Spectroscopic characterization of the metastable 4p pi (2)Pi(0)-valence states and the 5s (3)Sigma(+) Rydberg states of the CaAr, CaKr, and CaXe van der Waals molecules

The metastable Ca(4s4p P-3(J)). RG((3) Pi(0)-) states and the lowest-energy Ca(4s5s S-3(1)). RG((3) Sigma(+)) 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 Do and greater bond lengths R-0 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 (3) Sigma(+)) 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 omega(e) are actually higher for all the less strongly bound CaRG(3s4s (3) Sigma(+)) 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. (C) 1997 American Institute of Physics.