Showing papers by "Walter W. Duley published in 2000"

Detection of New Infrared Spectral Features in Hydrogenated Amorphous Carbon

Abstract: We have obtained high-resolution (1 cm-1) spectra of hydrogenated amorphous carbon (HAC) in both emission and absorption. Deconvolution of these spectra into Gaussian-Lorentzian components reveal two new features at 3098 cm-1 (3.23 μm) and 2890 cm-1 (3.46 μm) attributable to nonaromatic CCH2 and tertiary CH groups, respectively. In addition, relatively sharp lines (Δν ~ 25 cm-1), which we assign to CH2 groups in cyclohydrocarbon molecules such as hexahydropyrene, appear on heating HAC to 800 K. We propose that these molecules are the precursors of Hn polycyclic aromatic hydrocarbons seen in low-excitation protoplanetary nebulae. The profile of the 2890 cm-1 (3.46 μm) tertiary CH feature detected as a component of HAC is very similar to that observed in dark clouds where it has been attributed to interstellar diamond-like material. An absorption/emission feature at 3098 cm-1 (3.23 μm) should also be present in "transition" sources, i.e., regions where the composition of the carbonaceous gas/dust component is evolving from a saturated, CH2, CH3-rich material to less saturated aromatic material. The role of H atom reaction and UV irradiation in determining spectra in the 2.8-3.7 μm region is briefly discussed. On the basis of our laboratory data we find that the absence of CH2, CH3 absorption in dark clouds can be understood if H atom reactions with HAC are inhibited as a result of an increased rate of gas-phase reactions.

Normal Coordinate Analysis and Vibrational Spectra of Aromatic Carbon Nanostructures

Abstract: Theoretical infrared absorption spectra of aromatic ring molecules having up to 102 carbon atoms and with various edge hydrogenations have been obtained using a classic mechanical model and a simplified valence force field. Force constants have been adapted from those available for smaller molecules. Spectral line intensities are calculated in a double harmonic approximation with effective atomic charges obtained using a Huckel tight-binding Hamiltonian. Vibrational modes of clusters of like aromatic ring molecules are obtained through introduction of an interlayer force constant. These modes are predicted to occur in the wavelength range between 80 and 400 μm. We explore the effect of dehydrogenation on these spectra as well as hydrogenation of terminal C atoms in either aromatic or aliphatic form. Many spectra exhibit a quasi-continuum between 6 and 9 μm that arises from the overlap of many vibrational modes in this region. With CH2 edge groups, we find a new spectral feature near 16 μm and the aliphatic CH2 symmetric stretch vibration is found to shift from 3.5 to 3.3 μm when these groups are present at terminal sites on aromatic carbon skeletons. The relevance of calculated spectra to those of astronomical sources is briefly discussed.

Normal coordinate analysis and vibrational spectra of aromatic

Abstract: Theoretical infrared absorption spectra of aromatic ring molecules having up to 102 carbon atoms and with various edge hydrogenations have been obtained using a classic mechanical model and a simpli—ed valence force —eld. Force constants have been adapted from those available for smaller molecules. Spec- tral line intensities are calculated in a double harmonic approximation with eUective atomic charges obtained using a tight-binding Hamiltonian. Vibrational modes of clusters of like aromatic ring Hue ckel molecules are obtained through introduction of an interlayer force constant. These modes are predicted to occur in the wavelength range between 80 and 400 km. We explore the eUect of dehydrogenation on these spectra as well as hydrogenation of terminal C atoms in either aromatic or aliphatic form. Many spectra exhibit a quasi-continuum between 6 and 9 km that arises from the overlap of many vibrational modes in this region. With edge groups, we —nd a new spectral feature near 16 km and the ali- CH 2 phatic symmetric stretch vibration is found to shift from 3.5 to 3.3 km when these groups are CH 2 present at terminal sites on aromatic carbon skeletons. The relevance of calculated spectra to those of astronomical sources is brie—y discussed. Subject headings: ISM: lines and bandsISM: moleculesmolecular processes