Pulsed Discharge In Helium Research Articles

Activation Energy for the Three-Body Reaction of Helium Triplet Atom with Normal Helium

From experimental determinations of the rate constants for the reaction He(3S1)+2 He→He2(3Σu)+He, an activation energy, Ea=0.067 eV, was calculated. At 77°K, no contribution from this reaction was found in the afterglow of a pulsed discharge in helium; rather at 77°K the decay was found to be second order with a reaction constant about 2×10−9 cm3 atom−1·sec−1.

Electron Interactions in Cryogenic Helium Plasmas

Microwave-propagation and microwave-interaction techniques have been used to determine the electron collision frequency for momentum transfer in helium for electron energies in the vicinity of 0.001 eV. Measurements of the complex microwave conductivity and electron-energy relaxation rates have been performed in the afterglow of a pulsed discharge in helium in the pressure range 0.1 to 5 Torr submerged in a bath of liquid helium at 4.2\ifmmode^\circ\else\textdegree\fi{}K. Electron-radiation temperature measurements during plasma decay have demonstrated monotonically decreasing electron temperatures as a function of time. For times when an extrapolation of the electron temperature decay indicated near thermal equilibrium with the parent gas, a momentum-transfer cross section in the range 10\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ to 19\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ was determined. Measurements of electron-energy relaxation rates for atomic densities exceeding 2.3\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, where the electron de Broglie wavelength is becoming long in comparison to the average inter-scatterer spacing, indicate the limit of validity of binary-collision concepts.

Positive ion mobilities and conversion rates in a helium afterglow

An X-band (3 cm) microwave cavity method has been used to measure the electron loss rate in the afterglow of a pulsed discharge in helium. Further purification of spectroscopically pure gas samples was undertaken to give more reliable results and afterglow periods up to 60 msec were investigated. The positive ion reduced mobility μ0 (as deduced from a measurement of the ambipolar diffusion coefficient) was found to be constant at 16.7 ± 0.5 cm2 v-1 sec-1 in the pressure range 4-83 mmHg and this is attributed to the molecular ion He2+. Measurements at pressures below 4 mmHg indicated the presence of both atomic and molecular ions and an extrapolation to zero pressure yielded a figure of 10.6 cm2 v-1 sec-1 for the mobility of the atomic species He+. The frequency of conversion of atomic into molecular ions by three-body collisions was found to be 115p2 sec-1 in good agreement with theory.

Atomic Emission of the Helium Afterglow

The spectral emission from a dc pulsed helium discharge for pressures from 5 to 20 Torr has been investigated during the early afterglow using interference filters with gated photomultiplier. The radiation transmitted by the filters consists generally of both atomic lines and molecular bands and after the termination of the active discharge shows characteristically an initial rapid decrease in intensity followed by a much slower rate. The rapid decrease is due to the atomic radiation, the slower to molecular. By subtracting the molecular emission from the total emission transmitted through an interference filter, the atomic emission is obtained. From the decay curves of the atomic emission, which is the result of collisional-radiative recombination of He+, and the known diffusion loss of He+, the rate of conversion of He+ into He2+ is found to be 109±6 sec—1·Torr—2.

Spectral Emission of the Helium Afterglow

The spectral emission from a dc pulsed helium discharge for pressures from 5 to 30 Torr has been investigated during the early afterglow using a spectrometer with gated photomultiplier and from the early afterglow through the late afterglow using interference filters with gated photomultiplier. The afterglow spectrum was predominately that of molecular light which was attributed to the collisional-radiative recombination of molecular ions. The time-decay of molecular light at late times indicated ambipolar diffusion control by an ion with a Dap of 710±10 cm2/sec, which corresponds to a μ0 of 16.4±0.2 cm2/V·sec, in good agreement with values reported for the helium molecular ion employing other techniques.

Cross Sections for the De-Excitation of Helium Metastable Atoms by Collisions with Atoms

The lifetimes of metastable helium atoms $\mathrm{He}(2^{3}S)$ and $\mathrm{He}(2^{1}S)$ were measured by a time-resolved optical absorption technique in the afterglow of a pulsed helium discharge at 10-mm pressure with small concentrations of various impurity gases added. The following de-excitation cross sections in units of ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ have been determined for $\mathrm{He}(2^{3}S)$: Ne\char22{}0.28, Ar\char22{}6.6, Kr\char22{}10.3, Xe\char22{}13.9, ${\mathrm{N}}_{2}$\char22{}6.4, and ${\mathrm{H}}_{2}$\char22{}6.0. The uncertainties are estimated to be \ifmmode\pm\else\textpm\fi{}20% for the rare gases and \ifmmode\pm\else\textpm\fi{}50% for the molecular gases. $\mathrm{He}(2^{1}S)$ cross sections were measured to be Ne\char22{}4.1, Ar\char22{}55, Kr\char22{}64, and Xe\char22{}103, all times ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. The neon cross section is believed reliable to \ifmmode\pm\else\textpm\fi{}20% but the remaining singlet cross sections may be too high by as much as a factor 2 or 3. The cross sections refer to de-excitation by ionizing collisions except in the case of neon where neon excitation occurs. Competing destruction processes render the singlet cross-section analysis somewhat uncertain except in the case of neon.

Microwave Determination of the Probability of Collision of Electrons in Neon

A previously reported microwave method for determining the collision probability for momentum transfer of slow electrons has been modified so that a variation in average electron energy from 0.012 ev to 3 ev may be obtained. Measurements of the ratio of the real part to the imaginary part of the electron conductivity are performed in the afterglow of a pulsed helium discharge in a microwave resonant cavity. The average electron energy is varied by applying a microwave electric field in the afterglow and, under appropriate assumptions, the average electron energy is determined theoretically from this field. Measurements are also obtained by varying the gas temperature from 77\ifmmode^\circ\else\textdegree\fi{}K to 700\ifmmode^\circ\else\textdegree\fi{}K. The value of the collision probability for momentum transfer in helium is 18.3\ifmmode\pm\else\textpm\fi{}2 percent ${\mathrm{cm}}^{2}$/${\mathrm{cm}}^{3}$ per mm Hg from 0 to 0.75 electron volts and increases slowly to a peak value of 19.2\ifmmode\pm\else\textpm\fi{}2 percent at 2.2 ev.