The Stark effect in molecular Rydberg states is qualitatively different from the Stark effect in atomicRydberg states because of the anisotropy of the ion core and the existence of rotational andvibrational degrees of freedom. These uniquely molecular features cause the electric-field-induceddecoupling of the Rydberg electron from the body frame to proceed in several stages in a molecule.Because the transition dipole moment among the same-n* Rydberg states is much larger than thepermanent dipole moment of the ion core, the decoupling of the Rydberg electron from the ion coreproceeds gradually. In the first stage, analyzed in detail in this paper, f and N are mixed by theexternal electric field, while N~+is conserved. In the further stages, as external electric fieldincreases, N~+,n~*,andv~+are expected to undergo mixing. We have characterized these stages inn~*=13, v~+=1 states of CaF. The large permanent dipole moment of Car makes CaF qualitativelydifferent from the other molecules in which the Stark effect in Rydberg states has been described(H_2,Na_2, Lie, NO, and H_3) makes it an ideal testbed for documenting the competition betweenthe external and CaF~+ dipole electric fields. We use the weak-field Stark effect to gain access to thelowest-N rotational levels of f, g, and h states and to assign their actual or nominal N~+quantumnumber. Lowest-N rotational levels provide information needed to disentangle the short-range andlong-range interactions between the Rydberg electron and the ion core. We diagonalize an effectiveHamiltonian matrix to determine the e-characters of the 3≤e≤ 5 core-nonpenetrating ~2Σ+statesand to characterize their mixing with the core-penetrating states. We conclude that the mixing of thee=4,N–N~+=-4(g(-4))state with lower-e ~2Σ+states is stronger than documented in our previousmultichannel quantum defect theory and long-range fits to zero-field spectra.
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