Tip-modified Raman tensor of a porphine molecule

Abstract

© 2019 John Wiley & Sons, Ltd. It is well established that bringing a plasmonic tip into the proximity of a single molecule inevitably affects its state and vice versa. Due to the enhanced light–matter interaction, spectroscopic information on the molecule captured with tip-enhanced Raman spectroscopy (TERS) can noticeably differ on that coming from standard far-field Raman spectroscopy. In this work, we develop a theoretical approach for calculating Raman tensors of a single oblate molecule that takes near-field depolarization effects into account. Generally speaking, a tip-dressed molecule exhibits the modified polarizability and, therefore, the Raman tensors. It is reported that the TERS technique can be used to reconstruct Raman tensors of vibration modes of anisotropic single molecules. The symmetric and antisymmetric modes of the molecules nearby a plasmonic tip are enhanced differently. In order to unravel the Raman tensors, two near-field depolarization factors are necessarily introduced for the oblate molecule. As an example, we consider a free base porphine molecule as a test molecule to demonstrate the enhancement of intensities of Raman bands assigned to different symmetries. Also, we demonstrate an experimental way how to measure the depolarization factors using an ensemble of randomly oriented molecules. We believe that our finding will be beneficial to the interpretation of TERS measurements of single molecules.