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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Determination of diurnal average photodissociation rates

The calculation of diurnal average photolysis rates in models of planetary atmospheres can be done efficiently and easily by utilizing the results of empirical numerical fitting procedures. Analytic expressions are given for the ratio of the diurnal average differential photolysis rate to that at noon, as a function of optical depth, latitude and solar declination. The accuracy is better than + or - 6% for all altitudes, all seasons of the year and latitudes up to 60 deg.

Rundel, R. D.↗
NASA NTRS1980

Neutral cometary atmospheres. I - An average random walk model for photodissociation in comets

A method for constructing physically realistic photochemical models, taking into account the isotropic ejection of dissociated molecular fragments as well as radiation pressure acceleration, has been developed using Monte Carlo techniques. The effect of the isotropic ejection, as opposed to the radial motion arbitrarily assumed in Haser's model, is adequately described by a simple average random walk model. It is shown that measured radial (Haser) scale lengths are in fact only lower limits to a range of possible true scale lengths for a given brightness profile, which explains the current discrepancies between observed scale lengths and those predicted by photochemistry.

Combi, M. R.↗
NASA NTRS1980

A simplified method for the calculation of diurnally averaged photodissociation rates

Expressions are given for the calculation of the diurnally averaged transmission of solar flux from a series summation involving relatively few terms and readily available mathematical functions. It is shown that these expressions can be scaled to remove much of the dependence on latitude and solar declination so that at large optical depth, they become asymptotic to a function which depends only on the optical depth at noon and displays an inverse-square-root variation. From this variation, a simple analytic form containing one adjustable parameter is proposed. The functional dependence of this parameter on latitude and solar declination is determined by numerical fitting procedures. The accuracy of the analytic approximation is better than 4.5% for all altitudes, latitudes and seasons of the year.

Boughner, R. E.↗
NASA NTRS1982

Fluorescence yields from photodissociation of SO2 at 1060-1330 A

The absorption and fluorescence cross sections of SO2 were measured in the 1060-1330 A region using synchrotron radiation as the light source. Several absorption bands were observed and assigned tentatively to the Rydberg states converging to the SO2(+) states. It is shown that the apparent quantum yields for excitation wavelengths longer than 1200 A are small, and are presumably emitted from the O(1S) and S(1S) metastable species. At shorter excitation wavelengths, the fluorescence cross sections increase at the threshold wavelengths of possible SO2 dissociation processes.

Suto, M.↗
NASA NTRS1982

Photodissociation yields of CS2 at 1060-1520 A

Photoabsorption and fluorescence cross sections of CS2 were measured in the 1060-1520 A region using synchrotron radiation in order to provide information needed for modeling the CS abundance in the interstellar medium. The absorption in the 1060-1200 A region is smooth and continuous, except for the presence of a strong absorption band at 1117 A, while for wavelengths longer than 1200 A, the absorption spectrum shows the structure of Rydberg states. The fluorescence in the 1900-3000 A region begins to appear at 1335 A, and is mainly the CS(A 1Pi-Chi 1Sigma +) transition in the 2400-2800 A region. The fluorescence in the 1900-8000 A region begins to appear at 1490 A, and, in addition to the previous transition, is composed of the transitions of S(1S0-3P1) at 4589 A and of CS(d 3 Delta, a-prime 3Sigma +, and a 3Pi-Chi 1Sigma +) at 3000-4000 A. The quantum yields for the production of fluorescence are determined to have maxima of 13 and 7.5% at 1235 A for the fluorescence in the 1900-8000 and 1900-3000 A regions, respectively.

Day, R. L.↗
NASA NTRS1982

Fluorescence yields from photodissociation of OCS at 1060-1240 A

The cross sections for the production of fluorescence (3000-8000 A) from excited photofragments of OCS were determined in order to gain a better understanding of the role of carbonyl sulfide in interstellar photochemistry. Synchrotron radiation was used as the light source for the measurements in the 1060-1240 A region, as it is suitable for measuring the absorption cross section in the structured region. It was found that the quantum yields for the production of various CO triplet states are less than 1.5% at the observed excitation photon wavelengths.

Lee, L. C.↗
NASA NTRS1983

Photodissociation of NH3 at 106-200 nm

The absorption and fluorescence cross sections for NH3 are measured in the 106-200 nm region using synchrotron radiation as the light source. The threshold wavelengths for the production of the NH (b to X) and NH (c to a) emissions from NH3 dissociation are measured and compared with previous measurements. The heat of formation of NH determined from these thresholds agrees well with the value determined from thermochemical data. The process of dissociation of NH3 into NH2(2AL) and H2(S) has a significant quantum yield whose maximum at 134 nm is about twice the NH(c) production yield. All the vibronic levels of the B and C states produce the NH2(2A1) emission, contrary to previous theoretical interpretations.

Suto, M.↗
NASA NTRS1986

Application of an InGaAsP diode laser to probe photodissociation dynamics - I(asterisk) quantum yields from n- and i-C3F7I and CH3I by laser gain vs absorption spectroscopy

Initial measurements on I-asterisk yields of alkyl iodides at 266 nm are reported using gain vs. absorption spectroscopy with an InGaAsP diode probe laser. The results are 102 percent + or - 4 percent, 102 percent + or - 7 percent, and 73 percent + or - 4 percent for n-C3F7I, i-C3F7I, and CH3I respectively. Future prospects for the development of diode laser systems and for their use in dynamical studies are discussed.

Hess, W. P.↗
NASA NTRS1986

Photodissociation of metastable O2(a 1 Delta g): Implications for stratospheric O3

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.

Dalgarno, A.↗
NASA NTRS1987

Photoabsorption and photodissociation of molecules important in the interstellar medium

A windowless apparatus was constructed and used to measure the photoabsorption and fluorescence cross sections of molecules in the extreme ultraviolet region. Photoabsorption and fluorescence cross sections of H2O, D2O, H2S, D2s, and Co were measured in the 50 to 200 nm region. These quantitative data are currently needed for the determination of the formulation and destruction rates of molecules in the interstellar medium.

Lee, Long C.↗
NASA NTRS1989

Photoabsorption and photodissociation of molecules important in the interstellar medium

Photoabsorption and fluorescence cross sections of molecules important in the interstellar medium were measured in the 90 to 200 nm region using synchrotron radiation, excimer laser, and condensed discharge lamps as light sources. The quantitative spectroscopic data are currently needed for the modeling of formation and destruction rates of molecules by the interstellar radiation field. Fluorescences from excited photofragments produced by vacuum ultraviolet radiation of molecules are dispersed to identify the emitting species. The fluorescence data are useful for the identification of emission sources in interstellar clouds.

Lee, Long C.↗
NASA NTRS1988

CH3O(A-X) fluorescence from photodissociation of dimethyl ether

The paper reports the quantitative photoabsorption and fluorescence cross sections of dimethyl ether (DME) measured with synchrotron radiation. Determinations were also made of the absolute fluorescence cross sections, the fluorescence quantum yield, and the radiative lifetime.

Suto, Masako↗
NASA NTRS1988

Fluorescence yields from photodissociative excitation of HCOOH, HCOOCH3, and CH3COOH in the vacuum-ultraviolet region

The photoexcitation processes of HCOOH, HCOOCH3, and CH3COOH were studied in the vacuum-ultraviolet region by using synchroton radiation and a pulsed discharge lamp as light sources. The absorption and fluorescence cross sections of these molecules were measured in the 106-250-nm region. Fluorescences were detected from photoexcitation of HCOOH and HCOOCH3, but not from CH3COOH. Fluorescence produced at 123.9 nm was dispersed and identified as the excited OH and HCOO radicals. Fluorescence quantum yields of HCOOH and HCOOCH3 increase with decreasing excitation wavelengths with maxima of 5 and 0.3 percent at 106 nm, respectively.

Suto, Masako↗
NASA NTRS1990

Laboratory studies of photodissociation processes relevant to the formation of cometary radicals

The strength of the C2(d 3 Pi g yields a 3 Pi u) Swan band emission in the spectra of cometary comae identifies this species as a prominent constituent of the coma gas. It was previously suggested that the formation of cometary C2 proceeds via the secondary photolysis of the C2H radical. The detection of C2H in the interstellar medium and the recent analysis of the radial variation in C2(delta V=O) surface brightness of Comet Halley support the postulate that C2 is a third-generation molecule. Measurement of the C2 and C2H translational energy distributions produced from the multiphoton dissociation (MPD) of acetylene at 193 nm are identified . Time-resolved FTIR emission studies of the nascent C2H radical formed in the C2H2 yields C2H + H reaction verify that this species is produced both vibrationally and electronically excited. A survey of the internal energy distributions of the C2 fragments produced from the MPD of acetylene using a high intensity ArF laser is currently in progress in the laboratory. Recent experiments have focused on the measurement of rotational energy distribution for the C2(A 1 Pi u, a 3 Pi u) fragments. The C2(a 3 Pi u) detection capability is currently being improved by performing this experiment in a molecular beam, thus allowing for discrimination between initial emission and laser-induced fluorescence (LIF). Although the experiments performed to date provide considerable evidence in support of C2H yields C2 + H reaction, there is an important distinction to be made when comparing the laboratory conditions to those typically found in comets. The C2H radicals generated in the laboratory experiments are formed vibrationally and/or electronically excited. Any rotationally/vibrationally excited C2H present in cometary comae will quickly undergo radiative relaxation in the infrared to their lowest rotational and vibrational state. Experiments are currently under way to confirm the cometary formation of C2 via the VUV dissociation of cold C2H.

Urdahl, R. S.↗
NASA NTRS1989

Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres

The solution of the generalized two-stream approximation for radiative transfer in homogeneous multiple scattering atmospheres is extended to vertically inhomogeneous atmospheres in a manner which is numerically stable and computationally efficient. It is shown that solar energy deposition rates, photolysis rates, and infrared cooling rates all may be calculated with the simple modifications of a single algorithm. The accuracy of the algorithm is generally better than 10 percent, so that other uncertainties, such as in absorption coefficients, may often dominate the error in calculation of the quantities of interest to atmospheric studies.

Toon, Owen B.↗