24 research outputs found

### The Hoyle and associated excited states from the viewpoint of pocket resonances in alpha + 8Be reactions

We examine the production of the Hoyle and associated excited states from the
viewpoint of pocket resonances in the reaction of an $\alpha$-particle on a
ground state prolate $^8$Be nucleus within the optical model coupled-channel
framework. The predicted reaction cross sections, as a function of the
center-of-mass energy $E_{\rm cm}$, show prominent resonances, including the
Hoyle resonance. The positions and widths of these resonances are sensitive to
the target deformation ($\beta_2$ parameter) and the parity of the nuclear
surface potential $-$ deeper for the even-parity $L$ partial waves relative to
those for the odd-parity $L$ partial waves at the surface region because of the
Bose-Einstein exchange of the $\alpha$-bosons. Decomposing the reaction cross
sections to different partial waves, we find that the resonance energies and
widths reasonably agree with the available experimental data and previous
hyperspherical calculations for the $0_2^+$ (Hoyle state), $0_3^+$, $1_1^-$ and
$3_1^-$ states of $^{12}$C, except for the narrow theoretical width of the
$2_2^+$ state. Analyzing the wavefunctions and the resonance widths, we
identify the narrow and sharp $0_2^+$, $3_1^-$ and $2_2^+$ resonances as pocket
resonances -- resonances which occur below the potential barrier, while the
broad $0_3^+$ and $1_1^-$ resonances as above-the-barrier resonances. For
astrophysical applications, we also evaluate the astrophysical $S(E_{\rm
cm})$-factor for $E_{\rm cm}$ $<$ 1.0 MeV, for the fusion of $\alpha$+$^8$Be
into the $^{12}$C$(2^+)$ state based on our estimated $s$-wave $\alpha$+$^8$Be
reaction cross section and the associated $\gamma$- and $\alpha$-decay widths
for the decay of $^{12}$C excited states in the potential pocket.Comment: 15 pages, 9 figure

### Charge Transfer in Slow Collisions of O⁸⁺ and Ar⁸⁺ Ions with H(1s) Below 2 KeV/Amu

We calculated the charge-transfer cross sections for O⁸⁺ + H collisions for energies from 1eV/amu to 2keV/amu, using the recently developed hyperspherical close-coupling method. In particular, the discrepancy for electron capture to the n = 6 states of O⁷⁺ from the previous theoretical calculations is further analyzed. Our results indicate that at low energies (below 100eV∕amu) electron capture to the n=6 manifold of O7+ becomes dominant. The present results are used to resolve the long-standing discrepancies from the different elaborate semiclassical calculations near 100eV/amu. We have also performed the semiclassical atomic orbital close-coupling calculations with straight-line trajectories. We found the semiclassical calculations agree with the quantal approach at energy above 100eV/amu, where the collision occurs at large impact parameters. Calculations for Ar⁸⁺ + H collisions in the same energy range have also been carried out to analyze the effect of the ionic core on the subshell cross sections. By using diabatic molecular basis functions, we show that converged results can be obtained with small numbers of channels

### State-to-state rotational transitions in H$_2$+H$_2$ collisions at low temperatures

We present quantum mechanical close-coupling calculations of collisions
between two hydrogen molecules over a wide range of energies, extending from
the ultracold limit to the super-thermal region. The two most recently
published potential energy surfaces for the H$_2$-H$_2$ complex, the so-called
DJ (Diep and Johnson, 2000) and BMKP (Boothroyd et al., 2002) surfaces, are
quantitatively evaluated and compared through the investigation of rotational
transitions in H$_2$+H$_2$ collisions within rigid rotor approximation. The
BMKP surface is expected to be an improvement, approaching chemical accuracy,
over all conformations of the potential energy surface compared to previous
calculations of H$_2$-H$_2$ interaction. We found significant differences in
rotational excitation/de-excitation cross sections computed on the two surfaces
in collisions between two para-H$_2$ molecules. The discrepancy persists over a
large range of energies from the ultracold regime to thermal energies and
occurs for several low-lying initial rotational levels. Good agreement is found
with experiment (Mat\'e et al., 2005) for the lowest rotational excitation
process, but only with the use of the DJ potential. Rate coefficients computed
with the BMKP potential are an order of magnitude smaller.Comment: Accepted by J. Chem. Phy

### Peculiar Features of the Interaction Potential between Hydrogen and Antihydrogen at Intermediate Separations

We evaluate the interaction potential between a hydrogen and an antihydrogen
using the second-order perturbation theory within the framework of the
four-body system in a separable two-body basis. We find that the H-Hbar
interaction potential possesses the peculiar features of a shallow local
minimum located around interatomic separations of r ~ 6 a.u. and a barrier
rising at r~5 a.u. Additional theoretical and experimental investigations on
the nature of these peculiar features will be of great interest.Comment: 13 pages, 6 figure