A theoretical and experimental investigation of the H_3 system

Citation

Rodgers, Mary Theresa (1992) A theoretical and experimental investigation of the H_3 system. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ey99-a620. https://resolver.caltech.edu/CaltechTHESIS:09122011-095236680

Abstract

The H_3 system is the simplest triatomic neutral molecular species. It possesses only three electrons and three protons. As a result of its simplicity, the H_3 system has received a great deal of attention in ab initio quantum mechanical as well as experimental studies.

This dissertation consists of two parts. The first part is a theoretical investigation of the H_3 molecular system. Results of the ab initio quantum mechanical calculations for the lowest three electronic potential energy surfaces are given, as well as electronically nonadiabatic coupling elements between these states. The calculated nonadiabatic coupling elements compare well in some regions of configuration space with previous calculations performed on this system. Discrepancies in other regions can be attributed to the method of calculation. In our study these coupling elements were calculated by an ab initio method whereas analytic continuation was used in previous work. Calculation of the nonadiabatic coupling surfaces represents notable progress and will improve the fidelity of dynamics calculations of the H_3 system. All 3-D quantum mechanical theoretical investigations to date invoke the Born-Oppenheimer approximation and neglect nonadiabatic coupling of the nearby states. Although this is justified in many cases, the H_3 system exhibits a conical intersection near which this approximation breaks down. To obtain theoretical estimates of predissociative lifetimes of excited states of the H_3 system, accurate bound state wavefunctions and energies of the excited states of H_3 and accurate differential and integral cross sections in quantum mechanical scattering studies of the H + H_2 system above 2.75 eV, these nonadiabatic terms must be included.

The second part of this dissertation involves the development and characterization of an intense source of trihydrogen molecules. The ultimate goal of this work is to fully characterize the metastable H_3 molecules formed in this beam and to create a source of monoenergetic trihydrogen molecules whose translational energy would be continuously tunable from ~1-12 eV. Once developed, it could be utilized in crossed beam experiments and would enable many reactions to be studied that might not otherwise take place due to low reaction probability. The H_3 molecule in its 2p^2_zA^˝_2 electronic state is 5.85 eV higher and the 2p^2_x,_yE' repulsive ground state is 2.65 eV higher in energy than H + H_2 .^(17-20) Therefore, upon a vertical transition to the ground state, the 2p^2_zA^˝_2 state of H_3 will liberate about 3 eV of electronic energy with the remaining energy being channeled into vibration and rotation of the H + H_2 dissociated system. In a collision with another molecule, this energy could become available for reaction along with some fraction of the translational energy of these molecules (1-12 eV). This species can be expected to exhibit unusual dynamics, in that it may undergo novel chemical reactions as well as unique partitioning of the available energy into electronic, vibrational, rotational and translational energy of the products.

Thesis Files