Angle-Resolved Photoelectron Spectrometry Studies of the Autoionization of the2s22p2P<

Abstract

We have measured the angle-resolved energy dependence of the electrons emitted over the energy range of the triply excited 2s 2 2p 2 P lithium resonance using synchrotron radiation. We have also calculated the behavior of the angular distribution parameter b using the R-matrix approximation. Experimental and theoretical results are in good agreement and show deep minima in the 1s2p 1, 3 P ionic channels. The energy at which the minima occur does not coincide with the resonance energy, but is shifted towards higher energy. PACS numbers: 32.80.Hd, 32.80.Fb In this Letter, we present the results of a new experiment investigating the angular distribution of electrons emitted in autoionization of the lowest-energy triply excited state of atomic lithium. We made a critical comparison of the data with the results of advanced Rmatrix calculations performed in order to interpret the energy behavior of the angular distribution parameter b. Photons from the Advanced Light Source (ALS) photoexcited the 2s 2 2p 2 P hollow lithium state both on and off resonance. The electrons emitted were angularly analyzed with a cylindrical mirror analyzer (CMA) electron spectrometer. The experimental results reveal a pronounced energy dependence of the angular distribution parameter b in the Li 1 1s2p 1, 3 P´l satellite channels, in good agreement with theoretical prediction. Both experiment and theory show that the minima in the b profiles occur on the high energy side of the resonance.From the early 1960s [1], doubly excited states of helium have provided the ideal case for studying the threebody system of two correlated electrons interacting with the nucleus. Recent investigation on this system achieved very high resolution photoabsorption [2] and photoelectron [3] measurements. Experimental studies of helium helped to establish the theory of autoionization [4] and to introduce a new set of quantum numbers [5] describing doubly excited states in two-electron systems. The challenge to extend these investigations to triply excited states in neutral lithium started with their observation in collision experiments [6]. Subsequently, photoexcitation experiments [7][8][9][10] allowed accurate measurements of the excitation energies, identification of many of the resonances, and the determination of some partial photoionization cross sections for autoionization into the lowest continua of the Li 1 ion. Most recently, the investigation of this system received strong impetus from the use of a third generation storage ring delivering high-brightness photon beams in the photon energy range (140-200 eV). Highresolution partial cross sections were determined for triple excitation of lithium atoms [11,12] including a classification of many of the states into Rydberg series. The lowestenergy triply excited state in which all three electrons have the same principal quantum number ͑n 3͒ was also measured [13,14]. Theoretical calculations involving various approximations (R matrix [10-13], saddle point [15]) were carried out providing good agreement w...

K-Shell Photodetachment ofLi−: Experiment and Theory

“…This agreement was of significant value when performing the experiment in order to locate the resonance structure. It is interesting to note that this energy discrepancy between theory and experiment is the same as that found in the case of photoionization of neutral Li [29]. Our experimental and theoretical data are shown in Fig.…”

Universal scaling of resonances in vector correlation photoionization parameters

“…A shift of the resonance position in β with respect to the resonance in the cross section was noted a few times [17][18][19] and was clearly seen in some data [20,21]. In the present study we show that the above observations of broadening and energy shift are representative examples of a general scaling effect: the width and the shift of the resonances in the vector correlation parameters are universal quantities and are directly related to the resonance profile parameters in the integral photoionization cross section.…”

Laser-Induced Transitions between Triply Excited Hollow States

“…A triply excited state in lithium, also referred to as a hollow state, represents a fundamental case of highly correlated three-electron dynamics in a purely Coloumbic fourbody system, as attested by the continuing avalanche of related papers [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] over the last six years. Photoexcitation by synchrotron radiation has proven to be a very successful method to study energies and widths of such states.…”