Atomic-scale Stark-shift spectroscopy and microscopy of organic molecules

TL;DR

This paper extends Stark-shift spectroscopy to atomic-scale inhomogeneous electric fields, enabling nanoscale mapping of molecular charge redistribution and polarizability changes with high spatial resolution.

Contribution

It develops a theoretical framework for atomic-scale Stark shifts in inhomogeneous fields and demonstrates its application to molecular imaging with a scanning tunneling microscope.

Findings

Subnanometric mapping of charge redistribution in molecules.

Differentiation of linear and quadratic Stark effects.

Nanoscale Stark spectroscopy reveals excited-state charge dynamics.

Abstract

In conventional optical Stark-shift spectroscopy, molecules are exposed to spatially homogeneous static electric fields that shift the energies of their spectral lines. These shifts are attributed to the molecular electronic properties, such as variation of dipolar moment and polarizability of the molecule associated with photo(de)excitation. In realistic environments containing structural defects and nanoscale heterogeneities, however, molecules experience internal electric fields that vary strongly on the molecular scale, rendering the standard Stark selection rules inapplicable. Here we develop an extended theory of atomic-scale Stark shift, addressing such scenarios. Specifically, we present a detailed theoretical analysis of an experimentally relevant configuration where the atomically sharp tip of a light-assisted scanning tunneling microscope is used to controllably apply inhomogeneous electrostatic fields to representative molecular dyes spanning several molecular families. We decompose the total Stark shift into linear and quadratic contributions and show that they contain different information about the molecular properties. Concretely, spatial variations of the linear Stark shift as the tip scans across the molecule enable subnanometric mapping of the charge redistribution between ground and excited electronic states, with high sensitivity to molecular composition and chemical functionalization. The quadratic Stark contribution, in contrast, reflects changes in the conventional dipolar polarizability upon excitation. Together, these results establish nanoscale Stark-shift spectroscopy as a powerful tool for resolving excited-state charge dynamics in single molecules under realistic, strongly inhomogeneous electric fields.