Experimental and theoretical characterization of the BAr van der Waals complex: The X 2Π, A 2Σ+, and B 2Σ+ electronic states

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Abstract
The BAr van der Waals complex and its electronic transition correlating with the B atom 3s 2 S–2p 2 P transition have been characterized in a combined experimental and theoretical investigation. The experimental portion of the study consisted of the observation by laser fluorescence excitation of rotationally resolved bands of this molecule in a supersonic jet. Specifically, four bands of the (v’,0) progression of the B 2Σ+–X 2Π1/2 band system of each of the 11,10BAr isotopomers were observed and analyzed, where the upper state vibrational quantum numbers v’=4–7 were determined from the isotope splittings. Vibrational and rotational constants were obtained through fits to the observed transition wave numbers. These experimental results were compared with ab initio calculations of the X 2Π, A 2Σ+, and B 2Σ+ electronic states of BAr. In order to obtain the interactionenergies of the excited states of this weakly bound system accurately, multireference, internally contracted, configuration‐interaction calculations were carried out, with additional provision for the effect of higher order excitations. The information derived about these states from the experiment and calculations agree reasonably well. An interesting feature of the BAr B 2Σ+ state is the presence of a barrier in the potential energy curve.