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At least 109 records · Page 6
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DOE OSTI2025

Magnetic excitations from the hexagonal spin clusters in the 𝑆 = $\frac{1}{2}$ distorted honeycomb lattice antiferromagnet Cu 2 ⁢(pymca)⁢ 3 ⁢(ClO 4 )

Cu 2 ⁢(pymca) ⁢3 (ClO 4 ) (pymca: pyrimidine-2-carboxylate) consists of a slightly distorted honeycomb lattice of Cu 2+ spins, which shows no long-range magnetic order down to 0.6 K. A magnetization study revealed 1/3 and 2/3 plateau phases [A. Okutani et al., J. Phys. Soc. Jpn. 88, 013703 (2019)], which is not expected for regular honeycomb antiferromagnets. Inelastic neutron scattering experiments were performed using a powder sample to investigate the exchange interactions of this material. The spin excitations from the singlet ground state to the first three triplet states, predicted from the antiferromagnetic hexagonal spin cluster interacting with 3.9 meV, were observed. Using the exact diagonalization methods, the intercluster coupling was estimated from the excitation peak width to be about 20% of the intracluster interaction, which is consistent with the previously reported value. Finally, our exchange path model explains the anisotropic exchange interactions in the distorted honeycomb plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1977

MCSCF + Cl wavefunctions and properties of the X2-Pi and A2-Pi states of ClO

A multiconfiguration, self-consistent field plus configuration-interaction calculation has been performed on the X2-Pi and A2-Pi electronic states of ClO. The values of certain molecular properties computed from the wavefunctions agree well with those from experiment. The sum of the squares of the computed electronic transition moments between the X2-Pi and A2-Pi states at 3.07 bohr is 1.75 a.u., which corresponds to the experimental result of 1.58 plus or minus 0.20 a.u. The computed ground state dipole moment at the experimental equilibrium separation is 1.232 D, which compares well with two experimental results of 1.18 plus or minus 0.12 and 1.239 plus or minus 0.010 D. The value of the computed dissociation energy is 2.75 eV, which agrees well with the experimental value of 2.803 plus or minus 0.001 eV.

Arnold, J. O.↗
NASA NTRS1978

Rate constant for the reaction ClO + NO yields Cl + NO2

The rate constant for the reaction ClO + NO yields Cl + NO2 has been determined over the temperature range 226.7-415.4 K in a discharge flow system using a mass spectrometer as a detector. The results, expressed in the Arrhenius form, are compared with previous measurements.

Leu, M. T.↗
NASA NTRS1979

Remote measurement of ClO in the stratosphere

ClO has been detected in the stratosphere from observations of the solar spectrum in the infrared, in a small spectral interval near 12 micrometers. The observations were made with a balloon-borne laser heterodyne radiometer, launched from Palestine, Texas on September 20. By comparing high sun spectra with a number of sequential spectra taken during sunset, an altitude profile has been calculated in the 29-38 km altitude range. The results show a peak mixing ratio in excess of one ppb above 34 km, and a rapid decrease in mixing ratio with decreasing altitude below 34 km.

Menzies, R. T.↗
NASA NTRS1980

The influence of NO and ClO variations at twilight on the interpretation of solar occultation measurements

Measurement of short-lived photochemically-produced species in the stratosphere by solar occultation is difficult because the rapid variation of such species near the terminator introduces ambiguities in interpreting the measured absorption in terms of meaningful atmospheric abundances. These variations produce tangent path concentrations that are asymmetric relative to the tangent point, as opposed to the symmetrical distribution usually assumed in most inversion algorithms. Neglect of this asymmetry may yield an inverted profile that deviates significantly from the true sunset/sunrise profile. In the present paper, the influence of this effect on solar occultation measurements of ClO and NO is examined. The results show that average inhomogeneity factors, which measure the concentration variation along the tangent path and which can be calculated from a photochemical model, can indicate which species require more careful data analysis.

Boughner, R.↗
NASA NTRS1980

Kinetics study of the Cl/2P/ + Cl2O yields Cl2 + ClO reaction at 298 K

The kinetics of the Cl + Cl2O reaction, a possible source of ClO(2 Pi) radicals for atmospheric photochemical studies, are investigated at 298 K. The discharge flow/mass spectrometry and discharge flow/resonance fluorescence techniques were used to monitor the decay of C12O in the presence of excess concentrations of atomic chlorine and chlorine monoxide, respectively. The pseudo-first order rate constants obtained from both experiments are found to be in excellent agreement, averaging 9.8 + or - 0.8 x 10 to the -11th cu cm/molecule per sec. Results are consistent with the lower limit obtained by Edgecombe et al. (1957) but differ by a factor of 150 from those of Basco and Dogra (1971). The present value is also noted to be consistent with a lower value for the rate constant of the reaction of oxygen atoms with Cl2O.

Ray, G. W.↗
NASA NTRS1981

Pressure broadening of ClO by nitrogen

The temperature and pressure dependence of linewidths for the J = 11/2 - 9/2 and J = 15/2 - 13/2 transitions of ClO are determined by microwave spectroscopy. It is found that the widths for both transitions are the same within experimental error. The width parameter for nitrogen broadening is shown to be 3.35 + or - 0.17 MHz/torr at 317 K and 4.44 + or - 0.23 MHz/torr at 218 K.

Pickett, H. M.↗
NASA NTRS1983

Chlorine nitrate - The sole product of the ClO + NO2 + M recombination

A laser operating at 266 nm is used to photolyze the product of the ClO + NO2 + M reaction, and the yield of Cl atoms, measured by resonance fluorescence, is found to be identical with that from equivalent concentrations of ClONO2. This is interpreted as a strong indication that no other isomers are formed in the recombination reaction. Evidence for isomers is surveyed, and the apparent forward-and-reverse-rate constant discrepancy is explained as a probable uncertainty in the thermochemistry.

Margitan, J. J.↗
NASA NTRS1985

A critical analysis of CLO and O3 in the mid-latitude stratosphere

The Caltech one-dimensional photochemical model is used to analyze important questions concerning the upper stratospheric O3 and the key HO(x), NO(x), and ClO(x) free radicals. The model is described and first order effects of the inclusion of diffuse radiation in a spherical rather than plane parallel atmosphere at solar zenith angles close to 90 deg are assessed. A comparison is made between photochemical theory and observations for upper stratospheric O3, where local photochemistry rather than dynamics should control the O3 abundance. The photochemical equilibrium relation for odd oxygen is studied, including the important radicals involves. It is concluded that a significant model ozone deficit exists in the upper stratosphere. Possible causes for such a discrepancy are investigated in light of current model and experimental uncertainties. In particular, the observational ratio of atomic oxygen to zone appears to disagree with the model results.

Froidevaux, L.↗
NASA NTRS1986

Theoretical study of the electric dipole moment function of the ClO molecule

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2Pi using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. The band strength of the 1-0 fundamental transition is computed to be 12 + or - 2 per sq cm atm determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.

Pettersson, L. G. M.↗
NASA NTRS1987

Production of Cl2O2 from the self-reaction of the ClO radical

The species Cl2O2 has been generated in a gaseous flow system at 220-240 K by reacting Cl atoms with one of three different ClO precursors: O3, Cl2O, or OClO. The infrared spectra of the reactive mixture indicate that at least two different dimers are produced: a predominant form with bands centered at 1225 and 1057/cm attributed to ClOOCl, and a second form with a band at 650/cm attributed to ClOClO. The UV spectrum of the predominant form shows a maximum absorption cross section of about 6.5 x 10 to the -18th sq cm/molecule around 270 nm, with a wing extending beyond 300 nm. The implications of these results for the chemistry of the stratosphere are discussed.

Molina, L. T.↗
NASA NTRS1987

Kinetics of the BrO + ClO reaction and implications for stratospheric ozone

The first direct measurements of the rate constant and product distribution of the BrO + ClO reaction as a function of temperature is reported. It is shown that this reaction could account for a large fraction of the springtime ozone depletion over Antarctica and provide a source of chlorine dioxide of sufficient magnitude to explain the recent measurements of this species in the Antarctic atmosphere, provided that the stratosphere contains a sufficient quantity of bromine.

Hills, Alan J.↗
NASA NTRS1987

Determination of O2(a1Delta g) and O2(b1Sigma + g) yields in the reaction O + ClO yields Cl + O2 - Implications for photochemistry in the atmosphere of Venus

A discharge flow apparatus with a chemiluminescence detector was used to investigate the reaction O + ClO yields Cl + O2(asterisk), where O2(asterisk) = O2(a1Delta g) or O2(b1Sigma + g). It is found that the observed O2(a1Delta g) airglow of Venus cannot be explained in the framework of standard photochemistry using the experimental results obtained here and those reported in the recent literature. The possibility of an alternative source of O atoms derived from SO2 photolysis in the Venus mesosphere is suggested.

Leu, Ming-Taun↗
NASA NTRS1988

Kinetics and product studies of the BrO + ClO reaction - Implications for Antarctic chemistry

Discharge flow-mass spectrometry and flash photolysis-UV spectrometry were used to investigate the reaction of ClO with BrO over the temperature range of 220-400 K and the pressure range of 1-760 Torr. Rate constants were determined for: (1) Br + ClOO, (2) Br + OClO, and (3) BrCl + O2. It is found that the rate constants for the overall reaction and each reaction branch are inversely dependent on temperature and independent of pressure. For temperatures found in the Antarctic stratosphere, the rate coefficients for the channels yielding ClOO and OClO are a factor of 2-3 larger than previously estimated.

Sander, Stanley P.↗
NASA NTRS1988

Pulsed laser photolysis kinetics study of the O(3P) + ClO reaction

A pulsed laser photolysis technique was used to investigate the kinetics of the important stratospheric reaction O + ClO yields Cl + O2 in buffer gas over the temperature and pressure ranges of 231-367 K and 25-500 torr. The results indicate a lack of pressure dependence at 298 K over the 25-500 torr range.

Nicovich, J. M.↗
NASA NTRS1989

Rate of formation of the ClO dimer in the polar stratosphere - Implications for ozone loss

The gas-phase recombination of chlorine monoxide (ClO) has been investigated under the conditions of pressure and temperature that prevail in the Antarctic stratosphere during the period of maximum ozone (O3) disappearance. Measured rate constants are less than one-half as great as the previously accepted values. One-dimensional model calculations based on the new rate data indicate that currently accepted chemical mechanisms can quantitatively account for the observed O3 losses in late spring (17 September to 7 October). A qualitative assessment indicates that the existing mechanisms can only account for at most one-half of the measured O3 depletion in the early spring (28 August to 17 September), indicating that there may be additional catalytic cycles, besides those currently recognized, that destroy O3.

Sander, Stanley P.↗
NASA NTRS1990

Ultraviolet spectrum and chemical reactivity of the ClO dimer

The ClO dimer was prepared by photolysis (wavelength greater than 300 nm) of Cl2/Cl2O or Cl2/O3 mixtures or by photolysis of Cl2O alone. Temperatures were in the range 195-217 K, and experiments were carried out both in the gas phase and in the cryogenic solvents CF4, CO2, and N2O. Dimer cross sections in the range 190-400 nm are reported both in the gas phase and in the solvents. Results indicate that ClOOCl is the only dimer structure formed as a stable product. Upper limits of 1 x 10 to the -19th and 1 x 10 to the -20th cu cm/s are placed on the reactions of ClOOCl with O3 and with itself, respectively.

Demore, W. B.↗
NASA NTRS1990

Interpretation of aircraft measurements of NO, ClO, and O3 in the lower stratosphere

Results are presented from an October 17, 1988, flight of the NASA ER-2 stratospheric research aircraft. The flight sampled a nearly constant air mass at 20 km altitude, near 39 deg N latitude, from before sunrise until near noon. The instrumentation on board simultaneously measured NO, ClO, O3, temperature, and pressure. The measurements are combined with modeled photodissociation coefficients and known reaction kinetics to infer abundances of other important species, and the results are compared to previous estimates as a test for consistency in the understanding of the photochemical processes governing the species distributions.

Kawa, S. R.↗