Electron transport properties of an isolated quantum dot sandwiched between a metallic contact and a scanning tunneling microscopy tip are theoretically investigated. Keldysh-Green's function technique is used to calculate the tunneling current of an Anderson model with multiple energy levels. The spectral function of the quantum dot system (with arbitrary number of energy levels) embedded in a tunnel junction is derived and used to calculate the tunneling current spectra. Finally, the authors calculate the emission spectra due to the electron-hole recombination that occurs in the case of bipolar tunneling, where both electrons and holes are allowed to simultaneously tunnel into the quantum dot. The authors find dramatic changes in the emission spectra as the applied bias is varied. (C) American Vacuum Society.