High Resolution Measurements of Partial Photoionization Cross Sections in Hollow Lithium: A Critical Comparison with Advanced Many-Body Calculations

Abstract

Photoelectron data for hollow lithium states obtained with unprecedented high spectral resolution and sensitivity are presented. A critical comparison is made with the most recent theoretical results. Partial cross sections are measured providing the first definitive test of advanced ab initio calculations for this highly excited four-body atomic system. [S0031-9007(96)00241-4] PACS numbers: 32.80. Fb, 32.80.Hd In this Letter, we present photoelectron data for hollow lithium states obtained with unprecedented spectral resolution (0.019 eV) and sensitivity which allows a critical comparison with the most advanced recent theoretical calculations. The partial cross sections are measured and resonance profile parameters are determined. In addition, we have used the R-matrix approximation to calculate ab initio the partial photoionization cross sections into all continuum channels of the Li 1 ion over a wide photon energy range. The experimental results provide the first definitive test of such advanced calculations for this highly excited atomic system.The photoexcitation and decay dynamics of hollow lithium atoms, in which all three electrons are in excited states, have become the subject of intense recent experimental and theoretical interest. The single-photon threeelectron excitations depend entirely on electron-electron correlation interactions and so hollow lithium atoms constitute an ideal four-body Coulomb system for testing the most advanced atomic theories. Following the first photon-excited observation [1] of the lowest hollow atom resonance (1s 2 2s 2 S ! 2s 2 2p 2 P), two total ion yield photoion experiments at high [2] and medium [3] spectral resolution measured the autoionization of several hollow states in both the Li 1 and Li 21 decay channels. Photoelectron spectrometry [4] was also used with low resolution to study the decay of the hollow lithium 2s 2 2p state into the different continua of the Li 1 ion. The 0.5 eV spectral bandpass of the latest experiment however limited the critical nature of the comparison with theory.Double K-shell excitations in atomic helium have culminated in recent ultrahigh resolution measurements [5]. The third electron makes the theoretical and experimental study of hollow lithium states an even greater challenge than that of helium. Although triply excited states of lithium were first observed in collision experiments [6][7][8], only photon-excitation experiments provide the selectivity, sensitivity, and resolution required to unravel the many hollow lithium resonances. The dipole selection rules ensure that only 2 P o final states can be photoexcited from the ground state of the lithium atom. Photoelectron spectrometry combines the advantages of photon excitation with the unique capacity to measure separately the many decay channels and to provide insight into decay dynamics. The key role played by hollow atom states in ion-surface interactions has also been recognized [9]. 0031-9007͞96͞76(21)͞3915(4)$10.00

Experimental Observation and Theoretical Calculations of Rydberg Series in Hollow Lithium Atomic States

“…We present the variation of the partial cross sections, for photoionization into the singly excited level 1s2s 3 S (upper curve, left scale) and the doubly excited level 2s 2 1 S (lower curve, right scale) Li 1 states. Autoionization of the n 3 and n 4 terms into Li 1 ͓1s2s 3 S͔ has only a weak effect on the corresponding partial cross section, as expected from our ab initio calculations and suggested by our earlier measurements [5]. For n $ 5 the excitation energy of the 2s 2 1 S ionization limit is exceeded and well resolved electron lines are observed in this channel with intensities decreasing with increasing n; Rydberg states with n extending up to 8 are clearly evident.…”

“…At excitation energies lower than the 2s 2 1 S (151.66 eV), the n 3 and n 4 terms of the ͓͑2s2p 3 P͒ns 2 P͔ series only decay to the singly excited 1snᐉ states. These decay paths cause interferences with the direct photoionization route as reported earlier [5]. However, the n $ 5 terms of this series lie above the 2s 2 1 S excited state of Li 1 .…”

“…Simultaneous excitation of all three electrons can create hollow lithium states of the type nᐉn 0 ᐉ 0 n 00 ᐉ 00 with n $ 2 [1][2][3][4][5]. A calculation of some low-lying hollow lithium states in hyperspherical coordinates has been recently carried out [6] as a step towards the development of a classification scheme for triply excited states, analogous to the ͑K, T͒ A radial and angular correlation quantum numbers [7,8] used to describe the doubly excited states of helium [9].…”

“…The experimental setup and experimental procedure have been previously described [4,5]. A spectral resolution of 40 meV and an electron resolution DE͞E of 0.4% were used.…”

“…In fact, the authors specifically stated [3] "that the most striking aspect of the ion spectrum is the complete absence of any identifiable Rydberg series structure." Later measurements taking advantage of the final-state selectivity of photoelectron spectroscopy [4,5] suggested, however, that some of the observed structures could be identified as first members of Rydberg series. But their effect on partial cross sections for photoionization into final Li 1 ionic states with one vacancy in the 1s shell was very weak and no higher terms of any series were observed.…”