271 research outputs found
The 5.25 & 5.7 m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7
m (1905 and 1754 cm) that hitherto have been little studied,
but contain information about the astronomical PAH population that complements
that of the major emission bands. Here we report a study involving both
laboratory and theoretical analysis of the fundamentals of PAH spectroscopy
that produce features in this region and use these to analyze the astronomical
spectra. The ISO SWS spectra of fifteen objects showing these PAH features were
considered for this study, of which four have sufficient S/N between 5 and 6
m to allow for an in-depth analysis. All four astronomical spectra show
similar peak positions and profiles. The 5.25 m feature is peaked and
asymmetric, while the 5.7 m feature is broader and flatter. Detailed
analysis of the laboratory spectra and quantum chemical calculations show that
the astronomical 5.25 and 5.7 m bands are a blend of combination,
difference and overtone bands primarily involving CH stretching and CH in-plane
and CH out-of-plane bending fundamental vibrations. The experimental and
computational spectra show that, of all the hydrogen adjacency classes possible
on PAHs, solo and duo hydrogens consistently produce prominent bands at the
observed positions whereas quartet hydrogens do not. In all, this a study
supports the picture that astronomical PAHs are large with compact, regular
structures. From the coupling with primarily strong CH out-of-plane bending
modes one might surmise that the 5.25 and 5.7 m bands track the neutral
PAH population. However, theory suggests the role of charge in these
astronomical bands might also be important.Comment: Accepted ApJ, 40 pages in pre-print, 14 figures, two onlin
The Infrared Spectra of Very Large Irregular Polycyclic Aromatic Hydrocarbons (PAHs): Observational Probes of Astronomical PAH Geometry, Size and Charge
The mid-IR spectra of six large, irregular PAHs with formulae (C84H24 -
C120H36) have been computed using Density Functional Theory (DFT). Trends in
the dominant band positions and intensities are compared to those of large,
compact PAHs as a function of geometry, size and charge. Irregular edge
moieties that are common in terrestrial PAHs, such as bay regions and rings
with quartet hydrogens, are shown to be uncommon in astronomical PAHs. As for
all PAHs comprised solely of C and H reported to date, mid-IR emission from
irregular PAHs fails to produce a strong CCstr band at 6.2 um, the position
characteristic of the important, class A astronomical PAH spectra. Earlier
studies showed inclusion of nitrogen within a PAH shifts this to 6.2 um for PAH
cations. Here we show this band shifts to 6.3 um in nitrogenated PAH anions,
close to the position of the CC stretch in class B astronomical PAH spectra.
Thus nitrogenated PAHs may be important in all sources and the peak position of
the CC stretch near 6.2 um appears to directly reflect the PAH cation to anion
ratio. Large irregular PAHs exhibit features at 7.8 um but lack them near 8.6
um. Hence, the 7.7 um astronomical feature is produced by a mixture of small
and large PAHs while the 8.6 um band can only be produced by large compact
PAHs. As with the CCstr, the position and profile of these bands reflect the
PAH cation to anion ratio.Comment: accepted by Ap
The infrared spectra of very large, compact, highly symmetric, polycyclic aromatic hydrocarbons (PAHs)
The mid-infrared spectra of large PAHs ranging from C54H18 to C130H28 are
determined computationally using Density Functional Theory. Trends in the band
positions and intensities as a function of PAH size, charge and geometry are
discussed. Regarding the 3.3, 6.3 and 11.2 micron bands similar conclusions
hold as with small PAHs.
This does not hold for the other features. The larger PAH cations and anions
produce bands at 7.8 micron and, as PAH sizes increases, a band near 8.5 micron
becomes prominent and shifts slightly to the red. In addition, the average
anion peak falls slightly to the red of the average cation peak. The similarity
in behavior of the 7.8 and 8.6 micron bands with the astronomical observations
suggests that they arise from large, cationic and anionic PAHs, with the
specific peak position and profile reflecting the PAH cation to anion
concentration ratio and relative intensities of PAH size. Hence, the broad
astronomical 7.7 micron band is produced by a mixture of small and large PAH
cations and anions, with small and large PAHs contributing more to the 7.6 and
7.8 micron component respectively.
For the CH out-of-plane vibrations, the duo hydrogens couple with the solo
vibrations and produce bands that fall at wavelengths slightly different than
their counterparts in smaller PAHs. As a consequence, previously deduced PAH
structures are altered in favor of more compact and symmetric forms. In
addition, the overlap between the duo and trio bands may reproduce the
blue-shaded 12.8 micron profile.Comment: ApJ, 36 pages, 9 fig
The PAH hypothesis after 25 years
The infrared spectra of many galactic and extragalactic objects are dominated
by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 \mu m. The carriers of
these features remained a mystery for almost a decade, hence the bands were
dubbed the unidentified infrared (UIR) bands. Since the mid-80's, the UIR bands
are generally attributed to the IR fluorescence of Polycyclic Aromatic
Hydrocarbon molecules (PAHs) upon absorption of UV photons -- the PAH
hypothesis. Here we review the progress made over the past 25 years in
understanding the UIR bands and their carriers.Comment: 13 pages, 7 figures. To appear in the proceedings of IAU symposium
280 "The Molecular Universe
Properties of Polycyclic Aromatic Hydrocarbons in the Northwest Photon Dominated Region of NGC 7023. I. PAH Size, Charge, Composition, and Structure Distribution
Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer Infrared Spectrograph spectral map of the northwest photon dominated region (PDR) in NGC 7023 was analyzed exclusively using PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb). The 5-15 micron spectrum at each pixel is fitted using a non-negative-least-squares fitting approach. The fits are of good quality, allowing decomposition of the PAH emission into four subclasses: size, charge, composition, and hydrogen adjacency (structure). Maps tracing PAH subclass distributions across the region paint a coherent astrophysical picture. Once past some 20 seconds of arc from HD 200775, the emission is dominated by the more stable, large, symmetric, compact PAH cations with smaller, neutral PAHs taking over along the lines-of-sight toward the more distant molecular cloud. The boundary between the PDR and the denser cloud material shows up as a distinct discontinuity in the breakdown maps. Noteworthy is the requirement for PANH cations to fit the bulk of the 6.2 and 11.0 micron features and the indication of PAH photo-dehydrogenation and fragmentation close to HD 200775. Decomposition of the spectral maps into "principal" subclass template spectra provides additional insight into the behavior of each subclass. However, the general applicability of this computationally more efficient approach is presently undetermined. This is the first time the spectra of individual PAHs are exclusively used to fit the 5-15 micron region and analyze the spatial behavior of the aromatic infrared bands, providing fundamental, new information about astronomical PAH subpopulations including their dependence on, and response to, changes in local conditions
- …