Observation of the (2p,nd)1Podouble-excitation Ry

Abstract

Double-excitation Rydberg resonances of He below the TV =2 threshold of He + were studied with an energy resolution of =4 meV by photoionization using synchrotron radiation. All three Rydberg series of ] P° resonances accessible by photoabsorption, including the hitherto "missing" (2p,nd)*P° series, were resolved. This settles a long-standing discrepancy between experiment and theoretical predictions and establishes a firm basis for testing the accuracy of atomic multiconfiguration calculations.PACS 31.20.Tz, 31.50,+w, 32.80.Fb Doubly excited He is the archetypical example for autoionization phenomena, since it represents the most fundamental autoionizing atomic system, which is-due to its relative simplicity-also readily accessible to theoretical treatment. After the initial pioneering experimental [l] and theoretical [2] work, progress in this field was relatively slow due to severe spectroscopic limitations, in particular of available photon sources in the 60-to 80-eV range. In recent years, however, considerable attention has been paid to these states, as well as to related states in H~, from both the theoretical [3-7] and the experimental [8] point of view.Upon excitation of both electrons to states below the TV =2 ionization threshold of the He + , IP2, final states with six different symmetries 0S e ,*S e , { P°, 3 P°, ] D e , 3 D e ) can be populated, each containing up to three Rydberg series [3,9,10]. In photoexcitation, the dipole-selection rule allows only X P° final states, which reduces the number of observable Rydberg series to three [10]: Only two of them, the (sp,2n + ) x P° and the (sp,2n~) x P° series could be observed until now [3,8], while the third (2p,nd) l P° series was missing [11]. This situation has led to considerable confusion, since the various theoretical calculations are characterized by substantial discrepancies with respect to the energies and widths of the (2p,nd) ] P° states [5][6][7][11][12][13][14][15].In the present Letter we report on the first observation of the four lowest states of the (2p,nd) ] P° series by high-resolution photoabsorption spectroscopy using synchrotron radiation. With a resolving power of E/AE = 16000 combined with high photon flux, the (sp, 2n + ) x P° and (sp,2n -) l P° series were in addition resolved up to the n =20 and the n = 11 state, respectively. These results complete the identification of the simplest double-excitation states of He, i.e., the three Rydberg series of states accessible by photoexcitation below IP 2 . They also clarify a long-standing theoretical controversy concerning the correct energies of these states and provide a means to check the quality of existing as well as of future atomic multiconfiguration calculations for He.The measurements were performed with the highresolution SX700/II monochromator operated by the Freie Universitat Berlin at BESSY. With this beam line, a resolving power of E/AE> 10000 at hv = 60 eV had previously been achieved [8,16]. In the present work, the resolution at high photon flux could be further...

Toward a soft X-ray Fourier-transform spectrometer

“…Such values are crucial for the test of theories, which disagree by two orders of magnitude with respect to the width of the 2(-1,0)~ state, for example, the calculated values ranging from 4x1Q-5 to 3xiQ-3 meV [15,16]. Such experimental evidence as there is presently [14] agrees with neither of these values. As revealing as the latest high-resolution results have been they clearly demonstrate the need for even higher resolution.…”

“…This experiment challenges the theory to calculate adequate values for the position, shape, and intensity of these Rydberg lines, which for higher N are increasingly complicated by interchannel interferences. Further improvement of the resolution from 6 to 4 me V allowed the experimental detection of the lowest four members of the theoretically predicted 2(-1,0)~ Rydberg series for the first time [14]. These resonances are a sensitive test of the theory, which in some cases has failed to predict the right sequence of the 2(1,0);; and 2(-1,0)~ series.…”

Ultra-High Resolution Atomic and Molecular Spectroscopy at the Advanced Light Source

“…Doubly-excited autoionizing states of helium were first studied in the 1960's [3]. There was little progress thereafter until the early 1990's when high-resolution beam lines at synchrotron light sources were introduced [4]. A major advance in resolution was achieved on Beamline 9.0.1 at the ALS, where a resolving power of 64000 has been demonstrated by measurement of the very narrow 2p, 3d(2, -1 3 ) state of helium [2].…”

Atomic and Molecular Physics Using Synchrotron Radiation – the Early Years

“…Burke et al (1963) calculated 321 the position of these resonances, but no additional structure could be found with the resolution available at the time (E/AE ~_ 3500). It is interesting to note that it took almost 30 years for the 2pnd series to be observed with the SX700/II monochromator at BESSY (Domke, Remmers & Kaindl, 1992) …”