Showing papers by "Fred L. Walls published in 1976"

Cross-section measurements for electron-H3O+ recombination

Abstract: Cross sections for electron-${\mathrm{H}}_{3}$${\mathrm{O}}^{+}$ recombination have been measured over an electron energy range of 0.060-1.15 eV by using a trapped-ion technique to contain an ion sample at a temperature of \ensuremath{\simeq} 400 K. The cross section deduced from these measurements can be represented by $\ensuremath{\sigma}=4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}{E}^{\ensuremath{-}2.9}$ ${\mathrm{cm}}^{2}$ for $0.038lE\ensuremath{\le}0.110$ eV; $\ensuremath{\sigma}=4.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{E}^{\ensuremath{-}0.79}$ ${\mathrm{cm}}^{2}$ for $0.110lE\ensuremath{\le}0.420$ eV; and $\ensuremath{\sigma}=7.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}{E}^{\ensuremath{-}2.9}$ ${\mathrm{cm}}^{2}$ for $Eg0.420$ eV. Uncertainties are estimated to be at the \ifmmode\pm\else\textpm\fi{} 50% level. Recombination-rate coefficients were calculated from the deduced cross sections, assuming a Maxwellian electron velocity distribution and trial forms for the cross section below 0.038 eV. Assuming the theoretical ${E}^{\ensuremath{-}1}$ behavior for the cross section just above zero energy and comparing with the measured rate coefficient of Leu et al. at 540 K, we can deduce consistent cross sections below 0.038 eV to be $\ensuremath{\sigma}=6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}{E}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{2}$ for $0lEl0.023$ eV, and $\ensuremath{\sigma}=4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}{E}^{\ensuremath{-}2.9}$ ${\mathrm{cm}}^{2}$ for $0.023lEl0.038$ eV. The calculated rate coefficients have temperature dependence ${T}_{e}^{\ensuremath{-}0.5}$ at 50 K and ${T}_{e}^{\ensuremath{-}1.43}$ at ${10}^{5}$ K, and values of the coefficient are in good agreement with all direct measurements.

Time and Frequency

Abstract: In 1967 the General Conference on Weights and Measures adopted the cesium resonance frequency for the definition of the second. Universal Coordinated Time (UTC) has used a close approximation to the atomic second since 1972 (1). Time scales which refer to the rotation of the earth such as UTC are generated by inserting or leaving out seconds (leap seconds) at certain specified dates during the year, as necessary. This process is coordinated worldwide by the Bureau International de l’Heure (BIH). UTC is the de-facto basis for civil or legal time in most countries of the world (2). In addition to cesium beam standards, the atomic hydrogen maser has found use as primary frequency reference and clock.

Cross-section measurements for electron-H$sub 3$O$sup +$ recombination

Abstract: Cross sections for electron-H$sub 3$O$sup +$ recombination have been measured over an electron energy range of 0.060--1.15 eV by using a trapped-ion technique to contain an ion sample at a temperature of approx. =400 K. The cross section deduced from these measurements can be represented by sigma = 4.6 x 10$sup -18$E$sup -2$.$sup 9$ cm$sup 2$ for 0.038 0.420 eV. Uncertainties are estimated to be at the +- 50% level. Recombination-rate coefficients were calculated from the deduced cross sections, assuming a Maxwellian electron velocity distribution and trial forms for the cross section below 0.038 eV. Assuming the theoretical E$sup -1$ behavior for the cross section just above zero energy and comparing with the measured rate coefficient of Leu et al. at 540 K, we can deduce consistent cross sections below 0.038 eV to be sigma = 6.0 x 10$sup -15$E$sup -1$ cm$sup 2$ for 0 < E < 0.023 eV, and sigma = 4.6 x 10$sup -18$E$sup -2$.$sup 9$ cm$supmore » 2$ for 0.023 < E < 0.038 eV. The calculated rate coefficients have temperature dependence T$sup -0$.$sup 5$/sube/ at 50 K and T$sup -1$.$sup 43$/sube/ at 10$sup 5$ K, and values of the coefficient are in good agreement with all direct measurements. (AIP)« less