Effect of molecular structure on the infrared signatures of astronomically relevant PAHs

Abstract

Emission bands from polycyclic aromatic hydrocarbons (PAHs) dominate the mid-infrared spectra of a wide variety of astronomical sources, encompassing nearly all stages of stellar evolution. Despite their similarities, details in band positions and shapes have allowed a classification of PAH emission to be developed. It has been suggested that this classification is in turn associated with the degree of photoprocessing of PAHs. Over the past decade, a more complete picture of the PAH interstellar life-cycle has emerged, in which a wide range of PAH species are formed during the later stages of stellar evolution. After this they are photoprocessed, increasing the relative abundance of the more stable (typically larger and compact) PAHs. For this work we have tested the effect of the symmetry, size, and structure of PAHs on their fragmentation pattern and infrared spectra by combining experiments at the free electron laser for infrared experiments (FELIX) and quantum chemical computations. Applying this approach to the cations of four molecular species, perylene (C 20 H 12 ), peropyrene (C 26 H 14 ), ovalene (C 32 H 14 ) and isoviolanthrene (C 34 H 18 ), we find that a reduction of molecular symmetry causes the activation of vibrational modes in the 7-9 µm range. We show that the IR characteristics of less symmetric PAHs can help explain the broad band observed in the class D spectra, which are typically associated with a low degree of photoprocessing. Such large, nonsymmetrical irregular PAHs are currently largely missing from the NASA Ames PAH database. The band positions and shapes of the largest more symmetric PAH measured here, show the best resemblance with class A and B sources, representative of regions with high radiation fields and thus heavier photoprocessing. Furthermore, the dissociation patterns observed in the mass spectra hint to an enhanced stability of the carbon skeleton in more symmetric PAHs with respect to the irregular and less symmetric species, which tend to loose carbon containing units. Although not a direct proof, these findings are fully in line with the grandPAH hypothesis, which claims that symmetric large PAHs can survive as the radiation field increases, while their less symmetric counterparts are destroyed or converted to symmetric PAHs.

Structural investigation of doubly-dehydrogenated pyrene cations

“…IRMPD spectrum of Py + using FELIX and a Paul-type quadrupole ion trap (unpublished data). See Bouwman et al for experimental details 58. …”

Interstellar polycyclic aromatic hydrocarbons: Spectroscopy, photofragmentation and photoproducts

“…IR spectroscopy: We performed mid-IR action spectroscopy on a series of PAH cations ranging from small species such as naphthalene up to sizes that are astronomically relevant, with the largest species being dicoronylene (C 48 H 20 ). The PAHs studied in our experiments were selected to span various symmetry groups and degrees of compactness to investigate the effect of these parameters on the appearance of their respective IR spectra (Bouwman et al (2019)). From these studies we concluded that the most compact PAHs with the highest symmetry have IR spectra that best resemble the observed mid-IR bands, while PAHs with lower symmetry exhibit modes at frequencies that do not correspond to the main IR emission bands.…”

A Comparison of the Simulations and Observations for a Nearby Spiral Arm

“…Historically, this was one of the first hints of the spiral structure of galaxies. The dust itself can be best observed by studying atomic and molecular signatures in the spectra (Bouwman et al, 2019).…”