Charles H. Douglass

Merged beams at Minnesota

A new merged‐beam apparatus is described, which permits the measurement of the energy dependence of the absolute cross sections for ion–neutral and neutral–neutral reactions yielding ionic products over the energy range <0.010 to ∼10 eV. The apparatus has a somewhat different geometry than that of any previous merged‐beam instrument and incorporates several innovations which facilitate the rapid measurement of relative cross sections, the accurate determination of the beam overlap integral, and the high‐resolution measurement of the product energy distribution. The performance of the instrument is illustrated with data on the reaction H+2 + H2 → H+3 + H.

The dynamics of the reaction H+2+H2→H+3+H, with isotopic variations

We performed a merged‐beam study of the reactions HD++D2→HD+2+D, D+3+H, D+2+HD→HD+2+D, D+3+H, in which we measured the absolute reaction cross sections, branching ratios, and product energy distributions over the range of relative kinetic energy from ∼0.002 to 8 eV. Within the experimental uncertainty all three types of data are identical for the two reactant charge states at initial kinetic energies less than 0.5 eV, leading us to believe that adiabatic charge equilibration between the reactants occurs prior to reaction at these low energies. Diabatic behavior becomes increasingly dominanant at higher energies. The reaction mechanism is direct and products are formed with a broad distribution of translational energies. In low energy collisions, on the average, a small fraction of the reaction exothermicity and the reactant excitation energy is converted into translational energy of the products. At higher energies there is a net conversion of translational into internal energy. The reaction energetics ar...

The dynamics of the reaction D2++N→ND++D

We report the results of a merged‐beam study of the reaction D+2+N→ND++D over the range of relative kinetic energy from ∼0.005 to 10 eV. At low kinetic energies the reaction cross section is accurately proportional to the orbiting cross section calculated for the ion‐induced dipole potential plus an experimentally estimated r−6 term. The reaction probability for the N(4Su) ground state is estimated to be at least 71% for low‐energy orbiting collisions. The reaction mechanism is direct, with the ND+ product scattered preferentially forward with respect to the incident N atom velocity at initial kinetic energies as low as 0.031 eV. A deconvolution analysis of the measured product laboratory energy distributions shows a net conversion of internal to translational energy at initial kinetic energies less than 0.9 eV, and a net conversion of translational to internal energy at higher initial kinetic energies. We interpret the results in terms of the adiabatic electronic state correlations for NH+2 systems, taki...

The dynamics of the reaction D+2+O(3P) →OD++D, and the influence of the atomic quadrupole moment on the cross section at very low kinetic energies

We report the results of a merged‐beam study of the reaction D+2+O(3P) →OD++D over the range of relative kinetic energy from 0.002 to 27 eV. The data include accurate absolute values of the total reaction cross section and also high‐resolution measurements of the product ion laboratory energy distributions, which are deconvoluted to obtain estimates of the product energy and angle distributions in c.m. coordinates. The reaction mechanism is direct, with the OD+ product scattered preferentially in the direction of the incident O atom, and with large average internal excitation. The fact that O(3P) possesses an electrostatic quadrupole moment has important consequences for the interpretation of the low‐energy data in terms of the adiabatic state correlations of reactants, intermediates, and products. Our major conclusions are (1) that the average probabilities of reaction for low‐energy capture collisions are about 87% for approaches on Σ−–A2 potential energy surfaces and about 53% for approaches on Π–B1, B...

The dynamics of the reaction H+3+D2→HD+2+H2, and the effects of reactant vibrational excitation on the product energy and angle distributions

The dependence of the absolute total cross section and the product ion kinetic energy distribution on the reactant kinetic energy have been measured for the reaction H+3+D2→HD+2+H2 over the energy range from 0.002 to 11 eV, by means of the merged molecular beam technique. At low collision energies, the reaction occurs with high probability per close collision, and at all collision energies the product ion is scattered preferentially in the direction of the D2 reactant in center‐of‐mass coordinates. Because the reactants in these experiments are vibrationally excited, comparison of the present results with the crossed‐beam data of M. L. Vestal, C. R. Blakly, P. W. Ryan, and J. H. Futrell [J. Chem. Phys. 64, 2094 (1976)] reveals the influence of reactant vibration on the reaction dynamics.