Effects of temperature and other experimental variables on single moleculevibrational spectroscopy with the scanning tunneling microscope

Inelastic electron tunneling spectroscopy (IETS) was performed on single mo lecules with a variable temperature scanning tunneling microscope. The peak intensity, width, position, and line shape of single molecule vibrational spectra were studied as a function of temperature, modulation bias, bias po larity, and tip position for the (C-H,C-D) stretching vibration of acetylen e (C2H2,C2D2) on Cu(001). The temperature broadening of vibrational peaks w as found to be a consequence of Fermi smearing as in macroscopic IETS. The modulation broadening of vibrational peaks assumed the expected form for IE TS. Extrapolation of the peak width to zero temperature and modulation sugg ested an intrinsic width of similar to4 meV due primarily to instrumental b roadening. The inelastic tunneling cross section at negative bias was reduc ed by a factor of 1.7 for the C-H stretch mode. Low energy modes of other m olecules did not show such a reduction. There was no evidence of a tip-indu ced Stark shift in the peak positions. The spatial variation of the inelast ic signal was measured to determine the junction stability necessary for th e acquisition of single molecule vibrational spectra. (C) 2001 American Ins titute of Physics.