592 research outputs found
Laboratory photo-chemistry of pyrene clusters: an efficient way to form large PAHs
In this work, we study the photodissociation processes of small PAH clusters
(e.g., pyrene clusters). The experiments are carried out using a quadrupole ion
trap in combination with time-of-flight (QIT-TOF) mass spectrometry. The
results show that pyrene clusters are converted into larger PAHs under the
influence of a strong radiation field. Specifically, pyrene dimer cations
(e.g., [CHCH] or CH), will
photo-dehydrogenate and photo-isomerize to fully aromatic cations (PAHs) (e.g.,
CH) with laser irradiation. The structure of new formed PAHs
and the dissociation energy for these reaction pathways are investigated with
quantum chemical calculations. These studies provide a novel efficient
evolution routes for the formation of large PAHs in the interstellar medium
(ISM) in a bottom-up process that will counteract the top-down conversion of
large PAHs into rings and chains, and provide a reservoir of large PAHs that
can be converted into C and other fullerenes and large carbon cages
Laboratory gas-phase infrared spectra of two astronomically relevant PAH cations: diindenoperylene, CH and dicoronylene, CH
The first gas-phase infrared spectra of two isolated astronomically relevant
and large PAH cations - diindenoperylene (DIP) and dicoronylene (DC) - in the
5301800 cm (18.95.6 m) range - are presented. Vibrational
band positions are determined for comparison to the aromatic infrared bands
(AIBs). The spectra are obtained via infrared multiphoton dissociation (IRMPD)
spectroscopy of ions stored in a quadrupole ion trap (QIT) using the intense
and tunable radiation of the free electron laser for infrared experiments
(FELIX). DIP shows its main absorption peaks at 737 (13.57), 800 (12.50),
1001 (9.99), 1070 (9.35), 1115 (8.97), 1152 (8.68), 1278 (7.83), 1420 (7.04)
and 1550 (6.45) cm(m), in good agreement with DFT calculations that
are uniformly scaled to take anharmonicities into account. DC has its main
absorption peaks at 853 (11.72), 876 (11.42), 1032 (9.69), 1168 (8.56), 1300
(7.69), 1427 (7.01) and 1566 (6.39) cm(m), that also agree well
with the scaled DFT results presented here.
The DIP and DC spectra are compared with the prominent infrared
features observed towards NGC 7023. This results both in matches and clear
deviations. Moreover, in the 11.014.0 m region, specific bands can be
linked to CH out-of-plane (oop) bending modes of different CH edge structures
in large PAHs. The molecular origin of these findings and their astronomical
relevance are discussed
Laboratory photo-chemistry of covalently bonded fluorene clusters: observation of an interesting PAH bowl-forming mechanism
The fullerene C, one of the largest molecules identified in the
interstellar medium (ISM), has been proposed to form top-down through the
photo-chemical processing of large (more than 60 C-atoms) polycyclic aromatic
hydrocarbon (PAH) molecules. In this article, we focus on the opposite process,
investigating the possibility that fullerenes form from small PAHs, in which
bowl-forming plays a central role. We combine laboratory experiments and
quantum chemical calculations to study the formation of larger PAHs from
charged fluorene clusters. The experiments show that with visible laser
irradiation, the fluorene dimer cation -
[CHCH] - and the fluorene trimer cation -
[CHCHCH] - undergo
photo-dehydrogenation and photo-isomerization resulting in bowl structured
aromatic cluster-ions, CH and CH,
respectively. To study the details of this chemical process, we employ quantum
chemistry that allows us to determine the structures of the newly formed
cluster-ions, to calculate the hydrogen loss dissociation energies, and to
derive the underlying reaction pathways. These results demonstrate that smaller
PAH clusters (with less than 60 C-atoms) can convert to larger bowled
geometries that might act as building blocks for fullerenes, as the
bowl-forming mechanism greatly facilitates the conversion from dehydrogenated
PAHs to cages. Moreover, the bowl-forming induces a permanent dipole moment
that - in principle - allows to search for such species using radio astronomy.Comment: 8 pages, 7 figures, accepte
Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation
Interstellar polycyclic aromatic hydrocarbons (PAHs) are expected to be
strongly processed by vacuum ultraviolet photons. Here, we report experimental
studies on the ionization and fragmentation of coronene (C24H12), ovalene
(C32H14) and hexa-peri-hexabenzocoronene (HBC; C42H18) cations by exposure to
synchrotron radiation in the range of 8--40 eV. The results show that for small
PAH cations such as coronene, fragmentation (H-loss) is more important than
ionization. However, as the size increases, ionization becomes more and more
important and for the HBC cation, ionization dominates. These results are
discussed and it is concluded that, for large PAHs, fragmentation only becomes
important when the photon energy has reached the highest ionization potential
accessible. This implies that PAHs are even more photo-stable than previously
thought. The implications of this experimental study for the photo-chemical
evolution of PAHs in the interstellar medium are briefly discussed
Laboratory formation and photo-chemistry of fullerene/anthracene cluster cations
Besides buckminsterfullerene (C60), other fullerenes and their derivatives
may also reside in space. In this work, we study the formation and
photo-dissociation processes of astronomically relevant fullerene/anthracene
(C14H10) cluster cations in the gas phase. Experiments are carried out using a
quadrupole ion trap (QIT) in combination with time-of-flight (TOF) mass
spectrometry. The results show that fullerene (C60, and C70)/anthracene (i.e.,
[(C14H10)nC60]+ and [(C14H10)nC70]+), fullerene (C56 and C58)/anthracene (i.e.,
[(C14H10)nC56]+ and [(C14H10)nC58]+) and fullerene (C66 and C68)/anthracene
(i.e., [(C14H10)nC66]+ and [(C14H10)nC68]+) cluster cations, are formed in the
gas phase through an ion-molecule reaction pathway. With irradiation, all the
fullerene/anthracene cluster cations dissociate into monoanthracene and
fullerene species without dehydrogenation. The structure of newly formed
fullerene/anthracene cluster cations and the bonding energy for these reaction
pathways are investigated with quantum chemistry calculations.
Our results provide a growth route towards large fullerene derivatives in a
bottom-up process and insight in their photo-evolution behavior in the ISM, and
clearly, when conditions are favorable, fullerene/PAH clusters can form
efficiently. In addition, these clusters (from 80 to 154 atoms or ~ 2 nm in
size) offer a good model for understanding the physical-chemical processes
involved in the formation and evolution of carbon dust grains in space, and
provide candidates of interest for the DIBs that could motivate spectroscopic
studies.Comment: 10 pages, 8 figures, accepte
The X-ray reflector in NGC 4945: a time and space resolved portrait
We present a time, spectral and imaging analysis of the X-ray reflector in
NGC 4945, which reveals its geometrical and physical structure with
unprecedented detail. NGC 4945 hosts one of the brightest AGN in the sky above
10 keV, but it is only visible through its reflected/scattered emission below
10 keV, due to absorption by a column density of ~4\times10^24 cm-2. A new
Suzaku campaign of 5 observations spanning ~6 months, together with past
XMM-Newton and Chandra observations, show a remarkable constancy (within <10%)
of the reflected component. Instead, Swift-BAT reveals strong intrinsic
variability on time scales longer than one year. Modeling the circumnuclear gas
as a thin cylinder with the axis on the plane of the sky, we show that the
reflector is at a distance >30-50 pc, well within the imaging capabilities of
Chandra at the distance of NGC 4945 (1"~18 pc). Accordingly, the Chandra
imaging reveals a resolved, flattened, ~150 pc-long clumpy structure, whose
spectrum is fully due to cold reflection of the primary AGN emission. The
clumpiness may explain the small covering factor derived from the spectral and
variability properties.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in MNRA
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