Quantum mechanical studies of weakly bound molecular ions

The study of weakly bound complexes constitutes an active field of research. High-resolution experimental spectra are necessary to provide precise measurements of the intermolecular forces of these systems. Analogously, highly accurate quantum calculations are required to correctly describe weak intermolecular forces such as dispersion and induction. In this work, theoretical studies of three molecular ions, He3, Li-(H2), and LiH~, are described where each ion is weakly bound to their respective He^-|- He, Li~ +- H2, and LiH + -e - limits. For He3+, an analytical global ground state potential energy surface is developed from high quality ab initio calculations. The linear symmetric global minimum is found to be consistent with previous ab initio results (r^ = 2.340 a.u, and Dg = 0.175 eV). Numerical determination of the bound rovibrational levels reveals that (1) the ZPE is highly anharmonic, (2) a large number of banding states (v2 < 6) is supported, and (3) two quanta of pure asymmetric stretch (f 3 = 2) is not seen in our calculations implying that this state may be unbound. In the second study, an analytical potential energy surface is developed from high quality ab initio calculations for the electrostatic region of the Li~ -f H2 interaction. The Li~(H2) electrostatic complex is found to have a linear minimum energy structure with a De of 64.44 cm~-^ and numerical determinations of the bound levels indicate a Do of only ~7 cm~-^ for Li~(para-H2) and a considerably larger D0 of ~22 cm^-1 for Li-(ortho-B.2). Altogether, the Li-(para-H2) interaction is predicted to support 11 bound levels, whereas the Li~(ortho-H2) interaction is predicted to support 28 bound levels. Analogous results for the D2 and HD isotopomers are also reported. Finally, the photoelectron spectra of LiH" and LiD~ are determined from a first principles theoretical treatment. Satisfactory simulation of the experimental photoelectron spectra is accomplished by assuming a non-Boltzmann distribution of the anion vibrational levels and the discrepancy between the experimental (920 ± 80 cm-1) and theoretical (1176.1 cm~^) values of we of LiH- is resolved by a reassignment of the hot band transition region of the photoelectron spectrum.