ترغب بنشر مسار تعليمي؟ اضغط هنا

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

113   0   0.0 ( 0 )
 نشر من قبل Giuliano Malloci
 تاريخ النشر 2008
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of PAHs in several charge states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C24Hn, n less than 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. To assess the observational consequences of the latter hypothesis we have undertaken a systematic study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. We used state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time- dependent DFT to evaluate the electronic excited-state properties. We computed the absorption cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fullydehydrogenated counterparts. pi electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the pi-pi* collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.



قيم البحث

اقرأ أيضاً

The amount of deuterium locked up in polycyclic aromatic hydrocarbons (PAHs) has to date been an uncertain value. We present a near-infrared (NIR) spectroscopic survey of HII regions in the Milky Way, Large Magellanic Cloud (LMC), and Small Magellani c Cloud (SMC) obtained with AKARI, which aims to search for features indicative of deuterated PAHs (PAD or Dn-PAH) to better constrain the D/H ratio of PAHs. Fifty-three HII regions were observed in the NIR (2.5-5 {mu}m), using the Infrared Camera (IRC) on board the AKARI satellite. Through comparison of the observed spectra with a theoretical model of deuterated PAH vibrational modes, the aromatic and (a)symmetric aliphatic C-D stretch modes were identified. We see emission features between 4.4-4.8 {mu}m, which could be unambiguously attributed to deuterated PAHs in only six of the observed sources, all of which are located in the Milky Way. In all cases, the aromatic C-D stretching feature is weaker than the aliphatic C-D stretching feature, and, in the case of M17b, this feature is not observed at all. Based on the weak or absent PAD features in most of the observed spectra, it is suggested that the mechanism for PAH deuteration in the ISM is uncommon.
Mixtures of polycylic aromatic hydrocarbons (PAHs) have been produced by means of laser pyrolysis. The main fraction of the extracted PAHs were primarily medium-sized, up to a maximum size of 38 carbon atoms per molecule. The use of different extract ion solvents and subsequent chromatographic fractionation provided mixtures of different size distributions. UV-VIS absorption spectra have been measured at low temperature by matrix isolation spectroscopy and at room temperature with PAHs as film-like deposits on transparent substrates. In accordance with semi-empirical calculations, our findings suggest that large PAHs with sizes around 50 to 60 carbon atoms per molecule could be responsible for the interstellar UV bump at 217.5 nm.
Aromaticity is a well-known phenomenon in both physics and chemistry, and is responsible for many unique chemical and physical properties of aromatic molecules. The primary feature contributing to the stability of polycyclic aromatic hydrocarbons is the delocalised $pi$-electron clouds in the $2p_z$ orbitals of each of the $N$ carbon atoms. While it is known that electrons delocalize among the hybridized $sp^2$ orbitals, this paper proposes quantum walk as the mechanism by which the delocalization occurs, and also obtains how the functional chemical structures of these molecules arise naturally out of such a construction. We present results of computations performed for some benzoid polycyclic aromatic hydrocarbons in this regard, and show that the quantum walk-based approach does correctly predict the reactive sites and stability order of the molecules considered.
We present a new method to accurately describe the ionization fraction and the size distribution of polycyclic aromatic hydrocarbons (PAHs) within astrophysical sources. To this purpose, we have computed the mid-infrared emission spectra of 308 PAH m olecules of varying sizes, symmetries, and compactness, generated in a range of radiation fields. We show that the intensity ratio of the solo CH out-of-plane bending mode in PAH cations and anions (referred to as the 11.0 $mu$m band, falling in the 11.0-11.3 $mu$m region for cations and anions) to their 3.3 $mu$m emission, scales with PAH size, similarly to the scaling of the 11.2/3.3 ratio with the number of carbon atoms (N$_{mathrm{C}}$) for neutral molecules. Among the different PAH emission bands, it is the 3.3 $mu$m band intensity which has the strongest correlation with N$_{mathrm{C}}$, and drives the reported PAH intensity ratio correlations with N$_{mathrm{C}}$ for both neutral and ionized PAHs. The 6.2/7.7 intensity ratio, previously adopted to track PAH size, shows no evident scaling with N$_{mathrm{C}}$ in our large sample. We define a new diagnostic grid space to probe PAH charge and size, using the (11.2+11.0)/7.7 and (11.2+11.0)/3.3 PAH intensity ratios respectively. We demonstrate the application of the (11.2+11.0)/7.7 - (11.2+11.0)/3.3 diagnostic grid for galaxies M82 and NGC 253, for the planetary nebula NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. Finally, we provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to.
Polycyclic Aromatic Hydrocarbons (PAHs) are carbon-based molecules resulting from the union of aromatic rings and related species, which are likely responsible for strong infrared emission features (3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 microns). In this work, using a sample of Seyfert galaxies (DL<100 Mpc), we compare the circumnuclear (inner kpc) PAH emission of AGN and star-forming (SF) control samples, and we investigate the difference between the central and extended PAH properties. We employ newly developed PAH diagnostic model grids, derived from theoretical spectra, to compare the predicted and observed PAH ratios. We use Spitzer/InfraRed Spectrograph spectral data for a large sample of Seyfert galaxies and SF galaxies. In general we find that SF galaxies and powerful Seyfert galaxies are located in different regions of the PAH diagnostic diagram, which indicates that the size and charge of the PAH molecules but also the nature and hardness of the radiation field that excite them are different. Our work indicates that powerful AGN seem to favour larger PAH molecules (Nc>400) as well as neutral species. By subtracting the central from the total spectra we are able to compare the PAH emission in the central/extended region of a small sample of AGN. In contrast with the findings for central regions of AGN-dominated systems, we find that the extended emission of both Seyfert types has similar PAH molecular size distribution and ionized fraction of molecules than in central regions of SF galaxies (100< Nc< 300).
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا