The optical spectrum of a large isolated polycyclic aromatic hydrocarbon: hexa-peri-hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 Angstrom range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 Angstrom is estimated to have an oscillator strength of f=1.4x10^-3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4x10^12 cm^-2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.Comment: 9 pages, 1 figure. Fixed a typo on the frequency of the 'b' ban

Spectroscopic parameters for silacyclopropynylidene, SiC2_2, from extensive astronomical observations toward CW Leo (IRC +10216) with the Herschel satellite

A molecular line survey has been carried out toward the carbon-rich asymptotic giant branch star CW Leo employing the HIFI instrument on board of the Herschel satellite. Numerous features from 480 GHz to beyond 1100 GHz could be assigned unambiguously to the fairly floppy SiC$_2$ molecule. However, predictions from laboratory data exhibited large deviations from the observed frequencies even after some lower frequency data from this survey were incorporated into a fit. Therefore, we present a combined fit of all available laboratory data together with data from radio-astronomical observations.Comment: 7 pages, 1 figure, J. Mol. Spectrosc., appeared; CDMS links corrected (version 2; current version: 3; may be updated later this year

Theoretical study of the electronic spectra of small molecules that incorporate analogues of the copper-cysteine bond

The copper-sulphur bond which binds cysteinate to the metal centre is a key factor in the spectroscopy of blue copper proteins. We present theoretical calculations describing the electronically excited states of small molecules, including CuSH, CuSCH_3, (CH_3)_2SCuSH, (imidazole)-CuSH and (imidazole)_2-CuSH, derived from the active site of blue copper proteins that contain the copper-sulphur bond in order to identify small molecular systems that have electronic structure that is analogous to the active site of the proteins. Both neutral and cationic forms are studied, since these represent the reduced and oxidised forms of the protein, respectively. For CuSH and CuSH^+, excitation energies from time-dependent density functional theory with the B97-1 exchange-correlation functional agree well with the available experimental data and multireference configuration interaction calculations. For the positive ions, the singly occupied molecular orbital is formed from an antibonding combination of a 3d orbital on copper and a 3pπ orbital on sulphur, which is analogous to the protein. This leads several of the molecules to have qualitatively similar electronic spectra to the proteins. For the neutral molecules, changes in the nature of the low lying virtual orbitals leads the predicted electronic spectra to vary substantially between the different molecules. In particular, addition of a ligand bonded directly to copper results in the low-lying excited states observed in CuSH and CuSCH_33 to be absent or shifted to higher energies

Search for corannulene (C20H10) in the Red Rectangle

Polycyclic Aromatic Hydrocarbons (PAHs) are widely accepted as the carriers of the Aromatic Infrared Bands (AIBs), but an unambiguous identification of any specific interstellar PAH is still missing. For polar PAHs, pure rotational transitions can be used as fingerprints for identification. Combining dedicated experiments, detailed simulations and observations, we explored the mm domain to search for specific rotational transitions of corannulene (C20H10). We performed high-resolution spectroscopic measurements and a simulation of the emission spectrum of UV-excited C20H10 in the environment of the Red Rectangle, calculating its synthetic rotational spectrum. Based on these results, we conducted a first observational campaign at the IRAM 30m telescope towards this source to search for several high-J rotational transitions of (C20H10). The laboratory detection of the J = 112 <- 111 transition of corannulene showed that no centrifugal splitting is present up to this line. Observations with the IRAM 30m telescope towards the Red Rectangle do not show any corannulene emission at any of the observed frequencies, down to a rms noise level of Tmb = 8 mK for the J =135 -> 134 transition at 137.615 GHz. Comparing the noise level with the synthetic spectrum, we are able to estimate an upper limit to the fraction of carbon locked in corannulene of about 1.0x10(-5) relative to the total abundance of carbon in PAHs. The sensitivity achieved shows that radio spectroscopy can be a powerful tool to search for polar PAHs. We compare this upper limit with models for the PAH size distribution, emphasising that small PAHs are much less abundant than predicted. We show that this cannot be explained by destruction but is more likely related to the chemistry of their formation in the environment of the Red Rectangle.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA

a New Approach to Investigate PAH Derived Cations as DIB Carriers

International audiencePolycyclic aromatic hydrocarbons (PAHs) and their cations are considered as attractive candidates for the Diffuse Interstellar Bands, some discrete absorption features observed throughout the visible and near-infrared spectral range whose origin is still investigated. So far, not a single PAH species has been identified on the basis of a spectral agreement. This indicates either that the molecular diversity is very large or that the candidates that have been considered are not the correct ones. In particular, small/medium-sized PAHs are submitted to photodissociation under UV photons from stars. It is therefore of interest to characterize the spectroscopy of key breakdown products. These spectroscopic studies should be performed in conditions that mimic those found in interstellar space, that leads to additional experimental difficulties. We will describe the approach we are developing with the PIRENEA set-up and present results on 1-methylpyrene cation and derived species. Experimental measurements are guided by calculations based on density functional theory and its time-dependent implementation. F. Useli-Bacchitta, A. Bonnamy, G. Malloci, G. Mulas, D. Toublanc, and C. Joblin, Chem. Phys. 371 (2010), 16-23. Joint ANR project GASPARIM, ANR-10-BLAN-50

INTEGRAL FIELD SPECTROSCOPY OF THE RED RECTANGLE: UNRAVELING THE CARRIER OF THE RRBs IN 2D

Author Institution: IRAP; Universite de Toulouse, UPS; CNRS; 9 Av. Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France; Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia; Department of Chemical System Engineering, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan; School of Chemistry, The University of Sydney, New South Wales, 2006, AustraliaFollowing the initial detection of the C$_2$ Swan origin bands in the Red Rectangle, an unusual biconical protoplanetary nebula, a concerted effort has been undertaken in understanding this key astrophysical molecule from both the theoretical and experimental standpoint. In this talk we will present integral field observations on the Red Rectangle paying particular attention to the Swan Bands and the 5800{\AA} Red Rectangle band (RRB), an unassigned molecular emission feature. Integral field astronomy allows us to spectrally map the nebula, tracing the RRBs and C$_2$, and how they relate to one another across the object. By analysis of the C$_2$ Swan bands of the RR, and assuming that the RRBs arise from photoexcitation and emission, we relate the abundance of C$_2$ to the product of the oscillator strength and the column density of the RRB carrier

THE OPTICAL SPECTRUM OF THE SILICON TERMINATED CARBON CHAINS SiCnH

Author Institution: IRAP; Universite de Toulouse, UPS; CNRS; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France and Harvard-Smithsonian Center for Astrophysics; Cambridge, MA 02138, and School of Engineering & Applied Sciences; Harvard University, 29 Oxford St., Cambridge, MA 02138; Harvard-Smithsonian Center for Astrophysics; Cambridge, MA 02138, and School of Engineering & Applied Sciences; Harvard University, 29 Oxford St., Cambridge, MA 02138; Department of Chemistry, Virginia Tech, Blacksburg, VA 24061The gas phase optical spectra of the silicon terminated carbon chains, SiC$_n$H (n=3-5) formed in a silane acetylene discharge, have been investigated by R2C2PI and LIF/DF and will be reported here for the first time. Complementary to the experimental work, a theoretical investigation was undertaken with coupled cluster methods to garner a comprehensive understanding of the molecular structures and electronic properties of these systems. For the linear chains where there is an odd number of carbon atoms (SiC$_3$H and SiC$_5$H), the observed transitions are primarily from a $^2\Pi$ ground state to a $^2\Sigma$ state, but as in the case of isovalent carbon chains there are some Herzberg-Teller active modes from an excited $^2\Pi$ state. While a strong $\Pi-\Pi$ transition is predicted for SiC$_4$H, the spectrum is dominated by relatively dark sigma state which is vibronically coupled to the bright $^2\Pi$ state. In contrast to the odd carbon chains, which exhibit relatively sharp spectral features and lifetimes in the 10-100\,ns regime, SiC$_4$H shows broadened spectral features consistent with a ca. 10\,ps lifetime, and a subsequent long-lived decay ($>30$\,microseconds) which we tentatively interpret in terms of mixing with a nearby quartet state arising from the same electronic configuration, a process unavailable for the odd chains

UNRAVELING THE COMPLEX NEAR-UV SPECTRUM OF Si2_2C

Author Institution: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA; 02138, and School of Engineering & Applied Sciences, Harvard; University, 29 Oxford St., Cambridge, MA 02138; IRAP; Universite de Toulouse, UPS; CNRS; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4; France; Department of Chemistry, Virginia Tech, Blacksburg, VA 24061Though it has yet to be detected in space, there is good reason to believe that the disilicon carbide molecule, Si$_2$C, is an abundant constituent of circumstellar shells such as IRC+10216, in which the isovalent ionic ring SiC$_2$ is highly conspicuous. Si$_2$C exhibits a surprisingly complicated R2C2PI spectrum, which EOM-CCSD calculations indicate arises from a bent-to-linear transition involving several interacting electronic states in the region of vertical excitation. Some of the observed spectral features show well-resolved emission spectra when probed by LIF/DF, yielding vibrational frequencies in excellent agreement with \emph{ab initio} and matrix IR studies of the ground electronic state. Partially resolved rotational structure for these bands, which appear to be dominated by a vibronically allowed transition to a dark $A_2$ state, suggests a ground state bond angle significantly larger than predicted by theory. Owing to the presence of overlapping, unresolved bands, in addition to spectral contamination from Si$_2$ and SiC, further experimental characterization would appear ideally suited to a two-dimensional fluorescence approach