POSSIBILITY OF COHERENT MULTIPLE EXCITATION IN ATOM-TRANSFER WITH A SCANNING TUNNELING MICROSCOPE

We examine atom transfer resulting from coherent multiple excitation of the adsorbate-substrate bond caused by inelastic tunneling of a single electron (or hole) via a negative- (or positive-) ion resonance. At low biases and in particular for the transitions resulting in atom transfer, the rates of coherent multiple excitation are nonlinear and also highly asymmetrical with respect to the polarity of the bias. We establish a simple criterion for the regime in which this mechanism dominates over earlier proposed mechanisms for atom transfer resulting from vibrational heating by sequential (incoherent) inelastic resonance tunneling. In the case of the atomic switch, where a Xe atom is transferred between a Ni surface and a tip, the vibrational heating mechanism is found to dominate over the coherent mechanism. For other systems, such as Na adsorption on Cu or O2 adsorbed on Pt, the coherent mechanism is argued to play a role in bond breaking.