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Physical Review Letters

Resonant Two-Photon Detachment through the Lowest SingletDState in H−

A. Stintz, Xin Miao Zhao, Charlie E. M. Strauss, W.B. Ingalls, George A. Kyrala, David J. Funk, H. C. Bryant

Published Oct 16, 1995• 44 citations
Publisher

Abstract

We report the measurement of the two-photon detachment spectrum of the $^{1}D^{e}$ resonance of ${\mathrm{H}}^{\ensuremath{-}}$ just below the $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ threshold. The excess photon detachment resonance fits a Fano profile with energy 10.872(2) eV (relative to ${\mathrm{H}}^{\ensuremath{-}}$ ground state), a width of 0.0105(10) eV, and a shape parameter of $\ensuremath{-}8(2)$. To our knowledge, this describes the first time direct multiphoton excitation of a resonance has been observed in any negative atomic ion.

DOI
10.1103/physrevlett.75.2924

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arXiv:physics/9801032•Journal of Physics B Atomic Molecular and Optical Physics•Jun 14, 1998•16 citations
We report the measurement of the two-photon detachment spectrum of the 1De^{1}D^{e}1De resonance of H\ensuremath−{\mathrm{H}}^{\ensuremath{-}}H\ensuremath− just below the n=2n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2n=2 threshold. The excess photon detachment resonance fits a Fano profile with energy 10.872(2) eV (relative to H\ensuremath−{\mathrm{H}}^{\ensuremath{-}}H\ensuremath− ground state), a width of 0.0105(10) eV, and a shape parameter of \ensuremath−8(2)\ensuremath{-}8(2)\ensuremath−8(2). To our knowledge, this describes the first time direct multiphoton excitation of a resonance has been observed in any negative atomic ion.
Efficient excess-photon detachment to lowest nonvanishing order is observed with long (\ensuremath{\sim}0.7 ns) optical pulses and intensities of \ensuremath{\le}6\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 W/cm2{\mathrm{cm}}^{2}cm2. A Feshbach (``window'') resonance in ${\mathrm{Cs}}^{\mathrm{\ensuremath{-}}}$ suppresses single-photon detachment, while simultaneously enhancing the probability of absorption of one or more excess photons. The perspectives for future spectroscopic excess-photon detachment and studies of direct two-electron multiphoton detachment are briefly discussed.
We describe a method for calculating rates for multiphoton detachment or ionization of a two-electron system by a perturbative field. We have applied the method to ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$, and we present estimates of two- and three-photon detachment rates. In particular, we explore the 1^{1}1Se{\mathit{S}}^{\mathit{e}}Se and 1^{1}1De{\mathit{D}}^{\mathit{e}}De Feshbach resonances below the n=2 and n=3 excitation thresholds of H; these resonances lie in the ``excess-photon detachment'' regime, where one photon is already sufficient to detach the electron.
Four-photon double detachment of the helium negative ion is investigated experimentally and theoretically for photon energies where the transient helium atom is in the 1s2s3S1s2s\phantom{\rule{0.16em}{0ex}}^{3}S1s2s3S or 1s2p3Po1s2p\phantom{\rule{0.16em}{0ex}}^{3}P^{\mathrm{o}}1s2p3Po states, which subsequently ionize by absorption of three photons. Ionization is enhanced by intermediate resonances, giving rise to series of peaks in the He+{\mathrm{He}}^{+}He+ spectrum, which we study in detail. The He+{\mathrm{He}}^{+}He+ yield is measured in the wavelength ranges from 530 to 560 nm and from 685 to 730 nm and for various polarizations of the laser light. Double detachment is treated theoretically as a sequential process, within the framework of RRR-matrix theory for the first step and effective Hamiltonian theory for the second step. Experimental conditions are accurately modeled, and the measured and simulated yields are in good qualitative and, in some cases, quantitative agreement. Resonances in the double detachment spectra can be attributed to well-defined Rydberg states of the transient atom. The double detachment yield exhibits a strong dependence on the laser polarization which can be related to the magnetic quantum number of the intermediate atomic state. We also investigate the possibility of nonsequential double detachment with a two-color experiment but observe no evidence for it.
Multiphoton detachment of ${\mathrm{Te}}^{\mathrm{\ensuremath{-}}}$ is studied utilizing two optical fields of wavelengths \ensuremath{\sim}694 and \ensuremath{\sim}1064 nm, respectively. Three-photon detachment is resonantly enhanced when the Raman condition is satisfied for coupling of the ${\mathrm{Te}}^{\mathrm{\ensuremath{-}}}$ 5p5{\mathit{p}}^{5}p5 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​ and 2^{2}2P1/2{\mathit{P}}_{1/2}P1/2​ fine-structure states. The fine-structure splitting is determined to be 5004.7\ifmmode\pm\else\textpm\fi{}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which represents a 25-fold improvement over earlier measurements. The possibilities for extended spectroscopic measurements, as well as general perspectives for multiphoton experiments at high intensities, are briefly discussed.
The cross sections for resonant two-color, two-photon detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ with excitation of the degenerate H(2s) and H(2p) levels are calculated within a semiempirical adiabatic hyperspherical representation. The first photon, with energy ${\mathrm{\ensuremath{\omega}}}_{1}$=0.4017 a.u., is resonant with the well-known Feshback 1^{1}1Po{\mathit{P}}^{\mathit{o}}Po resonance below the H(n=2) threshold. The second photon, with energy ${\mathrm{\ensuremath{\omega}}}_{2}$\ensuremath{\ge}0.126 05 a.u., scans the energy region above the H(n=2) threshold over which long-range dipole-field-induced cross-section oscillations are predicted to occur. Such Gailitis-Damburg oscillations have not yet been observed experimentally. Results for various pairs of light polarization for the two photons are presented. Our resonant two-color, two-photon detachment cross sections are 8--9 orders of magnitude greater than the corresponding nonresonant, single-color, two-photon detachment cross sections obtained by Liu, Du, and Starace [Phys. Rev. A 43, 5891 (1991)]. Unmistakable evidence of long-range dipole-field effects is presented over the 5-meV energy range above the H(n=2) threshold. Furthermore, the differential cross sections for right- and left-circularly polarized, copropagating photons and especially the circular dichroism differential cross sections are shown to have nearly a full cycle of a greatly enhanced dipole-field-induced oscillation extending over the region from threshold to \ensuremath{\approxeq}34 meV above.
The photodetachment cross section of has been measured in the photon energy range 2.9-3.3 eV in order to investigate doubly excited states. Measurements were made channel specific by selectively detecting the residual He atoms left in a particular excited state following detachment. Three Feshbach resonances were found in the partial cross section: a resonance below the threshold and two resonances below the threshold. The measured energies of these doubly excited states are 2.959 260(6), 3.072(7) and 3.264 87(5) eV. The corresponding widths are found to be 0.20(2), 50(5) and 0.61(5) meV. The measured energies agree well with recent theoretical predictions for the , and states, respectively, but the widths deviate noticeably from calculations for and states.