SPECTROSCOPIC CHARACTERIZATION OF THE SINGLY EXCITED CAAR(4D-PI (3)PI(0)), CAAR(4D-DELTA (3)DELTA(1)) STATES AND THE DOUBLY-EXCITED CAAR(4P-PI-4P-PI (3)SIGMA(-)) STATE

The singly excited Ca(4s4d pi(3)D(J)).AT((3) Pi(0-)), and Ca(4s4d delt a(3)D(J)).Ar((3) Delta(1)) valence states and the unusual doubly excit ed Ca(4p pi 4p pi P-3(J)).Ar((3) Sigma(-)) valence state have been cha racterized via Resonance Enhanced Two-Photon Ionization (R2PI) spectro scopy of transitions from the long-lived Ca(4s4p pi P-3(J)).Ar((3) Pi( 0-)) metastable state of the CaAr van der Waals molecule. Because the Ar atom can approach along the nodal axis of the quite diffuse Ca(4d d elta) orbital with little repulsion, the CaAr(4s4d delta (3) Delta(1)) state is fairly strongly bound (D-0 = 609+/-50 cm(-1)) , but less bou nd than the CaAr+ ''core'' ion (D-0= 775+/-50 cm(-1)). The CaAr(il 4s4 d pi (3) Pi(0-)) state is less bound (D-0-450 +/- 50 cm(-1)), but has a bond strength much greater than that of the lower-energy CaAr(4s4p p i 3 Pi(0-)) state (D-0= 60 +/- 50 cm(-1)). The doubly excited CaAr(4p pi 4p pi (3) Sigma(-)) State is extremely strongly bound (D-0= 1277 +/ - 50 cm(-1)), and this is attributed to the lack of a valence Ca(4s) e lectron. The Ar atom can therefore approach with minimal repulsion alo ng the nodal axes of the two transversely aligned Ca(4p pi) electrons. The Ca(4p pi 4p pi (3) Sigma(-)) State is also strongly predissociate d, with a lifetime less than 10 ps. (C) 1997 American Institute of Phy sics.