Because of their unusual properties, Rydberg atoms, i.e., atoms with one electron occupying a highly excited orbit, have attracted the interest of many physicists. For example, due to their large size, such atoms are particularly sensitive to external fields. Fifty years ago, Segre and co-workers1 investigated the Stark effect and atomic diamagnetism of Rydberg states of alkalis with principal quantum numbers up to n = 30. During the last decade, Rydberg states have been studied extensively2 due to the availability of tunable laser radiation.
Stark spectra of barium Rydberg states above the classical field-ionization limit have been recorded at high density of states employing high-resolution laser spectroscopy. The spectra, taken at a binding energy of about \ensuremath{\varepsilon}\ensuremath{\approxeq}-55 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ with respect to zero-field threshold and at electric-field strengths F\ensuremath{\approxeq}100 V/cm exhibit dramatic variations in line profiles with increasing external field strength. The WKB quantum-defect theory of the Stark effect of nonhydrogenic atoms was used to analyze the recorded spectra, and excellent agreement between measured and calculated line profiles was achieved. Core-induced coupling of closed and open hydrogenic Stark channels was found to contribute dominantly to the linewidth of the recorded Stark resonances. In addition, a reformulation of this theory by McNicholl, Ivri, and Bergeman (unpublished) in terms of a configuration-interaction model was employed to analyze a particular avoided crossing of several Stark components. In this way recorded line profiles were parametrized within a limited model to obtain resonance energies, linewidths, and Fano q parameters.
Although the hydrogen atom has naturally served as the basis for extensive theoretical studies of the physics of highly excited atoms in strong external fields, experiments with this fundamentally important species in magnetic fields have not been reported, except for the most recent ones carried out in our laboratory.1,2 We report new results of studies on the spectroscopy of the atom in static homogeneous magnetic fields of strength B ≲ 6 T at excitation energies from high Rydberg states (n ≳ 20) into the ionization region (E ≲ +30 cm−1).
We have observed barium Rydberg states with principal quantum numbers up to
n=520. Such highly excited states allow quantitative in situ measurements of small electric fields on the order of tens of microvolts per centimeter. Because of the effective suppression of stray electric fields we have observed quasi-Landau resonances in external magnetic fields on the order of tens of gauss. The energy dependence of the spacing between neighboring quasi-Landau resonances was found to be in excellent agreement with calculations involving classical trajectories.
A theory of nonhydrogenic Stark spectra based on the hydrogen atom is specialized to quasidiscrete levels. Algebraic expressions for level positions ${\ensuremath{\epsilon}}_{r}$, widths ${\ensuremath{\Gamma}}_{r}$, and oscillator strengths
frF are derived in terms of matrices
HF and
hF, which represent the hydrogenic density of states in a Stark field
F. Core effects appear through zero-field quantum defects ${\ensuremath{\mu}}_{l}$ and dipole matrix elements
dl0. Normalized oscillator strengths
frF are defined which are independent of all
dl0 for $s\ensuremath{\rightarrow}p$ transitions. Isolated and interacting Stark manifolds with
m=0and1 are examined for systems with two nonnegligible ${\ensuremath{\mu}}_{l}$. Extensive comparisons are made with experimental Li spectra and matrix-diagonalization calculations of Zimmerman, Littman, Kash, and Kleppner [Phys. Rev. A 20, 2251 (1979); ZLKK]. For small fields
F<3n51 level positions are given analytically with respect to H levels of fixed
n;
m=0 intensity distributions do not appear hydrogenic in Li since ${\ensuremath{\mu}}_{0}\ensuremath{\sim}\frac{1}{2}$. At
F>3n51, degenerate parabolic eigenstates from different manifolds are coupled by the spherical core and avoid crossing. The upper levels disappear at
m=1 anticrossings in Li, as observed in ZLKK. For
m=0 the lower levels usually vanish instead. A full Stark map of calculated intensities
frF is presented for Li (
m=0) and agrees with experiment. Pseudocrossings occur at near triple degeneracies of hydrogen Stark states. Extensions to include
ls coupling are indicated. Experimental ionization rates in He are analyzed in a companion paper by van de Water, Mariani, and Koch [Phys. Rev. A 30, 2399 (1984)].