Electronic absorption spectra of hydrogenated protonated naphthalene and proflavine

TL;DR

This study uses time-dependent density functional theory to calculate the optical absorption spectra of hydrogenated polycyclic hydrocarbon cations, revealing spectral changes due to hydrogenation and potential links to interstellar diffuse bands.

Contribution

It provides new computational spectra for hydrogenated naphthalene and proflavine cations, highlighting hydrogenation effects on optical properties relevant to interstellar chemistry.

Findings

Hydrogenation significantly alters optical spectra of polycyclic hydrocarbon cations.

Some calculated spectral lines are close to observed interstellar diffuse bands.

Fully hydrogenated proflavine shows broad, featureless spectrum.

Abstract

We study hydrogenated cations of two polycyclic hydrocarbon molecules as models of hydrogenated organic species that form in the interstellar medium. Optical spectra of the hydrogenated naphthalene cation Hn-C10H8^+ for n=1,2, and 10, as well as of the astrobiologically interesting hydrogenated proflavine cation Hn-C13H11N3^+ for n=1 and 14, are calculated. The pseudopotential time dependent density functional theory is used. It is found that the fully hydrogenated proflavine cation H14-C13H11N3^+ shows a broad spectrum where positions of individual lines are almost lost. The positions of lines, their shapes, and intensities change in hydronaphthalene and hydroproflavine cations showing that hydrogen additions induce substantially different optical spectra in a comparison to base polycyclic hydrocarbon cations. One calculated line in the visible spectrum of H10-C10H8^+, and one in the visible spectrum of H-C13H11N3^+ are close to the measured diffuse interstellar bands. We also present positions of near-UV lines.