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Physical Review A

Nonhydrogenic Rydberg states in external fields as intermediate resonances in multiphoton ionization

Lambertus D. Noordam, M. P. de Boer, H. B. van Linden van den Heuvell

Published Jun 1, 1990• 6 citations
Publisher

Abstract

We report wavelength spectra of multiphoton ionization of rubidium, in which the intermediate state is a Rydberg state (35n45). The photoelectron yield is measured for a free atom, for an atom placed in an external electric field, and for an atom in a magnetic field. Both the magnetic- and the electric-field spectra show a dramatic decrease in yield above n\ensuremath{\approxeq}40. Our analysis centers on the field-induced mixing of states. The threshold occurs when the resonant state, which has a large quantum defect, mixes with the manifold. The analysis is also applied to experiments reported previously.

DOI
10.1103/physreva.41.6267

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We report wavelength spectra of multiphoton ionization of rubidium, in which the intermediate state is a Rydberg state (35n45). The photoelectron yield is measured for a free atom, for an atom placed in an external electric field, and for an atom in a magnetic field. Both the magnetic- and the electric-field spectra show a dramatic decrease in yield above n\ensuremath{\approxeq}40. Our analysis centers on the field-induced mixing of states. The threshold occurs when the resonant state, which has a large quantum defect, mixes with the manifold. The analysis is also applied to experiments reported previously.
Multiphoton ionization (MPI) of rubidium atoms at both low (atomic beam) and high (heat pipe) densities is studied using a tunable OPO laser. At high Rb densities ionization of the laser excited ns, np, and nd states occurs both through photoionization and collisional ionization. Excitation of the np states is found to be induced by the external electric field at both low and high densities. In addition, np signal is also seen at very low (E→0) fields in the heat pipe, providing evidence for collision mixing as well as field mixing. At low densities, signal for the high np states initially increases with applied field, but soon saturates (i.e. becomes field independent) while the signal for high nd states decreases with increasing field. At low Rb densities strong resonance features are observed in the energy region between the zero field limit (IP) and the field ionization limit. These features, as well as the field ionization threshold, are found to be dependent upon the angle between the laser polarization and the direction of the applied dc field. Evidence for tunneling through the barrier created by the −e2/r−eEr potential is also presented for ns and nd states.
We report on the observation and numerical calculation of ionization spectra of highly Stark-shifted Rydberg states of rubidium beyond the classical ionization threshold. In the numerical calculations, a complex absorbing potential (CAP) allows us to predict the energy levels and ionization rates of Rydberg states in this regime. Our approach of adjusting the CAP to the external electric field reduces the number of free parameters from one per resonance to a single one. Furthermore, we have measured the ionization spectra of magneto-optically trapped rubidium atoms which are excited to principal quantum numbers of 43 and 70 at various electric fields. The emerging ions are detected using an ion optics. We find good agreement between the numerically and experimentally obtained spectra.
We present theoretical results for the photoabsorption spectrum of an atom in parallel electric and magnetic fields, using the R-matrix method combined with quantum-defect theory. We introduce a radial basis set which is complete and orthonormal over a semi-infinite interval $[{r}_{0},\ensuremath{\infty}),$ to allow calculations to be performed for high Rydberg states in nonhydrogenic atoms without encountering problems due to linear dependence of the basis set. The nonhydrogenic character of the spectra is analyzed for Li and Rb, and a comparison is made with previous high-precision experiments which shows that the theoretical results agree very well with experiment.
We study the spectra of diamagnetic rubidium Rydberg atoms in strong magnetic and weak electric parallel fields, in the nnn-mixing regime. Our emphasis is on isolated pairs of near-degenerate, opposite-parity, diamagnetic states that become mixed by the weak electric field. Such level pairs allow for the generation of nondegenerate states with large, tunable permanent electric dipole moments and large optical excitation cross sections from the atomic ground state. We investigate how the dipole moments and the zero-electric-field energy defects of these level pairs can be tuned using small variations of the electric and magnetic fields. Using calculations, we explore the abundance of such level pairs over wide spectral regions for several magnetic quantum numbers. Applications of polar, diamagnetic Rydberg states in Rydberg-atom interaction experiments are briefly discussed.
Multiphoton ionization (MPI) via two-photon resonance with high-lying Rydberg levels is observed on Na and K atoms in modest (12--120 V/cm) dc electric fields. The field-induced MPI signals are manifested to such an extent that the nominally forbidden MPI via two-photon resonance with the np levels is induced at the same signal levels as or more intensely than the allowed MPI via the ns and nd Rydberg levels. The intensity inversion between the allowed and the forbidden MPI signals is observed on the K atoms in the wavelength region where the two-photon energy is very close to the field-ionization limit.
Rydberg states of the NO molecule have been probed by two-color resonantly enhanced multiphoton ionization in a magnetic field of 0.93 T. Penetrating s and d states show strong interactions with the intramolecular field while nonpenetrating f states show stronger interactions with the external magnetic field. Competition between the molecular core anisotropy and the external field anisotropy leads to a greater variety of situations than in a Rydberg atom, ranging from a “weak field” regime to a “strong field” regime. These different situations are illustrated by a multichannel quantum defect calculation on the np states of NO including the Zeeman effect. An extreme situation can be achieved in which the Rydberg molecule looses its molecular character and shows an almost pure atomic structure within each rotational channel.