Vibrational Frequency Used as Internal Clock Reference to Access Molecule-Metal Charge-Transfer Times

Dynamical charge transfer processes at molecule-metal interfaces proceed in the few fs timescale that renders them highly relevant to electronic excitations in optoelectronic devices. Yet, knowledge thereof is limited when electronic ground state situations are considered that implicate charge transfer directly at the fermi energy. Here we show that such processes can be accessed by means of vibrational excitations, with nonadiabatic electron-vibron coupling leading to distinct asymmetric line shapes. Thereby the characteristic timescale of this interfacial dynamical charge transfer can be derived by using the vibrational oscillation period as an internal clock reference.

Figure 1

Collection of infrared absorption spectra, obtained from $\mathrm{CuPc}/\mathrm{Ag}(111)$, $\mathrm{NTCDA}/\mathrm{Ag}(111)$, $\mathrm{PTCDA}/\mathrm{Ag}(111)$, $\mathrm{PTCDA}/\mathrm{Au}(111)$, and Tetracene/Ag(111) monolayers. The spectra were obtained at 80 K using a spectral resolution of $2{\mathrm{cm}}^{-1}$; individual curves are vertically offset for better display. The spectral region of C-H stretching modes has been omitted as the respective absorptions are negligibly small.

Figure 2

Infrared absorption spectra of (a) $\mathrm{CuPc}/\mathrm{Ag}(111)$, and (b) the relaxed $\mathrm{NTCDA}/\mathrm{Ag}(111)$ monolayers. The asymmetric lines have been fitted using Fano-type line shapes according to Langreth [11]. The derived parameters are indicated in the respective panels. The indicated irreducible representations refer to the $D_{4h}$ and the $D_{2h}$ symmetry groups of CuPc and NTCDA, respectively.

Figure 3

Infrared absorption spectra of NTCDA adsorbed on Au(111) (top), Ag(111) (bottom), and an AgAu(111) surface alloy created by annealing an $\mathrm{Ag}/\mathrm{Au}(111)$ monolayer to 580 K (center). Spectral resolution was $0.5{\mathrm{cm}}^{-1}$ for $\mathrm{NTCDA}/\mathrm{Au}(111)$, while $2{\mathrm{cm}}^{-1}$ was used otherwise.