Abstract
We devise a new kind of experiment that extends the technology of electron energy loss spectroscopy to probe (supra-)molecular systems: by using an electron beam in a configuration that avoids molecular damage and a very recently introduced electron optics setup for the analysis of the outcoming electrons, one can obtain information on the spatial features of the investigated excitations. Physical insight into the proposed experiment is provided by means of a simple but rigorous model to obtain the transition rate and selection rule. Numerical simulations of DNA G-quadruplexes and other biomolecular systems, based on time dependent density functional theory calculations, point out that the conceived new technique can probe the multipolar components and even the chirality of molecular transitions, superseding the usual optical spectroscopies for those cases that are problematic, such as dipole-forbidden transitions, at a very high spatial resolution.
Highlights
The origin of spectral lines is due to the absorption, emission, and scattering of a photon that modify the energy of the system, whereas the line shape can carry information about the dissipation of the energy absorbed, the interaction with the surroundings, and its influence in modulating the microscopic dynamics of chromophores.[1,4−6] not all of the electronic transitions can be probed in optical spectroscopic experiments because of different selection rules: being optically forbidden, the possibility to investigate the role of a given transition in the photophysical and/or photochemical activity of a molecular system is precluded
We describe how to modify the configuration of a trasmission electron microscopes (TEM)-energy-loss spectroscopy (EELS) apparatus and how to encode a quantum chemistry treatment of the molecular systems and its interaction with the structured wave of the swift electron to obtain orbital angular momentum (OAM) resolved EELS spectra, simulations of the expected experimental results will be presented in a number of paradigmatic cases considering the effects of the finite resolutions in both energy and OAM due to a nonideal setup
To be useful as a spectroscopic tool, the supporting substrate should not perturb significantly the electronic structure of the molecular systems: all the linear response calculations have been performed in vacuo
Summary
Understanding the electronic structure of matter is a formidable task that largely made use of optical spectroscopies and their corresponding selection rules; when probing optical excitations with nanometer resolution, one can obtain information on their dynamics and interactions down to the atomic scale.[1,2]The information acquired can range from the electronic structure and properties of a single molecule to the energy and electron transfer mechanism in complex systems, just to cite a few.[1,3] The origin of spectral lines is due to the absorption, emission, and scattering of a photon that modify the energy of the system, whereas the line shape can carry information about the dissipation of the energy absorbed, the interaction with the surroundings, and its influence in modulating the microscopic dynamics of chromophores.[1,4−6] not all of the electronic transitions can be probed in optical spectroscopic experiments because of different selection rules: being optically forbidden, the possibility to investigate the role of a given transition in the photophysical and/or photochemical activity of a molecular system is precluded. The loss function (i.e., the probability, per unit of transferred frequency, that the swift electron loses energy) is evaluated at the excitation energy of a given plasmonic resonance.[11] If spectroscopy carried out in the electron microscopes could be extended to the molecular and supramolecular systems, this technique could be used to determine the overall morphology and to follow the dynamics of electronic processes inside complex molecular aggregates: for instance, one could find at high spatial resolution where the different chromophores are located within the overall structure in proteins and pigment− protein complexes and study the processes leading to Received: January 14, 2021 Published: March 1, 2021
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