  In scanning tunneling microscopy induced luminescence (STML), the photon
counting is measured to reflect the single-molecule properties, e.g., the first
molecular excited state. The energy of the first excited state is typically
determined by a rising position of the photon counting as a function of the
bias voltage between the tip and the substrate. It remains a challenge to
determine the precise rise position of the current due to the possible
experimental noise. In this work, we propose an alternating current version of
STML to resolve the fine structures in the photon counting measurement. The
measured photon counting and the current at the long-time limit show a
sinusoidal oscillation. The zero-frequency component of the current shows knee
points at the precise voltage as the fraction of the detuning between the
molecular gap and the DC component of bias voltage. We propose to measure the
energy level with discontinuity of the first derivative of such zero-frequency
component. The current method will extend the application of STML in terms of
measuring molecular properties.
