Decay Of He Research Articles

Penning ionization widths by Fano-algebraic diagrammatic construction method.

We present an ab initio theory and computational method for Penning ionization widths. Our method is based on the Fano theory of resonances, algebraic diagrammatic construction (ADC) scheme for many-electron systems, and Stieltjes imaging procedure. It includes an extension of the Fano-ADC scheme [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)] to triplet excited states. Penning ionization widths of various He*-H2 states are calculated as a function of the distance R between He* and H2. We analyze the asymptotic (large-R) dependences of the Penning widths in the region where the well-established electron transfer mechanism of the decay is suppressed by the multipole- and/or spin-forbidden energy transfer. The R-12 and R-8 power laws are derived for the asymptotes of the Penning widths of the singlet and triplet excited states of He*(1s2s1,3S), respectively. We show that the electron transfer mechanism dominates Penning ionization of He*(1s2s 3S)-H2 up until the He*-H2 separation is large enough for the radiative decay of He* to become the dominant channel. The same mechanism also dominates the ionization of He*(1s2s 1S)-H2 when R < 5 Å. We estimate that the regime of energy transfer in the He*-H2 Penning ionization cannot be reached by approaching zero collisional temperature. However, the multipole-forbidden energy transfer mechanism can become important for Penning ionization in doped helium droplets.

The Decay of Forced Turbulent Coflow of He II Past a Grid

We present an experimental study of the decay of He II turbulence created mechanically, by a bellows-induced flow past a stationary grid in a 7×7 mm2 superfluid wind tunnel. The temporal decay L(t) originating from various steady-states of vortex line length per unit volume, L0, has been observed based on measurements of the attenuation of second-sound, in the temperature range 1.17 K<T<1.95 K. Each presented decay curve is the average of up to 150 single decay events. We find that, independently of T and L0, within seconds past the sudden stop of the drive, all the decay curves show a universal behavior lasting up to 200 s, of the form L(t)∝(t−t0)−3/2, where t0 is the virtual origin time. From this decay process we deduce the effective kinematic viscosity of turbulent He II. We compare our results with the bench-mark Oregon towed grid experiments and, despite our turbulence being non-homogeneous, find strong similarities.

Influences of the incident He* velocity on metastable de-excitation processes at a Cs-covered Si(1 0 0) surface

A Si(1 0 0) surface covered with cesium at saturation coverage were exposed to helium metastable-atom beams. The spectra of the electrons induced by metastable-atom de-excitation were detected as a function of the velocity of the incident He* in the velocity range 1–5 km/s. We discuss the influence of the incident He* velocity on the metastable de-excitation processes that take place at the Cs/Si(1 0 0) surface of very low work function. With increasing He* velocity, the relative weight of the peak from the decay of He* −(1s2s 2) in the spectrum was decreased, which means the auto-detachment process becomes more important than the Auger de-excitation process. The low-energy threshold of the spectrum shifted to higher energies with higher He* velocities. This implies that the surface electronic states are distorted by incident atoms, and, consequently, the surface potential barrier is increased.

Decay of counterflow He II turbulence in a finite channel: possibility of missing links between classical and quantum turbulence.

Decay of He II counterflow turbulence generated by applying a heat pulse to the closed end of a circular channel is investigated using second sound attenuation. Several orders of magnitude of decaying vortex line density display various regimes, starting with the inverse time decay predicted by Vinen towards distinct classical t(-3/2) power law occurring after saturation of the energy containing length scale by the size of the channel. The decaying counterflow turbulence displays a surprisingly close resemblance to recently reported decaying grid turbulence in He II, and both of them appear closely linked to classical grid generated turbulence.

Polarization study of the extreme-ultraviolet emission from helium following electron impact

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He $(1snp){}^{1}{P}^{o}$ states with wavelengths of 517 to 584 \AA{} has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the ${\mathrm{He}}^{+}$ decays with wavelengths between 256 and 1640 \AA{} have been measured. Particular attention has been paid to the $(2p){}^{2}{P}^{o}\ensuremath{\rightarrow}(1s){}^{2}S$ and $(3p){}^{2}{P}^{o}\ensuremath{\rightarrow}(1s){}^{2}S$ decays with wavelengths of 304 and 256 \AA{}, respectively. These have been measured from threshold (66 and 73 eV, respectively) to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 \AA{} ${\mathrm{He}}^{+}(2p){}^{2}{P}^{o}$ results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the ${(e}^{\ensuremath{-}}{,2e}^{\ensuremath{-}})$ reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with $L&gt;0$ contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 \AA{}, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.

Kinetic processes determining the time dependence of vuv emission in He

We present an interpretation of the data by Bartell, Hurst, and Wagner (BHW) on the time resolved vuv emission from He excited by pulses of fast charged particles. Here we argue that the very large term, linear in pressure, observed in the decay rate of the He(2 1P) population represents He(2 1P) + He(11S) →He(2 1S) + He(1 1S) and that the dominant mechanism for this reaction is strong rotational coupling of the adiabatic states 1Πg(1 1S+2 1P) and 1Σ+g(1 1S+2 1S) in the region of internuclear separation near the potential curve crossing at R?2.04 Å. The accurate prediction of the reaction rate, together with the fact that no other singlet states appear accessible at room temperature, leads to the conclusion that in nearly pure He most of the large He(2 1P) population is converted to He(2 1S) either by radiative transitions, the two-body process described above, or by the related predissociation of B 1Πg(1 1S+2 1P) when the molecule is formed in a high vibrational state. Thus, most of the continuum radiation comes from the A 1Σ+u and possibly the D 1Σ+u molecules, and the time dependence of radiation from the A 1Σ+u molecule is rate-limited by decay of He(2 1S) by various collisional processes.

Two-quantum processes in the coupled Hartree-Fock approximation

A method, based upon time-dependent Hartree-Fock theory, is developed for calculating the efficiencies of two-photon processes The lifetime for two-photon decay of He(2 1S) is calculated with the result 2 0 × 10-2 sec. Estimates for the cross section for the two-photon ionization of the ground state of helium are given for photon energies near the two-photon threshold. For incident photons at a wavelength of 1000 A the cross section is 5 × 10-31 I cm1 w-1, where I (w cm-2) is the intensity of the incident photons.

The half-life of He 6

The decay of He 6 was measured, with a plastic scintillation counter, over a period of 10 half-lives. The data were obtained by photographing two scalers, one of which registered the β counts while the other, operating on a 60 Hz signal, served as the clock. After suitable corrections for background, dead-time of the counting system, and pulse height dependence on counting rate, 34 separate runs were combined. A least squares fit to the data gives a value of T 1 2 = (0.797 ±0.003) sec .

The electron-neutrino angular correlation in beta decay of helium 6

The electron-neutrino angular correlation in decay of He 6 has been determined by measuring the distribution of electron energies and recoil velocities without selection of angle. The correlation parameter α was found to be −0.353 −0.053 +0.033, confirming current belief that the Gamow-Teller interaction is of axial vector type. The maximum admixture of a tensor interaction is given by |C T| 2/|C A| 2 < 0.08.