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Srivastava, S. K.

Publications and source records attributed to Srivastava, S. K..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Laboratory studies of UV emissions of H2 by electron impact - The Werner- and Lyman-band systems

The vacuum ultraviolet electron-impact-induced fluorescence emissions of H2 were studied for the Lyman and Werner band systems in the range of 120-170 nm, using an optical system containing a photomultiplier and a spectrometer, over an energy range from threshold to 400 eV. The emission cross sections for the Lyman and Werner transitions at 100 eV are determined. The cross-section ratio is in excellent agreement with theoretical calculations and experimental data for the optical oscillator strengths. The cross-section for cascading to the B state is stated as a percentage of the total emission cross section at both 100 and 300 eV, increasing substantially at 20 eV. The vibrational population distribution of the B state is found to be a function of electron-impact energy as the importance of cascading relative to direct excitation changes with electron-impact energy.

Ajello, J. M.↗
NASA NTRS1981

UV studies of electron impact excitation of CS2

The first measurements of emission cross sections of CS2 by electron impact over the wavelength interval 110-510 nm are reported. Absolute emission cross sections are obtained at 100 eV for all spectral features observed in this interval. Emission cross sections as a function of electron energy (0-125 eV) are reported for several of the principal electronic transitions.

Ajello, J. M.↗
NASA NTRS1981

Electron scattering by highly polar molecules. III - CsCl

Utilizing a crossed electron-beam-molecular-beam scattering geometry, relative values of differential electron scattering cross sections for cesium chloride at 5 and 20 eV electron impact energies and at scattering angles between 10 and 120 deg have been measured. These relative cross sections have been normalized to the cross section at 15 deg scattering angle calculated by the hybrid S-matrix technique. In the angular range between 0 and 10 deg and between 120 and 180 deg extrapolations have been made to obtain integral and momentum transfer cross sections. An energy-loss spectrum is also presented which gives various spectral features lying between the 4 and 10 eV regions in CsCl.

Vuskovic, L.↗
NASA NTRS1981

Excitation cross sections for krypton by electrons in the 15-100-eV impact-energy range

Differential, integral, and momentum-transfer cross sections have been determined for the excitation of the 24 lowest electronic states of Kr (some of the transitions are unresolved). The inelastic-scattering cross sections were normalized to the absolute scale with the help of the elastic-scattering differential cross sections (DCS's) which in turn were normalized with respect to absolute He DCS's. The impact energies were 15, 20, 30, 50, and 100 eV and the DCS's were obtained over the range of 5-135 deg scattering angles. The error limits associated with the differential, integral, and momentum-transfer cross sections have been estimated at 25%, 38%, and 46%, respectively.

Trajmar, S.↗
NASA NTRS1981

Cross section for Ly-alpha emission by electron impact on methane

Utilizing Lyman-alpha emission cross sections for H2 as secondary standards, absolute values of Lyman-alpha emission cross sections for CH4 have been obtained for electron impact energies varying from threshold to 100 eV. A crossed electron beam-molecular beam geometry was employed and the Lyman-alpha radiation was detected at 90 deg and 45 deg with respect to the incident electron beam by a solar blind photomultiplier in tandem with an oxygen filter. The results are compared with previous measurements. Appreciable differences among the various experimental data are found.

Orient, O. J.↗
NASA NTRS1980

Absolute elastic differential electron scattering cross sections for He - A proposed calibration standard from 5 to 200 eV

Absolute differential, integral, and momentum-transfer cross sections for electrons elastically scattered from helium are reported for the impact energy range of 5 to 200 eV. Angular distributions for elastically scattered electrons are measured in a crossed-beam geometry using a collimated, differentially pumped atomic-beam source which requires no effective-path-length correction. Below the first inelastic threshold the angular distributions were placed on an absolute scale by use of a phase-shift analysis. Above this threshold, the angular distributions from 10 to 140 deg were fitted using the phase-shift technique, and the resulting integral cross sections were normalized to a semiempirically derived integral elastic cross section. Depending on the impact energy, the data are estimated to be accurate to within 5 to 9%.

Register, D. F.↗
NASA NTRS1978

Absolute elastic differential electron scattering cross sections in the intermediate energy region. IV - CO

Using a crossed electron beam-molecular beam scattering geometry and a relative-flow technique, ratios of elastic differential cross sections of CO to those of He have been measured at electron impact energies of 3, 5, 7.5, 9.9, 15, 20, 30, 50, 75, and 100 eV. At each energy, an angular range of 15 to 130 deg has been covered. These ratios have been multiplied by previously known He elastic differential cross sections to obtain elastic differential cross sections for CO. Since pure rotational excitations were not resolved, the elastic differential cross sections are a sum of elastic and pure rotational excitations at room temperature. From a knowledge of differential cross sections (DCS), integral and momentum transfer cross sections have been calculated. Both the DCS and integral cross sections are compared at 50, 75, and 100 eV to a recent two-potential theory of e-molecule scattering. Present results show that the isoelectronic molecules CO and N2 have very similar magnitudes and shapes of their differential cross sections.

Tanaka, H.↗
NASA NTRS1978

Electron impact excitation of autoionising states of krypton

Energy-loss spectra of krypton in the region between 21 and 29 eV have been obtained at electron impact energies of 30, 60 and 100 eV. For each energy, the angular distribution of intensities has been measured at 5, 10 and 15 deg scattering angles. Assignments of spectral features found in this region are suggested and a comparison is made with previous measurements.

Srivastava, S. K.↗
NASA NTRS1977

Electron-impact excitation of the low-lying electronic states of HCN

The first study of the low-energy electron-impact excitation of low-lying electronic transitions in the HCN molecule is reported. Measurements were made at incident electron energies of 11.6 and 21.6 eV in the energy-loss range of 3-10 eV, and at scattering angles of 20-130 deg. Inelastic scattering spectra were placed on the absolute cross-section scale by determining first the ratio of inelastic-to-elastic scattering cross sections, and then separately measuring the absolute elastic scattering cross section. Several new electronic transitions are observed which are intrinsically overlapped in the molecule itself. Assignments of these electronic transitions are suggested. These assignments are based on present spectroscopic and cross-sections measurements, high-energy electron scattering spectra, optical absorption spectra, and ab initio molecular orbital calculations.

Chutjian, A.↗
NASA NTRS1977

Experimental differential and integral electron impact cross sections for the B /1-Sigma-u +/ state of H2 in the intermediate-energy region

Utilizing a crossed electron-beam-molecular-beam scattering technique, differential electron impact cross sections (DCS) for the excitation of the nu prime = 2 vibrational band of the B (1-Sigma-u +) state of H2 have been measured and are presented for the first time. These measurements were made at electron impact energies of 15, 20, 30, 40, 50 and 60 eV. At each energy, DCS between scattering angles of 10 and 135 deg were determined. They were then extrapolated to 0 and 180 deg scattering angles to obtain the integral cross sections. These integral cross sections and the Frank-Condon factor for the nu prime = 2 band were used to calculate the total cross sections for the excitation of the B (1-Sigma-u +).

Srivastava, S. K.↗
NASA NTRS1976

Photoabsorption spectrum of UF6 by electron impact

The photoabsorption cross section of UF6 in the wavelength region between 206.7 nm (2.5 eV) and 43.5 nm (28.5 eV) has been derived from the UF6 electron energy loss spectra at a 5-deg scattering angle and at incident electron energies of 75 and 100 eV. The shape of the resulting optical spectrum agrees well with available high-resolution photoabsorption measurements in the 400 nm (3.1 eV) to 200 nm (6.3 eV) region. Below 200 nm, where no photoabsorption data are available, there is strong absorption and appreciable structure present. Absolute values of absorption cross sections have been obtained by normalizing the present relative data to the recently measured photoabsorption value at 225.5 nm.

Srivastava, S. K.↗
NASA NTRS1976

Electron-impact excitation of UF6 at an electron energy of 20 eV in the energy-loss range of 0-10 eV

A technique combining electron impact excitation and optical absorption spectroscopy was applied to UF6. The crucial features of the experiment were that: (1) the electron optics was differentially pumped relative to the scattering chamber and (2) the target UF6 beam was condensed on a liquid nitrogen cold trap placed immediately above the scattering center. Energy loss spectra are presented at an incident electron energy of 20 eV and at scattering angles between 20 and 135 degrees. It is shown that no transitions are found below the first-detected feature at 3.0 eV and an optically forbidden excitation is found at 4.2 eV. A fairly strong optical absorption at 4.8 eV is observed to 'fill-in' at a scattering angle of 20 degrees but is practically absent at higher angles.

Chutjian, A.↗