Spectroscopic studies of molecules that challenge computational chemistry

The gas-phase electronic spectra of small Be-containing (Be2 +, BeOBe+, BeC), Hf-containing (HfF+ and HfS+) and actinide molecules (ThF, ThF+, UF and UF+) were recorded with variety of experimental techniques. Ab initio electronic structure calculations were used to facilitate the assignment of the observed spectra.;The ground electronic states of Be2+ and BeOBe + were probed using the PFI-ZEKE technique. The rotational symmetry selection rules were used to confirm the symmetry of the ground state as 2sumu in Be2+ and 2 sumg+ in BeOBe+. The ionization energies were refined over previous measurements. Results from recent theoretical calculations are in good agreement with the experimental data.;Electronic spectra for BeC were recorded over the range 30500-40000 cm -1. The vibronic structure consists of a series of bands with erratic energy spacings. Theoretical calculations indicate that the erratic vibronic structure results from strong interactions between the four lowest energy 3pi states. The predictions were sufficiently close to the observed structure to permit partial assignment of the spectra. Bands originating from the low-lying 15sum- state were also identified, yielding a 5sum- to X3sum - energy interval of 2305+/-80 cm-1.;The electronic spectra for HfS+ and HfF+ were recorded with PFI-ZEKE technique. Three electronic states were identified in HfS+ and four in HfF+. The ionization energies for neutral molecules, term energies and molecular constants for the ground and low-lying states of cations are reported. The results for HfF+ support the contention that HfF+ is a viable candidate for electron electric dipole moment measurements. The spacings between adjacent X1sum+ and 3Delta1 levels are found to be less favorable for the proposed studies of the fine structure constant.;The low-lying states of ThF+ and UF+ were probed PFI-ZEKE photoelectron spectroscopy. Vibronic progressions belonging to four electronic states of ThF+ and sixteen electronic states of UF+ were identified. The ground state in ThF+ is tentatively assigned as 1sum+. The 3Delta1 state of ThF+, of interest for investigation of the electron electric dipole moment, is just 315.0(5) cm -1 above the ground state. The observed patterns of low-lying electronic states of UF+ are consistent with ab initio calculations and ligand field theory predictions.