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Journal of Physics B Atomic and Molecular Physics

Highly-excited double Rydberg states of He-

Stephen J. Buckman, P. R. Hammond, Frank H. Read, GEORGE C. KING

Published Nov 14, 1983• 127 citations
Publisher

Abstract

By measuring the yield of metastable atoms resulting from electron-impact excitation of helium, in the range of incident electron energy from 19.8 to 24.6 eV, the authors have obtained evidence for He- resonances in which both the excited electrons have high values of the principal quantum number. The resonances occur in distinct groups, the lower members of which form Rydberg series characterised by effective core charges that are nonintegral. Classification schemes for these resonances are discussed.

DOI
10.1088/0022-3700/16/21/024

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Journal of Experimental and Theoretical Physics•Jan 1, 1991•0 citations
By measuring the yield of metastable atoms resulting from electron-impact excitation of helium, in the range of incident electron energy from 19.8 to 24.6 eV, the authors have obtained evidence for He- resonances in which both the excited electrons have high values of the principal quantum number. The resonances occur in distinct groups, the lower members of which form Rydberg series characterised by effective core charges that are nonintegral. Classification schemes for these resonances are discussed.
Long-lived, excited neutral particles, arising from the interaction of monochromatic synchrotron radiation with ground state He atoms, have been observed at photon energies close to the (N = 1,2,3 and 4) ionization thresholds. The measurements have been made using an unconventional experimental arrangement in which charged particles, responsible for the dominant signal in conventional photon impact studies, are prevented from reaching the detector. For , it appears that the formation, via photoexcitation, of relatively long-lived, doubly excited Rydberg states is a necessary step in the production of the observed signal. Four processes that might account for the production of the excited neutral particles are considered. The simplest, that atoms in the initial doubly excited Rydberg states are themselves directly observable, is considered unlikely. However, the lifetimes of the photoexcited double Rydberg states may be increased through the effects of electric fields present in the apparatus. Transitions from these double Rydberg states, occurring either as a result of collisions or by fluorescence, could result in metastable, singly excited neutral particles. The presence of signal related to the existence of long-lived doubly excited states could be significant in the interpretation of other photon impact measurements.
Introduction Series of atomic spectral lines had been studied by emission from excited vapors even before Balmer described the regularity of the visible lines of hydrogen in 1884. However, direct excitation had been restricted to the few lowest-lying levels, particularly those accessible by electric dipole transitions from the ground state using classical light sources. The advent of tunable lasers in the 1970s changed such spectroscopic limitations enormonsly, allowing excitation to a very much larger number of states. Atoms in highly excited states, typically with principal quantum numbers n ≥ 10, are called Rydberg atoms. They are oversize, fragile states of significant intrinsic interest. They are very rare in nature because of their size and fragility, but they are found in interstellar space (H I regions). One of the best sources for comprehensive descriptions of Rydberg atoms and references to the literature describing their exploration is Ref. [120]. It describes their characteristic properties and behavior both classically and quantum mechanically, and especially their strongly enhanced interactions with external fields. The first thing to note is that Rydberg atoms are usually characterized by a single, highly excited electron while all the other electrons remain in their original states. Thus their properties are very nearly classical, and the atoms are much closer to the ideal hydrogen-like model of Chap. 7, along with appropriate quantum defects (see Chap. 10). The spectroscopy of such states is especially interesting because the active electron is much less perturbed by the other electrons in the atom than when it is in lower states. Moreover, there are some special circumstances in which the energy of a Rydberg state is degenerate with an atomic state having two electrons excited to lower levels, and such configurations yield further new information. Second, Rydberg atoms do not usually decay back to the ground state by emission of characteristic radiation, so their study is mediated by other techniques that reveal new and interesting properties. Such properties arise because Rydberg atoms have a huge dipole moment and very low binding energy resulting from the large average distance between the excited electron and the nucleus-plus-core of the rest of the atom with net charge e . Third, transitions between Rydberg states are quite different from those connecting to the ground state for at least three reasons.
Rydberg states of atomic helium with principal quantum numbers ranging from n=20n=20n=20 to n=100n=100n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 23S12\phantom{\rule{0.16em}{0ex}}{{}^{3}S}_{1}23S1​ state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number $|{m}_{\ensuremath{\ell}}|=2$ were prepared in zero electric field and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg--Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-$|{m}_{\ensuremath{\ell}}|$ hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
Double Rydberg states were populated by triple electron capture in collisions of highly charged ions with atoms. The highest stabilization of states produced by triple capture at $v\ensuremath{\cong}0.2\mathrm{a}.\mathrm{u}.$ was found for binding energies of the projectile electrons near the limits of the $({2,n,n}^{\ensuremath{'}})$ and $({3,n,n}^{\ensuremath{'}})$ series. Population of triply excited double Rydberg states was confirmed by the emission of Rydberg transitions. Results are interpreted by two successive autotransfer into Rydberg states for $({3,n,n}^{\ensuremath{'}})$ states and of one-step two-electron autotransfer for $({2,n,n}^{\ensuremath{'}})$ states.