The electronic structure of Pa-2 has been investigated using a 10-elec
tron relativistic pseudopotential, Configuration interaction with four
active electrons and core-polarisation potential. Fine structure has
been taken into account through a semiempirical spin-orbit operator, P
otential energy curves have been determined for all the molecular stat
es dissociating into Ba(6s(2))+ Ba(6s(2); 6s5d; 6s6p) and spectroscopi
c constants have been obtained for bound states. Evidence for a doubly
excited nature of various excited states is pointed out. The ground s
tate of Pa-2 is found to be weakly bound (D-e = 0.202 eV) at long rang
e (R(c) = 9.22 a(0)) with an harmonic frequency omega(c) = 35 cm(-1).
The calculated energy for the transition (2)(1) Pi(g)-X(1) Sigma(g)(+)
is found to be in very good agreement with an infrared band observed
at 850 nm in the absorption spectrum of Pa-2 isolated in solid matrix.
Almost all present results are new for excited states.