n f Rydberg complexes of NO in a magnetic field, probed by double resonance multiphoton ionization

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Abstract
nf (v=1) Rydberg states of NO have been probed by double resonance multiphoton ionization in a 1 T external magnetic field. Due to the nonpenetrating character of the f orbitals, these Rydberg states are very sensitive probes of any external perturbation. As n increases, a decoupling of the angular momentum l of the Rydberg electron from the molecular frame occurs gradually, as the magnetic interaction becomes more and more important with respect to intramolecular forces. Up to n≂15, only the linear Zeeman perturbation has been taken into account. The rotational–electronic structure of the 7f and 15f states has been interpreted theoretically by considering the linear Zeeman perturbation in addition to the Coulombic interaction and the long range interaction due to the quadrupole moment and the polarizability of the ion core. The intensities and line positions of the transitions from the intermediate A 2Σ+,v=1 level to the 7f and 15f levels have been calculated. The alignment of the N, MS, MN Zeeman sublevels of the A state by the two‐photon pump excitation from the ground state as well as the polarization of both lasers have been taken into account in the calculations. A good agreement between the observed and the calculated transitions has been obtained. For the 7f levels, the electronic–rotational structure is well described in a coupled case (d) representation. For the 15f levels, the strong coupling of l to the field axis led to the first observation of the Paschen–Back effect in a molecule, within each rotational N+ series, with an accompanying drastic simplification of the spectra. This level is better described in a decoupled case (d) representation corresponding to a moderately strong field regime.