Excitation of Molecular Vibrational Modes with Inelastic Scanning Tunneling Microscopy Processes: Examination through Action Spectra of cis-2-Butene on Pd(110)

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection rules in terms of resonance tunneling.

Figure 1

(a) STM images of four equivalent orientations of a cis-2-butene molecule labeled $C_{\mathrm{UR}}$, $C_{\mathrm{DR}}$, $C_{\mathrm{UL}}$, and $C_{\mathrm{DL}}$, which are obtained by moving the molecule by injecting tunneling electrons. The suffixes UR, UL, DR, and DL correspond to the relative location of the head part, i.e., up-right, up-left, down-right, and down-left, with respect to the center of the molecule in the STM images, respectively. A trans-2-butene molecule (labeled $T$) was coadsorbed as a marker. The superimposed grid indicates the position of Pd atoms, which is determined from the STM image obtained with a molecular tip. The additional four dotted lines forming a rhombus correspond to the molecular long axes at each orientation. An asterisk represents the center of the large bright region of the molecule in each orientation, where the tunneling electrons were injected. (b) Proposed structure of cis-2-butene on Pd(110) at four equivalent adsorption orientations. (c) Current changes measured at sample bias voltage of 170 mV with the tip fixed over the center of the head of a cis-2-butene molecule. The inset shows the wide time range of the same data.

Figure 2

(a) Action spectra for described motions both of ${\mathrm{C}}_4{\mathrm{H}}_8$ (upper) and of ${\mathrm{C}}_4{\mathrm{D}}_8$ (lower). Data were taken under fixed tunneling current of 3 nA for ${\mathrm{C}}_4{\mathrm{H}}_8$ and of 2 nA for ${\mathrm{C}}_4{\mathrm{D}}_8$. (b) Magnification of the action spectrum for low barrier motion at around the threshold energy. Slight increase in the yield was observed around 115 mV for ${\mathrm{C}}_4{\mathrm{H}}_8$ and 95 mV for ${\mathrm{C}}_4{\mathrm{D}}_8$, as indicated by arrows, respectively.

Figure 3

STM-IETS spectra taken over (a) ${\mathrm{C}}_4{\mathrm{H}}_8$ and (b) ${\mathrm{C}}_4{\mathrm{D}}_8$. Solid lines and dashed lines represent vibrational spectra of the positive and negative bias range, respectively. The background spectrum taken over bare Pd(110) surface has been subtracted from each spectrum. All spectra were obtained at a tunneling gap set point of 0.25 nA and 20 mV, and $V_{\mathrm{rms}}=15\mathrm{mV}$ ac modulation at 797 Hz. Each spectrum is the average of 16 bias voltage ramps.

Figure 4

Molecular structure (left), and the shapes of HOMO (center) and LUMO (right) of a cis-2-butene molecule. The molecular orbitals (MO) were calculated using a semiempirical quantum-mechanical calculation package MOPAC.