The deposition of Mn-12-ac single molecule magnets on HOPG surfaces was investigated using microRaman spectrograph, scanning tunneling microscopy and spectroscopy (STM/S). The STM and Raman studies indicate that isolated Mn-12-ac molecules remain intact upon physically absorbing on HOPG surfaces. Reproducible STS curves reveal that Mn-12-ac molecules exhibits two unambiguous energy gaps, 2.34 and 0.87 eV, respectively. The former is corresponding to the minority-spin gap of the inner Mn4+, while the later is the majority-spin gap from the outer Mn3+ ions. These findings can be reasonably interpreted according to the former band structure calculations on Mn12O12 molecules. (C) 2012 Elsevier B. V. All rights reserved.