Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S32: Chemical Physics of Extreme Environments IFocus
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Sponsoring Units: DCP Chair: Arthur Suits, University of Missouri Room: 332 |
Thursday, March 17, 2016 11:15AM - 11:51AM |
S32.00001: Reaction Product Identification in Extreme Chemical Environments by Broadband Rotational Spectroscopy Invited Speaker: Brooks Pate Molecular rotational spectroscopy has several advantages for detection of reaction intermediates and products under extreme laboratory conditions. Rotational spectroscopy has high sensitivity to the molecular structure and provides high spectral resolution in low pressure environments. Furthermore, quantum chemistry provides accurate estimates of the spectroscopic parameters. As a result, rotational spectroscopy can identify molecular species in complex reaction mixtures without the need for chromatographic separation and without the need for a previously recorded ``library spectrum'' of the molecule. The application of chirped pulse Fourier transform rotational spectroscopy methods for the identification of molecules of astrochemical interest formed in pulsed discharge sources will be described including recent advances for high-throughput mm-wave spectroscopy. The set of reaction products created in the experiment can provide insight into the reaction mechanism. Reactions involving the CN radical will be discussed. These reactions can be barrierless making them candidates for interstellar gas reactions. The possibility that interstellar cyanomethanimine is produced by gas phase radical-neutral reactions instead of surface chemistry on grain-supported ices will be discussed using recent spatially resolved chemical images in Sagittarius B2 observed with the Jansky Very Large Array. [Preview Abstract] |
Thursday, March 17, 2016 11:51AM - 12:03PM |
S32.00002: Observation of $K$-dependent Reaction Rates in Pulsed Uniform Supersonic Flows by Chirped-Pulse Microwave Spectroscopy Bernadette Broderick, Nuwandi Ariyasingha, Arthur Suits Chirped-pulse Fourier-transform microwave spectroscopy was used to interrogate the reaction of Cl atoms with propyne in a pulsed uniform supersonic flow. The technique, termed ``CPUF'', utilizes broad-band microwave spectroscopy to extract structural information with MHz resolution and near universal detection, in conjunction with a Laval flow system, which offers thermalized conditions at low temperatures and high number densities. Previous studies have exploited this approach to obtain multichannel product branching fractions in a number of polyatomic systems, with isomer and often vibrational level specificity. This report highlights an additional capability of the CPUF technique: here, the state-specific reactant depletion is directly monitored on a microsecond timescale. In doing so, a clear dependence on the rotational quantum number $K$ in the rate of the reaction between Cl atoms and propyne is revealed. Future prospects for the technique will be discussed. [Preview Abstract] |
Thursday, March 17, 2016 12:03PM - 12:39PM |
S32.00003: Rotational Spectroscopic Studies and Observational Searches for HO$_3$ Invited Speaker: Susanna Widicus Weaver Interstellar chemistry is largely driven by reactions of unstable molecules that serve as reaction intermediates in terrestrial chemistry. One such class of compounds are weakly-bound clusters. These clusters could form in interstellar environments through radiative association reactions, but their identification and characterization in interstellar environments is limited by a lack of rotational spectral information. One such species is HO$_3$, which could be formed in the interstellar medium from O$_2$ and OH. HO$_3$ has been studied extensively in the infrared, and there are a few microwave spectral studies that have also been reported. However, no millimeter or submillimeter spectral information is available to guide astronomical observations. In this talk, we will present the laboratory characterization of $trans$-HO$_3$ and $trans$-DO$_3$ from 70 to 450 GHz using our newly developed fast sweeping technique. The molecular constants have been significantly refined, and additional higher order centrifugal distortion constants have been determined. We will also present an initial observational search for HO$_3$ in 32 star forming regions. Although no HO$_3$ lines have been detected thus far, strict upper limits can be placed on the HO$_3$ column density in these sources based on this analysis. [Preview Abstract] |
Thursday, March 17, 2016 12:39PM - 1:15PM |
S32.00004: \textbf{The Extreme Chemical Environments Associated with Dying Stars} Invited Speaker: Lucy Ziurys Mass loss from dying stars is the main avenue by which material enters the interstellar medium, and eventually forms solar systems and planets. When stars consume all the hydrogen burning in their core, they start to burn helium, first in their centers, and then in a surrounding shell. During these phases, the so-called ``giant branches,'' large instabilities are created, and stars begin to shed their outer atmospheres, producing so-called circumstellar envelopes. Molecules form readily in these envelopes, in part by LTE chemistry at the base of the stellar photosphere, and also by radical reactions in the outer regions. Eventually most stars shed almost all their mass, creating ``planetary nebulae,'' which consist of a hot, ultraviolet-emitting white dwarf surrounded by the remnant stellar material. The environs in such nebulae are not conducive to chemical synthesis; yet molecular gas exits. The ejecta from these nebulae then flows into the interstellar medium, becoming the starting material for diffuse clouds, which subsequently collapse into dense clouds and then stars. This molecular ``life cycle'' is repeated many times in the course of the evolution of our Galaxy. We have been investigating the interstellar molecular life cycle, in particular the chemical environments of circumstellar shells and planetary nebulae, through both observational and laboratory studies. Using the facilities of the Arizona Radio Observatory (ARO), we have conducted broad-band spectral-line surveys to characterize the contrasting chemical and physical properties of carbon (IRC$+$10216) vs. oxygen-rich envelopes (VY CMa and NML Cyg). The carbon-rich types are clearly more complex in terms of numbers of chemical compounds, but the O-rich variety appear to have more energetic, shocked material. We have also been conducting surveys of polyatomic molecules towards planetary nebulae. Species such as HCN, HCO$^{\mathrm{+}}$, HNC, CCH, and H$_{\mathrm{2}}$CO appear to be common constituents of these objects, and their abundances do not appear to vary with age. These results contradict the predictions of all chemical models. We have also been using millimeter-wave and Fourier transform microwave methods to measure rotational spectra of potential new interstellar molecules to complete the chemical inventories. The current results of these studies will be presented. [Preview Abstract] |
Thursday, March 17, 2016 1:15PM - 1:51PM |
S32.00005: Complex Mixture Analysis Using Rotational Spectroscopy Invited Speaker: Michael McCarthy Owing to its very high intrinsic resolution, exceeding ppm levels in supersonic jet sources, rotational spectroscopy is a powerful analytical tool to analyze complex mixtures that consist of both familiar and exotic molecules. We present here an experimental method to rapidly sort rotational lines in broadband spectra and assign them to individual chemical compounds in the cm-band. This method combines a chirped-pulse FT microwave (CP-FTMW) spectrometer with follow-up analysis using an automated cavity FTMW spectrometer with double resonance (DR) capabilities. The CP-FTMW spectrum acts as a filter, identifying only those regions of frequency space that contain molecular signal, and discarding the vast majority of frequency space that is devoid of molecular information. With superior sensitivity and resolution per unit time, a cavity spectrometer is then used for follow-up assays on these bright spectral lines, to group transitions which share common characteristics, such as elemental composition, etc. These groups can be further partitioned into smaller sub-groups by exhaustive DR experiments whereby only those rotational lines that share a common energy level from the same molecule are linked together. From these series of measurements and assays, rotational transitions of multiple, individual chemical compounds can be empirically sorted and identified, without the need for any theoretical guidance or input. Significant automation greatly enhances the overall efficiency, enabling rapid, exhaustive testing with little oversight. Examples illustrating the power of this methodology for rapid analysis of broadband spectra will be presented. [Preview Abstract] |
Thursday, March 17, 2016 1:51PM - 2:03PM |
S32.00006: Stretching molecules under extreme tensile strain: density functional theory versus multireference methods. Gary Kedziora, Stephen Barr, Rajiv Berry, James Moller, Timothy Breitzman A more refined understanding of how molecules behave under extreme tensile strain is desirable for modeling fracture initiation in polymers and other mecho-chemical studies. We investigated several quantum mechanical methods for use in multiscale models of highly strained polymers where bond breaking occurs. A small set of molecules and a protocol for stretching them were used as model test systems. The results from these tests using several functionals were compared with complete active space self-consistent field results. These test systems provide unique challenges for quantum mechanical models. Quantum mechanics is required for accurate bond breaking prediction because the results are dependent on the conformation and secondary electronic structure effects such as hyperconjugation. GGA methods with unrestricted solutions to the Kohn-Sham equations provide adequate results for our purposes even though there are some minor flaws based on the spin symmetry breaking. [Preview Abstract] |
Thursday, March 17, 2016 2:03PM - 2:15PM |
S32.00007: Mixed quantum/semiclassical studies of condensed-phase dynamics and spectroscopy Jeffrey A Cina, Philip A Kovac We report on theoretical and computational studies of molecular-level chemical dynamics and their time-resolved spectroscopic signatures for small molecules embedded in low-temperature crystalline-host environments. Our calculations are based on a mixed quantum mechanical/semiclassical theory, referred to as the variational fixed vibrational basis/Gaussian bath theory (v-FVB/GB), in which certain optically addressed coordinates driven to large-amplitude motion by laser pulses are treated fully quantum mechanically and a larger number of others executing small-amplitude motion are treated semiclassically. Model systems under investigation incorporate a dihalogen molecule isolated in a symmetrical cluster of rare-gas atoms, with the outer layer of host atoms bound together in a harmonic net that preserves the initial equilibrium structure, but emulates an extended medium by preventing dynamical reconstruction and host-atom evaporation. [Preview Abstract] |
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