Rovibrational structures of the pseudorotating lithium trimer Li-21(3): Rotationally resolved spectroscopy and ab initio calculations of the A E-2 ''<- X E-2(') 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, i sotope-selective rotationally resolved spectra of vibronic bands in the ele ctronic A E-2(')<--X E-2(') system have been recorded. The complex but comp letely resolved spectra could be analyzed using optical-optical double reso nance techniques in combination with accurate ab initio calculations which provide potential energy surfaces and rovibronic term values for both elect ronic states. A detailed comparison between experimental and theoretical re sults 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 demons trates excellent agreement. The coupling between pseudorotation and rotatio nal motion is shown to produce complex level patterns which, however, can b e accurately represented by an effective pseudo/rotation Hamiltonian in ter ms of rotational and coupling constants that directly provide structural in formation. For both electronic states the nuclear dynamics is adequately de scribed as a motion on a single adiabatic potential surface with a geometri c phase pi for closed loops around the conical intersection at D-3h geometr ies. (C) 2000 American Institute of Physics. [S0021-9606(00)00439-6].