  Achieving a high intensity in inelastic scanning tunneling spectroscopy
(IETS) is important for precise measurements. The intensity of the IETS signal
can vary up to a factor three for various tips without an apparent reason
accessible by scanning tunneling microscopy (STM) alone. Here, we show that
combining STM and IETS with atomic force microscopy enables carbon monoxide
front atom identification, revealing that high IETS intensities for CO/Cu(111)
are obtained for single atom tips, while the intensity drops sharply for
multi-atom tips. Adsorbing the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such
that it is elevated over the substrate strongly diminishes the tip dependence
of IETS intensity, showing that an elevated position channels most of the
tunneling current through the CO molecule even for multi-atom tips, while a
large fraction of the tunneling current bypasses the CO molecule in the case of
CO/Cu(111).
