Ultraviolet electronic spectroscopy of heavily substituted naphthalene derivatives

Insights on the potential double aromatic ring substructures of interstellar bump carriers

Institut des Sciences Moléculaires d’Orsay (ISMO), Université Paris-Saclay, CNRS, Bât. 520, Rue André Rivière, 91405 Orsay Cedex, France
e-mail: joffrey.frereux@gmail.com; emmanuel.dartois@universite-paris-saclay.fr; thomas.pino@universite-paris-saclay.fr

Received: 16 January 2023
Accepted: 18 July 2023

Abstract

Context. The so-called bump spectral signature observable on interstellar extinction curves, peaking at 217.5 nm, is commonly assigned to π^* ← π transitions from carbonaceous carriers, but the exact nature of the carbonaceous carriers remains debated.

Aims. To constrain the chemical structures associated with the bump carriers, we record and compare the UV spectra of a large variety of carbonaceous molecules to this interstellar feature.

Methods. Large carbonaceous molecules, such as polycyclic aromatic hydrocarbons (PAHs), were produced by a combustion process stabilized at low pressure under rich flame conditions. Species were extracted and probed through resonance enhanced multiphoton ionization spectroscopy coupled to a time-of-flight mass spectrometer. Masses and absorption profiles of the carbonaceous molecules were measured, and their spectra were compared to the bump feature.

Results. Species showing a specific mass progression starting at mass 128 u visible up to mass 394 u with a characteristic progression of +14 u present a common electronic absorption band profile peaking asymptotically around 220 nm. The first masses were assigned to a naphthalene C₁₀H₈ molecule and two of its derivatives: C₁₀H₇CH₃ and C₁₀H₇C₂H₅. The mass progression of +14 u is explained by successive H atom substitutions by CH₃ functional groups. This mass distribution was thus assigned to naphthalene derivatives with large aliphatic carbon substitution. This derivative family shows an electronic band assigned to S₃ ← S₀ transitions involving electron promotion within the π aromatic orbitals of the naphathlene chromophore. More importantly, after a few substitutions, the position of the band converges asymptotically to a value close to the interstellar bump signature, independent of the molecule size.

Conclusions. Based on the asymptotic behavior of the larger members in the species distribution, a similar band position is expected from double aromatic ring substructures within hydrogenated amorphous carbons (HACs). Similar to the conclusions of previous works, we find substituted naphthalene units as substructures of interstellar HACs to be good candidates as carriers of the bump feature.