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At least 181 records · Page 10
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NASA NTRS1986

Quantitative VUV spectroscopy of Cl2

The photoabsorption and fluorescence cross sections of Cl2 were measured in the 105-145 nm region using synchrotron radiation as a light source. The oscillator strengths for the major absorption bands were calculated from the measured absorption cross sections. The measured oscillator strengths for the 2 3Pi u and 2 1Pi u - X 1Sigma(+)g transitions agree quite well with the theoretical values. The absorption spectrum was analyzed in accord with the excited electronic states calculated by Peyerimhoff and Buenker (1981). The Rydberg series converging to the first and second ionization potentials were classified. The vibrational levels of the 2 1Sigma(+)u ionic state were determined up to v prime = 15 from the fluorescence excitation spectrum. It was observed that a band with peak at 108.3 nm produces VUV fluorescence, but it produces UV fluorescence only by a collisional excitation process.

Lee, L. C.↗
NASA NTRS1994

The abundant elements in interstellar dust

We explore the incorporation of the cosmically abundant species O, C, N, Mg, Si, Fe, and S into interstellar dust. Column densities based on Goddard High Resolution Spectrograph 3.5 km/s resolution measurements from the literature for eight individual absorbing regions toward five lines of sight are used. Corrections are applied as needed in order to account for recent improvements in oscillator strengths. In order to acquire the most accurate column densities, and check on the accuracy of the oscillator strengths, we compare column densities based on the very strong Lorentzian damped lines of C II, O I, N I, and Mg II with results for the weak lines of these species, and confirm the previously determined f-values for O I lambda 1335, C II lambda 2325, and N I lambda lambda 1159, 1160. New empirical f-values of 1.25 x 10(exp -3) and 6.25 x 10(exp -4), respectively, are derived for the Mg II weak doublet at 1239 and 1240 A. Assuming a cosmic reference abundance based on solar and B star values, we derive depletions and dust-phase abundances which suggest that more than 70% of the available Mg and Fe is incorporated into dust-grain cores, whereas only 35% of the silicon is. This implies that oxides are important constituents of the grain core population. Mg and Fe atoms are mantled onto grain cores in a ratio of 1.8 to 1, whereas approximately 4.0 Si atoms are in the mantle per Fe atom. Since Si is not expected to accrete onto silicate or graphite grains, other grain cores, perhaps oxides and/or metallic Fe, may provide mantling sites for this species. The abundances of Fe and Mg in mantles would imply that graphite grains must have a substantial coating unless oxides provide significant mantling sites for these species. The abundance of O and N in the dust phase as implied by the solar reference abundance values are difficult to reconcile with the fact that these elements are not expected to participate in mantle formation, and the 3.1 micrometer H2O ice feature is not seen in absorption toward stars similar to those studied. The B star reference abundances for O and N, however, imply that no mantling of these species has occurred. The dust-phase abundance for C implied by solar reference abundances agrees with predictions for the number of graphite grains needed to produce the 2175 A bump. B star reference abundances, however, suggest that the abundance of C in the dust phase is not always sufficient to produce the bump.

Sofia, Ulysses J.↗
NASA NTRS1996

Quantum Mechanical Determination of Potential Energy Surfaces for TiO and H2O

We discuss current ab initio methods for determining potential energy surfaces, in relation to the TiO and H2O molecules, both of which make important contributions to the opacity of oxygen-rich stars. For the TiO molecule we discuss the determination of the radiative lifetimes of the excited states and band oscillator strengths for both the triplet and singlet band systems. While the theoretical radiative lifetimes for TiO agree well with recent measurements, the band oscillator strengths differ significantly from those currently employed in opacity calculations. For the H2O molecule we discuss the current results for the potential energy and dipole moment ground state surfaces generated at NASA Ames. We show that it is necessary to account for such effects as core-valence Correlation energy to generate a PES of near spectroscopic accuracy. We also describe how we solve the ro-vibrational problem to obtain the line positions and intensities that are needed for opacity sampling.

Langhoff, Stephen R.↗
NASA NTRS2011

Initiating Molecular Growth in the Interstellar Medium via Dimeric Complexes of Observed Ions and Molecules

A feasible initiation step for particle growth in the interstellar medium (ISM) is simulated by means of ab quantum chemistry methods. The systems studied are dimer ions formed by pairing nitrogen containing small molecules known to exist in the ISM with ions of unsaturated hydrocarbons or vice versa. Complexation energies, structures of ensuing complexes and electronic excitation spectra of the encounter complexes are estimated using various quantum chemistry methods. Moller-Plesset perturbation theory (MP2, Z-averaged perturbation theory (ZAP2), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)), and density functional theory (DFT) methods (B3LYP) were employed along with the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Two types of complexes are predicted. One type of complex has electrostatic binding with moderate (7-20 kcal per mol) binding energies, that are nonetheless significantly stronger than typical van der Waals interactions between molecules of this size. The other type of complex develops strong covalent bonds between the fragments. Cyclic isomers of the nitrogen containing complexes are produced very easily by ion-molecule reactions. Some of these complexes show intense ultraviolet visible spectra for electronic transitions with large oscillator strengths at the B3LYP, omegaB97, and equations of motion coupled cluster (EOM-CCSD) levels. The open shell nitrogen containing carbonaceous complexes especially exhibit a large oscillator strength electronic transition in the visible region of the electromagnetic spectrum.

Bera, Partha P.↗
NASA NTRS1993

Ultraviolet transition probabilities in N II

Oscillator strengths were calculated for the ultraviolet transition array 2p sup 2 - 2p3s in the N II spectrum. Results obtained confirm that the 748 A intercombination line is usually strong as predicted by Fawcett (1987). The results of theoretical weighted oscillator strengths are considered to be reliable.

Ellis, David G.↗
NASA NTRS1996

Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high-energy range of the spectrum, and possesses a large oscillator strength), and seems to correlate with strong and broad DIBs. For noncompact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low-energy range of the spectrum, and possesses a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that (i) only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narow bands on the other can easily be interpreted in the context of the PAH proposal by a distribution of compact and noncompact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would reflect conditions within a cloud which locally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations in the ultraviolet range, is called for.

NASA Discipline Number 52-10↗
NASA NTRS1982

Laser-excitation technique for the measurement of absolute transition probabilities of weak atomic lines

A new technique is presented for the measurement of transition probabilities for weak allowed, intersystem, and forbidden lines. The method exploits the fact that oscillator strength is proportional to the number of stimulated absorptions and emissions produced by a narrow-band laser pulse of known energy which is in resonance with an atomic transition. The method is tested for a particular transition of Mg I with a known oscillator strength value and of appropriate magnitude. The number densities are measured using a Mach-Zehnder interferometer and the hook method for the lower level population and by measuring an absorption-equivalent width for the other. The apparatus consisted of a high-power tunable laser and a magnesium oven to produce excited Mg vapor, and a laser-plasma background continuum. The results are in good agreement with theoretical and other experimental data.

Kwong, H. S.↗
NASA NTRS1995

Assessment of the Polycyclic Aromatic Hydrocarbon-Diffuse Interstellar Band Proposal

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high energy range of the spectrum, and possess a large oscillator strength), and seems to correlate with strong and broad DIBs. In the case of non-compact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low energy range of the spectrum, and possess a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that: (i) Only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narrow bands on the other hand can easily be interpreted in the context of the PAH proposal by a distribution between compact and non-compact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would provide tracers of conditions within a cloud which globally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations of the ultraviolet range, is called for.

Salama, Farid↗
NASA NTRS1983

The low-lying 2-sigma-minus states of OH

The configuration-interaction method is used to determine the electronic wave functions of the two lowest 2-sigma-minus states of OH using four different atomic orbital basis sets. Potential energy curves, transition moments, oscillator strengths, and photodissociation cross sections are obtained. Electronic transition dipole moments connecting the excited 1 2-sigma-minus and 2(D)2-sigma-minus states with each other and with the ground chi-2-pi state are presented as functions of internuclear distance. The theoretical absorption oscillator strengths for the D-2-sigma-minus(v prime = 0) from chi-2-pi(v double prime = 0) transition are in good agreement with the empirical value derived from astronomical measurement. The photodissociation cross sections for absorption from the v prime = 0, 1, and 2 levels of the ground state into the continuum of the 1 2-sigma-minus state are calculated, and the interstellar and cometary photodissociation rates are derived.

Van Dishoeck, E. F.↗
NASA NTRS1995

Theoretical Studies of the Quasi-Bound Vibrational Levels in the B State of CH3

Consider the calculation of the oscillator strength of th B yielded by X electronic transition in CH3. The X state is strongly bound, but in the B state, CH3 is pre-dissociated by H atom tunneling through a potential energy barrier to form CH2+H. If we turn off pre-dissociation by making the potential outside of the barrier repulsive, the calculated 0-0 oscillator strength for CD3 from high quality ab initio potential and transition moment data along with accurate ro-vibrational wavefunctions is 0.0212, which is a factor of two greater than experiment. This discrepancy is largely due to the neglect of pre-dissociation. In this poster we explore techniques for including pre-dissociation in the calculations.

Schwenke, David W.↗
NASA NTRS1972

Photoabsorption by ground-state alkali-metal atoms.

Principal-series oscillator strengths and ground-state photoionization cross sections are computed for sodium, potassium, rubidium, and cesium. The degree of polarization of the photoelectrons is also predicted for each atom. The core-polarization correction to the dipole transition moment is included in all of the calculations, and the spin-orbit perturbation of valence-p-electron orbitals is included in the calculations of the Rb and Cs oscillator strengths and of all the photoionization cross sections. The results are compared with recent measurements.

Weisheit, J. C.↗
NASA NTRS1979

Molecules of significance in planetary aeronomy

This monograph is basically devoted to spectroscopic information of the molecules of planetary interest. Only those molecules have been dealt with which have been confirmed spectroscopically to be present in the atmosphere of major planets of our solar system and play an important role in the aeronomy of the respective planets. An introduction giving the general conditions of planets and their atmospheres including the gaseous molecules is given. Some typical planetary spectra is presented and supported with a discussion on some basic concepts of optical absorption and molecular parameters that are important to the study of planetary atmospheres. Quantities like dipole moments, transition probabilities, Einstein coefficients and line strengths, radiative life times, absorption cross sections, oscillator strengths, line widths and profiles, equivalent widths, growth curves, bond strengths, electronic transition moments, Franck-Condon factors and r-centroids, etc., are discussed. Spectroscopic information and relevant data of 6 diatomic (HF, HCL, CO, H2, O2, N2) and 6 polyatomic (CO2, N2), O3, HeO, NH3, CH4) molecules are presented.

Mohan, H.↗
NASA NTRS2024

Molecules in Cool Stars: The A 2 Π − X 2 Σ + and the B 2 Σ + − X 2 Σ + Band Systems of CaCl

Cool red giant stars exhibit absorption bands of calcium monochloride (CaCl) in the optical region. In this work, the bands of CaCl corresponding to the A 2 Π − X 2 Σ + transition with v' ≤ 4 and v'' ≤ 4, and B 2 Σ + − X 2 Σ + transition with v' ≤ 2 and v'' ≤ 2 are rotationally analyzed with PGOPHER, a multipurpose program for simulating and fitting molecular spectra. All bands were analyzed using high-resolution absorption spectra collected at the National Solar Observatory in 1985, calibrated with atomic lines from a high-resolution emission spectrum obtained in a separate hollow cathode experiment. The results are compared with previous laser-induced fluorescence measurements. Overall, the accuracy of spectroscopic constants for the A2Π and B2Σ+ states is improved. Scaled transition dipole moment calculations were used to obtain line strengths. Line lists are provided with Einstein A coefficients and oscillator strengths, which can be used to determine CaCl stellar abundances. Additionally, isotope splittings allowed us to estimate band origins for the 40 Ca 37 Cl isotopologue based on band-head positions.

Léo Lavy↗
NASA NTRS1975

Spectrophotometry of Comet Kohoutek /1973f = 1973XII/

Spectrum scans have been obtained of Comet Kohoutek with a Fourier spectrometer at heliocentric distances from 0.55 to 1.05 AU. The useful spectral range is roughly 3800-8400 A and the resolution is about 10-15 A at 5000 A. Large variations occurred in the gas-to-dust ratio (pre- versus postperihelion) and in the relative strength of the NH2 bands (with heliocentric distance). There is a smaller change in the ratio of CN to C2 emission. The Swan bands of C2 show the customary anomaly in strengths implying highly non-Boltzmann distributions among vibrational levels. The red and violet CN system fluxes are in slightly better agreement with laboratory data on oscillator strengths than results on previous comets. Other species measured include Na, (O I), CH, and C3.

Ahearn, M. F.↗
NASA NTRS1984

Studies of extreme-ultraviolet emission from Rydberg series of H2 by electron impact

Calibrated laboratory measurements of H2 bands in the 70-170-nm region using crossed neutral and electron beams have allowed the estimation of the cross sections for H2 bands which account for virtually all of the H2 EUV band emission. It is found, in general, that the internal population structure of the directly excited EUV band systems, B-X, B'-X, C-X, D-X, and D'-X, is modeled accurately by theoretical collision strengths. Moreover, the analysis of the measurements indicates that the same constant factor at a given electron energy at or above 100 eV, relates the theoretical oscillator strengths of the systems to the cross sections to within about 15 percent. The higher-Rydberg series band systems measured have been found to be a significant factor in the analysis of EUV planetary spectra. The prominence of the E,F-B contribution at low electron impact energies is also of astrophysical importance and further work is recommended to better define the excitation parameters of the E,F system.

Ajello, J. M.↗
NASA NTRS1980

Spectrophotometry of Comet West

Postperihelion observations of Comet West (1975n = 1976 VI) have been made with a Fourier transform spectrometer at heliocentric distances from 0.57 to 1.68 AU. Measurements were made of the emission bands of C2, CN, C3, CH, and NH2, as well as the emission lines of Na D and forbidden (O I), and the flux in the continuum in nine different bandpasses. Several ratios of the band strengths of CN have been used to determine the two free parameters in the fluorescence equilibrium model of CN of Danks and Arpigny (1973). From the values of the parameters it is inferred that the vibrational transition probability for the ground electronic state is between 0.025 and 0.075 per sec and that the ratio of oscillator strengths between the (0-0) bands of the violet and red systems is between 25 and 30. When corrected for field-of-view effects, NH2 shows no systematic variation in abundance relative to C2 while CH shows a small increase. The cometary continuum is found to be slightly redder than the solar continuum, consistent with results for other bright, dusty comets. The equivalent width of the Delta u = 0 sequence of C2 shows a marked decrease at r(H) = 1.2 AU.

Ahearn, M. F.↗