Impact of Ice Structure on Ultrafast Electron Dynamics in ${\mathrm{D}}_2\mathrm{O}$ Clusters on Cu(111)

The structure of ${\mathrm{D}}_2\mathrm{O}$ clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.

Figure 1

(a),(b): STM images of 1 BL (nominal coverage) amorphous ${\mathrm{D}}_2\mathrm{O}/\mathrm{Cu}(111)$ as deposited at 85 K (A; 0.2 V, 1 pA) and after annealing at 120 K (B; 0.1 V, 9 pA). As apparent from the high resolution images (insets) both structures are disordered, i.e., amorphous. The line scans (c) depict the apparent height $h$ in nm and the real height in BL before and after annealing. (e),(f): 2PPE spectra of 0.8 BL ${\mathrm{D}}_2\mathrm{O}/\mathrm{Cu}(111)$ at different pump-probe delays (right axis). Features at energies $>3.3\mathrm{eV}$ result from the Cu(111) surface (SS) and image potential state (IPS). The peak in the spectra of porous ice (e) originates from solvated electrons ($e_s$) and its intensity decreases after transition to compact clusters (f). (d) In 2PPE a UV pulse excites metal electrons that are transferred to the ice conduction band (CB). A time-delayed VIS pulse probes the transient electron population.

Figure 2

(a) Dependence of the apparent height of porous amorphous ice clusters on tunnelling voltage. (b)–(d) Line scans at the voltages indicated in (a) for tunneling through the band gap at 0.3 V (b), at the lower band edge of the conduction band at 2.9 V (c), and through the conduction band of the ice clusters at 3.5 V (d); $I=500\mathrm{fA}$, 0.8 BL.

Figure 3

Transient shift of the peak maximum of $e_s$ in porous and compact amorphous clusters. Inset: Population decay of $e_s$. The population (symbols) is determined by integrating $E=2.58–3.17\mathrm{eV}$. Fits (solid lines) consist of an exponential decay to positive delays convolved with the laser pulses’ cross correlation envelope (dashed line) [8].

Figure 4

Energetic shift of $e_s$ as a function of Xe exposure ($\Delta t=0\mathrm{fs}$). $1.6\times {10}^{-7}\mathrm{mbar}\mathrm{s}$ correspond to 1.0(5) ML (dashed line) calibrated on bare Cu(111). Inset (top): Schematic representation of the titration experiment. Inset (bottom): Spectra of compact clusters on Cu(111) before and after coadsorption of 2 ML Xe.