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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

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Calculations of inner-shell photoionization of 1s 2 2p 2 P o and 1s 2 3p 2 P o Li excited states have been performed using recent developments of the R-matrix code with a 29-term target representation for incident photon energies up to 165 eV, with particular emphasis on multiple electron processes. Partial and total cross sections are given for a number of excitation processes including the lower members of 19 hollow atom resonance series. The results are compared with recent experimental measurements of the lowest 2s2p 2 2 L e resonance of even-parity hollow lithium states produced by photoexcitation of laser-excited lithium atoms. Experiment and theory are in excellent agreement on a relative scale. The enhancement of the shake-up process increases with initial state excitation, up to 500% over that of the ground state for 1s 2 3p 2 P o . Good agreement is also obtained for K-shell photoionization when comparing branching ratios with available experimental results.

We have measured, using electron spectroscopy, the lowest-energy doubly hollow lithium triply excited (3l3lЈ3lЉ) 2 P state. Energies, widths, and partial cross sections have been measured and calculated using the saddle-point technique and the R-matrix approximation. Our results show good agreement between experimental and theoretical data for the energy and the width of the doubly hollow state. ͓S1050-2947͑97͒50408-2͔PACS number͑s͒: 32.80. Hd, 32.80.Fb In a hollow atomic or ionic state, the first inner shell ͑the K shell͒ is empty. Correlation effects can produce simultaneous excitation of all three electrons in lithium atoms and create hollow lithium states of the type (nlnЈlЈnЉlЉ) with nу2. After the first observation of the lowest-energy triply excited state ͓1͔, several experiments involving the use of photoion ͓2-4͔ and photoelectron ͓5-8͔ spectrometries have contributed to provide the energies of a number of (nlnЈlЈnЉlЉ) states with at least one of n, nЈ, or nЉ equal to 2, as well as the partial cross sections for photoionization of atomic lithium into various (nlnЈlЈ) final states of the Li ϩ ion (nϭ1 or 2, nЈу2). Several Rydberg series have been measured and identified in the many observed hollow lithium states ͓8,9͔. Theoretical calculations using the R-matrix approximation have provided results for the partial photoionization cross sections ͓5-9͔ that are generally in very good agreement with the experimental data, whereas the saddlepoint technique ͓10-12͔ has given very accurate calculations of the energies of a number of hollow lithium states, sometimes within one or two hundredths of an eV of the measured values.The demands on theoretical and experimental techniques increase at higher photon energies, for which triply excited states with all three electrons having principal quantum numbers equal to 3 or above can be created. We propose the name doubly hollow state for a triply excited state in which both K and L shells are empty. All n, nЈ, and nЉ have values that are higher than 2 for such a triply excited (nlnЈlЈnЉlЉ) state. An earlier report on an experiment with ion detection in the Li 2ϩ channel ͓4͔ mentioned some analogous features, but no data were shown at that time. Following submission of the present work, we received an article ͓13͔ in which one figure shows a profile of the first (3l,3lЈ,3lЉ) state recorded in the Li 2ϩ ionic channel. At excitation energies higher than the lowest doubly excited 2s 2 1 S Li ϩ limit ͑151.66 eV͒, triply excited states can decay to either singly or doubly excited states of the Li ϩ ion. Decay path into (1snl) Li ϩ ionic states causes interferences with the direct photoionization route as reported earlier ͓5,6͔. Decay to doubly excited (2l2lЈ) Li ϩ ionic states produces emission of a low-energy electron. This Li ϩ state subsequently decays to the ground state of the doubly charged Li 2ϩ ion, with emission of a high-energy electron. By measuring the photon-energy dependence of the intensity of these high-energy electrons, one determines a partial cross sec...