Singlet-triplet transitions of physisorbed molecule O-2 in scanning tunneling microscope

Theory is developed of inelastic spin-dependent electron tunneling through the nanocontacts containing single adsorbed molecules. This theory consiste ntly takes into account the subbarrier elastic and inelastic electron scatt ering including exchanging one. Estimates are made for the probabilities of different transitions in the adsorbed molecule, which interacts with the t unneling current. At low temperatures, the threshold features of conductivi ty were shown to have the shape of discontinuities (conductance jumps), who se scale depends on two parameters: the relative probability of inelastic t unneling and the ratio of the spontaneous relaxation to electron-current-in duced transition rates. The general analytical expressions are obtained for the shape of the STM conductance threshold features. The STM threshold fea tures can carry the information about the relaxation time of single electro n-excited adsorbed particles. As an example, the excitation of the singlet state in the physically adsorbed oxygen molecule is analyzed in details. Ou r calculations show that the singlet-triplet relaxation time for a single m olecule O-2 adsorbed on semiconductor surface can be determined by varying the distance between the surface and the STM tip. (C) 2001 Elsevier Science B.V. All rights reserved.