Laboratory Determination of the Infrared Band Strengths of Pyrene Frozen in Water Ice: Implications for the Composition of Interstellar Ices

Abstract

Broad infrared emission features (e.g., at 3.3, 6.2, 7.7, 8.6, and 11.3 µm) from the gas phase interstellar medium have long been attributed to polycyclic aromatic hydrocarbons (PAHs). A significant portion (10%−20%) of the Milky Way's carbon reservoir is locked in PAH molecules, which makes their characterization integral to our understanding of astrochemistry. In molecular clouds and the dense envelopes and disks of young stellar objects (YSOs), PAHs are expected to be frozen in the icy mantles of dust grains where they should reveal themselves through infrared absorption. To facilitate the search for frozen interstellar PAHs, laboratory experiments were conducted to determine the positions and strengths of the bands of pyrene mixed with H 2 O and D 2 O ices. The D 2 O mixtures are used to measure pyrene bands that are masked by the strong bands of H 2 O, leading to the first laboratory determination of the band strength -2for the CH stretching mode of pyrene in water ice near 3.25 µm. Our infrared band strengths were normalized to experimentally determined ultraviolet band strengths, and we find that they are generally ∼50% larger than those reported by Bouwman et al. based on theoretical strengths. These improved band strengths were used to reexamine YSO spectra published by Boogert et al. to estimate the contribution of frozen PAHs to absorption in the 5−8 µm spectral region, taking into account the strength of the 3.25 µm CH stretching mode. It is found that frozen neutral PAHs contain 5%−9% of the cosmic carbon budget, and account for 2%−9% of the unidentified absorption in the 5−8 µm region.

A multifunctional setup to record FTIR and UV-vis spectra of organic molecules and their photoproducts in astronomical ices

“…It is very likely that this mismatch is due to an underestimate of the used band strengths as calculated by density functional theory (DFT) methods. This is in line with conclusions by Hardegree-Ullman et al, 56 who find theoretical pyrene band strengths 50% larger than experimentally derived by Bouwman et al 57 Whereas the neutral precursor signals, though weak, are clearly visible, no IR features can be assigned to modes of the triphenylene cation. Based on the computed band strengths, we can estimate whether we should be able to.…”

Observations of the Icy Universe

“…j HCOO − and/or CH 3 CHO may be carriers of the 7.41 µm ice band . k Using column density of C-C bonds derived in Hardegree-Ullman et al (2014) and assuming each PAH species contains 50 carbon atoms.…”

“…The C-H stretch mode of PAH species, detected by its 3.29 µm emission throughout the Galaxy, is thus a good candidate (Sellgren et al, 1994). Indeed, the strengths of other absorption features in YSO spectra, in particular in the 5-7 µm region, are consistent with PAHs embedded in the ices (Hardegree-Ullman et al, 2014). No convincing overall laboratory fit is available, however, due to the heterogeneity of PAH species, including size distributions and ionization within the ice ).…”

“…Maximum values of a factor of 2 larger are measured. Frozen PAH species are likely an important reservoir of (refractory) carbon in the ices, at 18% of the volatile budget, at least, using absolute band strength measurements (Hardegree-Ullman et al, 2014) of neutral pyrene, but this needs to be re-assessed when the degree of ionization of the frozen PAHs is constrained (Keane et al, 2001b;Bouwman et al, 2011). NH 3 , NH + 4 , and OCN − account for ∼11% of the available N (Öberg et al, 2011).…”

Electronic spectra of oxygen containing polycyclic hydrocarbon cations and the protonated analogues

“…Therefore, laboratory exploration of PAHs on cosmic ice analogs upon ultra-violet radiation became a topic in astrochemistry, and several experiments were carried out by varying the temperature, size of aromatics, and using ice of different molecules (e.g., H 2 O, O 2 , CO 2 , HCN, NH 3 ). [7][8][9][10][11][12] The results demonstrate that ices modify the organics and even create new systems during radiation. In the case of H 2 O mantels, formation of ketones and alcohols of the corresponding PAHs is predominant.…”