Ultrafast Electron Transfer Dynamics at NH₃/Cu(111) Interfaces: Determination of the Transient Tunneling Barrier

Abstract

Electron transfer (ET) dynamics at molecule-metal interfaces plays a key role in various fields as surface photochemistry or the development of molecular electronic devices. The bare transfer process is often described in terms of tunneling through an interfacial barrier that depends on the distance of the excited electron to the metal substrate. However, a quantitative characterization of such potential barriers is still lacking. In the present time-resolved two-photon photoemission (2PPE) study of amorphous NH 3 layers on Cu(111) we show that photoinjection of electrons is followed by charge solvation leading to the formation of a transient potential barrier at the interface that determines the ET to the substrate. We demonstrate that the electrons are localized at the ammonia-vacuum interface and that the ET rate depends exponentially on the NH 3 layer thickness with inverse range parameters beta between 1.8 and 2.7 nm (-1). Systematic analysis of this time-resolved and layer thickness-dependent data finally enables the determination of the temporal evolution of the interfacial potential barrier using a simple model description. We find that the tunneling barrier forms after tau E = 180 fs and subsequently rises more than three times faster than the binding energy gain of the solvated electrons.

A Dynamic Landscape from Femtoseconds to Minutes for Excess Electrons at Ice−Metal Interfaces

“…55 The observed decay times are all faster than 500 pssfar too fast to explain residence times of minutes. Extrapolation of the thickness dependent decay times in amorphous ammonia layers to values reported for crystalline ice structures results in a distance of 16 nm between the excess electron and the metal.…”

Electron Dynamics at Polar Molecule–Metal Interfaces: Competition between Localization, Solvation, and Transfer

“…Here, the electrons always localize at the NH 3 -vacuum interface, as demonstrated by Xe overlayer experiments similar to the ones shown in Figure 16.3 [31]. Assuming that d can be varied systematically and supposing that the resulting transfer probabilities are measured, it would be possible to extract the b and thereby gain information about the tunneling potential.…”

Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales

“…This process of electron solvation (step 2) is accompanied by a binding energy gain and the constriction of the excess electron's wave function that is thus becoming increasingly localized. The molecular rearrangement leads to the formation of an interfacial potential well, 40 comparable to small polaron formation, that leads to a decreased wave function overlap of the excess charge with the metal. In other words, the electron is increasingly screened from the substrate.…”

“…As shown in ref. 40 and mentioned further above, the solvated electrons reside at the NH 3 -vacuum interface in the case of amorphous ammonia on Cu(111). This occurrence leads to the observation of residence times of the excited electrons that depend exponentially on the ammonia layer thickness (cf.…”

“…Details on this analysis can be found in ref. 40, however, in the present context, only the resulting exponential dependence is needed to extrapolate to electron residence times on minute timescales as discussed in the following. As mentioned earlier in this section, the increase of excess electron lifetimes that goes along with crystallization may result from (i) an enhancement of the molecular screening and (ii) an increased electron-metal distance due to the morphology change.…”