Gas-phase electronic action absorption spectra of protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs)

Abstract

Context. Extended red emission (ERE) denotes a broad unassigned feature extending from 540 to 800 nm observed in many regions of the interstellar medium (ISM), and is thought to originate from photoluminescence of cosmic dust. However, definitive assignment of specific carriers remains to be achieved. Aims. Our aim is to investigate the photoabsorption spectra of astrophysically relevant protonated oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs) to probe their ability to absorb photons in the near-ultraviolet (UV) and visible (vis) spectral region and to search for any low-lying electronic states that may account for the ERE. Methods. Gas-phase electronic action absorption spectra of the protonated OPAHs were recorded in the spectral range of 200−700 nm using the ELISA ion-storage ring. Additional time-dependent density functional theory (TD-DFT) calculations were performed to compute excited state transitions that complement the experimental spectra. Results. A set of five protonated (O)PAHs was considered, namely pentacene and the four oxygen-functionalized PAHs, pentacenequinone, pentacenetetrone, anthraquinone, and phenathrenequinone. All pentacene-related species show a main absorption band between 400 and 500 nm, while the smaller OPAHs, anthraquinone and phenanthrenequinone, generally absorb further to the blue compared to the pentacenes. Interestingly, pentacenequinone and phenanthrenequinone exhibit wide absorption plateaus towards the red side of their main absorption band(s), which places them among the potential candidates to contribute to ERE. Additional photodissociation mass spectra reveal the formation of smaller functionalized PAHs and small oxygen-bearing species. Conclusions. Our results demonstrate the ability of OPAHs to absorb in the UV/vis spectral region. Among the four studied OPAHs, two revealed very broad absorption characteristics at wavelengths up to 700 nm, which makes them suitable candidates to contribute to a part of the ERE spectrum. Moreover, these two OPAHs, pentacenequinone and phenanthrenequinone, could dissociate efficiently into oxygen-bearing molecules and smaller functionalized PAHs in photon-dominated regions (PDRs) of the ISM.

Role of Acetylene in the Chemical Evolution of Carbon Complexity

“…463,464 Functionalized PAHs with oxygen-bearing groups can be a source of small oxygen-bearing molecules under conditions of UV irradiation in the ISM. 465,466 Recently, it has been shown that PAHs can also be catalysts for many other hydrogenation reactions in the ISM, resulting in the synthesis of molecules such as H 2 O and HCO. 467 Similarly, under certain conditions, PAHs can play a role in the synthesis of benzene from simple precursors.…”

“…The active development of carbocatalysis in the last decade has shown that important catalytic processes can proceed even without the participation of metals. Several decades ago, PAHs were noted to be appropriate for H 2 formation in PDRs. , Functionalized PAHs with oxygen-bearing groups can be a source of small oxygen-bearing molecules under conditions of UV irradiation in the ISM. , Recently, it has been shown that PAHs can also be catalysts for many other hydrogenation reactions in the ISM, resulting in the synthesis of molecules such as H 2 O and HCO . Similarly, under certain conditions, PAHs can play a role in the synthesis of benzene from simple precursors.…”