potential energy surfaces and reaction dynamics studies of small triatomic systems: o+h2, oh+h and oh+d

Abstract

This dissertation is focused on the interaction of open-shell atoms and molecules. We have studied the systems in the title by means of electronic structure and statistical reaction dynamics methods.

We present an ab initio study of the O(3P) + H2 system. In particular, we have calculated potential energy surfaces for the van der Waals region of the interaction and derived and calculated the spin-orbit coupling matrix in the diabatic representation.

The rest of the dissertation is comprised of statistical, coupled-states dynamics studies. Cross sections are calculated by the coupled-states (CS) statistical method including the full open-shell character of the systems. All electronic and spin-orbit couplings are included.

We report state-to-state and overall thermal rate constants for the isotope exchange D(2S) + OH(2Pi) -> OD(2Pi) + H(2S) for 0 K < T < 500 K. We predict a reaction rate constant of 14.22x10-11 cm3 molecule-1 s-1 at T=100 K and 10.78x10-11 cm3 molecule-1 s-1 at T=300 K. At lower temperatures, (T ~ 50 K), the value rises to k(T)=15x10-11 cm3 molecule-1 s-1. A negative temperature dependence in the rate constant is observed. The state-resolved cross sections and rate-constants predict a significant propensity toward formation of the OD Pi(A') \Lambda-doublet level and the ground spin-orbit manifold, F1.

This dissertation is also concerned with the study of vibrational and rotational relaxation of OH(^2\Pi) by collision with H atoms. Four potential energy surfaces (PESs) ({1,3}A' and {1,3}A") describe the interaction of OH(X2Pi) with H atoms. Of these, three are repulsive, while one (1A') correlates with the deep H2O well. Consequently, rotationally- and ro-vibrationally-inelastic scattering of OH in collisions with H can occur by scattering on the repulsive PESs, in a manner similar to the inelastic scattering of OH by noble gas atoms, or by collisions which enter the H2O well and then re-emerge. We report state-to-state cross sections and thermal rate constants for the collisions. At 300 K, we predict large (~1x10-10 cm3 molecule-1 s-1) vibrational relaxation rates out of both v=2 and v=1, comparable to earlier experimental observations.

This surprisingly fast relaxation results from capture into the H2O complex. There also exists a significant propensity toward formation of OH in the Pi(A') Lambda-doublet level. We also report state-resolved cross sections and rate constants for rotational excitation within the OH v=0 manifold. Collisional excitation from the F1 to the F2 spin-orbit manifold leads to an inverted Lambda-doublet population.