Engineering topics
Publications and source records attributed to Dalgarno, A..
Transition probabilities for the second negative band system of O2(+) are computed using the dipole transition moment presented by Wetmore et al. (1984). Vibrational levels v double prime = 0 - 54 of the X2Pi(g) ground state and v prime = - 33 of the excited A2Pi(u) state are included. Franck-Condon factors for ionization-excitation of O2 to O2(+) are also presented.
The line radiations emitted in the cooling gas behind a fast dissociative shock are studied. The intensities emitted in high rotational transitions of the neutral molecules CO, SiO, HCN, CN, NO, and SO are estimated, as well as in rovibrational transitions of the molecular ions HeH(+) and OH(+) in radio recombination lines of atomic hydrogen and in fine-structure transitions of C, C(+), O, and Si(+). The predictions are compared with the observed intensities of line emission from the Orion-KL region. For Orion-KL the observations do not exclude, but probably do not require, the presence of a fast dissociative shock. Emission from SiO in high-J rotational states and from vibrationally excited OH(+), HeH(+), HeH(+), and SO(+) may be detectable from dissociative shocks under suitable conditions of preshock density and shock velocity; such emission may prove to be a useful diagnostic probe of fast shock activity.
The physical and chemical processes that operate in the cooling gas behind a fast, dissociative, single-fluid shock propagating in a dense interstellar cloud are discussed. The treatment extends previous theoretical work on fast molecular shocks by including the effects of the conversion of Ly-alpha photons into radiation of the two-photon continuum and into H2 Lyman band emission lines, the effects of CO photodissociation following line absorption, and the formation and destruction of molecules containing the elements nitrogen, silicon, and sulphur, and of the complex hydrocarbons. Abundance profiles for the molecular species of interest are presented. After molecular hydrogen begins to reform, by means of gas phase and grain surface processes, the neutral species OH, H2O, O2, CO, CN, HCN, N2, NO, SO, and SiO reach substantial abundances. The molecular ions HeH(+), OH(+), SO(+), CH(+), H2(+), and H3(+), are produced while the gas is still hot and partially ionized. Emissions from them provide a possible diagnostic probe of fast molecular shocks.
The transition dipole moments of the c 3 Pi u - a 3 Sigma g +, i 3 Pi g - c 3 Pi u, and i 3 Pi g - b 3 Sigma u + transitions of molecular hydrogen were calculated over a wide range of internuclear distances R. Calculations of the emission spectra are nearing completion. Substantial progress was made in the calculation of the absorption coefficient of a colliding pair of hydrogen atoms in the binary encounter approximation. Detailed calculations were performed on the continuum emission arising from the excitation of the a 3 sigma g + state of H2 by the impact of electrons of various energies. Each curve is labelled by the electron energy in electron volts. The strong emission near Lyman alpha is interesting in that it is produced by excitation of molecular hydrogen.
The effects of the presence of a substantial component of large molecules on the chemistry of diffuse molecular clouds are explored, and detailed models of the zeta Persei and zeta Ophiuchi clouds are constructed. The major consequence is a reduction in the abundances of singly charged atomic species. The long-standing discrepancy between cloud densities inferred from rotational and fine-structure level populations and from the ionization balance can be resolved by postulating a fractional abundance of large molecules of 1 x 10 to the -7th for zeta Persei and 6 x 10 to the -7th for zeta Ophiuchi. If the large molecules are polycyclic aromatic hydrocarbons (PAH) containing about 50 carbon atoms, they contain 1 percent of the carbon in zeta Persei and 7 percent in zeta Ophiuchi. Other consequences of the possible presence of PAH molecules are discussed.
Recent theoretical and observational investigations of photochemical processes in the atmospheres of the planets and their satellites are reviewed. Particular attention is given to the CO2-dominated atmospheres of Mars and Venus, the hydrogen-dominated atmospheres of the Jovian planets, the SO2 atmosphere of Io, and the massive atmospheres of Titan and Triton. The principal reaction paths involved are listed and briefly characterized, and numerical data on atmospheric compositions are given in tables.
The rates of photodissociation of the CH radical from absorption of solar radiation are calculated to lie between 0.00416 and 0.0267/sec for heliocentric velocities between -42 and 42 km/sec at 1 AU from the Sun. For CD, the corresponding rates lie between 0.00433 and 0.0131/sec. The v'=0 level of the C2 sigma(+) is the main channel of CH as well as CD dissociation and, in each case, contributes over 77 percent of the total dissociation. The CH and CD radicals may be detectable in comets through absorption lines of the C2 sigma(+) - X2Di(r) electronic bands near 3143.2 and 3142.1 A, respectively.
Energetic N(+) ions are produced in interstellar clouds by reactions of He(+) ions and N2 with sufficient energy to overcome the exoergicity of the reaction of N(+) with H2 and thereby from NH(+) ions. The reacting N(+) ions also slow down by elastic collisions with the ambient H2 molecules. The equilibrium energy distributions of the N(+) ions in a gas with temperature between 10 and 70 K are derived and the effective reaction rate coefficients are calculated. At 30 K, the rate coefficient is 1.6 x 10 to the -10th cu cm/sec.
Calculations are carried out of the elastic scattering and excitation exchange cross-sections in collisions of O(D-1) and O(P-3) atoms, which determine the degree of thermalization of the O(D-1) atoms produced by dissociative recombination in the thermosphere. The effective elastic scattering and excitation exchange cross-sections are calculated to be 1.55 x 10 to the -15th and 6.25 x 10 to the -16th sq cm, respectively, at a relative collision energy of 1.0 eV. The mutual diffusion coefficient between O(D-1) and O(P-3) atoms is also presented.
The rate coefficient for the quenching of O(1D) by O(3P) has recently been calculated by Yee et al. (1985). Their results indicate that quenching by atomic oxygen should not be ignored in the analysis of the 6300 A emission airglow. Data obtained by the Visible Airglow Experiment on board the AE satellites have been reanalyzed to determine the quenching rate of O(1D) by atomic oxygen. The results of this analysis are presented.
Multiconfiguration variational calculations of the electronic wave functions of the a 3Sigma(+)g and b 3Sigma(+)u states of molecular hydrogen are presented, and the electric dipole transition moment between them (of interest in connection with stellar atmospheres and the UV spectrum of the Jovian planets) is obtained. The dipole moment is used to calculate the probabilities of radiative transitions from the discrete vibrational levels of the a 3Sigma(+)g state to the vibrational continuum of the repulsive b 3Sigma(+)u state as functions of the wavelength of the emitted photons. The total transition probabilities and radiative lifetimes of the levels v prime = 0-20 are presented.
Ab initio calculations of the B3Sigma sub u(-) potential energy curve of O2 are reported together with the transition moment connecting the B3Sigma sub u(-) and the ground electronic states. Photodissociation cross sections are presented over the wavelength region 127-152 nm. Small adjustments to the potential energy curve and the transition moment lead to a model of the absorption which agrees well with the experimental cross sections and which reproduces the structural features observed on the high-energy edge. The shoulder at 135.6 nm is attributed to the lowest 3Pi sub u state and the auxiliary maxima to transitions into the B3Sigma sub u(-) state.
The potential energy curves of Saxon and Liu (1977) are used to calculate nuclear radial wave functions for O2 in the a 1 Delta g and C 3 Delta u states. Franck-Condon factors are evaluated for both bound-bound and bound-free transitions. Observational values for the radiative lifetimes of discrete vibrational levels of the C 3 Delta u state are used to place constraints on the cross section for photodissociaation of a 1 Delta g. It is argued that the cross section for the dissociation of O2(a 1 Delta g) through the C 3 Delta u state is smaller by four orders of magnitude than values required by Frederick and Cicerone (1985) to resolve the apparent discrepancy between computed and measured concentrations for O3 in the upper atmosphere.
The altitude variation of the Atmosphere Explorer optical glow intensity suggests that two different processes are responsible for the glow. One, dominant for altitudes above 180 km, has an emission brightness proportional to the ambient atomic oxygen density whereas the other, dominant at altitudes below 160 km, produces an emission whose intensity is proportional to the product of the densities of any of N2, O2 or NO. The first mechanism apparently has two components, one from the surface recombination of O and H and the other from a process similar to that producing the Shuttle glow. Unless the efficiency of the second mechanism is much enhanced on the Shuttle its contribution to the Shuttle glow is negligible.
Recent analyses of the Atmosphere Explorer data are discussed in which it is demonstrated that the satellite glows have two components, one at high altitudes which is consistent with excitation in single collisions of atmospheric oxygen atoms with the vehicle surface and the other at low altitudes which is consistent with double collisions of nitrogen molecules. Contrary to an earlier suggestion, the low-altitude data are not consistent with collisions of oxygen molecules. The separation of the two components strengthens the conclusion that the high-altitude glow arises from vibrationally excited OH molecules produced by a formation mechanism that is different from that leading to the normal atmospheric OH airglow. The spectrum is consistent with association of oxygen and hydrogen atoms at sites on the surface into the vibrational levels of OH. The low-altitude glow is consistent with the green mechanism but there are difficulties with it. The shuttle glows are different and have the spectral appearance of emission from NO2. The characteristics of the shuttle glows and the satellite glows will be contrasted and a tentative resolution of the differences in the Atmosphere Explorer and shuttle glows will be offered.
The densities and vibrational distributions of N2(+) in the X2sigma g +, A2Pi u and B2sigma u + states in the daytime terrestrial ionosphere are computed for both low and high solar activity. Altitude profiles of the relative populations of the vibrational levels of N2(+) X2sigma g + are presented. The fraction of vibrationally excited N2(+) varies from 5 percent at 100 km to 50 percent at 450 km. Several models are examined in which loss of N2(+)(v) is enhanced for v greater than 0 and in which various assumptions are made about the vibrational distributions produced in charge transfer reactions. The N2(+) densities are significantly reduced if N2(+)(v) reacts with O at rates which are near gas-kinetic. Vibrational distributions of the A2pi u and B2sigma u + states are also presented, as well as volume emission rates and integrated overhead intensities of the major bands of the Meinel and first negative systems.
From published potential energy curves and transition dipole moments, there are obtained by numerical integration of the equations of nuclear motion the vibrational eigenfunctions of the X 1Sigma(g)(+) and B-prime 1Sigma(u)(+) states of H2. The probabilities of radiative transitions from the discrete vibrational levels of the excited B-prime 1Sigma(u)(+) electronic state of H2 to the discrete and continuum vibrational levels of the ground X 1Sigma(g)(+) electronic state are calculated. The Franck-Condon factors are also presented.
The optical glow induced by spacecraft-environment interaction under daytime conditions is analyzed using photometric data obtained by the Visible Airglow Experiment on board the Atmospheric Explorer Satellite. Because of the increased ambient oxygen density, the glow is more intense in the daytime. The daytime production efficiency, meausured in Rayleighs per oxygen atom, is similar to the nighttime value. No dependence of the glow intensity on the ambient plasma density was found. The production efficiency increased with time throughout the duration of the spacecraft in orbit.