SPECTROSCOPIC CHARACTERIZATION OF THE EXCITED MG(3S3D(3)D(J))CENTER-DOT-AR((3)PI), MG(3S3D(3)D(J))CENTER-DOT-AR((3)DELTA), AND MG(3S4P(3)P(J))CENTER-DOT-AR((3)PI) VAN-DER-WAALS STATES

The Mg(3s3d pi D-3(J)). Ar((3) Pi), Mg(3s3d delta D-3(J)). Ar((3) Delt a), and Mg(3s4p pi P-3(J)). Ar((3) Pi) states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of tr ansitions from the long-lived Mg(3s3p pi P-3(J)). Ar((3) Pi(0+,0-)) me tastable states of the MgAr van der Waals molecule. Because the outer excited Mg(3d) and Mg(4p) orbitals are (similarly) quite diffuse and t he Ar atom can approach along the nodal axes of each of the aligned or bitals, minimizing repulsion, the Mg(3s3d delta D-3(J)). Ar((3) Delta) and Mg(3s4p pi P-3(J)). Ar((3) Pi) states are both very strongly boun d (D-0=1140+/-40, 1250+/-60 cm(-1), respectively), approaching the bon d energy of the MgAr+ ''core'' ion (for which D-0=1240+/-40 cm(-1)). I n contrast, the Mg(3s3d pi D-3(J)). Ar state is more weakly bound (D-0 =290+/-40 cm(-1)), although it has a greater bond strength than the Mg (3s3p pi P-3(J)). Ar((3) Pi(0-)) lower state (for which D-0 is 160+/-4 0 cm(-1)). The effective spin-orbit constant of the Mg(3s3d pi D-3(J)) . Ar((3) Pi) state is much larger than expected from the Mg(3s3d D-3(J )) multiplet splittings, and also increases in magnitude as upsilon' d ecreases; possible reasons for this are discussed. It is suggested tha t the predissociation of the Mg(3s3d pi D-3(J)). Ar((3) Pi(0+)) state [the Mg(3s3d pi D-3(J)). Ar((3) Pi(0-)) state does not predissociate] is caused by a curve-crossing with the + repulsive Mg(3s3d sigma D-1(2 )). Ar((1) Sigma(0+)(+)) state, which lies at a lower asymptotic energ y because of substantial mixing of Mg(3p3p D-1(2)) character into its wave function. (C) 1996 American Institute of Physics.