Cross Sections For He Research Articles

Extensive L2 calculation of partial photoionization cross sections of He in the 4lnl' resonance region.

We present a fully-[ital L][sup 2]-integrable calculation of the partial photoionization cross sections of He between the [ital N]=3 and 4 thresholds. The energy positions, total and partial autoionization widths, and Fano and Starace parameters for the 4[ital lnl][prime] doubly excited states lying in this region are also provided. We have found that only one series of resonances with large autoionization widths is observed in the total-cross-section spectrum. On the other hand, several resonance series are exhibited in the partial [ital N]=3 cross sections. Our results also show that the 2[ital p] and 3[ital p] cross sections dominate the [ital N]=2 and 3 ones, respectively. Good agreement with the existing experimental data is found.

Determination of rank 4 multipoles and of the partial cross sections for He(3(1)D) excitation by electron impact.

We report the first measurements of a rank 4 state multipole for He(${3}^{1}$D) excitation by electron impact with a polarized photon-photon coincidence technique. Our results allow, for the first time, the extraction of partial cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{m}}$ for He(${3}^{1}$${\mathit{D}}_{\mathit{m}}$) excitation (m=0, \ifmmode\pm\else\textpm\fi{}1, \ifmmode\pm\else\textpm\fi{}2) which are not otherwise available. The present results put stringent tests on existing theories and, for example, point in the theoretical calculations to an overestimation of the partial cross section ${\mathrm{\ensuremath{\sigma}}}_{2}$.

Absolute cross section for D( 4He, D) 4He forward scattering

The D( 4He, D) 4He differential cross section for forward scattering of deuterons by helium ions has been measured, with an estimated precision of ± 5%, in the range 1–3 MeV at the scattering angles of 20°, 25°, 30°, 35° and 40°. Deuterated polystyrene (C 8D 8) n films spun on Si wafers were used for the measurements, enabling the forward scattering cross section to be determined directly with reference to the 4He backscattering cross section of carbon (which was, in turn, referenced to the Rutherford cross section for 4He on Cu). The D( 4He, D) 4He cross section exhibits a resonance at 2.135 ± 0.01 MeV, with ∼ 60 keV FWHM, the amplitude of which is strongly dependent on scattering angle. For the rest of the energy range, the cross section is featureless, and can be fitted well by a simple polynomial.

Gaussian type orbital basis sets for the calculation of continuum properties in molecules: The photoionization cross section of H2

The now well-established L2 techniques applied to the description of the electronic continuum of small molecules and generally implemented with suitable oscillating basis sets are here adapted and used in connection with Gaussian type orbital (GTO) functions. As a test, a K-matrix approach associated with extended sets of GTOs is employed to compute with satisfactory accuracy partial wave phase shifts and both the integral and differential photoionization cross sections of the hydrogen molecule. It is apparently possible to greatly extend the range of applicability of the usual L2 techniques beyond the limited set of molecular systems amenable to the one center expansion approximation.

Quantum and classical calculations of transport and relaxation cross sections in He–CO mixtures

Rigorous quantum and classical calculations for transport and relaxation properties of the gaseous mixture of He with CO have been carried out over a broad range of temperatures. The interaction potential employed was the one recently suggested from molecular beam experiments (W. Dilling, Ph.D. thesis, University of Göttingen, Germany, 1985) and the calculations were carried out with the correct dynamical couplings between tensorial basis sets in the quantum case, leading to the usual close-coupled (CC) equations. Classical trajectory (CT) calculations were also carried out and compared with the CC results. A combined use of CC and CT results, together with an appropriate choice of partition functions, is found to produce rather good agreement with the experimental findings for diffusion and mixture viscosity coefficients. The present results suggest ways for further testing the original potential energy surface and for extending the multiproperty analysis of it started earlier on with the scattering data of Dilling and the pressure broadening data of Green, Boissoles, and Boulet [J. Quant. Spectrosc. Radiat. Transfer 39, 33 (1988)].

State-selected chemiionization cross sections of He*(2 1,3S)+Xe for collision energies from 0.003 to 6 eV

State-selected chemiionization cross sections of He*(2 1,3<i>S</i>)+Xe for collision energies from 0.003 to 6 eV

State-selected chemiionization cross sections of He* (2 1,3S)+Ar for collision energies from 0.003 to 5.5 eV

In addition to the well-known thermal minimum, a hyperthermal minimum around 0.8 eV has been found in the singlet chemiionization cross section. The hyperthermal triplet ionization curve shows the well-known behavior in the form of a saturation and a subsequent decline. The experimental data are compared with fully quantum-mechanical optical potential calculations.

Transport and relaxation cross sections for He–N2 mixtures: A test of a multiproperty interaction

Full quantum calculations of several transport and relaxation cross sections have been carried out for the title system, using the multiproperty potential energy surface obtained earlier by Gianturco et al. [Chem. Phys. 109, 417 (1986)] which has been recently shown to be one of the most reliable descriptions of the He–N2 interaction. A rigorous close coupling (CC) treatment of the scattering problem turns out to be the only realistic choice for evaluating viscomagnetic relaxation data and for anisotropy-dependent nonequilibrium properties. A comparison of the present calculations with classical trajectory (CT) estimates is carried out for several transport cross sections as function of both the collision energy and the temperature of the mixture. It is clearly shown by the present results that classical treatments markedly differ from the quantum calculations at collision energies which are comparable with the average well depth of the chosen interaction. On the other hand, CT and CC values are nearly coincident at higher collision energies for all the computed properties. Thus, their combined use in evaluating the temperature dependence of diffusion and viscosity coefficients reveals the high quality of the present multiproperty interaction vis à vis the experimental findings examined in this work.

The hardness factor in rotational inelastic scattering

Inelastic cross-sections for He + HF collisions have been calculated using an exponential repulsive potential in which the exponent can be increased to approach the hard shape limit. Convergence to the hard shape results is slow, particularly for a transition 0 → 3 which is near the threshold. We attribute slow convergence to the different boundary conditions that are applied in the soft and hard calculations.

Deexcitation of He(2 1P) in collisions with diatomic molecules

Deexcitation cross sections of He(2 1P) by N2 and O2 have been measured using a pulse radiolysis method in a region of the mean collisional energy between 18 and 38 meV. Fairly large deexcitation cross sections (≂100 Å2) by N2 and the negative slope of the curve of cross section vs energy are interpreted in terms of the energy transfer cross section based on a dipole–dipole interaction. Validity of theoretical formula based on a semiclassical approach with rectilinear trajectories, i.e., the Watanabe–Katsuura formula, is discussed. The deexcitation cross section of He(2 1P) by O2 and its collisional energy dependence are not very different from that by N2. Optical model calculation of the deexcitation cross section has been also presented for collisions between He(2 1P) and M (M=H2 or N2) with an isotropically averaged complex potential optimized through a fitting procedure to the experimental cross sections. It has been found by the model calculation that an electron exchange interaction is not negligible in the deexcitation process.

Deexcitation cross sections of He (2 3S, 2 1S, and 2 1P) by molecules containing group-IV elements

Deexcitation cross sections of He(2 3S), He(2 1S), and He(2 1P) by C2H6, Si2H6, CF4, SiF4, and SiCl4 have been measured at a mean collisional energy corresponding to room temperature (295 K) and compared systematically with those by CH4, SiH4, and GeH4 (previously reported) to understand general features of the cross section values dependent on both the electronic states of excited helium atoms and the target molecules containing group-IV elements and to substantiate further the deexcitation mechanism. In the de-excitation of the metastable atoms, He(2 3S) and He(2 1S), the magnitude of the cross sections has mainly been ascribed to a short-range electron exchange interaction. The obtained cross section values for He(2 3S) are compared with a semiempirical formula and it is concluded that the number of the equivalent electrons which can be ionized with the excitation energy of He(2 3S) and the lowest ionization potential of the target molecule are of great importance in determination of the cross section values. Systematic comparisons of the deexcitation cross section values of the resonant atom, He(2 1P), with theoretical ones have shown an importance of not only deexcitation transition mainly ascribed to a long-range dipole–dipole interaction between He(2 1P) and a target molecule but also a bent trajectory caused by an attractive interaction potential between them to determine the cross section values.

Electron capture into He+ (41) states in collisions of He2+ on Li

Absolute cross sections for He II (n=4 to n=3) visible light emission subsequent to charge changing collisions (0.8-9.75 keV amu-1) of He2+ with Li have been measured. The respective l state selective He+ (41) electron capture cross sections have been determined from the intensity of the He II (n=4 to n=3) transition along the ion beam path by means of a deconvolution technique based on the lifetimes of the He+ (41) states.

Optical model calculation of the total Penning ionization cross sections for He(2 1P)–Ar, Kr, and Xe

Quantum mechanical calculation of the total Penning ionization cross sections for He(2 1P) –Ar, Kr, and Xe collisions has been made in the region of the collisional energy from 0.1 meV to 1 eV with an optical complex potential approach for an electronic transition. The real part of an optical potential at long range is expressed in an explicit dispersion form and that in a repulsive region is expressed in a shielded Coulomb form. We have determined a repulsive potential by an excellent fit with our experimental cross sections. The imaginary part of the optical potential at long range is expressed by a dipole–dipole autoionization width, and at a small interatomic distance an electron exchange width is also considered to account an exponentially dispersive increase of the autoionization width. Good agreement between the present calculation and the experimental data has been obtained both in the energy dependence and in the absolute cross section values. The real part of the optical potential for He(2 1P) –Ar, Kr, or Xe is analogous to that for He(2 1S) –Ar, Kr, or Xe, respectively. It has been found that the width for the electron exchange effectively increases the total Penning ionization cross section, particularly in the case of He(2 1P) –Xe.

Polarization propagator calculations of generalized oscillator strengths and stopping cross sections of He

We describe the first application of the polarization propagator method to the calculation of generalized oscillator strengths (GOS's). Both first-order (RPA) and second-order polarization propagator results are reported for the test case of the He atom. We show that fulfillment of the Bethe sum rule in finite Gaussian basis sets will always deteriorate for sufficiently large momentum transfers q, but that it is possible to obtain a satisfactory sum rule for all q by grafting on a limiting hydrogenlike expansion for the GOS's for large values of the excitation energies, E n − E 0. Even though the polarization propagator calculation gives the main part of the stopping cross section we will demonstrate that in order to obtain the correct result for large velocity of the incoming projectile it is essential to include the GOS's obtained from the limiting expansion.

Total and partial ionization cross sections of NH3 by electron impact

Values of cross sections for the production of positive ions resulting from electron collisions with NH3 at impact energies ranging from threshold to 1 keV have been measured using a crossed-beams apparatus. These cross sections have been normalized by using the relative flow technique and the previously determined cross sections of He and Ne. The measured values are compared with previous data available in the literature. The threshold energy for the production of each ion has been measured. Total ionization cross sections as a function of electron impact energies have been deduced from the measured partial ionization cross sections.

State-selected chemiionization cross sections of He*(2 1,3S) + H 2 in the energy range from 0.1 to 1.6 eV

More details of the recently developed double-arc source which enables the production of He*(2 1,3S) in the superthermal energy range are presented. With this source state-selected chemiionization cross sections of He*(2 1,3S) + H 2 have been measured from 0.1 to 1.6 eV collision energy. The measurements are compared with theoretical results.

Inert gas ion beam formation at the tandem accelerator in Alma-Ata (abstract)

At the Cockroft–Walton-type 2×1 MV tandem accelerator in Alma-Ata, a method of obtaining inert gas ion beams by means of injection of neutral particles into a first accelerating tube has been developed. The procedure of injected beam intensity control is described. The neutralization cross section of He+ and Ar+ ions on the various gaseous targets has been estimated. Recently obtained results in comparison with analogous data obtained elsewhere are presented.

Polarization effects on resonance enhanced two-photon dissociation of H2 by the X→(E F+G K+I) transition via intermediate B and C states

The generalized resonance enhanced two-photon dissociation (RETPD) cross sections of H2 are calculated for crossed and circular (same- and opposite-sense) polarizations of the two-photon fields using the reduced density operator formalism for the transitions X 1Σ+g(vg=0, Jg=0–4) →(ω1,ε̂1)VUVB 1Σ+u (C 1Πu)[vi=0, 3, Ji =0(1)–3]→(ω2,ε̂2)UV/visible [(EF+GK)1Σ+g+I 1Πg), Ef,Jf]. The wavelength (λ2) of the second photon is so chosen that ionization from the resonant B(C) state is energetically forbidden. The effects of polarizations on the wavelength and quantum number dependences of RETPD profiles have been studied. For vi=3 of both B and C states, a strong polarization dependence of RETPD cross sections is observed. The same- and opposite-sense circular polarizations have about the same RETPD profiles for the B state except at the shape resonance positions due to the final I state quasibound level, while the profiles are quite different for the C state.

Negative-ion formation from positive ion by two-electron transfer from an alkali-metal target

To examine two-electron transfer producing negative ions from positive ions, an apparatus using an alkali-metal target has been developed. Positive ions formed by electron impact in an ion source collided with alkali-metal vapor in a target chamber. Negative ions formed by two-electron transfer have been analyzed with a single-focusing mass spectrometer. Precursor positive ions and product negative ions have been identified from the apparent masses in the negative-ion spectra. Using the target density dependence of the negative-ion intensity, the processes of negative-ion formation were determined to be double-electron transfer in one collision or successive single-electron transfer in two collisions. The cross section of these processes has been estimated from the peak area of positive- and negative-ion spectra and the target density evaluated using the vapor-pressure curve as a function of the temperature of the alkali-metal cell. A cross section for He−-ion formation in the Cs target with successive single-electron transfer have been evaluated to be 1.40×10−30±0.2 cm4 at a collision energy of 2.0 keV, which showed the fair agreement with the values reported by Donnally and Thoeming [Phys. Rev. 159, 87 (1967)]. The double-electron transfer cross section for C2−-ion formation from a C2+ ion with a Cs target has been evaluated as 7.03×10−18±0.1 cm2 at 3.0 keV. For O−-ion formation from O2+ ions, four broad peaks were observed due to the kinetic energy releases at dissociation which were 7.3, 3.4, 1.1, and 0.014 eV at full width at half maximum. Since the kinetic energy releases of the broad peaks corresponded with those by Peterson and Bae [Phys. Rev. A 30, 2807 (1984)], the peak extents have been explained as the dissociations of the excited neutral formed from exothermic neutralization. This two-electron transfer from an alkali-metal target offers a good advantage for the determination of the dissociation mechanism for excited neutral particles by means of the negative-ion detection method.

High resolution neutron spectroscopy for helium isotopes

A high resolution fast neutron time-of-flight spectrometer is described, neutron time-of-flight spectra are taken using a specially designed TDC in connection to an on-line computer. The high time-of-flight resolution of 5 ps/m enables the study of the total cross section of 4He for neutrons near the 23+ resonance in the 5He nucleus. The resonance parameters were determined by a single level Breit-Winger fit to the data.