Momentum and energy dissipation of hot electrons in a Pb/Ag(111) quantum well system

The band structure of multilayer systems plays a crucial role for the ultrafast hot carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited electrons in a highly ordered Pb monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy with femtosecond visible light pulses, we show that the electron dynamics of the Pb/Ag(111) quantum well system is largely dominated by two types of scattering processes: (i) isotropic intraband scattering processes within the quantum well state (QWS) and (ii) isotropic interband scattering processes from the $p_z$-like QWS into the Pb $p_{x/y}$ band. In the latter case, the Pb QWS acts as an electron source for the momentum space refilling process of the Pb $p_{x/y}$ band. This conclusion is confirmed by the modification of the band structure and the quasiparticle dynamics of the Pb/Ag(111) bilayer film after the adsorption of PTCDA. We find both an adsorption-induced suppression of the QWS itself as well as of the refilling process into the Pb $p_{x/y}$ band. Our study hence demonstrates the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovers a new possibility to selectively tune and control scattering processes occurring in quantum (well) materials by the adsorption of organic molecules.

Figure 1

Left: Energy vs momentum photoemission map of the excited states of the Pb/Ag(111) bilayer system along the ${\overline{\mathrm{M}}}^{\prime }\overline{\mathrm{\Gamma }}$ direction. The dispersions of the Pb-derived QWS and the Pb $p_{x/y}$ band are illustrated in the band structure sketch on the right side.

Figure 2

Tr-2PMM data set of the Pb ML on Ag(111) system obtained with an incidence angle of $4^{\circ }$ (nearly normal incidence) and a photon energy $\hslash \omega =3.1\mathrm{eV}$ [see panel (a)]. (b) $E$ vs $k_{\parallel }$ cut along the ${\overline{\mathrm{M}}}^{\prime }\overline{\mathrm{\Gamma }}\overline{\mathrm{M}}$ direction. Constant energy intensity (c) and lifetime (d) maps for $E-E_F=2.65, 2.85,$ and $3.0\mathrm{eV}$ starting from the bottom to the top. (e) Exemplary autocorrelation traces of different regions in momentum space (dots) marked in (c), together with the two-pulse autocorrelation of the fs laser pulses (gray solid line). (f) Corresponding $E$ vs $k_{\parallel }$ lifetime map. (g) Energy-dependent quasiparticle lifetime obtained from the momentum averaged photoemission yield. In (b) and (d) The band dispersion of the Pb $p_{x/y}$ band is indicated with green dotted lines as a guide to the eye.

Figure 3

Tr-2PMM data set of the Pb ML on Ag(111) system obtained in grazing incidence geometry (${65}^{\circ }$ with respect to surface normal), $p$-polarized light, and $\hslash \omega =3.1\mathrm{eV}$ [see panel (a)]. (b) $E$ vs $k_{\parallel }$ cut along the ${\overline{\mathrm{M}}}^{\prime }\overline{\mathrm{\Gamma }}\overline{\mathrm{M}}$ direction. Constant energy intensity (c) maps for $E-E_F=2.65, 2.85$, and $3.0\mathrm{eV}$ starting from the bottom to the top. The band dispersion of the Pb QWS and the Pb $p_{x/y}$ band are indicated by the green dashed and dotted lines that are guides to the eyes. (d) Exemplary autocorrelation traces for the quantum well state (blue), the Pb $p_{x/y}$ band (green) and the background (red). The corresponding regions are marked in (c). The data are shown as circles, the fit curve (black) and the laser cross correlation (grey) as solid lines.

Figure 4

Constant energy (a) and $E$ vs $k_{\parallel }$ lifetime maps (b) of the tr-2PMM data shown in Figs. 3 and 3. The band dispersion of the Pb QWS and the Pb $p_{x/y}$ band are indicated as green dashed and dotted lines, respectively. (c) Sketch of the inter- and intraband scattering processes dominating the hot electron dynamics of the Pb/Ag(111) bilayer system.

Figure 5

Exemplary normalized autocorrelation curves extracted from the $k$-space integrated photoemission yield of a tr-2PMM experiment of the PTCDA/Pb/Ag(111) surface. As light source, a monochromatic laser setup with $p$-polarized light and 3.1 eV photon energy was used. The fits based on optical Bloch equations are plotted as black solid lines. The measured data in both panels are the same and presented as black circles. The data in panel (a) were fitted with one autocorrelation revealing a lifetime of $\tau =14.1\mathrm{fs}$ while the fit curve in region (b) is the sum of two separated autocorrelation curves, referring to the Pb and the PTCDA states. The autocorrelation trace of the PTCDA layers yields a lifetime of 50.2 fs and contributes 33% to the fitting envelope, the one of the Pb layer yields a lifetime of 8.3 fs with a weighting of 67% ($A$ = 0.33).

Figure 6

Tr-2PMM data of a PTCDA monolayer film on a Pb/Ag(111) bilayer ($p$-polarized light, grazing incidence, $\hslash \omega =3.1\mathrm{eV}$). (a) CE intensity and lifetime maps of the PTCDA and Pb derived states for $E-E_F=2.65\mathrm{eV}$. (b) Momentum averaged lifetimes of PTCDA (red) and Pb (blue) -derived states of the PTCDA/Pb/Ag(111) multilayer structure. As a reference, the momentum averaged lifetimes of the bare Pb/Ag(111) bilayer are depicted in green and black for grazing (GI, p-pol) and normal incidence (NI) geometry.