			15min:

WHERE MILLIMETER WAVE SPECTRA ARE SENSITIVE TO SMALL ELECTRIC FIELDS: HIGH RYDBERG STATES OF XENON AND THEIR HYPERFINE STRUCTURES.

MARTIN SCHÄFER, MATTHIAS RAUNHARDT AND FRÉDÉRIC MERKT, ETH Zürich, Laboratorium für Physikalische Chemie, Wolfgang-Pauli-Strasse 10, 8093 Zürich, Switzerland.

In the range 0--45~cm^-1 below the ionization limit, the separation between adjacent electronic states (Rydberg states with principal quantum number n>50) of atoms and molecules is smaller than 2~cm^-1. In order to resolve the fine or hyperfine structure of these states, it is necessary to combine high-resolution vacuum ultraviolet (VUV) laser radiation, which is required to access the Rydberg states from the ground state, with millimeter wave radiation. \footnoteC. Fabre, P. Goy, S. Haroche, J. Phys. B: Atom. Mol. Phys. \textbf10, L183--189 (1977). F. Merkt, A. Osterwalder, Int. Rev. Phys. Chem. \textbf21, 385--403 (2002). M. Schäfer, M. Andrist, H. Schmutz, F. Lewen, G. Winnewisser, F. Merkt, J. Phys. B: At. Mol. Opt. Phys. \textbf39, 831--845 (2006). Such double-resonance experiments have been used to study the hyperfine structure of high Rydberg states of ⁸³Kr \footnoteM. Schäfer, F. Merkt, Phys. Rev. A , \textbf74, 062506 (2006)., H₂ \footnoteA. Osterwalder, A. Wüest, F. Merkt, Ch. Jungen, J. Chem. Phys. , \textbf121, 11810--11838 (2004)., or D₂ \footnoteH. A. Cruse, Ch. Jungen, F. Merkt, Phys. Rev. A \textbf77, 04502 (2008)..

Millimeter wave transitions (240--350 GHz) between n\ell (52\leq n\leq64, \ell\leq3) Rydberg states of different xenon isotopes were detected by pulsed field ionization followed by mass-selective detection of the cations. Because of the high polarizability of high-n Rydberg states (\propto n⁷, sim 10⁴ MHz cm² V^-2 for n~ 50), it is necessary to reduce the electric stray fields to values of the order of mV/cm (or less) in order to minimize the (quadratic) Stark shift of the millimeter wave transitions. Some p and d Rydberg states of Xe are nearly degenerate and efficiently mixed by small stray fields, making it possible to observe transitions forbidden by the Delta \ell=\pm1 selection rule or transitions exhibiting a linear Stark effect, which is typical for the degenerate high-\ell Rydberg states.

Multichannel quantum defect theory (MQDT) was used to analyze the millimeter wave data and to determine the hyperfine structures of the ²P_3/2 ground electronic states of ¹²⁹Xe⁺ and ¹³¹Xe⁺.