Recent Progress in DIB Research: Survey of PAHS and DIBS

The spectra of several neutral and ionized PAHs isolated in the gas phase at low temperature have been measured in the laboratory under experimental conditions that mimic interstellar conditions and are compared with an extensive set of astronomical spectra of reddened, early type stars [1, 2]. The comparisons of astronomical and laboratory data provide upper limits for the abundances of specific neutral PAH molecules and ions along specific lines-of-sight. Something that is not attainable from infrared observations alone. We present the characteristics of the laboratory facility (COSmIC) that was developed for this study and discuss the findings resulting from the comparison of these unique laboratory data with high resolution, high S/N ratio astronomical observations. COSmIC combines a supersonic free jet expansion with discharge plasma and high-sensitivity cavity ringdown spectroscopy and provides experimental conditions that closely mimic the interstellar conditions. The column densities of the individual neutral PAH molecules and ions probed in these surveys are derived from the comparison of these unique laboratory data with high resolution, high S/N ratio astronomical observations. The comparisons of astronomical and laboratory data lead to clear and unambiguous conclusions regarding the expected abundances for PAHs of various sizes and charge states in the interstellar environments probed in the surveys. Band profile comparisons between laboratory and astronomical spectra lead to information regarding the molecular structures and characteristics associated with the DIB carriers in the corresponding lines-of-sight. These quantitative surveys of neutral and ionized PAHs in the optical range open the way for unambiguous quantitative searches of PAHs and complex organics in a variety of interstellar and circumstellar environments

A search for interstellar anthracene toward the Perseus anomalous microwave emission region

We report the discovery of a new broad interstellar (or circumstellar) band at 7088.8 +- 2.0 \AA coincident to within the measurement uncertainties with the strongest band of the anthracene cation (C$_{14}$H$_{10}$$^+$) as measured in gas-phase laboratory spectroscopy at low temperatures (Sukhorukov et al.2004). The band is detected in the line of sight of star Cernis 52, a likely member of the very young star cluster IC 348, and is probably associated with cold absorbing material in a intervening molecular cloud of the Perseus star forming region where various experiments have recently detected anomalous microwave emission. From the measured intensity and available oscillator strength we find a column density of N$_{an^+}$= 1.1(+-0.4) x 10$^{13}$ cm$^{-2}$ implying that ~0.008% of the carbon in the cloud could be in the form of C$_{14}$H$_{10}$$^+$. A similar abundance has been recently claimed for the naphthalene cation (Iglesias-Groth et al. 2008) in this cloud. This is the first location outside the Solar System where specific PAHs are identified. We report observations of interstellar lines of CH and CH$^+$ that support a rather high column density for these species and for molecular hydrogen. The strength ratio of the two prominent diffuse interstellar bands at 5780 and 5797 \AA suggests the presence of a ``zeta'' type cloud in the line of sight (consistent with steep far-UV extinction and high molecular content). The presence of PAH cations and other related hydrogenated carbon molecules which are likely to occur in this type of clouds reinforce the suggestion that electric dipole radiation from fast spinning PAHs is responsible of the anomalous microwave emission detected toward Perseus.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

The optical spectroscopy of extraterrestrial molecules

The ongoing quest to identify molecules in the interstellar medium by their electronic spectra in the visible region is reviewed. Identification of molecular absorption is described in the context of the elucidation of the carriers of the unidentified diffuse interstellar bands while molecular emission is discussed with reference to the unidentified Red Rectangle bands. The experimental techniques employed in undertaking studies on the optical spectroscopy of extraterrestrial molecules are described and critiqued in the context of their application.Comment: 21 pages, 9 figures, Invited review Australian Journal of Chemistry, accepted for publicatio

Plasma structure in a pulsed discharge environment

A pulsed slit discharge nozzle (PDN) has been developed in our laboratory to generate molecular ions in an astrophysically relevant environment. The free cold molecular ions are formed through soft (Penning) ionization of the neutral precursor molecules seeded in a supersonic free jet and are probed with cavity ringdown spectroscopy. An attempt is made to characterize the nature and the structure of the plasma that is generated in these experiments to optimize the yield of formation of ions in the jet. The experimental conditions are characterized by a strong pressure gradient in a short discharge zone. We find that the plasma generated in the PDN source is best characterized as an intense abnormal glow discharge and that its structure is reduced to a negative glow and to dark zones near the electrodes. We have calculated the parameters (length, thickness and cathode voltage fall) that are associated with the Crookes dark space and the negative glow in the plasma. We have also estimated the electron temperature (Te) and density (ne) in the plasma. All these parameters are required to optimize the yield of formation of ions and radicals in the jet expansions, a key requirement in our experiments

The Visible Absorption Spectrum of 12C2H2 III. The Region 14500-17000 cm-1

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JET-DISCHARGE CAVITY RINGDOWN SPECTROSCOPY OF SELECTED IONIZED POLYCYCLIC AROMATIC HYDROCARBONS

$^{a}$D. Romanini et al., Chem. Phys. Lett. 303 (1999) 165Author Institution: NASA-Ames Research Center; Space Science Division, NASA-Ames Research Center; Space Science Division, Los Gatos ResearchThe gas-phase spectroscopy of selected ionized polycyclic aromatic hydrocarbons (PAHs) is investigated in order to decisively test the contribution of this family of molecular ions to interstellar spectra. Following our preliminary report of the detection of the naphthalene ion in the gas $phase^{a}$ we present and discuss, here, the electronic absorption spectra of naphthalene $(C_{10}H^{+}_{8})$, phenanthrene $(C_{14}H^{+}_{10})$, and pyrene $(C_{16}H^{+}_{10})$ cations in the visible to NIR range. This study has been carried out using an ultrasensitive and versatile technique: Cavity Ring Down Spectroscopy (CRDS). The harsh physical conditions of the interstellar medium - low temperature and strong VUV radiation - have been simulated by coupling the pulsed-CRDS spectrometer to a supersonic slit jet and to an ionizing electronic discharge