Resonant molecular transitions in second harmonic generation spectroscopy of Fe-octaethylporphyrin adsorbed on Cu(001)

TL;DR

This study uses second harmonic generation spectroscopy and electronic structure calculations to analyze the resonant molecular transitions of Fe-octaethylporphyrin on Cu(001), revealing charge-transfer effects and interface properties.

Contribution

It demonstrates the use of surface SHG combined with coupled cluster calculations to probe electronic interactions at molecule-metal interfaces, highlighting resonant enhancement mechanisms.

Findings

SHG response is modified at 2.15-2.35 eV photon energy.

Resonant enhancement of SHG due to molecular transitions at ≥2 eV.

Charge-transfer character influences the nonlinear susceptibility.

Abstract

Metal-organic molecular adsorbates on metallic surfaces offer the potential to both generate materials for future (spin-)electronics applications as well as a better fundamental understanding of molecule-substrate interaction, provided that the electronic properties of such interfaces can be analyzed and/or manipulated in a targeted manner. To investigate electronic interactions at such interfaces, we measure optical second harmonic generation (SHG) from iron-octaethylporphyrin (FeOEP) adsorbed on Cu(001), and perform electronic structure calculations using coupled cluster methods including optical excitations. We find that the SHG response of FeOEP/Cu(001) is modified at 2.15-2.35 eV fundamental photon energy compared to the bare Cu(001) surface. Our polarization-dependent analysis shows that the $\chi_{zzz}^{(2)}$ non-linear susceptibility tensor element dominates this modification. The first-principles calculations confirm this effect and conclude a resonantly enhanced SHG by molecular transitions at $\hbar\omega \geq 2$ eV. We show that the enhancement of $\chi^{(2)}_{zzz}$ results from a strong charge-transfer character of the molecule-substrate interaction. Our findings demonstrate the suitability of surface SHG for the characterization of such interfaces and the potential to employ it for time-resolved SHG experiments on optically induced electronic dynamics.