Ionization of metastable He(1s2s) atoms by alkalated surfaces

We calculate the ionization probability for singlet and triplet metastable He(1s2s) atoms scattered from potassium covered surfaces, on the basis of the resonant tunnelling process. We do not assume the trajectory approximation, but we do assume classical motion on the potential energy curves, with crossing between neutral and ionic curves at a given point calculated from electronic tunnelling probabilities. Energy spectra of the He⁺ ions are also calculated. The results are found to be somewhat sensitive to tunnelling probability as a function of distance from the surface. Results are compared with experimental ionization probabilities measured by Roussel. The agreement is found to be fair if the tunneling rate is parameterized according to the theory of Janev et al. A simple interpretation is given.     

Collision-energy-resolved Penning ionization electron spectroscopy of styrene, 2-vinylpyridine, and 4-vinylpyridine with He*(23S) metastable atoms

Collisional ionization of styrene (phenylethylene), 2-vinylpyridine, and 4-vinylpyridine with metastable He*(2³S) atoms were studied by means of collision-energy/electron-energy resolved two-dimensional Penning ionization electron spectroscopy. Collision energy dependence of partial ionization cross-sections, which reflects the anisotropic interactions between a He*(2³S) atom and the target molecules, indicates that attractive interaction for the out-of-plane access of a He*(2³S) atom to phenyl group is stronger than that for the out-of-plane access to vinyl group. Moreover, it was found for vinylpyridines that the attractive interaction around π electrons became weaker than that for styrene, and that the attractive interaction for the in-plane access to the nitrogen atom is stronger than that for out-of-plane π-directions. However, in 2-vinylpyridine, the hydrogen atom of vinyl group prevents a He*(2³S) atom from approaching to the nitrogen atom along in-plane directions, and thus the attractive interactions around the nitrogen atom were shielded by the vinyl group. The experimentally observed anisotropic interactions were qualitatively supported with ab initio model interaction potential calculations between a Li (He*(2³S)) atom and the target molecule. Concerning with electronic structures of investigated molecules, the assignment of Penning ionization electron spectrum for 4-vinylpyridine was discussed on the basis of different behavior of collision-energy dependence of partial ionization cross-sections, and the satellite ionization band in Penning ionization electron spectra was also reported for styrene.     

ELECTRON MOMENTUM SPECTRA OF EXCITED He(2~1S) AND He(2~3S)

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Population of the states in collisions of mixed-state (1s21S,1s2s3S) B3+ and C4+ ion beams with He and H2 targets

Auger KLn lines are observed in high-resolution electron spectra obtained in collisions of mixed-state (1s²¹S,1s2s³S) He-like beams of 4 MeV B³⁺ with H₂ and 6 MeV C⁴⁺ with He targets. Supporting atomic structure calculations show these lines to correspond to doubly excited states, which can be readily populated by electron transfer to the component of the mixed-state beam. They thus provide indirect evidence for the existence of the corresponding KLn quartet states, similarly produced, even though their weak Auger decay does not allow for their direct observation in the electron spectra. These KLn quartet states mostly decay in a cascade chain of strong radiative E1 transitions, eventually terminating at the state, which is thus additionally enhanced. An upper limit on the state population due to cascades is obtained by assuming a statistical production of KLn quartet to doublet states followed by a 100% cascade feeding of the state. Our estimated upper limit is supported by our absolute cross section measurements and corresponding three-electron atomic orbital close coupling calculations in progress. Results to date are presented and discussed.     

Large-space cluster model calculations for the 3He(3He,2p)4He and 3H(3H,2n)4He reactions

The ³He(³He,2p)⁴He and ³H(³H,2n)su4He reactions are studied in a microscopic cluster model. We search for resonances in the ³He+³He and ⁴He + p + p channels using methods that treat the two- and three-body resonance asymptotics correctly. Our results show that the existence of a low-energy resonance or virtual state, which could influence the ⁷Be and ⁸B solar neutrino fluxes, is rather unlikely. Our calculated ³He(³He,2p)⁴He and ³H(³H,2n)⁴He cross sections are in a good general agreement with the experimental data.     

Microscopic study of the low-energy3He(3He, 2p)4He and3H(3H, 2n)4He fusion cross sections

We have calculated the³He(³He, 2p)⁴He and³H(³H, 2n)⁴He reaction cross sections at low energies within the microscopic multichannel resonating group method. For both reactions, we find good agreement with experiment. For the³H(³H, 2n)⁴He reaction, our calculated energy dependence reproduces that of each individual low-energy experimental data set, except for a normalization constant. Using this fact, we derive at a low-energy³H(³H, 2n)⁴He rate by taking the averaged mean of these fits.This work has been supported in part by the National Science Foundation, Grants PHY86-04197 and PHY88-17296.     

Ionization of O2 by metastable He(23 S, 21 S) at thermal collision energy

We investigate Penning ionization of O₂ by He(2³ S, 2¹ S) by means of electron spectroscopy. We give a detailed analysis of the reaction mechanism which completes and partly corrects earlier interpretations given by other authors. It becomes apparent that symmetry properties of the reaction partners play an important role.     

Primary populations of metastable antiprotonic (4)He and (3)He atoms

Initial distributions of metastable antiprotonic (4)He and (3)He atoms over principal (n) and angular momentum (l) quantum numbers have been deduced using laser spectroscopy experiments. The regions n = 37-40 and n = 35-38 in the two atoms account for almost all of the observed fractions [(3.0 +/- 0.1)% and (2.4 +/- 0.1)%] of antiprotons captured into metastable states.     

The He(1s, 2s) + Ne interaction potential

A well depth D = 0.79 meV at R = 11.56 au is calculated for the excited He(1s, 2s) atom interacting with Ne by using a non-local pseudopotential method in good agreement with the experimental values. The pseudopotential I use also contains polarization effects in an adiabatic form. The contribution of the polarization to the well depth is calculated to be 0.42 meV.     

The (p, 2p) reaction on 2H, 3He and 4He

The (p, 2p) cross sections on ²H, ³He and ⁴He were measured at 65, 85 and 100 MeV. The experimental data show rather strong energy and target mass dependence, particularly for ⁴He.     

Electron stimulated desorption ion angular distributions of H2O and NH3 on Ru(001)

The recent experiments of Madey et al. on the Electron Stimulated Desorption Ion Angular Distributions (ESDIAD) of H₂O and NH₃ on a Ru(001) surface are discussed using the reflection approximation for Franck-Condon ionization. In this approximation, ESDIAD data mirrors, the adsorbate wave function and therefore equilibrium bond angles as well as vibrational information can be extracted from it. It is also shown that image potential distortions of the desorbed ion trajectory from chemical bond directions are easily calculated and could be quite large although considerable uncertainty exists.     

The reaction3He(d,p)4He with a polarized target

The left-right asymmetry for the³He(d, p)⁴He reaction has been measured with a polarized³He target at a bombarding energy of 12 MeV. The experimental results are compared with proton polarization measurements for unpolarized target and beam. The construction of the optically pumped³He target is described. A special design has been chosen to overcome the difficulty that the target contamination caused by the bombarding particles destroys the polarization. The target can easily be purified without removing it from the experimental set-up.     

2H( 3He, t)2p reaction at 2 GeV

The exclusive ²H( ³He, t)2p reaction has been studied at 2 GeV for energy transfers up to 500 MeV and triton angles up to 3.4^°. The protons were measured in the large acceptance magnetic detector DIOGENE, in coincidence with the forward tritons detected in a dedicated magnetic arm. The energy transfer spectra extend well above the pion threshold. However, in the region of Δ excitation, the yield is less than 10% of the inclusive ²H( ³He, t) cross-section, which indicates the small contribution of the ΔN ↦ NN process. The angular distributions of the two protons in their center of mass have been analysed as a function of energy transfer and triton angle and a Legendre polynomial decomposition has been achieved. These data have been compared to a model based on a coupled-channel approach for describing the NN and NΔ systems. Received: 21 October 2002 / Accepted: 15 November 2002 / Published online: 11 March 2003 RID="a" ID="a"e-mail: ramstein@ipno.in2p3.fr RID="†" ID="†"Deceased. RID="†" ID="†"Deceased. RID="d" ID="d"Present adress: School of Engineering, J?nk?ping University, P.O. Box 1026, S-551 11 J?nk?ping, Sweden. Communicated by M. Gar?on     

The (3He,n) reaction in the lower 2s1d shell

The two-proton transfer reaction (³He, n) has been measured on four gas targets, ¹⁶O, ¹⁸O, ²⁰Ne, and ²²Ne, with an incident ³He energy of 18.3 MeV. The data were taken with a neutron time-of-flight spectrometer in the angular range 0° to 40° c.m. The results for selected transitions are compared to DWBA predictions using shell-model spectroscopic amplitudes. In addition, the data for the ¹⁸O(³He, n)²⁰Ne reaction are compared to predictions of the SU(3) strong coupling model.     

Winkelverteilung der Kernphotoreaktion3He (γ,p)2H

The angular distribution of the photonuclear reaction³He(γ,p)²H was measured with an exitation energyE _x=16 to 27 MeV using a 32.5 MeV betatron. In agreement with a theoretical calculation of Böschet al. we found for the coefficients of the distributionf(θ)=b (a/b+sin²θ(1+β·cosθ+γ·cos²θ)) the values:a/b≦0.14; β=0.77 (+0.14; ?0.07); γ≦0.30. This was the first attempt to use spark chambers as a detection device for photonuclear reactions in the low energy range. We found it to be a promising new facility.