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.
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