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Physical Review A

Dielectronic recombination for Ca+ via 4s→4p excitation

Jim Williams

Published May 1, 1984• 46 citations
Publisher

Abstract

Dielectronic recombination has been directly observed with a crossed, modulated electron and calciumion beams technique. The stabilizing photons (about 397 nm) and the 500-eV Rydberg-state neutral atoms were detected in coincidence following electron-impact excitation in the vicinity of doubly excited ${\mathrm{Ca}}^{**}$ resonant states below the ${\mathrm{Ca}}^{+}(4p^{2}P)$ excited ionic state. A peak ${\mathrm{Ca}}^{+}(4P^{2}P_{\frac{3}{2}})$ excitation cross section of 1.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ was measured simultaneously with the peak dielectronic recombination cross section of 1.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ which is about double the Burgess-Mertz theoretical value.

DOI
10.1103/physreva.29.2936

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Henrik Q. Lorensen, Harold Parks, Eileen M. Spain +3 more

The formation of ${\mathrm{Ca}}^{+}$ ions is measured in an effusive Ca beam by exciting through either the 4s4${\mathit{p}}^{1}$${\mathit{P}}_{1}$ or the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{1}$ state to highly excited Ca*(4${\mathit{snd}}^{1}$${\mathit{D}}_{2}$) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{1}$ state, we detect a large increase in ${\mathrm{Ca}}^{+}$ ions for n=25, compared to other high-n Rydberg states. It had been reported (Fabrikant [Phys. Rev. A 48, R3411 (1993)]; McLaughlin and Duquette) that ``ground-state--Rydberg-state'' collisions cause this large increase in ion production and that the result provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of the original experiment in which the same Rydberg states are accessed through the 4s4${\mathit{p}}^{1}$${\mathit{P}}_{1}$ level instead of the 4s5${\mathit{p}}^{1}$${\mathit{P}}_{1}$ level shows no enhancement of ions for any particular Rydberg level. With a three-laser experiment we show that the enhanced ionization effect for the 25d state is due to the 3${\mathit{d}}_{3/2}$9${\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25${\mathit{d}}^{1}$${\mathit{D}}_{2}$) state by the 808-nm photon involved in the original experiment by McLaughlin and Duquette. The existence of this resonance close to the experimentally observed energy is also established by R-matrix calculations. \textcopyright{} 1996 The American Physical Society.

Physical Review A•Aug 1, 1996•5 citations
Dielectronic recombination has been directly observed with a crossed, modulated electron and calciumion beams technique. The stabilizing photons (about 397 nm) and the 500-eV Rydberg-state neutral atoms were detected in coincidence following electron-impact excitation in the vicinity of doubly excited Ca∗∗{\mathrm{Ca}}^{**}Ca∗∗ resonant states below the Ca+(4p2P){\mathrm{Ca}}^{+}(4p^{2}P)Ca+(4p2P) excited ionic state. A peak Ca+(4P2P32){\mathrm{Ca}}^{+}(4P^{2}P_{\frac{3}{2}})Ca+(4P2P23​​) excitation cross section of 1.7 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−15{10}^{\ensuremath{-}15}10\ensuremath−15 cm2{\mathrm{cm}}^{2}cm2 was measured simultaneously with the peak dielectronic recombination cross section of 1.8 \ifmmode\times\else\texttimes\fi{} 10\ensuremath−17{10}^{\ensuremath{-}17}10\ensuremath−17 cm2{\mathrm{cm}}^{2}cm2 which is about double the Burgess-Mertz theoretical value.
We have observed electron impact ionization of highly excited sodium Rydberg atoms in nsnsns and ndndnd states, n=35−−51n=35--51n=35−−51, below E=2  eVE=2\text{ }\text{ }\mathrm{e}\mathrm{V}E=2  eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from ${\ensuremath{\sigma}}_{35d}\ensuremath{\approx}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ to ${\ensuremath{\sigma}}_{50d}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\text{ }\text{ }{\mathrm{c}\mathrm{m}}^{2}$. The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and ${\ensuremath{\sigma}}_{n}$ follows a power law in nnn. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.
This volume offers a timely look at Rydberg states of atoms in external fields and dielectronic recombination. Each topic provides authoritative coverage, presents a fresh account of a flourishing field of current atomic physics and introduces new opportunities for discovery and development. Topics considered include electron-atom scattering in external fields; observations of regular and irregular motion as exemplified by the quadratic zeeman effect and other systems; Rydberg atoms in external fields and the Coulomb geometry; crossed-field effects in the absorption spectrum of lithium in a magnetic field; precise studies of static electric field ionization; widths and shapes of stark resonances in sodium above the saddle point; studies of electric field effects and barium autoionizing resonances; autoionization and dielectronic recombination in plasma electric microfields; dielectronic recombination measurements on multicharged ions; merged beam studies of dielectronic recombination; Rydberg atoms and dielectronic recombination in astrophysics; and observations on dielectronic recombination.
Experimental details are presented for the measurement of the effects of extrinsic electric fields on cross sections and Rydberg product-state distributions for dielectronic recombination of Mg+{\mathrm{Mg}}^{+}Mg+ with electrons. A new type of Rydberg state detector is described. Experimental results for dielectronic recombination in the presence of fields are presented for three field values, and these results are compared with theoretical calculations.
Relative yields of Ca+{\mathrm{Ca}}^{+}Ca+ and ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions from thermal-energy collisions between highly excited Ca∗{\mathrm{Ca}}^{\mathrm{*}}Ca∗ 4snd 1^{1}1D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ and a ground-state Ca+{\mathrm{Ca}}^{+}Ca+ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. We observe a large increase in positive and negative ions for n=25 only. This result strongly suggests that the binding energy of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ is 24\ifmmode\pm\else\textpm\fi{}1.4 meV.
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.
The formation of Ca+{\mathrm{Ca}}^{+}Ca+ ions is measured in an effusive Ca beam by exciting through either the 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ or the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state to highly excited Ca*(4snd1{\mathit{snd}}^{1}snd1D2{\mathit{D}}_{2}D2​) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state, we detect a large increase in Ca+{\mathrm{Ca}}^{+}Ca+ ions for n=25, compared to other high-n Rydberg states. It had been reported (Fabrikant [Phys. Rev. A 48, R3411 (1993)]; McLaughlin and Duquette) that ``ground-state--Rydberg-state'' collisions cause this large increase in ion production and that the result provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of the original experiment in which the same Rydberg states are accessed through the 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ level instead of the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ level shows no enhancement of ions for any particular Rydberg level. With a three-laser experiment we show that the enhanced ionization effect for the 25d state is due to the 3${\mathit{d}}_{3/2}999{\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25d1{\mathit{d}}^{1}d1D2{\mathit{D}}_{2}D2​) state by the 808-nm photon involved in the original experiment by McLaughlin and Duquette. The existence of this resonance close to the experimentally observed energy is also established by R-matrix calculations. \textcopyright{} 1996 The American Physical Society.