Engineering Papers⌕ Search

Engineering topics

Lee, L. C.

Publications and source records attributed to Lee, L. C..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Fluorescence yields from photodissociative excitation of chloromethanes by vacuum ultraviolet radiation

The photoabsorption and fluorescence cross sections of chloromethanes were measured in the 105-220 nm region using synchrotron radiation as a light source. The fluorescence threshold for CCl4 is at 152 nm with a maximum yield of 3 percent at 113 nm. The fluorescence results from the CCl2(A-X) system. For CHCl3, the fluorescence threshold is at 155 nm with a maximum yield of 0.6 percent at 110 nm. For CH2Cl2, the threshold is at 137 nm with a maximum yield of 0.35 percent at 107 nm. The fluorescence yield of CH3Cl is very small with an upper limit of 0.02 percent. The photodissociation processes are discussed in accord with the fluorescence data observed. Vibrational structures in CHCl3 and CH3Cl2 are observed and classified into progressions.

Lee, L. C.↗
NASA NTRS1986

Fluorescence from VUV excitation of formaldehyde

The 105-180-nm photoabsorption and fluorescence cross sections of H2CO are determined experimentally using a synchrotron-radiation light source and the apparatus described by Lee (1980). The results are presented in tables and graphs and discussed. VUV emission with threshold wavelength 140.3 nm is detected and attributed to the A 1Pi - X 1Sigma(+) system of CO, and UV emission from the HCO (B-X) system is found below a 147.5-nm threshold, the H-HCO dissociation energy having an upper limit of 3.61 + or - 0.03 eV. At 116 nm, the maximum quantum yields of the VUV and UV fluorescences are about 1.6 and 0.23 percent, respectively.

Suto, Masako↗
NASA NTRS1986

Quantitative photoabsorption and fluorescence study of HCl in vacuum ultraviolet

The photoabsorption and fluorescence cross sections of HCl were measured in the 106-185 nm region. Sharp absorption bands appear at wavelengths shorter than 135 nm, and fluorescence occurs at several excited states. The fluorescence cross sections are generally quite small, indicating that the excited states are strongly predissociative. The molecular processes for producing the VUV and UV fluorescences are investigated, and the Rydberg characteristics of the strong absorption bands are discussed.

Nee, J. B.↗
NASA NTRS1986

Photoabsorption and photodissociation of molecules important in the interstellar medium

In the period from May 15, 1985 to May 14, 1986, the photoabsorption and photodissociation cross sections of the interstellar radical of SO and the interstellar molecules of HCl, H2CO, and CH3CN were measured and the results were reported in scientific papers. In the meantime, a windowless apparatus is used to measure the photoabsorption and photodissociation cross sections of CO in the 90-105 nm region. The optical data obtained in this research program are needed for the determination of the formation and destruction rates of molecules and radicals in the interstellar medium. Accomplishments in this research period are summarized below.

Lee, L. C.↗
NASA NTRS1986

Quantitative VUV spectroscopy of Cl2

The photoabsorption and fluorescence cross sections of Cl2 were measured in the 105-145 nm region using synchrotron radiation as a light source. The oscillator strengths for the major absorption bands were calculated from the measured absorption cross sections. The measured oscillator strengths for the 2 3Pi u and 2 1Pi u - X 1Sigma(+)g transitions agree quite well with the theoretical values. The absorption spectrum was analyzed in accord with the excited electronic states calculated by Peyerimhoff and Buenker (1981). The Rydberg series converging to the first and second ionization potentials were classified. The vibrational levels of the 2 1Sigma(+)u ionic state were determined up to v prime = 15 from the fluorescence excitation spectrum. It was observed that a band with peak at 108.3 nm produces VUV fluorescence, but it produces UV fluorescence only by a collisional excitation process.

Lee, L. C.↗
NASA NTRS1986

Vacuum ultraviolet photoabsorption study of SO

The photoabsorption of SO was investigated by producing the radicals from a pulsed discharge of a trace of SO2 in Ar. The photoabsorption cross section of SO in the 115-135 nm region was measured. A repulsive potential curve is constructed from the observed photoabsorption spectrum.

Nee, J. B.↗
NASA NTRS1986

Collisional tearing instability in the current sheet with a low magnetic Lundquist number

An MHD model is used to calculate the growth rate of the collisional tearing mode instability in a current layer with a Lundquist number in the range 3-100,000. The maximum growth rate is found to be a function of the Alfven transit time in the current sheet and the Lundquist number. When the Lundquist number is below 50, the growth rate asymptotically approaches a value of about -0.075/transit time. The discovery of the dependence of the asymptotic value is considered to be of use in studies of the current sheet in the diffusion region of a magnetic reconnection configuration, where the Lundquist number will be in the range 2-100.

Lee, L. C.↗
NASA NTRS1986

Measurements of HO2 chemical kinetics with a new detection method

Research for the period from December 1, 1985 to May 31, 1986 is discussed, i.e., the reaction rate constant of HO2+O3 has been measured with a discharge-flow-tube apparatus. The HO2 radical was detected by the OH(A-X) photofragment emission produced from photodissociative excitation of HO2 at 147 nm. In the meantime, the optical emissions produced by the vacuum ultraviolet excitation of chemical species in the flow tube were investigated and used to examine the possibility for their interference with the HO2 detection. The research results are summarized below.

Lee, L. C.↗
NASA NTRS1986

Quantitative photoabsorption and fluorescence study of H2O and D2O at 50-190 nm

The photoabsorption cross sections and the fluorescence quantum yields of H2O and D2O were measured in the 50-190 nm region using synchrotron radiation as a light source. The oscillator strengths for the Rydberg states of H2O and D2O were determined from the absorption cross sections measured. The processes for the production of fluorescence from the excited species H(asterisk) (n greater than 2), D(asterisk) (n greater than 2), OH(asterisk) (A) and OD(asterisk) (A) are discussed. The upper limit for the dissociation energy of D(D-OD) was determined, from the threshold of the OD(A-X) fluorescence, to be 5.14 + or - 0.01 eV. The upper limit for the cross section of visible fluorescence from the excited H2O(+) ions was determined to be 2 x 10 to the -19th sq cm. A comparison between the photoexcitation spectra of H2O and D2O is made.

Lee, L. C.↗
NASA NTRS1985

Photoabsorption cross section of CH3CN - Photodissociation rates by solar flux and interstellar radiation

The photoabsorption cross section of CH2CN vapor was measured in the 106-180 nm region using synchrotron radiation as a light source. The cross section and the quantum yield for the production of CN (A, B-X) fluorescence were measured and were used to infer the photodissociation cross section of CH3CN. The cross sections were used to calculate the photodissociation rates of CH3CN by the solar flux and by the interstellar radiation. In both the stratosphere and the troposphere, the solar photodissociation of CH3CN is negligible in comparison with chemical degradation.

Suto, M.↗
NASA NTRS1985

Measurements of HO2 chemical kinetics with a new detection method

In this research program, HO2 was detected by the OH(A-X) photofragment from dissociative excitation of HO2 at 147 nm. This detection method was applied to measure the reaction rate constant of HO2 + O3. This reaction rate constant is needed for the understanding of stratospheric chemistry. Since C12 was used in the flow system, photoexcitation of C12 may produce fluorescence to interfere with the measurements. Thus, the photoexcitation process of C12 in the vacuum ultraviolet region was also examined in this research period using synchrotron radiation as a light source. The research results are summarized.

Lee, L. C.↗
NASA NTRS1985

Photoabsorption and photodissociation of molecules important in the interstellar medium

The photoabsorption and photodissociation cross sections of several interstellar molecules and radicals in the 105 to 210 nm region were measured. The research results accomplished are briefly described. Photoabsorption cross sections of OD and CN, and photoabsorption and photodissociation of HCl, and photoabsorption and photodissociation cross sections of CH3OH are discussed.

Lee, L. C.↗
NASA NTRS1985

Photodissociation rates of OH, OD, and CN by the interstellar radiation field

The photoabsorption cross sections for OH, OD, and CN in the vacuum ultraviolet region are measured. The cross sections for the hydroxyl radicals are of the order of 10 to the -17th sq cm, but the photoabsorption for CN is so low that only an upper limit of 2 x 10 to the -18th sq cm is obtained. The molecular photodissociative processes are discussed. The photodissociation cross sections are inferred from the photoabsorption cross sections. On the basis of the measured data, the photodissociation rates by the interstellar radiation field are computed and discussed.

Nee, J. B.↗
NASA NTRS1985

A note on the nature of the distant geomagnetic tail magnetopause and boundary layer

In the present comparison of recent plasma and magnetic field measurements of the distant geomagnetic tail magnetopause and boundary layer with numerical simulation results for an 'open' boundary, most aspects revealed are consistent with simulation results for conditions in which the normal magnetic field component at the magnetopause is generally small. On the basis of these results, the simulations of Swift and Lee (1982, 1983) are judged to be able to furnish a theoretical basis for understanding many aspects of the distant geomagnetic tail magnetopause and mantle boundary layer.

Gosling, J. T.↗
NASA NTRS1984

Photoabsorption cross section of OD at 115-180 nm

The photoabsorption cross sections of OD in the 115-180 nm region were measured. The OD radicals were produced from a pulsed discharge in a mixture containing a trace of D2O in a few Torr of argon. Results are compared with the photoabsorption of OH previously measured.

Nee, J. B.↗
NASA NTRS1984

Photoabsorption cross sections of OH at 115-183 nm

The absorption spectrum for OH was obtained in the 115-183 nm region. The OH radicals were produced by a pulse discharge of trace H2O in few torr of Ar. Absorption cross sections were obtained by calibration with absorption of the OH (X 2 Pi to A 2 Sigma +) transition. The features in the absorption spectrum are correlated with the excited states 1 2 Sigma -, D 2 Sigma -, 1 2 Delta, B 2 Sigma + and possibly others calculated by van Dishoeck, Langhoff, and Dalgarno. The measured cross sections are comparable with the calculated values.

Nee, J. B.↗
NASA NTRS1984

Photodissociation rates of molecules by the interstellar radiation field

The photodissociation rates of CO, NO, H2O, HCN, NO2, SO2, CS2, OCS, NH3, CH4, H2O2, and C2H2 dissociated by the interstellar ultraviolet background in the solar neighborhood are calculated for the 106-200 nm wavelength region. The processes for photodissociation of various interstellar molecules into fragments are discussed.

Lee, L. C.↗