Detection and Characterization of Anharmonic Overtone Vibrations of Single Molecules on a Metal Surface

Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) is a powerful technique used to characterize the vibration and spin states at the single-molecule level. While IETS lacks hard selection rules, historically it has been assumed that vibrational overtones are rarely seen or even absent. Here we provide definitive experimental evidence that the hindered rotation overtone excitation of carbon monoxide molecules adsorbed on Ag(110) can be detected with STM-IETS via isotope substitution. We also demonstrate that the anharmonicity of the overtone excitation can be characterized and compared between adsorption sites and find evidence of anisotropy in the vibrational anharmonicity for CO adsorbed on the $[1\overline{1}0]$ step edge.

Figure 1

STM topography and IETS of ${}^{12}\mathrm{C}{}^{16}\mathrm{O}$ molecules adsorbed on the terrace and tip. (a) Constant-current topography of a single CO molecule with feedback set point $100\mathrm{mV}/0.1\mathrm{nA}$. (b) Constant-current topography of the Ag(110) surface obtained after transferring the CO molecule to the STM tip with feedback set point $10\mathrm{mV}/0.3\mathrm{nA}$, image size $20\times 20\AA$. (c) STM-IETS of CO adsorbed on the Ag(110) surface (top), CO adsorbed on the STM tip (middle), and the Ag(110) surface taken with bare tip as a background measurement (bottom). Each spectrum is the average of 40 passes, taken with $2{\mathrm{mV}}_{\mathrm{rms}}$ bias modulation at 471 Hz. Spectra in the inset were taken with finer bias step size and average of 80 passes. (d) Schematic diagram showing the four possible vibration modes of CO on the surface. The degeneracy of the HT and HR modes can be lifted by the surface anisotropy as in the present Letter.

Figure 2

STM topography and IETS of the three CO isotopes on the terrace. (a) High-resolution plot of the HR doublet low-energy peak for all three isotopes. (b) Constant-current topography of three CO isotopes arranged together by tip manipulation. Isotopes were identified by measuring IETS peak positions prior to tip manipulation and were otherwise indistinguishable in STM topography. (c) STM-IETS of the HT doublet (left) and HR doublet (right) for all three isotopes. All HT spectra were taken with $300\mu {\mathrm{V}}_{\mathrm{rms}}$ bias modulation at 311.11 Hz, averaged over 80 passes, while all HR spectra were taken with $600\mu {\mathrm{V}}_{\mathrm{rms}}$ bias modulation, averaged over 40 passes. All peak positions displayed are the average of positive and negative bias, and peak position indicators for ${}^{12}\mathrm{C}{}^{16}\mathrm{O}$ are included for all spectra to aid comparison.

Figure 3

STM-IETS for the three CO isotopes on the terrace and tip. (a) IETS measurements of the HR overtone for all three isotopes adsorbed on the terrace taken with bare Ag tip. Each spectrum was taken with 1.5 mV bias modulation at 471 Hz, averaged over 160 passes. (b) Schematic diagram illustrating the anharmonicity in vibrational energy level spacing for the HR mode. A harmonic potential is displayed in blue and a model anharmonic potential is displayed in red. Both potentials are functions of the vibration coordinate $\theta$, visualized as the angle the Ag-C bond axis makes with the surface normal for conceptual simplicity. (c) STM-IETS of the HR (left) and HR overtone (right) for each CO isotope adsorbed on the tip. For all spectra in (a) and (c), peak positions are displayed as the average of positive and negative bias, and peak position indicators for ${}^{12}\mathrm{C}{}^{16}\mathrm{O}$ are included to aid comparison.

Figure 4

STM topography and IETS of the three CO isotopes adsorbed on the $[1\overline{1}0]$ step edge. (a) STM-IETS of the HR (top) and HR overtone (bottom) for the ${}^{12}\mathrm{C}{}^{16}\mathrm{O}$ isotope. (b) Plot of HR overtone (HRO) vs low-energy HR peak position for all measurements made in this Letter. Isotope is indicated by color and adsorption geometry by shape. Deviations from the $y=2x$ line provide a visual measure of the anharmonicity in the HR overtone. (c) Plot of HRO vs high-energy HR peak position. Note the absence of the tip geometry due to the degeneracy of the tip HR and HRO peaks. (Inset) Constant-current topography of three CO isotopes transferred to the step edge by tip manipulation. The identity of each isotope is indicated. For all spectra, peak positions are displayed as the average of positive and negative bias positions. Error bars are given by the peak fit standard errors in the peak position [34].