Engineering topics

Huo, Winifred M.

Publications and source records attributed to Huo, Winifred M..

49 records · Page 3

NASA NTRS1991

Rotational and vibrational effects in the E 1Sigma(+)g-X 1Sigma(+)g two-photon transitions of H2, HD, and D2

This paper reports a theoretical study of the dependence of the E 1Sigma(+)g v(E) = 0, J-X 1Sigma(+)g, v(X),J transitions in H2, HD, and D2 on the initial vibrational and rotational quantum numbers v(X) and J. The results demonstrate the concerted nature of two-photon transitions. The magnitude of the two-photon transition moment increases with v(X) between 0 and 2, then decreases rapidly at higher v(X). The J dependence of the two-photon transition moment is most strongly affected by the changes in vibrational wave functions caused by centrifugal distortion of the vibrational potential. It is shown that the J variation can be used as a probe of resonant tunneling in the double-well potential of the E,F state.

Huo, Winifred M.↗

NASA NTRS1991

Quantitative determination of H2, HD, and D2 internal-state distributions by (2+1) resonance-enhanced multiphoton ionization

Results are presented of an experimental study of (2+1) resonance-enhanced multiphoton ionization (REMPI) detection of H2, HD, and D2 via the E,F two-photon transition, in which the relationship between ion signals and quantum-state populations was determined by calibration against a thermal effusive source. Vibrational and rotational correction factors were obtained for 102 rovibrational levels for v-double-prime = 0, 1, and 2 and for J-double-prime ranging from 0 to 17. These correction factors make it possible to convert ion signals into relative quantum-state populations without relying on a theoretical description of the (2+1) REMPI process.

Rinnen, Klaus-Dieter↗

NASA NTRS1990

Application of the Schwinger multichannel formulation to electron-impact excitation of the b 3Sigma(+) state of CO

The Chi 1Sigma - b 3Sigma(+) transition of CO is described using the Schwinger multichannel method in the two-state approximation, in an energy range from threshold to 20 eV. The resonance structure is analyzed by performing a partial-wave decomposition and the resonance positions are established based on the typical discontinuous behavior in the K-matrix elements as well as the pi radian change in the partial-wave eigenphases. The resonance behavior is related to the concept of core-excited resonances and the 'grandparent' model of resonances. The results are related to the formation of the negative ion by carrying out bound-state calculations on the (5 sigma)(3 s sigma)-squared core-excited Rydberg state of CO(-), and the position is found to agree well with the low-energy resonance positions.

Weatherford, Charles A.↗

NASA NTRS1990

Electron collision cross sections involving excited states

The vibrational excitation/deexcitation of vibrationally and rotationally hot N2 by electron impact, pertinent to the reactions in the flow field around a hypersonic space vehicle entering the planetary atmosphere, is calculatively investigated. It is found that the resonance position shifts to a lower energy when N2 is rotationally hot. Electron impact excitations between electronic excited states of N2 are studied using a multichannel treatment. The results show that the assumption that excited-excited cross sections are the same as ground-excited ones, and result in the same final state, is invalid. A more reliable set of data on N2 is provided.

Huo, Winifred M.↗

NASA NTRS1989

Optical Stark effect in the four-wave mixing and stimulated Raman spectra of N2

The influence of the optical Stark effect on spectral line shapes in four-wave-mixing Raman spectroscopy (FWMRS) and stimulated Raman spectroscopy (SRS) is investigated experimentally and theoretically. Using an experimental setup capable of rapid alternation between the simultaneous measurement of coherent Stokes Raman spectroscopy and inverse Raman spectroscopy at low and high intensities, together with a sophisticated frequency reference scheme, it was possible to perform a rather direct comparison between Stark-broadened and non-Stark-broadened spectra of both classes of Raman spectroscopies. The results demonstrate that SRS spectra show more Stark shift and broadening than their FWMRS counterparts. A discrepancy with theoretical results is pointed out, and an attempt is made to explain it.

Moosmuller, H.↗

NASA NTRS1988

Electron-CF4 elastic scattering in the static-exchange approximation

Differential, integral, and momentum-transfer cross sections for the elastic scattering of electrons by CF4 from 5 to 35 eV are reported. The calculations were carried out at the fixed-nuclei, static-exchange level usign the Schwinger variational principle. Analyses of the partial channel amplitudes show four broad shape resonances; two in T2, one in A1, and one in E symmetries. The positions of these resonances agree with experimental data from total-cross-section measurements, dissociative attachment studies, and low-energy electron excitation spectra, the only exception being the 8.9-eV structure in the total-cross-section data. No experimental measurements on differential cross sections are available, but comparing with the corresponding static-exchange calculations for the e-CH4 system, the e-CF4 cross sections generally are less backward peaked.

Huo, Winifred M.↗

NASA NTRS1988

The rate parameters for coupled vibration-dissociation in a generalized SSH approximation

A theoretical study of vibrational excitations and dissociations of nitrogen undergoing a nonequilibrium relaxation process upon heating and cooling is reported. The rate coefficients for collisional induced vibrational transitions and transitions from a bound vibrational state into a dissociative state have been calculated using an extension of the theory originally proposed by Schwarz (SSH) et al. (1952). High-lying vibrational states and dissociative states were explicitly included but rotational energy transfer was neglected. The transition probabilities calculated from the SSH theory were fed into the master equation, which was integrated numerically to determine the population distribution of the vibrational states as well as bulk thermodynamic properties. The results show that: (1) the transition rates have a minimum near the middle of the bound vibrational levels, causing a bottleneck in the vibrational relaxation and dissociation rates; (2) high vibrational states are always in equilibrium with the dissociative state; (3) for the heating case, only the low vibrational states relax according to the Landau-Teller theory; (4) for the cooling case, vibrational relaxation cannot be described by a rate equation; (5) Park's (1985, 1988) two-temperature model is approximately valid; and (6) the average vibrational energy removed in dissociation is about 30 percent of the dissociation energy.

Sharma, Surendra P.↗

NASA NTRS1988

Radiative processes in air excited by an ArF laser

The emission spectrum of air that is excited by an ArF laser has been investigated experimentally and theoretically to determine the conditions under which fluorescence from O2 can be used for the measurement of temperature in aerodynamic flows. In addition to the expected fluorescence from O2, the spectrum from excitation with an intense laser beam is shown to contain significant contributions from the near-resonant Raman fundamental and overtone bands, the four-photon fluorescence excitation of C produced from ambient CO2, and possibly the three-photon excitation of O(2+). The nature of the radiative interactions contributing to these additional features is described.

Laufer, Gabriel↗

NASA NTRS1987

Schwinger multichannel study of the 2Pi(g) shape resonance in N2

The results of a study on electron-target correlations in the 2Pi(g) shape resonance of elastic e-N2 scattering, using the Schwinger multichannel formulation, are reported. The effects of basis set, orbital representation, and closed-channel-configurations are delineated. The different roles of radial and angular correlations are compared.

Huo, Winifred M.↗

NASA NTRS1987

Correlation effects in elastic e-N2 scattering

The Schwinger multichannel formulation has been applied to study the role of electron correlation in low-energy e-N2 scattering. For the five nonresonant partial-wave channels studied here, angular correlation is found to be much more important than radial correlation. The calculated total and differential cross sections agree well with experiment except for the differential cross sections at 1.5 eV.

Huo, Winifred M.↗

NASA NTRS1987

The production of excited molecular species by electron impact in high temperature nonequilibrium air

Low-energy electron-impact excitation cross sections of N2 and CO are calculated using the Schwinger multichannel formulation, and calculations are extended to the important threshold region where experimental measurement becomes difficult. Cross sections are used to determine the excitation rate coefficients for the production of the important radiative states of N2 at a variety of electronic, rotational, and vibrational temperatures. Excitation of these states requires at least 6.2 eV, and the rate coefficients increase with the electronic temperature. The dependence on the vibrational temperature is mainly determined by the shape of the potential function for the vibrational motion. A strong resonance feature near threshold is found for the excitation of CO, in agreement with previous measurements.

Huo, Winifred M.↗

NASA NTRS1987

Application of the Schwinger multichannel formulation to electron-impact excitation of the B 1Sigma(+)u state of H2

The paper reports cross sections for electron-impact excitation of the X 1Sigma(+)g - BISigma(+)u transition in H2 for collision energies of 15, 20, and 30 eV. For this dipole-allowed transition with its associated long-range potential, the contributions of the more strongly scattered low-angular-momentum partial waves to the cross section were obtained from a two-state Schwinger multichannel calculation, and a modified Born-closure scheme was used to include the contributions from the remaining weakly scattered partial waves. Agreement between the calculated differential cross sections and available experimental data is encouraging.

Gibson, Thomas L.↗

NASA NTRS1987

(2 + 1) resonant enhanced multiphoton ionization of H2 via the E,F 1Sigma(+)g state

In this paper, the results of ab initio calculations of photoelectron angular distributions and vibrational branching ratios for the (2 + 1) resonant enhanced multiphoton ionization (REMPI) of H2 via the E,F 1Sigma(+)g state are reported, and these are compared with the experimental data of Anderson et al. (1984). These results show that the observed non-Franck-Condon behavior is predominantly due to the R dependence of the transition matrix elements, and to a lesser degree to the energy dependence. This work presents the first molecular REMPI study employing a correlated wave function to describe the Rydberg-valence mixing in the resonant intermediate state.

Rudolph, H.↗