  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.
