For the open shell van der Waals molecule, CAr, the potential energy curves (PECs) for the B 3Π and 1 5Σ states, the B 3Π–1 5Σ spin–orbit coupling and the B 3Π fine structure splitting are determined using multireference configuration interaction wave functions as large as ∼8 million configuration state functions. The B 3Π state is strongly bound, with De=5100 cm−1.Re(B 3Π)=3.7a0 and is considerably shorter than Re(X 3Σ)=6.07 a0. The PEC for the repulsive 1 5Σ state crosses that of the B 3Π state at Rx(1 5Σ,B 3Π)=3.31a0 leading to spin–orbit induced predissociation. The B 3Π–1 5Σ spin–orbit coupling is the result of valence-Ryberg mixing in the B 3Π state and is considerably enhanced by the heavy atom effect. The heavy atom effect is also reflected in a marked decrease in the fine structure splitting of the B 3Π state with increasing vibrational level. The implications of these results for using CAr(B 3Π) in laser induced fluorescence detection of CAr(X 3Σ,v) are discussed.

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