Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E26: The Chemical Physics of Molecules in Space IIFocus Live
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Sponsoring Units: DCP Chair: Sergio Ioppolo; Queen Mary Univ London Ryan Fortenberry, Univ. of Mississippi |
Tuesday, March 16, 2021 8:00AM - 8:12AM Live |
E26.00001: Probing the Dissociation of Interstellar Polycyclic Aromatic Hydrocarbons Jordy Bouwman The presence of interstellar polycyclic aromatic hydrocarbons (PAHs) is inferred from their characteristic mid-infrared (IR) emission bands that are observed towards a large number of galactic and extragalactic sources. It has been derived that PAHs constitute up to 15% of the total cosmic carbon budget and thus play an important role in cosmochemical cycle of matter. Energetic processing of interstellar PAHs occurs in photon dominated regions and may result in ionization and/or dissociation of the aromatic species. Observational and laboratory data suggest that dissociation of large interstellar polyaromatics eventually leads to fullerene formation. Our group characterizes PAH dissociation processes by applying state-of-the-art experimental physicochemical techniques and combining the results with quantum chemical computations. Using this approach, we obtain insight into the dissociation mechanisms at the molecular level of detail. In this talk I will review our recent results and will emphasize the challenges that lie ahead of us. |
Tuesday, March 16, 2021 8:12AM - 8:24AM Live |
E26.00002: Photoionization Dynamics of the C60 fullerene in an astrophysical context Helgi Hrodmarsson, Gustavo Garcia, Harold Linnartz, Laurent Nahon The cosmic omnipresence of C60 serves as testament to the fact that large complex molecules can exist in space. Now with four of the Diffuse Interstellar Bands (DIBs) having been assigned to transitions in its cationic counterpart, C60+, the need to fully comprehend the photoionization dynamics of C60 is evident. We inspected the photoionization dynamics of the C60 buckminsterfullerene with VUV synchrotron radiation using a double-imaging photoelectron photoion coincidence spectrometer. We present the data as a 2D photoelectron matrix which contains a wealth of spectroscopic data which relates to (i) the threshold photoelectron spectrum (TPES) which is compared to data relevant to the DIBs, (ii) the kinetic photoelectron distribution at the Lyman-α line which is relevant to the dominant heating source in the ISM, and (iii) the absolute photoionization cross section of C60 up to approx. 10.5 eV. The TPES implies that the symmetry of the ground state is different than previous theoretical models predict. |
Tuesday, March 16, 2021 8:24AM - 8:36AM Live |
E26.00003: Quantized translational motion of 3He and 4He atoms in He@C60 endofullerene. Tanzeeha Jafari, Anna Shugai, Urmas Nagel, Toomas Room, George Bacanu, Gabriela Hoffman, Malcolm H Levitt, Richard Whitby We studied the translational motion of He atom trapped inside the molecular cage of C60 molecule using infrared (IR) spectroscopy. The T dependence of the IR absorption spectra of 3He@C60 and 4He@C60 powder samples was measured between 5 and 300K. At the lowest T a single absorption line was observed at 81 cm-1 (97 cm-1 for 3He), but as the T increased four new lines appeared above the first transition. We fitted the IR spectra with an anharmonic spherical oscillator model and assumed a rigid C60 cage. The line frequencies and intensities were reproduced with the harmonic (V2R2) and anharmonic (V4R4) potential and the induced dipole moment (A1R) terms, where R is the displacement of the He atom from the C60 cage center. The fit gave equal, within the error bars, potential and dipole parameters for 3He@C60 and 4He@C60. The translational energies will be compared to the C60 encapsulated translational energies of H2, HD, D2, HF, and H2O. Our results are the test bed of theories describing the interaction of inert gases with carbon surfaces. |
Tuesday, March 16, 2021 8:36AM - 9:12AM Live |
E26.00004: New Frontiers in Cosmic Carbon Invited Speaker: Brett McGuire Since the mid 1980s, ubiquitous infrared emission in our galaxy and others has led to the conclusion that as much as 25% of interstellar carbon is sequestered in large aromatic species. If true, the physics and chemistry along the cycle of star- and planet-formation may be dominated by these molecules, as they would serve not only as reservoirs of reactive carbon, but as charge balance carriers for entire molecular clouds, catalytic surfaces of molecular hydrogen formation, and as the seeds of interstellar dust. In this talk, I will discuss recent advances in our studies of aromatic chemistry, enabled by novel applications of Bayesian analysis and matched filtering. I will present the first detections of individual large aromatic species in space, confirming a 30+ year hypothesis, describe its impact on our current understanding of the evolution of cosmic carbon, and look forward at where this new window on aromatic chemistry can take us. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E26.00005: Examining Clumpy Cyanide Chemistry in the Inner Envelope of IRC+10216 Mark A. Siebert, Anthony Remijan In recent years, many questions have arisen regarding gas phase chemistry of CN-bearing molecules in the carbon-rich winds of evolved stars. To address them, there is a need for high angular resolution interferometric work covering many emission lines which trace different regions of these objects. We used archival band 7 ALMA observations to image high-J transitions of several cyanide species in the inner envelope (<8”) of the carbon star IRC+10216. The emission maps of HC3N, CH3CN, and NaCN show compact morphologies comprising various arcs and loops, with significant enhancement in dense clumps at an angular distance of ~3” (350 AU) from the star. Considering the proposed formation mechanisms of these molecules, our results are consistent with the theory that anisotropic photochemistry is occurring in warm (~200 K) knots present in the inner regions of this circumstellar envelope. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E26.00006: The First Mid-Infrared Detections of HNC and H13CN in the Interstellar Medium Sarah Nickerson, Naseem Rangwala, Sean Colgan, Curtis DeWitt, Xinchuan Huang, Kinsuk Acharyya, Maria Drozdovskaya, Ryan Fortenberry, Eric Herbst, Timothy J Lee We present the first mid-infrared (MIR) detections of HNC and H13CN in the interstellar medium, and numerous HCN transitions. Our observations span 12.8 to 22.9 µm towards the hot core Orion IRc2, obtained with the Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA/EXES). 5 km/s resolution distinguishes individual rovibrational transitions of the three molecules, allowing direct measurement of their excitation temperatures, column densities, and relative abundances. HNC and H13CN share temperatures of 100 K with a local standard of rest velocity of -7 km/s. HCN shows two velocity components at -7 km/s at 165 K, and 1 km/s at 309 K. |
Tuesday, March 16, 2021 9:36AM - 10:12AM Live |
E26.00007: The Fate of Carbon in the Interstellar Medium: Experiment, Theory and Observations Invited Speaker: Lucy M Ziurys Carbon is the fourth most abundant element in the Galaxy. It is created in stars, and enters the |
Tuesday, March 16, 2021 10:12AM - 10:24AM Live |
E26.00008: Exploring molecular formation and growth upon ionization of van der Waals clusters using ab-initio molecular dynamics Tamar Stein, Partha Bera, Timothy J Lee, Martin P Head-Gordon Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM)1. The mechanism for their formation in the ISM's low-temperature environment is, as of yet, nonetheless a mystery. Understanding the growth mechanism of complex molecules such as PAHs and nitrogen-based PAHs (PAHN) is a long-standing challenge that has been drawing much attention for several decades. |
Tuesday, March 16, 2021 10:24AM - 10:36AM Live |
E26.00009: Multidisciplinary investigation of benzene (C6H6) condensation in Titan’s South Pole David Dubois, Laura Iraci, Erika Barth, Farid Salama, Sandrine Vinatier, Ella Sciamma-O'Brien Following the northern spring equinox in August 2009, Titan’s global atmospheric circulation reversed within the next two years. This event increased the mixing ratios of benzene (C6H6) at the South pole. Simultaneously, a strong cooling with temperatures dropping below 120 K favored the condensation of hydrocarbon molecules at unusually high altitudes (>250 km). Cassini detected for the first time an IR spectral signature consistent with the presence of C6H6 ice in the South Pole at these high altitudes. Current laboratory data, however, is insufficient to allow models to reproduce the formation of this high-altitude cloud system. |
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