Hack Sung Kim

Peripherally Hydrogenated Neutral Polycyclic Aromatic Hydrocarbons as Carriers of the 3 Micron Interstellar Infrared Emission Complex: Results from Single-Photon Infrared Emission Spectroscopy

Infrared emission spectra of five gas-phase UV laser-excited polycyclic aromatic hydrocarbons (PAHs) containing aliphatic hydrogens are compared with the main 3.3 microns and associated interstellar unidentified infrared emission bands (UIRs). We show that neutral PAHs can account for the majority of the 3 microns emission complex while making little contribution to the other UIR bands; peripherally hydrogenated PAHs produce a better match to astrophysical data than do those containing methyl side groups; 3.4 microns plateau emission is shown to be a general spectral feature of vibrationally excited PAHs containing aliphatic hydrogens, especially those containing methyl groups; and finally, hot-band and overtone emissions arising from aromatic C-H vibrations are not observed in laboratory emission spectra, and therefore, in contrast to current assignments, are not expected to be observed in the UIRs.

Single-Photon Infrared Emission Spectroscopy of Gaseous Polycyclic Aromatic Hydrocarbon Cations: A Direct Test for Proposed Carriers of the Unidentified Infrared Emission Bands

We describe the measurement of infrared emission from excited gas-phase polycyclic aromatic hydrocarbons (PAHs) cations by an electron-impact ion beam reflectron system coupled to our ‘‘ single-photon infrared emission ’’ (SPIRE) spectrometer. This experiment provides for direct comparison of laboratory infrared emission spectra of gaseous ionized PAHs with the ‘‘ unidentified infrared emission bands ’’ (UIRs), the origin of which is still debated. We present results for the pyrene cation (C16H10 þ ) and the dehydrogenated pyrene cations (C16H8 þ and C16H6 þ ) showing general agreement between the corresponding SPIRE bands and UIR features, although it is not a detailed match. We also discuss possible contributions from the pyrene dimer cation ðC16H10Þ2 þ . Subject headings: dust: extinction — infrared: ISM — ISM: lines and bands — methods: laboratory — molecular data — techniques: spectroscopic

Resonance raman spectroscopic study of alumina-supported vanadium oxide catalysts with 220 and 287 nm excitation.

We present detailed resonance Raman spectroscopic results excited at 220 and 287 nm for alumina-supported VO(x) catalysts. The anharmonic constant, harmonic wavenumber, anharmonic force constant, bond dissociation energy, and bond length change in the excited state for double bonded VO and single bonded V-O were obtained from fundamental and overtone frequencies. Totally symmetric and nontotally symmetric modes could be discerned and assigned on the basis of the overtone and combination progressions found in the resonance Raman spectra. Selective resonance enhancement of two different vibrational modes with two different excitation wavelengths was observed. This allowed us to establish a linear relationship between charge transfer energy and VO bond length and, consequently, to assign the higher-energy charge transfer band centered around 210-250 nm in the UV-vis spectra to the VO transition.

Role of Cr3+/Cr6+ redox in chromium-substituted Li2MnO3·LiNi1/2Mn1/2O2 layered composite cathodes: electrochemistry and voltage fade

The effect of redox-active Cr substitution on the electrochemistry and voltage fade of a lithium-rich “layered–layered” composite cathode material has been investigated. A series of Cr-substituted 0.5Li2MnO3·0.5LiNi1/2Mn1/2O2 powder samples (i.e., Li1.2Ni0.2−2/xMn0.6−2/xCrxO2, where x = 0, 0.05, 0.1, and 0.2) was synthesized via the sol–gel method. X-ray diffraction data confirmed the incorporation of Cr ions into the lattice structure. While similar initial charge capacities (∼300 mA h g−1) were obtained for all of the cathode samples, the capacity contribution from the Li2MnO3 activation plateau (at 4.5 V vs. Li) decreased with increasing Cr content. This finding suggests suppressed oxygen loss that triggers cation migration and voltage fade in subsequent cycles. Continued investigation revealed that the Cr substitution mitigates the voltage fade on charge but not discharge. The resulting insignificant effect of Cr substitution on mitigating voltage fade, in spite of decreased Li2MnO3 activation, is attributed to the additional instability caused by Cr6+ migration to a tetrahedral site, as evidenced by ex situ X-ray absorption spectroscopy. Our results provide the framework for a future redox active cation substitution strategy by highlighting the importance of the structural stability of the substituent itself.

An ion beam reflectron/single-photon infrared emission spectrometer for the study of gas-phase polycyclic aromatic hydrocarbon ions: Testing proposed carriers of the unidentified infrared emission bands

We describe the design and performance of an electron-impact ion beam reflectron system coupled to our “single-photon infrared emission spectrometer” for measurement of infrared emission from excited gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This experiment provides for direct comparison of laboratory infrared emission spectra of gaseous ionized PAHs with the “unidentified infrared emission bands,” the origin of which is still debated.

Infrared spectra of sodium and benzene in solid argon

Abstract An infrared matrix isolation study has been performed for sodium and benzene mixtures in solid argon at 9 K. In contrast with the earlier report, sodium was evidence to form complexes with benzene. Although the exact nature of the complexes is not certain, ab initio quantum mechanical calculation indicated clearly that the 1:1 NaC 6 H 6 complex should be as stable as the 1:1 LiC 6 H 6 complex. The tentatively assigned peak frequencies for the 1:1 NaC 6 H 6 complex were also inv fair agreement with the theoretical values.

Infrared Spectra of Benzene in Solid Argon

Abstract Highly isolated benzene molecules with near zero matrix shift have been prepared in the argon matrix at 9 K. By correlating the peak frequencies in the matrix state with those in other states, uncertainties could be resolved for the Fermi triad in the C-H stretching region of gas phase spectrum. The gas phase frequencies inferred from the matrix data were found in good agreement with those from the supersonic jet experiment. Based on the concentration and the temperature dependence, the peaks in the matrix spectra were classified in terms of monomeric, dimeric, and multimeric species.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.