DOUBLY-EXCITED VALENCE STATES OF NEUTRAL VAN-DER-WAALS MOLECULES - MG(3P-PI,3P-PI P-3(J))CENTER-DOT-AR((3)SIGMA(-))

The first of a new class of doubly excited valence states of van der W aals molecules, Mg(3p pi,3p pi(3)P(J)). Ar((3) Sigma(-)), has been dis covered anti characterized by resonance two-photon ionization spectros copy. This state has been found to be quite strongly bound (D-0=2850 /- 100 cm(-1) and to have a very short bond length (R(0)=2.41+/-0.02 A ngstrom). The analogous singly excited state, Mg(3s3p pi(3)P(J)) . Ar( (3) Pi(0-)), in stark contrast, has a very much smaller bond strength (D-0=160+/-40 cm(-1)) and a longer bond length (R(0)=3.66+/-0.02 Angst rom). In fact, the new Mg(3p pi 3p pi(3)P(J)). Ar state is more than t wice as bound than even the Mg(3s S-2(1/2)). Ar+ ground-state ion (D-0 =1240+/-40 cm(-1), R(0)=2.82 Angstrom). The strong bonding is due both to the transverse alignment of the Mg(3p pi) electrons and particular ly to the lack of a Mg(3s) electron. The resulting loss of Mg(3s sigma )/Ar(3p sigma)(2) repulsive forces facilitates close approach of the A r atom. This is turn allows attractive Mg(3p pi)(2)/Ar dispersive forc es and Mg2+/Ar ion/induced-dipole type forces to continue to much smal ler internuclear distances, coating a strong, short van der Waals bond . Possible mechanisms for the parity-dependent predissociation and the apparent ''spin-spin'' type splittings observed for the Mg(3p pi,3p p i(3)p(J)). Ar((3) Sigma(-)) State are also discussed. (C) 1996 America n Institute of Physics.