Spectroscopic characterization of the metastable 4pπ 2Π− valence states and the 5s 3Σ+ Rydberg states of the CaAr, CaKr, and CaXe van der Waals molecules

The metastable Ca(4s4p 3PJ)⋅RG(3Π0−) states and the lowest-energy Ca(4s5s 3S1)⋅RG(3Σ+) Rydberg states (RG=Ar, Kr, Xe) have been characterized by means of resonance-enhanced two-photon ionization spectroscopy (R2PI). All of the Ca⋅RG states have smaller bond energies D0 and greater bond lengths R0 than their Mg⋅RG analogs. This is rationalized as being due to repulsive forces setting in at greater internuclear distances R for the larger Ca atom. Similar to the analogous MgRG states studied earlier, the CaRG(4s5s 3Σ+) states are almost as strongly bound as the CaRG+ “core” ions, indicating quite efficient penetration of the diffuse Ca(5s) Rydberg orbital by the RG atoms. The vibrational frequencies ωe are actually higher for all the less strongly bound CaRG(3s4s 3Σ+) states than for their analogous CaRG+ ions, consistent with “narrower” potential curves due to possible small maxima in the Rydberg state potentials at large R because of the difficulty of the RG atoms penetrating the outer lobe of the Ca(4s) orbital.