Ab Initio Linear and Pump-Probe Spectroscopy of Excitons in Molecular Crystals

TL;DR

This paper demonstrates ab initio calculations of excitonic spectra in molecular crystals using EOM-CCSD, successfully predicting experimental absorption peaks and exploring multi-exciton states relevant to singlet fission.

Contribution

It extends high-accuracy ab initio spectroscopy methods to periodic molecular crystals, capturing multi-exciton states beyond current approaches.

Findings

Predicted first exciton absorption at 4.4 eV in naphthalene crystal

Successfully simulated pump-probe spectra matching experimental features

Captured multi-exciton states with double-excitation character

Abstract

Linear and non-linear spectroscopies are powerful tools used to investigate the energetics and dynamics of electronic excited states of both molecules and crystals. While highly accurate \emph{ab initio} calculations of molecular spectra can be performed relatively routinely, extending these calculations to periodic systems is challenging. Here, we present calculations of the linear absorption spectrum and pump-probe two-photon photoemission spectra of the naphthalene crystal using equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD). Molecular acene crystals are of interest due to the low-energy multi-exciton singlet states they exhibit, which have been studied extensively as intermediates involved in singlet fission. Our linear absorption spectrum is in good agreement with experiment, predicting a first exciton absorption peak at 4.4 eV, and our two-photon photoemission spectra capture the behavior of multi-exciton states, whose double-excitation character cannot be captured by current methods. The simulated pump-probe spectra provide support for existing interpretations of two-photon photoemission in closely-related acene crystals such as tetracene and pentacene.