Laser-Induced Electronic and Vibronic Dynamics in the Pyrene Molecule and its Cation

TL;DR

This study uses first-principles simulations to explore the ultrafast electronic and vibrational dynamics of pyrene and its cation, revealing harmonic motion in the neutral and anharmonic effects in the cation driven by electronic coherence.

Contribution

It provides a detailed first-principles analysis of pyrene's ultrafast vibronic dynamics, highlighting differences between neutral and cationic states and offering insights into their spectral responses.

Findings

Nuclear motion is mainly harmonic in neutral pyrene.

Strong anharmonic oscillations occur in the cation due to electronic coherence.

Results enhance understanding of ultrafast vibronic processes in polycyclic aromatic hydrocarbons.

Abstract

Among polycyclic aromatic hydrocarbons, pyrene is widely used as an optical probe thanks to peculiar ultraviolet absorption and infrared emission features. Interestingly, this molecule is also an abundant component of the interstellar medium, where it is detected via its unique spectral fingerprints. In this work, we present a comprehensive first-principles study on the electronic and vibrational response of pyrene and its cation to ultrafast, coherent pulses in resonance with their optically active excitations in the ultraviolet region. The analysis of molecular symmetries, electronic structure, and linear optical spectra is used to interpret transient absorption spectra and kinetic energy spectral densities computed for the systems excited by ultrashort laser fields. By disentangling the effects of the electronic and vibrational dynamics via \textit{ad hoc} simulations with stationary and moving ions, and, in specific cases, with the aid of auxiliary model systems, we rationalize that the nuclear motion is mainly harmonic in the neutral species, while strong anharmonic oscillations emerge in the cation, driven by electronic coherence. Our results provide additional insight into the ultrafast vibronic dynamics of pyrene and related compounds and set the stage for future investigations on more complex carbon-conjugated molecules.