Scaling in Molecular Electronic Spectroscopy: Core-Nonpenetrating Rydberg States

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The Experimental Physical Chemistry Program supports Prof. Robert W. Field of Massachusetts Institute of Technology in his continuing investigation of `scaling laws,` a priori known functions of quantum numbers. Such scaling laws arise when the coupling between parts of a molecule becomes sufficiently weak. For example, many properties of hydrogenic eigenstates can be expressed as simple functions of Z, n and l. Many atom-based scaling relationships remain useful and instructive, even for polyatomic molecules; for core-nonpenetrating molecular Rydberg states, in which the coupling between Rydberg electron and the molecular ion-core is very weak, the resemblance to hydrogenic systems is quite strong. Field will undertake various types of experiments involving either resonance-enhanced multiphoton ionization or multiple resonance spectroscopies in an effort to develop experimental methods for spectroscopic studies of core-nonpenetrating molecular Rydberg states. These studies will yield information on the electric anisotropies and polarizabilities of molecular ion cores. A few comments about the practical significance of these experiments are warranted. Why should we care about the electric multipole moments of cations? All +1 cations have the same monopole, so this dominant aspect of the interaction of the cation with the outside world is independent of the molecule. Ion-specific long-range interactions, on the other hand, derive from the higher multipoles (and their normal coordinate displacement derivatives). These multipoles and derivatives control the mechanisms and rates of resonant long-range ion-molecule, ion-electron, and possibly ion-surface energy and angular momentum exchange. The information obtainable from systematic studies of core-nonpenetrating Rydberg states would yield valuable insights into a wide variety of processes occurring in plasmas.