Electrons tunneling from a scanning tunneling microscope tip to individual CO molecules on Cu(111) can cause their hopping from the surface to the tip if the bias exceeds a threshold of 2.4 V. Polarization- and time-resolved two-photon photoemission identifies the underlying elementary process as intermediate population of a CO $2{\pi }^{*}$-derived level, which exhibits an ultrashort lifetime of 0.8–5 fs. From an isotope effect of ${2.7}_{-0.5\mathrm{}}^{+0.3\mathrm{}}$ it can be calculated that $\approx 0.05\%$ of the tunneling current transiently occupies this level while a desorption of the excited molecule occurs only in $5\times {10}^{-9\mathrm{}}$ of the cases.

• Received 10 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2004