999 resultados para HE IMPLANTATION


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Under coronal conditions, the steady state rate-equations are used to calculate the inter-stage line ratios between Li-like Is(2)2p(P-2(3/2))-> 1s(2)2s -> ((2) S-1/2) and He-like 1s2p (P-1(1))-> 1s(2) (S-1(0)) transitions for Ti in the electronic temperature ranges from 0.1 keV to 20 keV. The results show that the. temperature sensitivities are higher at the electronic temperature less than 5000 eV and the temperature sensitivities will decrease with the increase of electronic temperature.

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High-order harmonics and single attosecond pulse generation by using an infrared laser pulse combined with attosecond pulse trains (APT) interacting with He+ have been investigated. We show that the ionization for different instant time intervals can be controlled by altering the time delay between the APT and the infrared pulse. Consequently, APT can be used as a tool to control the efficiency of high-order harmonics emitted at different times. By choosing appropriate APT and time delay, the driving pulse width for single attosecond pulse generation can be extended up to six optical cycles. (c) 2007 Optical Society of America.

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The single ionization of an He atom by intense linearly polarized laser field in the tunneling regime is studied by S- matrix theory. When only the first term of the expansion of the S matrix is considered and time, spatial distribution, and fluctuation of the laser pulse are taken into account, the obtained momentum distribution in the polarization direction of laser field is consistent with the semiclassical calculation, which only considers tunneling and the interaction between the free electron and external field. When the second term, which includes the interaction between the core and the free electron, is considered, the momentum distribution shows a complex multipeak structure with the central minimum and the positions of some peaks are independent of the intensity in some intensity regime, which is consistent with the recent experimental result. Based on our analysis, we found that the structures observed in the momentum distribution of an He atom are attributed to the " soft" collision of the tunneled electron with the core.

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Reactions produced by the He3 bombardment of the He3 have been investigated for bombarding energies from 1 to 20 MeV using a tandem Van de Graaff accelerator. Proton spectra from the three-body reaction He3(He3, 2p)He4 have been measured with a counter telescope at 13 angles for 9 bombarding energies between 3 and 18 MeV. The results are compared with a model for the reaction which includes a strong p-He4 final-state interaction. Alpha-particle spectra have been obtained at 12 and 18 MeV for forward angles with a magnetic spectrometer. These spectra indicate a strongly forward-peaked mechanism involving the 1S0 p-p interaction in addition to the p-He4 interaction. Measurements of p-He4 and p-p coincidence spectra at 10 MeV confirm these features of the reaction mechanism. Deuteron spectra from the reaction of He3(He3, d)pHe3 have been measured at 18 MeV. A triton spectrum from the reaction He3(He3, t)3p at 20 MeV and 40 is interpreted in terms of a sequential decay through an excited state of the alpha particle at 20.0 MeV. No effects are observed which would indicate an interaction in the residual (3p) system. Below 3 MeV the He3(He3, 2p)He4 reaction mechanism is observed to be changing and further measurements are suggested in view of the importance of this reaction in stellar interiors.

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The (He3, n) reactions on B11, N15, O16, and O18 targets have been studied using a pulsed-beam time-of-flight spectrometer. Special emphasis was placed upon the determination of the excitation energies and properties of states with T = 1 (in Ne18), T = 3/2 (in N13 and F17) and T = 2 (in Ne20). The identification of the T = 3/2 and T = 2 levels is based on the structure of these states as revealed by intensities and shapes of angular distributions. The reactions are interpreted in terms of double stripping theory. Angular distributions have been compared with plane and distorted wave stripping theories. Results for the four reactions are summarized below:

1) O16 (He3, n). The reaction has been studied at incident energies up to 13.5 MeV and two previously unreported levels in Ne18 were observed at Ex = 4.55 ± .015 MeV (Γ = 70 ± 30 keV) and Ex = 5.14 ± .018 MeV (Γ = 100 ± 40 keV).

2) B11 (He3, n). The reaction has been studied at incident energies up to 13.5 MeV. Three T = 3/2 levels in N13 have been identified at Ex = 15.068 ± .008 MeV (Γ ˂ 15 keV), Ex = 18.44 ± .04, and Ex 18.98 ± .02 MeV (Γ = 40 ± 20 keV).

3) N15 (He3, n). The reaction has been studied at incident energies up to 11.88 MeV. T = 3/2 levels in F17 have been identified at Ex = 11.195 ± .007 MeV (Γ ˂ 20 keV), Ex = 12.540 ± .010 MeV (Γ ˂ 25 keV), and Ex = 13.095 ± .009 MeV (Γ ˂ 25 keV).

4) O18 (He3, n). The reaction has been studied at incident energies up to 9.0 MeV. The excitation energy of the lowest T = 2 level in Ne20 has been found to be 16.730 ± .006 MeV (Γ ˂ 20 keV).

Angular distributions of the transitions leading to the above higher isospin states are well described by double stripping theory. Analog correspondences are established by comparing the present results with recent studies (t, p) and (He3, p) reactions on the same targets.