Observation of high-lying resonances in theH−ion

Abstract

Several series of doubly excited resonances have been observed in H ~ photodetachment, appearing as dips in the production cross section of excited neutral hydrogen. At least three resonances are clearly visible converging on each of the thresholds /! ™5, 6, 7, and 8. The data support recent theoretical calculations for the energies of doubly excited resonances and verify a Rydberg-dipole formula for the main resonance positions.PACS numbers: 32.80.FbThe dipole potential of an excited hydrogen atom polarized by a second electron will support series of resonances associated with the relevant hydrogenic state. The resonances that lead to the threshold of the outgoing channel H(n)+e~ can be observed in the continuum above the H(« -1 ) + e ~ threshold. These doubly excited resonances may be classified by "+" and " -" quantum numbers, * which emphasize whether the wave function is approximately odd (for -) or even (for 4-) under radial exchange ri«-*/*2. By combining a modified Rydberg formula 2 for the energy of the lowest resonance in each series with a recursion formula 3 for the energies of successive resonances within a series, we obtain an analytic expression for the energies (in eV) of all of the +type X P resonances in H ~ associated with all of the hydrogenic thresholds: 4Here £+ is the double-detachment threshold energy, R is the Rydberg constant, a n is a parameter associated with the hydrogenic threshold of principal quantum number n, TJ = 1 -a includes a screening factor, n is a quantum defect, and m =/*, n + 1, n +2,. . .. The cross section in the vicinity of a resonance at energy EQ and of width T, which is embedded in a continuum, is described by a Fano profile 5 o = o b + o a (q + e) 2 /(\+e 2 ) ,where e=*2(E -EQ)/T.

Fundamental Aspects of Atomic Photoionization with High-Brightness Light Sources

“…Keeping only the lowest "+" states converging to each threshold H(n), they were able to interpret the doubly excited resonance structures converging to the n = 4-8 thresholds that were observed in the photodetachment measurements of Harris et al [14] Sadeghpour and Greene [19] interpreted these observed resonances as the doubly excited states supported by the lowest "+" adiabatic hyperspherical radial potentials, as shown in Figure 1.…”

“…Clearly, however, the simple single-configuration picture presented above gives a qualitative, order-of-magnitude understanding of why the Be + 1s 2 2s ionic state cross section is so small. Namely, the transition matrix element to the 2s state is subject to extensive cancellations, as revealed in (14), whereas the transition matrix elements to the np states are not, as revealed in (13).…”

“…The interpretation given by Sadeghpour and Greene for the observed photodetachment spectra [14] implies that all of the other allowed levels supported by the many other 1 P O adiabatic hyperspherical potentials converging to each H(n) threshold are not populated in the photodetachment process. Sadeghpour and Greene justified their interpretation by noting that the states corresponding to the lowest + channels converging to each H(n) threshold have no nodes in the angle θ 12 between the two electrons.…”

“…Only recently, however, through the use of lasers and of high-brightness synchrotron light, has it been possible to obtain experimental spectra on their doubly excited states converging to thresholds higher than n = 2. Thus, Harris et al [14] observed doubly excited spectra of H -converging to the n = 4-8 thresholds of H + . Similarly, Domke et al [15] recently reported the high-resolution photoionization study of the doubly excited He states below the n = 2-7 thresholds of He + .…”

Experimental studies of partial photodetachment cross sections in K−below the K(7 2P) threshold

“…The properties of such states are substantially influenced by the effects of electron-electron correlation. A great amount of theoretical and experimental effort has been devoted to the prototype negative ion, H − [1][2][3][4][5][6][7]. Here, two electrons move in the pure Coulomb field of the nucleus; a case resembling the helium atom.…”