Vibronic Interactions in Sodium Trimers

With a single s-electron in the outer shell, alkali atoms are the building blocks of the simplest metal clusters. While aggregates of several dozen atoms have been found to resemble finite size samples of electrical conductors, trimers or tetramers offer the opportunity for detailed studies of interactions that are typical for highly symmetric metal lattices but can hardly be disentangled in the large clusters. Our work examines vibronic couplings in two- and three-state electronic systems of sodium trimers. Spectroscopic information was collected through cw laser excitation followed by laser ionization and mass selective detection in a molecular beam experiment. A significant experimental advantage was gained by utilizing optical–optical double resonance methods with probing through the A state of the molecule. Narrowband Resonant Two-Photon Ionization (RTPI) detection through this long-lived state is extremely efficient and can accurately probe the population of individual rotational ground state levels. Scanning another narrowband laser through the short-lived B-X and B'-X transitions leads to the depletion of these probed levels and allowed for the first high resolution spectroscopy of the coupled B/B' system including rotational assignments. Theoretical analysis of the spectra within the framework of a Jahn–Teller formalism suggests that the B'-X system corresponds to transitions into the middle part of a three-surface potential that is the consequence of both Pseudo Jahn–Teller and Jahn–Teller interactions while the B state represents the lower part