Ultrafast Dynamics of Electronic Resonances in Molecules Adsorbed on Metal Surfaces: A Wave Packet Propagation Approach

Abstract

We present a wave packet propagation-based method to study the electron dynamics in molecular species in the gas phase and adsorbed on metal surfaces. It is a very general method that can be employed to any system where the electron dynamics is dominated by an active electron and the coupling between the discrete and continuum electronic states is of importance. As an example one can consider resonant moleculesurface electron transfer or molecular photoionization. Our approach is based on a 1 computational strategy allowing to incorporate ab initio inputs from Quantum Chemistry methods, such as Density Functional Theory, Hartree-Fock and Coupled Cluster.Thus, the electronic structure of the molecule is fully taken into account. The electron wave function is represented on a 3D grid in spatial coordinates, and its temporal evolution is obtained from the solution of the time dependent Schrödinger equation. We illustrate our method with an example -electron dynamics of anionic states localized on organic molecules adsorbed on metal surfaces. In particular, we study resonant charge transfer from π * orbitals of three vinyl derivatives (acrylamide, acrylonitrile, and acrolein) adsorbed on a Cu(100) surface. Electron transfer between these lowest unoccupied molecular orbitals and the metal surface is extremely fast, leading to a decay of the population of the molecular anion on the femtosecond timescale. We detail how to analyze the time-dependent electronic wave function in order to obtain the relevant information on the system: the energies and lifetimes of the molecule-localized quasi-stationary states, their associated resonant wavefunctions, and the population decay channels. In particular, we demonstrate the effect of the electronic structure of the substrate on the energy and wave vector distribution of the hot electrons injected into the metal by the decaying molecular resonance.

Effect of a Dielectric Spacer on Electronic and Electromagnetic Interactions at Play in Molecular Exciton Decay at Surfaces and in Plasmonic Gaps

“…Similar results are obtained for the molecule/substrate systems characterized by very different atomistic and electronic structures. Thus, for the molecules adsorbed on a metal surface, the fast RET with Γ RET ≃ 0.5 eV corresponds to a very short lifetime of the electron population of the π* orbital τ = 1/Γ RET ≃ 1 fs typical for such systems. ,, According to our calculation, the monolayer (1 ML) of NaCl introduced between the molecule and the metal reduces the Γ RET by nearly 2 orders of magnitude. Considering that the band gap of NaCl (and thus its effect on RET) is fully developed for the film thickness ≥2 ML, − we fit the WPP results for the 2 and 3 ML coverage with an exponential dependence Γ RET ∝ e –γ N ML .…”

“…In the absence of a spacer, the RET typically proceeds at femtosecond time scales. ,, It is orders of magnitude faster than the radiative and nonradiative intramolecular de-excitation, and the luminescence is quenched even for the most optimal plasmonic gap configuration in a STM setup , (see below). We thus first address the reduction of the RET by the ionic crystal NaCl spacer layer as this is a necessary condition for the other processes to be operative.…”

“…To trace the dynamics of the RET, we apply a wave packet propagation (WPP) approach. , The method allows for a complete characterization of the molecule-localized π* resonance, including its energy, wave function, and RET rate. In brief, one explicitly solves the time-dependent Schrödinger equation for the electron active in the transitions between the molecule and the substrate.…”

“…The π* resonance is thus inside the projected band gap of Cu(100), located between 1.6 and 7.7 eV . In such a situation, the RET along the surface normal is not possible (white region in the metal below the molecule), and the electron escapes from the molecule at a finite angle with respect to the z -axis. ,, Note that the symmetry of the ZnPP molecule also imposes the nodal structure of the wave function along the ( x = 0, z )-axis. However, this symmetry effect is specific to ZnPP.…”

“…Atomic and molecular adsorbates on metal surfaces represent canonical examples of a discrete state–continuum interaction where the electronic orbitals localized at the adsorbate hybridize with the continuum of electronic states of the substrate. This leads to extremely short, at the femtosecond (fs) scale, lifetimes of the adsorbate-localized states. − The excited electron promptly escapes from the adsorbate into the unoccupied electronic states of the metal valence band via energy-conserving resonant electron transfer (RET). ,,,,,, Similarly, the excited state can be quenched by RET from the occupied electronic states of the metal into the adsorbate. − …”