Slow Predissociation Research Articles

A galvanostatic study of the electrolytic reduction silica in molten cryolite

The electrodeposition of silicon from SiO 2 dissolved in molten cryolite has been investigated using chronopotentiometry at nickel working electrodes. The reduction mechanism involves the slow predissociation of a silica complex to form an electroactive species: ▪ where the equilibrium constant K = k f / k b = 1.36, k f = 31.0 s −1 and k b = 23.0 s −1. The preceding slow chemical reaction and subsequent silicon-nickel interaction caused curvature in the E vs ln (τ 1 2 − t 1 2 )/ t 1 2 plots and deviation from the expected value of n = 4. The diffusion coefficient for the electroactive species was found to be 6.4 × 10 −5 cm −2/s at 1293 K.

Kinetic energy distributions in the fragmentations of metastable H 2S, HDS and D 2S ions

The distribution of kinetic energy release associated with H 2 loss from metastable H 2S +− ions has been measured in the coordinate system moving with the velocity of the center of mass and compared with those for the analogous reaction in HDS +− and D 2S +−. The distributions have similar forms but are displaced relative to one another by amounts which correspond to the differences in zero point energies of the respective products. The results are in agreement with the occurrence of fragmentation of the metastable ions by slow spin-forbidden predissociation involving a transition from the 2A 1 state of H 2S +− to the repulsive 4A 2 state. The similarities in the energy distributions seem to rule out tunnelling as a contributor to the curve crossing process. The energy release distributions also allow the estimation of the energy of the classical crossing point.

Thermochemistry and energy partitioning in the charge separation reactions of doubly charged triatomic ions

Ion kinetic energy and appearance potential measurements have been used to study the unimolecular charge separation reactions of the triatomic ions CO22+, COS2+, CS22+, N2O2+, NO22+ ·, and SO22+. The experimentally determined metastable peak shapes have been compared with calculated peak shapes including calculations that determine both the average kinetic energy release and the distribution about this average. The product ion states have been assigned from the thermochemical data, and in all the reactions total spin and angular momentum are found to be conserved. There is evidence that the metastable peaks are associated with maxima on single potential surfaces rather than with a slow predissociation. For CO2++ and COS++ almost all the available energy is partitioned into translational energy of product separation.

Kinetic energy distributions of fragment ions in the mass spectrum of isocyanic acid

Kinetic energy distributions for the HCO + and NH + fragment ions produced by electron impact from isocyanic acid, HNCO, have been studied by a deflection method. The kinetic energy distribution observed for HCO + is consistent with formation by slow predissociation via a repulsive quartet state of the parent ion. The distribution observed for NH + is of the shape expected for a dissociation without significant reverse activation energy but with a spread of excitation energies available for the formation of the fragment ion. Quantitative disagreement between the observed deflection curve and quasi-equilibrium theory prediction may be due to the formation of some NH + ions in an excited electronic state.