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At least 19 records
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NASA NTRS2016

On the Detectability of CO Molecules in the Interstellar Medium via X-Ray Spectroscopy

We present a study of the detectability of CO molecules in the Galactic interstellar medium using high-resolution X-ray spectra obtained with the XMM-Newton Reflection Grating Spectrometer. We analysed 10 bright low mass X-ray binaries (LMXBs) to study the CO contribution in their line of sights. A total of 25 observations were fitted with the ISMabs X-ray absorption model which includes photoabsorption cross-sections for Oi, Oii, Oiii and CO. We performed a Monte Carlo (MC) simulation analysis of the goodness of fit in order to estimate the significance of the CO detection. We determine that the statistical analysis prevents a significant detection of CO molecular X-ray absorption features, except for the lines of sight towards XTE J1718-330 and 4U 1636-53. In the case of XTE J1817-330, this is the first report of the presence of CO along its line of sight. Our results reinforce the conclusion that molecules have a minor contribution to the absorption features in the O K-edge spectral region. We estimate a CO column density lower limit to perform a significant detection with XMM-Newton of N(CO) greater than 6 x 10(exp 16) per sq cm for typical exposure times.

ISM: structure↗
NASA NTRS1978

Low energy ionizing collisions between N2 and CO beam molecules and CO, N2, NO, CH4, and CO2 target molecules

Absolute total negative charge production cross sections for N2 + CO, CO + N2, CO + CO, N2 + NO, N2 + CH4, and N2 + CO2 collisions are reported, and a simple model of collisions is discussed. The cross sections were measured to within about 1 eV of their thresholds. Specific reaction channels were investigated by referring to mass spectrometric identification of the product ions scattered in the forward direction, and these product ion identifications were used to explain characteristic structures in the total charge production cross sections in the near-threshold regions. The extent of the importance of dissociative ionization and 'simple' ionization in the studied collisions at low energy is considered, and charge transfer cross sections for (CO)+ + CO, CO(+) + CH4, and N2(+) + CH4 are presented.

Utterback, N. G.↗
NASA NTRS1975

Proton transfer reactions from H3/+/ ions to N2, O2, and CO molecules

The rate constants for proton transfer from H3(+) ions to N2, O2, and CO have been measured as function of hydrogen-buffer-gas partial pressure. The rate constant for proton transfer from H3(+) to N2 shows a very large pressure dependence, increasing from 1.0 by 10 to the -9th power cu cm/s at low H2 partial pressures to 1.7 by 10 to the -9th power cu cm/s at high H2 partial pressures. The rate constants for proton transfer from H3(+) to O2 and CO are constant with partial pressure of H2. The roles of excess vibrational energy in H3(+) ions and of equilibrium between forward and back reactions are discussed. Back reaction is observed only for the reaction of H3(+) ions with O2, and an equilibrium constant of 2.0 (plus or minus 0.4) at 298 K has been determined. From these data, the proton affinity of O2 is deduced to be 0.47 (plus or minus 0.11) kcal/mole higher than that of H2.

Kim, J. K.↗
NASA NTRS1978

Collisional excitation of interstellar molecules - Linear molecules CO, CS, OCS, and HC3N

Rates for excitation of CO, CS, OCS, and HC3N by collisions with He atoms and H2 molecules are presented. These have been obtained from extensive theoretical calculations using methods of known reliability. In general, rates necessary for modeling interstellar molecular clouds with kinetic temperatures to 100 K have been included. Methods for fitting these rates in more compact form are discussed.

Green, S.↗
NASA NTRS1981

A theoretical study of selected singlet and triplet states of CO molecule

The results of the configuration calculations of six singlet electronic states and one triplet electronic state of CO are presented. The potential energy curves, spectroscopic constants, and electron transition moments are calculated, along with electronic dipole moment functions for three states. The self consistent field and configuration calculations used to obtain the electronic wave functions are described. The theoretical results are found to be in good agreement with the experimental measurements, and in the case of the dipole moment function calculations, preferable to them.

Cooper, D. M.↗
NASA NTRS1986

Carbon monoxide production in low energy oxygen ion bombardment of pyrolytic graphite and Kapton surfaces

The results of an investigtion of the interaction of low energy oxygen ions with pyrolytic graphite and Kapton surface are reported. CO molecules emitted from the surface as a result of the ion bombardment were detected by a mass spectrometer. Because the ion-induced signals were small compared to that arising from the CO background pressure in the vacuum system, the ion beam was modulated and the modulated component of the CO signal measured with a lock-in amplifier. The quantum yield (CO molecules emitted per incident oxygen ion) for graphite rose from 1.9 at 4.5 eV ion energy to 6.6 at 465 eV. Comparable yields were obtained for Kapton. The large size of the yields suggests contributions to the reaction process from the background O2 molecules in the vacuum system.

Horton, C. C.↗
NASA NTRS1978

Theoretical study of CO chemisorption on nickel and copper surfaces

A comparative study of the chemisorption of CO on Ni(001) and Cu(001) surfaces has been performed. The study is based on an analysis of the electronic structure of (M)5CO clusters obtained from self-consistent field X alpha scattered-wave calculations. The electron orbital formed principally by the 5 sigma orbital on CO was found to be mainly responsible for the bonding of the CO molecule to the metal surface for both the CO/Ni and CO/Cu systems. The different occupation of the antibonding 7 a sub 1 orbital in the two clusters is believed to be the major reason for the large difference observed in the measured heats of adsorption of CO on Ni and Cu surfaces. It was found also that metal atoms transfer electronic charge to the antibonding pi(asterisk)-like orbital of CO. A possible correlation between the amount of the charge transfered and the relative ease of dissociation of CO molecules on metal surfaces is discussed.

Yu, H. L.↗
NASA NTRS1977

Cluster calculation of carbon monoxide on a Cu surface using the self-consistent-field-X-alpha-scattered-wave method

Two geometries of CO chemisorbed on a Cu(100) surface have been studied using the self-consistent-field-X-alpha-scattered-wave method. Both geometries assume the CO molecule is bonded to the Cu surface with the C end down; one geometry is in the fourfold-symmetric hole site and one is directly on top of a Cu atom. The calculated valence density of states for the fourfold-symmetry case gives a satisfactory interpretation of the two main peaks below the copper d band in the UV photoemission spectra (UPS). A third weaker peak of the UPS data can be correlated with a CO molecule adsorbed directly on top of a copper atom.

Yu, H. L.↗
NASA NTRS2018

Comet C/2013 V5 (Oukaimeden): Evidence for Depleted Organic Volatiles and Compositional Heterogeneity as Revealed through Infrared Spectroscopy

We obtained high-resolution (lambda divided by delta times lambda equals approximately 25,000) pre-perihelion spectra of Comet C/2013 V5 (Oukaimeden) using NIRSPEC at Keck II on UT 2014 September 5-6, and CSHELL (Cryogenic Echelle Spectrograph) at the NASA-Infrared Telescope Facility on September 11-13, altogether spanning a range in heliocentric distance R (sub h) equals 0.789-0.698 astronomical units. We report water production rates, and production rates and abundance ratios relative to co-measured H2O for eight trace molecules: CO, H2CO, CH3OH, CH4, C2H2, C2H6, HCN, and NH3. Our measured water production rates from NIRSPEC ( Near Infra-Red Spectrograph) and CSHELL observations remained relatively constant and were close to those from SOHO (Solar and Heliospheric Observatory) / Solar Wind Anisotropies observations that encompassed our dates, suggesting H2O production dominated by release directly from or within approximately 2000 kilometers of the nucleus. All trace volatiles were depleted relative to their respective median abundances among comets, excepting NH3, which was consistent with its median abundance. Most surprising were pronounced increases in abundance ratios for CH3OH (by 51 percent relative to simultaneously measured H2O) and especially C2H6 (by 87 percent) between September 5 and 6. On September 5, C2H6 was severely depleted, consistent with its lowest abundance yet measured for any comet. It also tracked the spatial profile of H2O, suggesting C2H6 was associated with a polar ice phase dominating gas production. On September 6, C2H6 was only moderately depleted and was spatially distinct from H2O, suggesting both polar- and nonpolar-dominated ice phases contributed to the activity then. Our results are consistent with a nonhomogeneous volatile composition for C/2013V5, implying differential processing of its constituent ices.

DiSanti, Michael A.↗
NASA NTRS2011

The Carnegie Hubble Program: The Leavitt Law at 3.6 microns and 4.5 microns in the Large Magellanic Cloud

The Carnegie Hubble Program (CHP) is designed to improve the extragalactic distance scale using data from the post-cryogenic era of Spitzer. The ultimate goal is a determination of the Hubble constant to an accuracy of 2%. This paper is the first in a series on the Cepheid population of the Large Magellanic Cloud, and focuses on the period-luminosity relations (Leavitt laws) that will be used, in conjunction with observations of Milky Way Cepheids, to set the slope and zero-point of the Cepheid distance scale in the mid-infrared. To this end, we have obtained uniformly-sampled light curves for 85 LMC Cepheids, having periods between 6 and 140 days. Period- luminosity and period-color relations are presented in the 3.6 micron and 4.5 micron bands. We demonstrate that the 3.6 micron band is a superb distance indicator. The cyclical variation of the [3.6]-[4.5] color has been measured for the first time. We attribute the amplitude and phase of the color curves to the dissociation and recombination of CO molecules in the Cepheid s atmosphere. The CO affects only the 4.5 micron flux making it a potential metallicity indicator.

Scowcroft, Victoria↗
NASA NTRS1975

Conversion of laser energy to gas kinetic energy

Techniques for the gas phase absorption of laser radiation for conversion to gas kinetic energy are discussed. Absorption by inverse Bremsstrahlung, in which laser energy is converted at a gas kinetic rate in a spectrally continuous process, is briefly described, and absorption by molecular vibrational rotation bands is discussed at length. High pressure absorption is proposed as a means of minimizing gas bleaching and dissociation, the major disadvantages of the molecular absorption process. A band model is presented for predicting the molecular absorption spectra in the high pressure absorption region and is applied to the CO molecule. Use of a rare gas seeded with Fe(CO)5 for converting vibrational modes to translation modes is described.

Caledonia, G. E.↗
NASA NTRS1994

Thermal desorption of CO and H2 from degassed 304 and 347 stainless steel

Thermal desorption spectroscopy (TDS), along with Auger electron spectroscopy, was used to study the desorption of H2 and CO from baked 304 and 347 stainless-steel samples exposed only to residual gases. Both 347 and 304 samples gave identical TDS spectra. The spectra for CO contained a sharp leading peak centered in the temperature range 410-440C and an exponentially increasing part for temperatures higher than 500C, with a small peak around 600C appearing as a shoulder. The leading peak followed a second-order desorption behavior with an activation energy of 28+/-2 kcal/mol, suggesting that the rate-limiting step for this peak is most likely a surface reaction that produces the CO molecules in the surface layer. The amount of desorbed CO corresponding to this peak was approximately 0.5X10(exp 14) molecules/cm(exp 2) . The exponentially rising part of the CO spectrum appeared to originate from a bulk diffusion process. The TDS spectrum for H2 consisted of a main peak centered also in the temperature range 410-440C, with two small peaks appearing as shoulders at approximately 500 and 650C. The main peak in this case also displayed a second-order behavior with an activation energy of 14+/-2 kcal/mol. The amount of desorbed H2, approximately 1.9X 10(exp 15) molecules/cm(exp 2) , appeared to be independent of the concentration of hydrogen in the bulk, indicating that the majority of the desorbed H2 originated from the surface layer.

Rezaie-Serej, S.↗
NASA NTRS1978

Nonequilibrium chemistry in shocked molecular clouds

The gas-phase chemistry is studied behind a 10-km/s shock propagating into a dense molecular cloud. The principal conclusions are that: the concentrations of certain molecules (CO, NH3, HCN, N2) are unperturbed by the shock; other molecules (H2CO, CN, HCO(+)) are greatly decreased in abundance; and substantial amounts of H2O, HCO, and CH4 are produced. Approximately 1 million yr (independent of the density) must elapse after shock passage before chemical equilibrium is attained.

Iglesias, E. R.↗
NASA NTRS1984

Vibrational and rotational excitation of CO in comets. Part 1: Non-equilibrium calculations. Part 2: Results of the calculation for standard bright comet, comet Iras-Araki-Alcock and comet Halley

The vibrational and rotational excitation of the CO molecule in cometary comae were investigated using a model which includes IR vibrational pumping by the solar flux, vibrational and rotational radiative decay, and collisional coupling among rotational states. Steady state was not assumed in solving the rate equations. The evolution of a shell of CO gas was monitored as it expanded from the nucleus into the outer coma. Collisional effects were treated using a kinetic temperature profile derived from theoretical work on the coma energy balance. The kinetic temperature was assumed to be extremely cold in the inner coma; this has significant consequences for the CO excitation. If optical depth effects are ignored, only low J transitions will be significantly excited in comets observed at high spatial resolution. Ground-based observations of CO co-vibrational and rotational transitions will be extremely difficult due to lack of sensitivity and/or terrestrial absorption. However, CO should be detectable from a large comet with favorable observing geometry if the CO is a parent molecule present at the 10% level (or greater) relative to H2O. Observations using cooled, spaceborne instruments should be capable of detecting CO emission from even moderately bright comets.

Chin, G.↗
NASA NTRS1994

Radiative excitation of molecules near powerful compact radio sources

In a recent paper, Barvainis & Antonucci searched for and failed to detect CO J = 1 goes to 0 absorption from the obscuring torus in the nearby powerful radio galaxy Cygnus A. We show that a plausible explanation for the lack of absorption (assuming that the ionization parameter within the torus is low enough for the gas to be molecular) is that radiative excitation of the CO molecules by the nonthermal radio continuum increases the excitation temperature of the lower rotational levels substantially, reducing the optical depths. The excitation temperature may approach the brightness temperature of the radio source at high enough flux-to-density ratios. Heating of the gas by the nonthermal excitation may also be important. We discuss the region of parameter space in which this excitation mechanism will be important and the implications for observations of obscuring tori.

Maloney, Philip R.↗
NASA NTRS1972

Preferential orientation of CO adsorption on Ni as determined by extended Hueckel calculations.

The adsorption of CO on the nickel surface has been calculated using a modified extended Hueckel technique for a number of different orientations of the CO with respect to the nickel surface. The calculations show that double site adsorption with the CO molecule normal to the surface and the carbon atom closest to the metal gives the most stable configuration (2.57 eV). The single site absorbs with an energy of 2.39 eV. However, other configurations also give fairly large bonding energies. The CO normal to the surface with oxygen closest to the metal gives a heat of adsorption of 0.37 eV. Two-center adsorption with the CO axis parallel to the surface and the molecule symmetrically placed between the nearest neighbors gives a chemisorption energy of 1.34 eV. These values compare with an experimental value of 1.98 eV.

Robertson, J. C.↗
NASA NTRS1991

Gas and dust in the Ophiuchus region

The interstellar gas and dust in the Ophiuchus molecular cloud region are studied through H I, CO, and IRAS observations. The generally good global correlation of H I with the infrared intensity and dust optical depth breaks down on the smaller scales that can be distinguished in this nearby region. The presence of an intense ultraviolet field and shocks caused by the early-type stars can account for this difference. Integrated emission from the CO molecule correlates with the dust optical depth. This correlation is used to derive the conversion factor between W(CO) and the atomic hydrogen column density. The region close to the early-type stars in Upper Scorpius shows a considerable amount of infrared emission, but negligible CO emission.

De Geus, E. J.↗