm 2$, and $148\pm 7\mathrm{mV}$, even in the absence of corresponding fundamental vibrational modes. The mechanism of lateral CO hopping on Ag(110) was explained by the multistep excitation of overtone modes of $v(M\text{−}\mathrm{C})$ based on the numerical fitting of the action spectra, the nonlinear dependence of the hopping rate on the tunneling current, and the hopping barrier obtained from thermal diffusion experiments.

Figure 1

Schematic illustrations of five types of action spectra: (a) Single-mode excitation; (b) multiple-mode excitation; (c) combination-mode excitation; (d) single-overtone-mode excitation; and (e) multiple-overtone-mode excitation.

Figure 2

(a) Topographic STM image of five CO molecules on an Ag(110) surface ($5\times 5{\mathrm{nm}}^2$, scale $\mathrm{bar}=1\mathrm{nm}$, $V_s=100\mathrm{mV}$, $I_t=1\mathrm{nA}$). Inset-image obtained from a CO-terminated tip shows atomic rows along the $[1\overline{1}0]$ direction of Ag(110), represented by blue solid lines. The CO molecule hopped along the $[1\overline{1}0]$ direction by injecting tunneling electrons, as marked by “$\mathbf{A}$.” (b) Tunneling current monitored during the injection of tunneling electrons ($V_s=250\mathrm{mV}$, $I_t=1\mathrm{nA}$) over the center of the “$\mathbf{A}$” CO molecule with open feedback loop. (c) Action spectra for lateral hopping of a CO molecule as a function of sample bias voltage under constant tunneling currents ($I_t$) of 1 (filled triangle) and 20 nA (filled circle). Solid lines indicate the spectral fitting results of the experimental data. The inset shows the hopping rate ($R_{\text{hop}}$) of a CO molecule as a function of tunneling current ($I_t$) for sample bias voltages 90 (open triangle), 140 (open circle), 180 (filled triangle), and 280 mV (filled circle). The dashed lines in the inset represent least squares fits to the data, and correspond to the powers ($n$) in nonlinear power-law dependence, $R\propto I^n$.

Figure 3

Schematic representation of vibrational excitations of multiple overtone modes originating from the metal-molecule stretching mode, $v(M\text{−}\mathrm{C})$: (a) First, (b) second, and (c) fourth overtone modes. Lateral hopping of a CO molecule is induced by an intermode coupling between overtone modes of $v(M\text{−}\mathrm{C})$ and the reaction-coordinate (RC) mode, i.e., the hindered-translation (HT) mode.