  The role of the tip in inelastic electron tunneling spectroscopy (IETS)
performed with scanning tunneling microscopes (STM) is theoretically addressed
via first-principles simulations of vibrational spectra of single carbon
monoxide (CO) molecules adsorbed on Cu(111). We show how chemically
functionalized STM tips modify the IETS intensity corresponding to adsorbate
modes on the sample side. The underlying propensity rules are explained using
symmetry considerations for both the vibrational modes and the molecular
orbitals of the tip and sample. This suggests that single-molecule IETS can be
optimized by selecting the appropriate tip orbital symmetry.
