Volume 130, Issue 3, 21 January 2009
Index of content:
130(2009); http://dx.doi.org/10.1063/1.3065970View Description Hide Description
The determination of the dissociative recombination rate coefficient of has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho- and para-. A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of in the diffuse interstellar medium, where has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched para- using a supersonic expansion source that produced ions with . We observed an increase in the low energy recombination rate coefficient of the enriched para- by a factor of in comparison to produced from normal . The ratio of the rate coefficients of pure para- to that of pure ortho- is inferred to be at low collision energies; the corresponding ratio of the thermal rate coefficients is at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of .
Vibrationally inelastic collisions of : A comparison of quantum mechanical, quasiclassical, and experimental results130(2009); http://dx.doi.org/10.1063/1.3065668View Description Hide Description
A detailed comparison of quantum mechanical (QM) and quasiclassical trajectory(QCT) integral and differential cross sections (DCSs) as well as opacity functions is presented in this work for the vibrationally inelastic collisions of at 1.72 eV collision energy. These results are also compared with the experimental differential cross sections by Greaves et al. [Nature (London)454, 88 (2008)]. The agreement between QCT and QM results is fairly good but some differences are appreciable, and it is shown that the experimental results are in a somewhat better agreement with the calculated QM DCS. The present results and their analysis confirm that the vibrational excitation takes place by elongation of the D–D bond in a “tug-of-war” mechanism, where the incoming H atom and one of the D atoms compete for the formation of a bond with the other D atom, as proposed by Greaves et al. It is also found that these collisions may give rise to the formation of short-lived collision complexes that can be traced back to the presence of relatively deep wells in the potential surface when the original D–D bond is stretched. The analysis of the trajectories into reveals that most of them cross at least twice the reaction barrier via a recrossing mechanism.
130(2009); http://dx.doi.org/10.1063/1.3056412View Description Hide Description
The solid-solid coexistence of a polydisperse hard sphere system is studied by using the Monte Carlo simulation. The results show that for large enough polydispersity the solid-solid coexistence state is more stable than the single-phase solid. The two coexisting solids have different composition distributions but the same crystal structure. Moreover, there is evidence that the solid-solidtransition terminates in a critical point as in the case of the fluid-fluid transition.