New Rydberg-Rydberg Transitions of the ArH and ArD Molecules.

We report observations and analyses of bands in the spectra of the ArH and ArD molecules due to emission from the 4f Rydberg complexes. For ArD, the observed bands are 4f --> 5s and 4f --> 3dlambda for all three components lambda = 0, 1, and 2 of the 3d state. These give direct information on all 14 spin-orbit components of the 4f complex. For ArH, the data is more limited because the levels of the 5s state are broadened by predissociation, and only the lambda = 2 component of the 4f --> 3dlambda transitions is seen clearly, although a number of lines could be assigned to the 4f --> 3dpi band, which is strongly overlapped by the 6s --> 4p band. The 4f complexes of both isotopes are good example of Hund's case (d) coupling. The 4f orbitals are essentially non-core-penetrating, so that spin-orbit coupling is negligible for them, and the zero-rotation splitting agrees well with calculations from the ab initio quadrupole moment and anisotropic polarizability of the core. The orbit-rotation coupling parameter xi4f gives a value of the core electric dipole moment that is in reasonable agreement with ab initio and previous experimental values. A summary of the presently known states of ArH and ArD is presented. Copyright 1998 Academic Press.