Threshold structures in the multiphoton detachment yield from theH−ion

Abstract

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...

Multiphoton detachment ofH−

“…Our two-photon cross sections are in close agreement with those obtained by Adelman [21] (corrected, as pointed out by Geltman [4], by a factor of 2) [32] (n =2, 3,4), and the generalized cross sections calculated by Crance [33] Geltman's "best-phase" approximation results [4]. Dasheddotted curve: Geltman's "model" approximation results [4].…”

“…, either 0 or 1), that the shape of cross sections at threshold will be qualitatively different for even-and odd-photon detachment [4]. [1,3]. This is partly due to the fact that the ac Stark shift of the ground state and the laser intensitydependent effects are not included in these perturbative calculations.…”

Multiphoton detachment ofH−. II. Intensity-dependent photodetachment rates and threshold behavior—complex-scaling generalized pseudospectral method

“…Overall, it appears that this recent study [5] provides the consistent results with the accuracy for higher-order o"(n ) 3) comparable to that of lower orders (n =2, 3). Detailed discussion of this previous work is given in Ref.…”

“…Further, the perturbative simulation contains a free parameter in the laser profile to fit the experimental data. Our present study indicates that nonperturbative treatment is required and capable of describing satisfactorily the process of multiphoton detachment of H in the Los Alamos experiments [3]. …”

“…Both the experimental [3] and theoretical works [4,5] indicated that the process of multiphoton detachment of H under the experimental conditions of Ref. [3] cannot be described adequately by perturbation theory. Intensity-dependent threshold shifts and photodetachment rates must be considered.…”