Takashi Mii

Theory of vibrational tunneling spectroscopy of adsorbates on metal surfaces

Abstract A theoretical model for studying elementary processes in vibrational scanning tunneling microscopy/spectroscopy is presented. General formulae for the elastic and inelastic tunneling currents are derived on the basis of the Anderson Hamiltonian coupled to a vibrational degree of freedom. Our treatment includes the vibrational damping due to electron–hole pair excitations. It is found that the non-equilibrium situation plays a crucial role for opening of the inelastic channel and excitations of adsorbate phonons. Elementary processes for both elastic and inelastic tunneling currents are clarified in a qualitative manner. The distribution function of adsorbate phonons is derived and may be of great importance for the understanding of the recently observed current-induced dynamical motions of adsorbates.

Contribution to a theory of vibrational scanning tunneling spectroscopy of adsorbates: Nonequilibrium Green's function approach

Abstract Formal theory of a phonon-assisted tunneling between a metal surface with an adsorbate and a metal scanning tip through an adsorbate (i.e., vibrational scanning tunneling spectroscopy (STS)) has been presented on the basis of the self-consistent nonequilibrium Keldyshs diagram technique. Within this approach, it is possible to take into account a finite lifetime of vibrational excitation, and the balance between elastic and opening inelastic tunneling channels. The relaxation of the vibrational degrees of freedom is included into the phonon Keldysh–Green function and plays a very important role both in the elastic and the inelastic tunneling through the adsorbate. The tunnel current (the total as well as the inelastic one) is calculated in terms of the one-particle nondiagonal substrate–adsorbate Keldysh–Greens function supplemented with the vibrational lifetime and filling factors of adsorbate orbital and phonons. The general expression of the inelastic tunnel current and conductance via the excitation of phonons is also presented in terms of the phonon occupation number and the phonon damping function in the stationary condition.

Influence of pulse duration on time-resolved sum-frequency generation of surface vibrations

The influence of the pulse duration on time-resolved sum-frequency generation (SFG) is studied on the basis of the density matrix formulation for non-linear optical processes. A general expression is derived for a sum-frequency polarization whose decay as a function of the time delay between the pump (infrared) and probe (visible) pulses provides a direct measure of the vibrational dephasing time of adsorbates. It is clearly demonstrated how the transient SFG intensity is influenced by the pulse duration and under what conditions the temporal resolution is estimated from the rising edge of the SFG signal, and its decay is described by a single exponential with a time constant of the dephasing. The coherent mixing effect between sequent pulses in the transient SFG signal near zero time delay is also discussed.

Coherent optical effect on time-resolved vibrational SFG spectrum of adsorbates

Abstract We present a theory to study the influence of the coherent mixing between pump-infrared and probe-visible pulse on a time-resolved sum-frequency generation (TR-SFG) spectrum for vibrations at surfaces. The general formula of the time-dependent and its Fourier transform of the SFG polarization and its Fourier transform allows us to calculate the time-resolved vibrational SFG spectrum and the transient characteristics of the SFG intensity as a function of the delay time t d between the pump-infrared and probe-visible pulse. It is found the coherent optical effect manifests itself in the broadening and narrowing of the SFG spectrum with the intrinsic width of T 2 at negative and positive t d , respectively, being in qualitative agreement with recent experimental results. The influence of the coherent mixing on the transient behavior of the SFG intensity is also discussed in conjunction to the T 2 determination.

Theory of the relation between inelastic scanning tunneling spectroscopy of adsorbates and their vibrational deexcitation

Abstract We show how the rate for controlled excitation of adsorbate vibrational modes by scanning tunneling microscope can be quantitatively related to the lifetimes of these modes when deexciting by electron–hole pair mechanism. To this end we present a Green function formalism that accounts for the two processes in a unified way. A model calculation for an adsorbate like CO is studied and compared with existing experimental results.

Theory of photon-assisted tunneling through quantum dot

Abstract Based on the two-channel Anderson Hamiltonian with the non-equilibrium electron distribution under an external electric field, we develop the theory of stationary photon-assisted tunneling current via strongly correlated energy levels in a quantum dot. The non-equilibrium (Mahanthappa–Baksi–Keldysh) Green function formalism is employed to study the Kondo effect and Coulomb blockade phenomenon in the presence of an external field. The Coulomb interaction is treated within the second order perturbation in the self-energy. The change of electron distribution function in the quantum dot due to emission and absorption of photons is taken into account and is found to play a very important role for the suppression of the Kondo effect. We show that an opening of new tunneling channel through photon side band accounts for a Coulomb staircase in the electric current.

Theory of time-resolved two-photon photoemission spectroscopy from metal surfaces

Abstract We present a theory of energy and time-resolved two-photon photoemission (TR-2PPE) spectra from metal surfaces on the basis of the density matrix formulation. A system consists of an initial occupied (bulk and surface) states, intermediate unoccupied states such as an image potential state and a hot electron state in bulk, and a final photoelectron state. A focus is placed on how the pulse duration of the excitation laser influences the transient 2PPE signal and how the relaxation time of the intermediate states is determined using laser pulses with much longer pulse duration. The present analysis is successfully applied to the recent TR-2PPE experiments for Ag and Cu surfaces. The energy-resolved 2PPE spectrum is also calculated to demonstrate that when it is measured with a ultrashort laser pulses the width of the “one” photoemission peak from the intermediate state is a function of the delay time between pump and probe pulses, in addition to the relaxation time of the system and pulse duration.