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High spatial resolution observations of comet Bradfield (1979X) with the IUE spectrograph have made it possible to tentatively identify a feature at 2972 A as the 1S-3P forbidden transition of oxygen atoms. It is shown here that the most likely source of this emission is the photodissociation of water molecules, mainly by solar Lyman-alpha photons, but that a significant contribution of CO2 cannot be excluded.
Strong neutral carbon emission at 610 microns (492 GHz) has been detected from a bright-rimmed cloud abutting the H II region NGC 1977. The similarity of velocity and width between (C-13)O and C I lines suggests that both lines originate in the same region. A model for the density and temperature structure of the cloud, based on (C-13)O and (C-12)O observations, has been used to estimate the carbon abundance. The abundances of both C I and (C-13)O increase with depth into the cloud away from the rim. The carbon abundance reaches its peak value nearer the rim than does the (C-13)O abundance. This variation in the relative abundance distributions of CO and C I confirms the importance of photodissociation in the chemistry of molecular clouds, and of the C I line to studies of the interaction of hot stars with clouds.
It has been hypothesized that chlorine compounds in the stratosphere could photodissociate and release atomic chlorine which catalytically destroys ozone. For an accurate assessment of the possibility of stratospheric ozone depletion, it will be necessary to know the chlorine budget of the entire atmosphere, including the troposphere and the stratosphere. Most chlorine species in the atmosphere are emitted at the surface and diffuse upward. Measurements involving chlorine are reviewed and photochemical calculations are discussed. It is found that between 0.1-1.0 ppbv of inorganic chlorine is present at the surface and about 0.02 ppbv (parts per billion by volume) at the tropopause. Most of the inorganic chlorine is believed to be in the form of HCl, and thus natural in origin. For organic chlorine the corresponding numbers are 1.9 ppbv and 1.7 ppbv, respectively. At the surface approximately 32% of the organic chlorine is natural and 68% anthropogenic.
A two-dimensional time dependent radiative-photochemical-dynamical model of the stratosphere which takes into account the feedback effect of temperature on ozone concentration is used to investigate the possible changes in the seasonal and latitudinal variations of temperature, ozone, and other minor constituents associated with the UV flux variations from the solar minimum to the solar maximum. The transport model determines the zonal wind, non-zonal eddy transports, and eddy momentum fluxes, while the photochemical system includes the important reactions affecting the concentrations of ozone and other relevant trace constituents in the stratosphere. Experiments are carried out by changing the position of the sun every day and evaluating the daily average photodissociation integrals for the relevant minor constituents and ozone heating. Values are found for the ambient stratosphere and a stratosphere perturbed by solar cycle variations, and a rough comparison is made with observed ozone and temperature trends.
The operating characteristics of a flashlamp-pumped IBr laser were investigated to evaluate its suitability for solar-pumped laser applications. A peak power of 350 W/sq cm at 2.7 microns was achieved at 12-torr IBr pressure. At 500-J flashlamp energy, the IBr output saturated; a gain of 0.17% per cm was measured for IBr. Neon was found to be effective for enhancing the recombination of the photodissociation products. With neon as a buffer gas, the laser pulse length was extended to 53 microsec. The termination of the laser pulse, within the flashlamp pulse, is thought to be due to the temperature rise in the gas. Increasing the IBr initial temperature decreased the lasing output. At 300 C, output dropped to approximately one-half the room temperature value. The dominant quencher is thought to be atomic iodine. IBr was found to couple better to the flashlamp energy than C3F7I.
A model atmosphere of Venus for altitudes between 100 and 178 km is presented for the dayside and nightside. Densities of CO2, CO, O, N2, He, and O2 on the dayside, for 0800 and 1600 hours local time, are obtained by simultaneous solution of continuity equations. These equations couple ionospheric and neutral chemistry and the transport processes of molecular and eddy diffusion. Photodissociation and photoionization J coefficients are presented to facilitate the incorporation of chemistry into circulation models of the Venus atmosphere. Midnight densities of CO2 CO, O, N2, He, and N are derived from integration of the continuity equations, subject to specified fluxes. The nightside densities and fluxes are consistent with the observed airglow of NO and O2(1 Delta). The homopause of Venus is located near 133 km on both the dayside and nightside.
Interstellar UV radiation cannot penetrate into the interior of dense clouds, and cosmic ray ionization is thought to be the sole driver of the gas phase chemistry. However, cosmic ray energy deposition also involves electronic excitation of the absorbing gas. It appears, therefore, possible that emissions resulting from these excitations might maintain a significant flux of chemically effective UV photons in the interior of dense clouds. The present investigation is concerned with this possibility, taking into account a simplified approach. This approach involves a consideration of Lyman and Werner band photons of molecular hydrogen and their relevance to C and CO chemistry. The results of this investigation suggest that a chemically significant flux of UV photons may exist inside dense clouds due to cosmic ray excitation of the various band systems of hydrogen. These photons would recover C I from the CO reservoir in dense clouds via photodissociation at an order of magnitude faster rate than that possible in connection with the reaction of CO with He(+).
The distribution of atomic and molecular hydrogen in the expanding circumstellar envelopes of cool evolved stars is discussed. The main concern is to evaluate the effects of photodestruction of H2 by galactic UV radiation, including shielding of the radiation by H2 itself and by dust in the envelope. One of the most important parameters is the H/H2 ratio which is frozen out in the upper atmosphere of the star. For stars with photospheric temperatures greater than about 2500 K, atmospheric models suggest that the outflowing hydrogen is mainly atomic, whereas cooler stars should be substantially molecular. In the latter case, photodissociation of H2 and heavy molecules contribute to the atomic hydrogen content of the outer envelope. The presented estimates indicate that atomic hydrogen is almost at the limit of detection in the C-rich star IRC + 10216, and may be detectable in warmer stars. Failure to detect it would have important implications for the general understanding of circumstellar envelopes.
The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N2, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N2 excitation and dissociation rates and the ionization rates of the principal atmospheric consituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source.
The phosphine photochemistry on Saturn is studied with a 1D photochemical model. The PH3 concentration is rapidly depleted with height (scale height 3.5 km) in the upper troposphere. Formation of P, a probable precursor of P4, (a potential red chromophore in the atmosphere), is highly improbable unless the rate constant for the recombination reaction PH + H2 + M yields PH3 + M is less than 10 to the -41st cm exp 6/molecule-squared sec. Coupling of PH3 and hydrocarbon photochemistry, specifically the C2H2 catalyzed photodissociation of CH, is important. Column production rates of the organophosphorus compounds CH3PH2 and HCP of 3 x 10 to the 8th/sq cm sec are predicted, with potentially observable column densities of greater than 1 x 10 to the 17th/sq cm.
A solar pumped laser is described in which the lasant is a gas that will photodissociate and lase when subjected to sunrays. Sunrays are collected and directed onto the gas lasant to cause it to lase. Applications to laser propulsion and laser power transmission are discussed.
Current theoretical models of Saturn's ionosphere are difficult to reconcile with the ionospheric electron density profiles obtained from the Pioneer and Voyager radio occultation observations and the large diurnal variation of maximum ionospheric electron density deduced from studies of Saturn lightning discharges. A model of Saturn's ionosphere is proposed in which water plays a major role as a minor constituent present by virtue of downward diffusion from an external source. This model of the Saturn ionosphere is a classical 'F2' type layer resulting from the photodissociative production of H(+) from H2 and rapid chemical loss due to a series of charge exchange reactions with water. A planet-wide influx of about 4x10 to the 7th power molecules/sec/sq cm of water from the rings is consistent with the observed ionospheric electron densities and estimates of influx due to micrometeoride bombardment of the rings. An enhanced influx of water occurs at latitudes (-38 deg, +44 deg) magnetically connected to the inner edge of Saturn's B ring which results from an electromagnetic erosion process contributing substantially to the (local) upper atmosphere water content. Present day influx at these latitudes is possibly as large as 2x10 to the 9th power molecules/sec/sq cm.
Voyager 1 and 2 sensor data are analyzed in order to derive the composition, energy spectra, and spatial distribution of energetic ions in the Saturn magnetosphere. In order of abundance, the major species are H, H2(+), He, H3(+), C, and O. The fluxes of all species decreased inside the orbit of Dione, and nearly vanished in the 'slot' region within the orbit of Tethys. Both satellite absorption and precipitation due to pitch angle scattering may be important loss processes in that region. In the outer magnetosphere, photodissociation rapidly destroys a large fraction of the H2(+) ions, but dissociation by impact with neutral H atoms is faster for H2(+) ions in the lowest vibrational state. The ground state lifetime of about 23 days places a limit of about 10-100 days on the mean overall residence time for energetic ions in Saturn's magnetosphere.
Synchrotron radiation has been used to measure the excitation function for producing fluorescence in the 1050-1800 A region through photodissociative excitation of N2 in the 400-620 A region and the photoabsorption cross section of N2 in the 600-670 A region. From the fluorescence excitation function there is found (1) a molecular state(s) in the 21-22.5 eV region which has not been observed in absorption and photoionization spectra, (2) structures which correlate with known two-electron excited Rydberg states, and (3) a decrease in fluorescence cross section at photon energies greater than 23.8 eV, which indicates the existence of new competing processes. The observed molecular state(s) as well as the weak absorption structure in the 19-21 eV region are tentatively assigned as a Rydberg series leading to the D2Pi(g) of N2(+). The newly observed Rydberg states in the 21-22.5 eV region may account for the unusual behavior recently reported in photoelectron spectroscopy results.
Synchrotron radiation light source measurements were conducted in the 105-155 nm region to obtain the photoabsorption and fluorescence cross sections of C2H2, and the quantum yield for producing the C2H-asterisk fluorescence from C2H2's photodissociation was measured in the 106-136.5 nm region. The products of the radiative lifetime quenching rate constant increase with excitation wavelengths. The data obtained on lifetimes, quenchings, and fluorescence spectra point to an upper state of the C2H-asterisk fluorescence that is well bound, while the lower state is a repulsive or weak-bound state.
The rates and altitudes for the dissociation of atmospheric constituents on Titan are calculated for solar ultraviolet radiation, the solar wind, Saturn magnetospheric particles, the Saturn co-rotating plasma, and cosmic rays. Laboratory experiments show that a variety of simple gas phase organic molecules and more complex organic solids called tholins are produced by such irradiations of simulated Titanian atmospheres. Except for ultraviolet wavelengths longward of the methane photodissociation continuum, most dissociation events occur between about 3100 and 3600 km altitude, corresponding well to the region of EUV opacity detected by Voyager. For a wide variety of simple to moderately complex organic gases in the Titanian atmosphere, condensation occurs below the top of the main cloud deck at about 2825 km. It is proposed that such condensates, begining with CH4 at about 2615 km comprise the principal mass of the Titan clouds. There is a distinct tendency for the atmosphere of Titan to act as a fractional distillation device, molecules of greater complexity condensing out at higher altitudes.
The diffuse 157.74 micron (CII) emission from the Galaxy was sampled at several galactic longitudes near the galactic plane including complete scan across the plane at (II) = 2.16 deg and (II) = 7.28 deg. The observed (CII) emission profiles follow closely the nearby (12)CO (J=1to0) emission profiles. The (CII) emission probably arises in neutral photodissociation regions near the edges of giant moleclar clouds (GMC's). These regions have densities of approximately 350 cm(-3) and temperatures of approximately 300 K, and amount to 4x10(8) solar mass of hydrogen in the inner Galaxy. The total 157.74 micron luminosity of the Galaxy is estimated to be 6x10(7) solar luminosity. Estimates were also made of the galactic emission in other far-infrared (FIR) cooling lines. The (CII) line was found to be the dominant FIR emission line from the galaxy and the primary coolant for the warm neutral gas near the galactic plane. Other cooling lines predicted to be prominent in the galactic spectrum are discussed. The 145.53 micron (OI) emission line from the Orion nebula was also measured.
The cometary hydrogen particle-trajectory model was used successfully to analyze observations of Comet P/Encke. The Pioneer Venus Orbiter Ultraviolet Spectrometer observed the comet at 1216A (hydrogen Lyman-alpha) on 15 April 1984, when the comet was .58 AU from the Sun and 1.02 AU from Venus. The analysis implies a production rate at .58 AU of 3.3 x 10 to the 28th power/sec of the water molecules which photodissociate to produce the observed hydrogen.