Laser photodetachment on a relativistic H beam has revealed several series of resonances in which both electrons are excited. These "bound states in the continuum" appear as dips in the partial production cross sections of various excited states of neutral hydrogen. We present here a detailed account of the experimental observation of the resonances and the results of fits of the measured yields to sets of Fano profiles. A simple analytic formula for the energy-level spacing in this quantum-mechanical three-body system is verified.In this paper we report the observation of several series of high-lying 'P resonances in the photodetachment cross section of H . The energy levels of these doubly excited states are shown to obey a simple and elegant analytic formula. We discuss here in greater depth the results presented earlier. '
Multiphoton detachment from H ~ has been studied using linearly polarized light for photon energies from 0.15 to 0.39 eV. Characteristic threshold structures were observed. The electron detachment rates are estimated at peak laser intensities from 2 to 12 GW/cm 2 in the laboratory frame. The two-photon detachment data reveal intensity-dependent shifts of the H ~ electron detachment threshold energy.PACS numbers: 33.80.Rv, 33.80.Eh Multiphoton detachment (MPD) from H ~ provides a window on fundamental multiphoton physics. 1 Structures in the continuum 2 far above the detachment threshold need not be considered for moderate laser fields. Because high-power lasers have had limited wavelength tunability, with the exception of the measurements using relativistic Doppler shifts, the general behavior of the MPD rate (cross section) versus photon energy has been explored only in theory. 3 Comparisons between theory and experiment involving multiphoton ionization (MPI) of neutral atoms are usually complicated by intensity-induced shifts, resonances, and the laser intensity distribution. In addition, subtle gauge problems 4 call for experimental data. Thus, it is desirable to study a simple system like H ~, using a tunable laser, to obtain an unobstructed view of MPI processes.The first observation of MPD in H ~ was reported recently. 5 However, 6 difficulties precluded detailed data analysis. Here, we report new observations of MPD of H "~ in moderate laser fields. These data reveal clear threshold phenomena occurring near photon energies where a transition from one order of absorption to another can take place (e.g., N-photon absorption to 7V + 1). Double-electron detachment from H ~~ was not observed. Briefly, 5 ' 6 an 800-MeV beam of H ", at the Los Alamos Clinton P. Anderson Meson Physics Facility (LAMPF), was made to intersect with a focused CO2 laser beam. The H ~ beam, roughly 3 mm in diameter, consisted of 0.25-ns micropulses, with a separation of 5 ns. A transversely excited atmospheric CO2 oscillator-amplifier, including a low-pressure gain cell in the oscillator, was operated in the TEMoo, single-axial mode with wavelength of 10.6 //m. Linearly polarized, temporally smooth laser pulses had a duration of -130 ns, full width at half maximum (FWHM), and a l-/is long tail. The measured intensity distribution in the focal region of a 25.4-cm focal-length lens demonstrated that the beam was nearly diffraction limited. The beam diameter at the focus was 110 ± 5 jjm (FWHM), corresponding to a transit time of 0.24 ps in the ion's frame when the beams cross at 90°, and a Rayleigh range of 2.6 mm. The highest peak laser intensity in the experiment was estimated to be 12 GW/cm 2 (±10%) in the laboratory frame. The photon energy (iTiab) was Doppler shifted to Zs c . m . in the center-of-mass (cm.) frame of the H ~ ion according to £c.m. -yO +j8cosa)£ , i ab ,where a is the angle of intersection (a =0 when head on) and p (0.842) and 7 (1.85) are the usual relativistic parameters. The photon-energy spread in the ion's frame i...
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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