Ab initio studies of ground and excited electronic states of MgAr,CdAr, and BeAr

Summary The ground state (X¹⁺) and several excited state (A³,c³+,C¹,D¹⁺, andE³⁺) potential energy surfaces for the diatomic molecules MgAr, CdAr, and BeAr have been computed using complete active space self-consistent field (CASSCF) wavefunctions and valence double- and triplezeta quality basis sets augmented with polarization and diffuse functions. Pump-and-probe laser experiments have examined the quenching, of excited singlet states of metal-rare gas complexes such as CdXe to produce triplets that dissociate to³P_Jmetal atoms. This quenching, which is detected for CdXe but not for CdAr or MgAr, is thought to occur via a crossing or strong coupling of a repulsive triplet curve correlating to the underlying³P state of the metal, with an attractive singlet curve that correlates to the higher¹P state of the metal. The present work indicates that the attractiveC¹ and repulsivec³⁺ curves of MgAr and CdArdo not intersect in the energetically accessible region of theC¹ surface, unlike the corresponding curves for the CdXe diatom. These data are consistent with the absence of³P_J Cd atoms in the MgAr and CdAr experiments, respectively. However, an alternative quenching mechanism involving vibronic coupling between theC¹ vibrational eigenstates and the continuum eigenstates of the underlying repulsive³⁺ surface may be operative; this possibility is examined qualitatively and predicted to be unlikely for MgAr (due to small spin-orbit coupling) and CdAr (due to unfavorable vibronic factors). BeAr, which has yet to be probed experimentally, is predicted to be bound by 770 and 900 cm^–1 in theD¹⁺ state (which has metal 2s2p character) and theE³⁺ state (which has Rydberg metal 2s3s character), respectively, and to display interesting potential curve intersections.Dedicated to Prof. Klaus Ruedenberg