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The scanning-tunneling-microscope-induced luminescence emerges recently as an incisive tool to measure the molecular properties down to the single-molecule level. The rapid experimental progress is far ahead of the theoretical effort to the observed phenomena. Such incompetence leads to a significant difficulty in quantitatively assigning the observed feature of the fluorescence spectrum to the structure and dynamics of a single molecule. This work is devoted to revealing the microscopic origin of the molecular excitation via inelastic scattering of the tunneling electrons in the scanning tunneling microscope. The theory proposed here excludes the inelastic electron scattering as the origin of the observed larger photon-counting rate at the positive bias than that at the negative bias voltage. [ABSTRACT FROM AUTHOR]