Rovibrational structures of the pseudorotating lithium trimer²¹Li₃: Rotationally resolved spectroscopy and ab initio calculations of the A²E″←X²E′ system

Based on resonant two-photon ionization of ²¹Li₃ with cw lasers and mass-selective detection of ²¹Li₃⁺ ions by a quadrupole mass spectrometer, isotope-selective rotationally resolved spectra of vibronic bands in the electronic A ²E″←X²E′ system have been recorded. The complex but completely resolved spectra could be analyzed using optical-optical double resonance techniques in combination with accurate ab initio calculations which provide potential energy surfaces and rovibronic term values for both electronic states. A detailed comparison between experimental and theoretical results for the (v′_s = 0, v′_b = 0, v′_a = 0)←(v″_s = 0, v″_b = 0, v″_a = 0) band demonstrates excellent agreement. The coupling between pseudorotation and rotational motion is shown to produce complex level patterns which, however, can be accurately represented by an effective pseudo/rotation Hamiltonian in terms of rotational and coupling constants that directly provide structural information. For both electronic states the nuclear dynamics is adequately described as a motion on a single adiabatic potential surface with a geometric phase π for closed loops around the conical intersection at D_3h geometries.