Dependence of the H2-H2 Interaction on the Monomer Bond Lengths: Steps Toward an Accurate ab initio Estimate

We compute the vibrational coupling between two H₂ molecules from ab initio quantum chemical calculations of the H₂-H₂ potential carried out at the full configuration interaction level of theory using the atom-centered aug-cc-pVTZ basis set for hydrogen. We compare the full configuration interaction results with those obtained using two variants of coupled cluster theory and find that a fully iterative treatment of connected triples may be required to estimate the H₂-H₂ vibrational coupling accurately using coupled cluster theory.     

Disentangling the reaction mechanisms of weakly bound nuclei

Complete and incomplete fusion cross sections for the ⁹Be + ¹⁴⁴Sm reaction have been measured at near-barrier energies, using the delayed X-ray detection technique. At above-barrier energies these show a suppression of complete fusion for this weakly bound projectile on an intermediate mass target. The suppression factor, attributed to ⁹Be break-up, was deduced from a comparison of complete fusion yields with coupled-channels calculations, and appears consistent with measured incomplete fusion product yields. At ∼10%, it is considerably smaller than the value of ∼30% previously found for a ²⁰⁸Pb target. Simultaneous measurements of elastic and inelastic scattering permit a clearer picture of the reaction mechanisms.     

First and second derivative matrix elements for the stretching,bending, and torsional energy

Matrix elements for the first and second derivatives of the internal coordinates with respect to Cartesian coordinates are reported for stretching, linear, nonlinear, and out-of-plane bending and torsional motion. Derivatives of the energy with respect to the Cartesian coordinates are calculated with the chain rule. Derivatives of the energy with respect to the internal coordinates are straightforward, but the calculation of the derivatives of the internal coordinates with respect to the Cartesian coordinates can be simplified by the following two steps outlined in this article. First, the number of terms in the analytical functions can be reduced or will vanish when the derivatives of the bond length, bond angle, and torsion angle are reported in a local coordinate system in which one bond lies on an axis and an adjacent bond lies in the plane of two axes or is projected onto perpendicular planes for linear and out-of-plane bending motion. Second, a simple rotation transforms these derivatives to the appropriate orientation in the space-fixed molecular coordinate system. Functions of the internal coordinates are invariant with respect to translation and rotation. The translational invariance and the symmetry of the second derivatives for a system with L atoms are used to select L-1- and L(L-1)/2-independent first and second derivatives, respectively, of which approximately half of the latter vanish in the local coordinate system. The rotational invariance permits the transformation of the simplified derivatives in the local coordinate system to any orientation in space. The approach outlined in this article simplifies the formulas by expressing them in a local coordinate system, identifies the most convenient independent elements to compute, from which the dependent ones are calculated, and defines a transformation to the space-fixed molecular coordinate system.     

Dipole-bound anions of highly polar molecules: ethylene carbonate and vinylene carbonate

Results of experimental and theoretical studies of dipole-bound negative ions of the highly polar molecules ethylene carbonate (EC, C3H4O3, mu=5.35 D) and vinylene carbonate (VC, C3H2O3, mu=4.55 D) are presented. These negative ions are prepared in Rydberg electron transfer (RET) reactions in which rubidium (Rb) atoms, excited to ns or nd Rydberg states, collide with EC or VC molecules to produce EC- or VC- ions. In both cases ions are produced only when the Rb atoms are excited to states described by a relatively narrow range of effective principal quantum numbers, n*; the greatest yields of EC- and VC- are obtained for n*(max)=9.0+/-0.5 and 11.6+/-0.5, respectively. Charge transfer from low-lying Rydberg states of Rb is characteristic of a large excess electron binding energy (Eb) of the neutral parent; employing the previously derived empirical relationship Eb=23/n*(max)(2.8) eV, the electron binding energies are estimated to be 49+/-8 meV for EC and 24+/-3 meV for VC. Electron photodetachment studies of EC- show that the excess electron is bound by 49+/-5 meV, in excellent agreement with the RET results, lending credibility to the empirical relationship between Eb and n*(max). Vertical electron affinities for EC and VC are computed employing aug-cc-pVDZ atom-centered basis sets supplemented with a (5s5p) set of diffuse Gaussian primitives to support the dipole-bound electron; at the CCSD(T) level of theory the computed electron affinities are 40.9 and 20.1 meV for EC and VC, respectively.     

The He-LiH potential energy surface revisited. II. Rovibrational energy transfer on a three-dimensional surface

We use our rigid rotor He-LiH potential energy surface [B. K. Taylor and R. J. Hinde, J. Phys. Chem. 111, 973 (1999)] as a starting point to develop a three-dimensional potential surface that describes the interaction between He and a rotating and vibrating LiH molecule. We use a fully quantum treatment of the collision dynamics on the current potential surface to compute rovibrational state-to-state cross sections. We compute excitation and relaxation vibrational rate constants as a function of temperature by integrating these cross sections over a Maxwell-Boltzmann translational energy distribution and summing over Boltzmann-weighted initial rotational levels. The rate constants for vibrational excitation of LiH are very small for temperatures below 300 K. Rate constants for vibrational relaxation of excited LiH molecules, however, are several orders of magnitude larger and show very little temperature dependence, suggesting that the collisions that result in vibrational relaxation are governed by long-range attractive interactions.     

Predominant time scales in fission processes in reactions of S, Ti and Ni with W: zeptosecond versus attosecond

The inhibition of fusion by quasifission is crucial in limiting the formation of superheavy elements in collisions of heavy nuclei. Time scales of ～10(-18) s inferred for fissionlike events from recent crystal blocking measurements were interpreted to show either that quasifission itself is slower than previously believed, or that the fraction of slow fusion-fission is higher than expected. New measurements of mass-angle distributions for (48)Ti and (64)Ni bombarding W targets show that in these reactions quasifission is the dominant process, typically occurring before the system formed after contact has made a single rotation, corresponding to time scales of ≤10(-20) s.     

Limit to high-spin isomeric fission

The possibility that high-spin isomers in heavy nuclei might undergo fission has been investigated experimentally. The partial fission lifetime of the 34 μs, I = 34 isomer in ²¹²Fr is found to be at least one hour.     

Correlated oscillations in the excitation functions of deep-inelastic collisions: evidence for nuclear pulsars?

Excitation functions for the strongly dissipative collision ¹⁹ F(E _lab = 135–140 MeV)+⁸⁹ Y with an energy step of 250 keV in the lab. system have been measured. The data are consistent with previous measurements and exhibit oscillations, which are interpreted as an indication of quantum chaotic phenomena in dissipative heavy-ion collisions. The Fourier component of the energy autocorrelation of the total projectile-like angle-integrated dissipative yield has been calculated and is found to have a pulsing behaviour with a time interval between sequential pulses equal to the period of the coherent nuclear rotation. This provides another possibility of experimentally detecting quantum chaos in dissipative heavy-ion collisions.     

Influence of entrance channel properties on heavy-ion reaction dynamics

In the fusion of heavy nuclei, there is a distribution of fusion barrier energies resulting from coupling between intrinsic motion and internal degrees of freedom. Precise experimental measurements of excitation functions have allowed the extraction of the distributions by taking the second derivative using a point-difference method. In the case of statically deformed nuclei, experimental data shows that the different fusion barrier energies correspond to different physical configurations of the colliding nuclei, the latter affecting the subsequent dynamical trajectories over the potential energy surface, influencing the ultimate reaction products, as for example in quasi-fission. The fusion barrier distribution is also valuable in understanding the fusion of weakly bound nuclei, enabling a reliable prediction of the expected fusion cross-sections, and thus the determination of fusion suppression factors at above-barrier energies. Received: 1 May 2001 / Accepted: 4 December 2001