Rovibrational structures of the pseudorotating lithium trimer 21Li3:: Rotationally resolved spectroscopy and ab initio calculations of the A 2E←X 2E′ system

Based on resonant two-photon ionization of Li-21(3) with cw lasers and mass-selective detection of Li-21(3)+ ions by a quadrupole mass spectrometer, isotope-selective rotationally resolved spectra of vibronic bands in the electronic A E-2(')<--X E-2(') 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 (upsilon (s)'=0, upsilon (b)'=0, upsilon (a)'=0)left arrow>(*) over bar *(upsilon (s) =0, upsilon (b) =0, upsilon (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 pi for closed loops around the conical intersection at D-3h geometries. (C) 2000 American Institute of Physics. [S0021-9606(00)00439-6].