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At least 487 records · Page 27
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NASA NTRS1986

Infrared absorption and emission characteristics of interstellar PAHs

The mid-infrared interstellar emission spectrum with features at 3.28, 6.2, 7.7, 8.7 and 11.3 microns is discussed in terms of the Polycyclic Aromatic Hydrocarbon (PAH) hypothesis, which is based on the suggestive, but inconclusive comparison between the interstellar emission spectrum with the infrared absorption and Raman spectra of a few PAHs. The fundamental vibrations of PAHs and PAH-like species which determine the IR and Raman properties are discussed. Interstellar IR band emission is due to relaxation from highly vibrationally excited PAHs excited by ultraviolet photons. The excitation/emission process is described and the IR fluorescence from one PAH, chrysene, is traced. Generally, there is sufficient energy to populate several vibrational levels in each mode. Molecular vibrational potentials are anharmonic and emission from these higher levels will fall at lower frequencies and produce weak features to the red of the stronger fundamentals. This process is also described and can account for some spectroscopic details of the interstellar emission spectra previously unexplained. Analysis of the interstellar spectrum shows that PAHs contain between 20 and 30 carbon atoms are responsible for the emission.

Barker, J. R.↗
NASA NTRS1987

The effect of the isotopic composition of oxygen on the non-mass-dependent isotopic fractionation in the formation of ozone by discharge of O2

Isotopic fractionation processes in the formation of O3 by 20-kV 60-kHz discharge of O2 in a fused silica container at 77 K are investigated experimentally, with a focus on the effects of changes in the isotopic composition of O2, the O2 pressure, the degree of O3 formation, and the discharge geometry on the isotope abundances in the postdischarge O2 and O3. The results are presented in tables and graphs and compared with the predictions of theoretical models involving vibrational anharmonicity and symmetry effects on O3 predissociation. In experiments using O2 enriched with O-17 and O-18, the heavy isotope is enriched in the residual O2 and not in the O3, whereas the opposite is true when atmospheric isotope abundances are used. It is inferred that the predissociation rate depends on the symmetry and mass of the O3 molecules and not on self-shielding or symmetry effects in the reactant O2. The potential value of the present findings for studies of anomalous isotope abundances in meteorites is indicated.

Yang, Jongmann↗
NASA NTRS1987

The effect of pressure and excitation energy on the isotopic fractionation in the formation of ozone by discharge of O2

Isotopic fractionation processes in the formation of O3 by discharge (at 20 kV and 3 MHz) of O2 in a fused silica container at 77 K are investigated experimentally, with a focus on the effects of changes in the isotopic abundances in the predischarge O2, the O2 pressure, and the excitation energy on the isotope abundances in the postdischarge O2 and O3. The results are presented in extensive tables and graphs and characterized in detail, and good general agreement is found with data obtained using 60-kHz excitation (Yang and Epstein, 1987). It is shown that O-17 and O-18 enrichment of the residual O2 (when the predischarge gas is enriched) or of the O3 (when atmospheric isotope abundances are used) can occur independently of one another. It is inferred that the rate of anharmonic predissociation of vibrationally hot O3 depends on the symmetry and mass of the O3 molecules. The significance of the present findings for studies of anomalous oxygen isotope abundances in meteorites is indicated.

Yang, Jongmann↗
NASA NTRS1987

Infrared absorption and emission characteristics of interstellar PAHs

The mid-infrared interstellar emission spectrum with features at 3.28, 6.2, 7.7, 8.7 and 11.3 microns is discussed in terms of the Polycyclic Aromatic Hydrocarbon (PAH) hypothesis, which is based on the suggestive, but inconclusive comparison between the interstellar emission spectrum with the infrared absorption and Raman spectra of a few PAHs. The fundamental vibrations of PAHs and PAH-like species which determine the IR and Raman properties are discussed. Interstellar IR band emission is due to relaxation from highly vibrationally excited PAHs excited by ultraviolet photons. The excitation/emission process is described and the IR fluorescence from one PAH, chrysene, is traced. Generally, there is sufficient energy to populate several vibrational levels in each mode. Molecular vibrational potentials are anharmonic and emission from these higher levels will fall at lower frequencies and produce weak features to the red of the stronger fundamentals. This process is also described and can account for some spectroscopic details of the interstellar emission spectra previously unexplained. Analysis of the interstellar spectrum shows that PAHs contain between 20 and 30 carbon atoms and are responsible for the emission.

Allamandola, L. J.↗
NASA NTRS1987

Infrared emission from interstellar PAHs

The mid-IR absorption and Raman spectra of polycyclic aromatic hydrocarbons (PAHs) and the mechanisms determining them are reviewed, and the implications for observations of similar emission spectra in interstellar clouds are considered. Topics addressed include the relationship between PAHs and amorphous C, the vibrational spectroscopy of PAHs, the molecular emission process, molecular anharmonicity, and the vibrational quasi-continuum. Extensive graphs, diagrams, and sample spectra are provided, and the interstellar emission bands are attributed to PAHs with 20-30 C atoms on the basis of the observed 3.3/3.4-micron intensity ratios.

Allamandola, L. J.↗
NASA NTRS1989

Influence of afterbodies on AOTV flows

The role of vibration-dissociation coupling in flows representative of AOTV's with afterbodies is discussed. A physical model designed to reflect the behavior of anharmonic diatomic molecules at high temperatures was developed and incorporated into the direct simulation Monte Carlo method of Bird (1976). The model leads to reduced dissociation and increased convective heating for flows with finite catalytic surfaces.

Olynick, David P.↗
NASA NTRS1989

New emission features in the 11-13 micron region and their relationship to polycyclic aromatic hydrocarbons

Moderate-resolution spectra of NGC 7027, HD 44179, IRAS 2182+5050, and BD +30 deg 3639 are presented, showing that the 11.3-micron feature actually peaks at 11.22 microns. Evidence is found for new emission features near 11.9 and 12.7 microns, supporting an origin from PAHs. Also, the observed asymmetry of the 11.3-micron band is consistent with the anharmonicity expected in the C-H out-of-plane bending mode in PAHs. The analysis of the 11-13-micron emission suggests that the molecular structures of the most intensely emitting free PAHs vary between the high-excitation environment in NGC 7027 and the low-excitation but high-flux environment close to HD 44179. In addition, a series of regularly spaced features between 10 and 11 microns is detected in the spectrum of HD 44179, suggesting that a simple polyatomic hydride is present in the object's gas phase.

Witteborn, F. C.↗
NASA NTRS1989

A high level Ab initio study of the anionic hydrogen-bonded complexes FH-CN-, FH-NC-, H2O-CN- and H2O-NC-

HF, H2O, CN- and their hydrogen-bonded complexes were studied using state-of-the-art ab initio quantum mechanical methods. A large Gaussian one particle basis set consisting of triple zeta plus double polarization plus diffuse s and p functions (TZ2P + diffuse) was used. The theoretical methods employed include self consistent field, second order Moller-Plesset perturbation theory, singles and doubles configuration interaction theory and the singles and doubles coupled cluster approach. The FH-CN- and FH-NC- and H2O-CN-, H2O-NC- pairs of complexes are found to be essentially isoenergetic. The first pair of complexes are predicted to be bound by approx. 24 kcal/mole and the latter pair bound by approximately 15 kcal/mole. The ab initio binding energies are in good agreement with the experimental values. The two being shorter than the analogous C-N hydrogen bond. The infrared (IR) spectra of the two pairs of complexes are also very similar, though a severe perturbation of the potential energy surface by proton exchange means that the accurate prediction of the band center of the most intense IR mode requires a high level of electronic structure theory as well as a complete treatment of anharmonic effects. The bonding of anionic hydrogen-bonded complexes is discussed and contrasted with that of neutral hydrogen-bonded complexes.

Lee, Timothy J.↗
NASA NTRS1989

Wentzel-Kramers-Brillouin method in the Bargmann representation

It is demonstrated that the Bargmann representation of quantum mechanics is ideally suited for semiclassical analysis, using as an example the WKB method applied to the bound-state problem in a single well of one degree of freedom. For the harmonic oscillator, this WKB method trivially gives the exact eigenfunctions in addition to the exact eigenvalues. For an anharmonic well, a self-consistent variational choice of the representation greatly improves the accuracy of the semiclassical ground state. Also, a simple change of scale illuminates the relationship of semiclassical versus linear perturbative expansions, allowing a variety of multidimensional extensions.

Voros, A.↗
NASA NTRS1989

A high-level ab initio study of the anionic hydrogen-bonded complexes FH-CN(-), FH-NC(-), H2O-CN(-), and H2O-NC(-)

HF, H2O, CN- and their hydrogen-bonded complexes were studied using state-of-the-art ab initio quantum mechanical methods. A large Gaussian one particle basis set consisting of triple zeta plus double polarization plus diffuse s and p functions (TZ2P + diffuse) was used. The theoretical methods employed include self consistent field, second order Moller-Plesset perturbation theory, singles and doubles configuration interaction theory and the singles and doubles coupled cluster approach. The FH-CN- and FH-NC- and H2O-CN-, H2O-NC- pairs of complexes are found to be essentially isoenergetic. The first pair of complexes are predicted to be bound by approx. 24 kcal/mole and the latter pair bound by approximately 15 kcal/mole. The ab initio binding energies are in good agreement with the experimental values. The two being shorter than the analogous C-N hydrogen bond. The infrared (IR) spectra of the two pairs of complexes are also very similar, though a severe perturbation of the potential energy surface by proton exchange means that the accurate prediction of the band center of the most intense IR mode requires a high level of electronic structure theory as well as a complete treatment of anharmonic effects. The bonding of anionic hydrogen-bonded complexes is discussed and contrasted with that of neutral hydrogen-bonded complexes.

Lee, Timothy J.↗
NASA NTRS1990

Vibrational frequencies for Be3 and Be4

The harmonic frequencies of Be3 and Be4, evaluated by multireference configuration-interaction (MRCI) and singles and doubles coupled-cluster (CCSD) methods, are reported, as are IR intensities evaluated by using the double harmonic approximation. A comparison of the results shows that the CCSD method augmented with a correction for connected triple excitations, CCSD(T), can be a viable alternative to MRCI for determining the ground-state properties of small Be clusters (other than the dimer). Results of normal mode analyses indicate that the anharmonic vibrational frequencies of Be3 and Be4 are reasonably consistent at the CCSD, CCSD(T), or MRCI levels of treatment.

Rendell, Alistair P.↗
NASA NTRS1990

The vibrational frequencies of ozone

The singles and doubles coupled-cluster (CCSD) method is used, together with the recently proposed CCSD(T) and CCSDT-1 methods, to determine the equilibrium structure and harmonic frequencies of ozone at various levels, on the basis of large one-particle basis sets. It is demonstrated that f-type functions are required in order to obtain quantitative accuracy for the equilibrium structure and harmonic frequencies. The CCSD(T) level of theory, in conjunction with a (4s 3p 2d 1f) atomic natural orbital set, yields an equilibrium geometry and harmonic frequencies that are in excellent agreement with experimental results; an effort has accordingly been made to determine the cubic and quartic force constants, and evaluate the anharmonic corrections to the fundamental vibrational frequencies.

Lee, Timothy J.↗
NASA NTRS1989

Spectral structure near the 11.3 micron emission feature

If the 11.3 micron emission feature seen in the spectra of many planetary nebulae, H II regions, and reflection nebulae is attributable to polycyclic aromatic hydrocarbons (PAHs), then additional features should be present between 11.3 and 13.0 microns. Moderate resolution spectra of NGC 7027, HD 44179, BD+30 deg 3639, and IRAS 21282+5050 are presented which show evidence for new emission features centered near 12.0 and 12.7 microns. These are consistent with an origin from PAHs and can be used to constrain the molecular structure of the family of PAHs responsible for the infrared features. There is an indication that coronene-like PAHs contribute far more to the emission from NGC 7027 than to the emission from HD 44179. The observed asymmetric profile of the 11.3 micron band in all the spectra is consistent with the slight anharmonicity expected in the C-H out-of-plane bending mode in PAHs. A series of repeating features between 10 and 11 microns in the spectrum of HD 44179 suggests a simple hydride larger than 2 atoms is present in the gas phase in this object.

Witteborn, Fred C.↗
NASA NTRS1990

Vibrations in small Mg clusters

The equilibrium geometries, binding energies, and harmonic frequencies of Mg3 and Mg4 have been determined using large atomic natural orbital basis sets in conjunction with high levels of electron correlation. The correlation treatments comprise multireference configuration interaction (MRCI), singles and doubles coupled-cluster (CCSD) theory and the CCSD(T) extension that includes a perturbational estimate of connected triple excitations. As with previous studies of small Be clusters, the CCSD(T) method is found to reproduce the MRCI results with a remarkable degree of accuracy and at a fraction of the computational cost. Using the CCSD(T) method, full quartic force fields for Mg3 and Mg4 have been determined and anharmonic analyses have been performed using second-order perturbation theory. Vibration-rotation interaction constants and centrifugal distortion constants have also been determined.

Lee, Timothy J.↗
NASA NTRS1991

Theoretical characterization of the potential energy surface for the reversible reaction H + O2 yields HO2(asterisk) yields OH + O. III - Computed points to define a global potential energy surface

Computed energies and geometries are reported which, combined with previously published calculations, permit a global representation of the potential energy surface for the reaction H + O2 yields HO2(asterisk) yields OH + O. These new calculations characterize the potential energy surface (PES) for all H atom angles of approach to O2 and for the region of the inner repulsive wall. The region of the T-shaped H-O2 exchange saddle point is connected with the constrained energy minimum (CEM) path, and a new collinear H-O2 exchange saddle point is characterized which lies only 9 kcal/mol above the H + O2 asymptote. A vibrational analysis which utilizes local cubic and quartic polynomial representations of the PES along the CEM path has been carried out. Optimal geometries, energies, and harmonic frequencies are reported along with anharmonic analyses for the O2 and OH asymptotes and for the HO2 minimum region of the PES.

Walch, Stephen P.↗
NASA NTRS1991

Linear and circular digital spectral analysis of serial data

Two methods of digital spectral analysis of unevenly sampled data are developed and illustrated here. One method uses a linear function of time (or space), the other uses circular functions. The circular method turns out to be essentially equivalent to a least-squares sine-wave analysis. The linear, anharmonic method uses only the field of real numbers and elementary algebraic operations, and hence it can be made computationally very fast and accurate. Both methods are very general, properly handling all kinds of time series ranging from simple series consisting only of the times of events to complicated series consisting of pulses with long duty cycles. The two methods are here applied to the analysis of annual mean relative sunspot numbers.

Stothers, Richard B.↗
NASA NTRS1993

Quantized discrete space oscillators

A quasi-canonical sequence of finite dimensional quantizations was found which has canonical quantization as its limit. In order to demonstrate its practical utility and its numerical convergence, this formalism is applied to the eigenvalue and 'eigenfunction' problem of several harmonic and anharmonic oscillators.

Uzes, C. A.↗
NASA NTRS1992

Nonlinear acoustics and the thermal expansivity of glass

The paper shows the intrinsic relationship between acoustical nonlinearity and the thermal expansivity of crystalline solids (a thermal measure of anharmonicity) from consideration of the 'static' radiation field generated by the vibrating lattice (atomic) sources of the crystal. A modification of the theory to account for long-range structural disorder in glass is proposed and applied to an explanation of the zero thermal expansivity at the cross-over temperature in silicate and titanium silicate glasses. Experimental evidence which validates the essential features of the theory is presented. An application of nonlinearity measurement, based on these results, to the processing of ULE glass, where the addition of titanium in various amounts is used to establish the temperature at which the zero thermal expansivity occurs, is suggested.

Cantrell, John H.↗