SPECTROSCOPIC CHARACTERIZATION OF THE (3)DELTA(4D), (3)PI(4D), (3)SIGMA(-DER-WAALS MOLECULE()(4D), AND (3)PI(5P) RYDBERG STATES OF THE MGARVAN)

The Mg(3s4d sigma(3)D(J)). Ar((3) Sigma(+)), Mg(3s4d pi(3)D(J)). Ar(3 Pi), Mg(3s4d delta(3)D(J)). Ar((3) Delta), and Mg(3s5p pi(3)P(J)). Ar( 3 Pi) Rydberg states have been characterized via Resonance Enhanced Tw o-Photon Ionization (R2PI) spectroscopy of transitions from the long-l ived Mg(3s3p pi(3)P(J)) . Ar(3 Pi(0+,0-)) metastable states of the MgA r van der Waals molecule. The 4d pi, 4d delta, and 5p pi states are al l strongly bound (D-0=1230+/-50, 1200+/-40, 1270+/-50 cm(-1), respecti vely). These bond energies are very similar to that of the ground-stat e MgAr+ ''core'' ion (D-0=1240+/-40 cm(-1)), indicating very effective penetration of transversely aligned, diffuse Rydberg electron clouds on the Mg atom by the Ar atom, even for the low-lying n=4,5 Rydberg st ates. The Mg(3S4d sigma(3)D(J)). Ar((3) Sigma(+)) State was substantia lly less bound, D-0=800+/-40 cm(-1), showing there is still some resid ual Mg(4d sigma)/Ar(3p sigma)(2) electron-electron repulsion preventin g penetration of the axially aligned Mg(4d sigma) Rydberg electron clo ud by the Ar atom. Successful computer simulations of the rotational s tructure of several of the vibrational transitions to the 4d Delta and 4d Sigma states, assuming Hund's case ''b'' upper-state character, re sulted in R(0)', values of 2.80+/-0.04 Angstrom and 2.90+/-0.05 Angstr om, respectively (compared to R(0)=2.82+/-0.01 Angstrom determined pre viously by others for the MgAr+ ion). Because the 4d pi and 5p pi stat es have similarly large bond strengths as well as similar asymptotic M g(4d(3)D(J)) and Mg(5d(3)P(J)) atomic energies, they have ''mixed'' 4d pi/5p pi character. (C) 1997 American Institute of Physics.