Experimental and theoretical study of the electronic spectrum of the BAr2 complex: Transition to the excited valence B(2s2p(2) D-2) state

The 2s2p(2) D-2 <-- 2s(2)2p P-2 valence transition in the BAr2 cluster is i nvestigated in a collaborative experimental and theoretical study. Laser fl uorescence excitation spectra of a supersonic expansion of B atoms entraine d in Ar at high source backing pressures display several features not assig nable to the BAr complex. Resonance fluorescence is not observed, but inste ad emission from the lower 3s state. Size-selected fluorescence depletion s pectra show that these features in the excitation spectrum are primarily du e to the BAr2 complex. This electronic transition within BAr2 is modeled th eoretically, similarly to our earlier study of the 3s <-- 2p transition [M. H. Alexander , J. Chem. Phys. 106, 6320 (1997)]. The excited potential ene rgy surfaces of the fivefold degenerate B(2s2p(2) D-2) state within the ter nary complex are computed in a pairwise-additive model employing diatomic B Ar potential energy curves which reproduce our previous experimental observ ations on the electronic states emanating from the B(D-2)+Ar asymptote. The simulated absorption spectrum reproduces reasonably well the observed fluo rescence depletion spectrum. The theoretical model lends insight into the e nergetics of the approach of B to multiple Ar atoms, and how the orientatio n of B p-orbitals governs the stability of the complex. (C) 2000 American I nstitute of Physics. [S0021-9606(00)01210-1].