I. Dabrowski et al., NEW RYDBERG-RYDBERG TRANSITIONS OF THE ARH AND ARD MOLECULES .1. EMISSION FROM NP STATES OF ARD, The Journal of chemical physics, 104(21), 1996, pp. 8245-8257
The ground electronic state of argon hydride has a repulsive potential
apart from a long-range van der Waals minimum, but the Rydberg excite
d states have bound potentials similar to those of the ion ArH+. These
states can be described approximately in terms of united-atom quantum
numbers nl. We report here rotational analyses of the bands 5p-->5s,
5p-->6s, and 6p-->5s of ArD, which help to further characterize the np
Rydberg series. In ArH the bands 5p-->5s and 6p-->5s have broad lines
because of predissociation in the lower state, and 5p-->6s is difficu
lt to analyze without further information. The present data are fitted
with a Hund's case (d) effective Hamiltonian. In previous work the 4p
state was found to have a very small sigma-pi splitting, but this doe
s not hold for the higher np states, and is probably due to an acciden
tal cancellation between electrostatic and polarizability contribution
s. On the other hand, the spin-orbit coupling decreases monotonically
with n. Features of the rotational levels are discussed in terms of th
e high-J limiting quantum numbers l(J)=N-R and s(J)=J-N, where R=N+, i
n particular the effect of spin-orbit coupling on the levels with (l(J
),s(J)) =(-1,1/2) and (0,-1/2), which produces a tendency to Hund's ca
se e behavior in 4p, and a sharp avoided crossing in 6p. The correspon
ding avoided crossing in 5p would occur beyond the present range of ob
served J values.