The excitation of molecular vibration by means of the inelastically tunneled electrons from the tip of a scanning tunneling microscope (STM) can lead to various dynamical processes at surfaces. The vibrational spectrum of a single molecule provides useful information not only for the chemical identification of the molecule but also for investigating how molecular vibration can couple with the relevant dynamical processes.
Inelastic electron tunneling spectroscopy with the STM (STM-IETS) has been mainly used for obtaining vibrational spectrum of individual molecules. STM-IETS detects the vibrational modes of a single molecule by measuring the total conductance change resulting from both elastic and inelastic electron tunneling. So far, however, selection rules for STM-IETS are not fully established yet. Moreover, the STM-IETS is not applicable to some molecules showing mobile character when they are vibrationally excited by inelastically tunneled electrons during measurement. The response of vibrationally mediated molecular motion to applied bias voltage, namely an "action spectrum", can reveal vibrational modes that are not visible in STM-IETS, because the molecular motion is induced via only inelastic tunneling. Thus, the action spectrum would be a candidate for detecting which vibrational mode is actually excited and associated with molecular motions.
Here, I discuss the usefulness and selection rules of the two types of single molecule vibrational spectroscopic methods by considering the action spectroscopy as an alternative vibrational spectroscopic method for the STM-IETS through the study of vibrationally mediated molecular motions with a low-temperature STM.
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