"Spectrum-only" assignment of core-penetrating and core-nonpenetrating Rydberg states of calcium monofluoride

Rydberg states of calcium monofluoride in the n* = 17–20 region have been observed by ionization-detected optical–optical double-resonance spectroscopy via the D 2 Σ + v = 1 intermediate state. All members of the six core-penetrating Rydberg series in the n* = 17–20 region and several components of the 17f and 17g core-nonpenetrating Rydberg states have been assigned. While the assignment of core-penetrating Rydberg states is straightforward without use of an effective Hamiltonian model, "spectrum-only" assignment of core-nonpenetrating states is complicated because strong l-uncoupling causes the core-nonpenetrating states to evolve rapidly from Hund's case (b) to Hund's case (d) coupling. We describe "spectrum-only" assignment procedures, developed in the spirit of Gerhard Herzberg, that can be used to assign optical–optical double-resonance spectra of core-penetrating and core-nonpenetrating Rydberg states using only information contained in the spectrum rather than predictions derived from an effective Hamiltonian model. The ambiguities that arise in the assignment of each class of states are discussed in detail.Key words: CaF, electric quadrupole moment, Rydberg states, laser spectroscopy.