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Journal of Physics B Atomic Molecular and Optical Physics

Optical selectivity calculation of calcium isotopes in a double-resonance ionization scheme

Manda Sankari, Perumal Kumar, Manda V. Suryanarayana

Published Oct 26, 2000• 8 citations
Publisher

Abstract

Optical selectivities of calcium isotopes have been calculated for the 4s 2 1 S 0 4s4p 1 P 1 4s4d 1 D 2 →Ca + double-resonance ionization scheme using the spectral simulation (SS) approach. Optical selectivities obtained by the SS approach are in good agreement with the results obtained by the density matrix formalism. Studies on the effect of laser detuning and linewidth on optical selectivities indicate that lasers with 50 MHz linewidth for both the excitation steps can yield an optical selectivity of 2.2 × 10 5 , which is adequate for selective excitation of the 41 Ca isotope for certain applications.

DOI
10.1088/0953-4075/33/21/331

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The Review of Laser Engineering•Jan 1, 2007•0 citations
Optical selectivities of calcium isotopes have been calculated for the 4s 2 1 S 0 4s4p 1 P 1 4s4d 1 D 2 →Ca + double-resonance ionization scheme using the spectral simulation (SS) approach. Optical selectivities obtained by the SS approach are in good agreement with the results obtained by the density matrix formalism. Studies on the effect of laser detuning and linewidth on optical selectivities indicate that lasers with 50 MHz linewidth for both the excitation steps can yield an optical selectivity of 2.2 × 10 5 , which is adequate for selective excitation of the 41 Ca isotope for certain applications.
Isotopic selectivity calculations are carried out for minor calcium isotopes against the major isotope 40 Ca for the single-resonance two-step and double-resonance three-step photoionization schemes with narrow-band lasers by using spectral simulation (SS) and modified spectrum (MS) approaches. The results of these calculations are compared with the density matrix (DM) results reported in the literature. It is noted that the values of isotopic selectivity from the SS approach do not agree with those from the DM approach whereas the MS approach, considering hole burning in the Doppler-broadened atomic spectrum, predicts selectivity values which are in good agreement with the DM results. It is argued that one can adequately use the simple MS approach rather than the complex DM approach for the calculation of isotopic selectivity of multi-step photoionization with single-frequency lasers.
Lineshapes for double-resonance three-photon ionization have been evaluated using the density matrix formalism. Experimental conditions were incorporated by considering realistic distributions for atomic velocity and angular divergence, as well as spatial laser intensity profiles. Results for the 4s2 1S0→4s4p 1P1→4s4d 1D2→Ca+ ionization scheme of calcium are discussed and compared with experimental measurements. The calculations accurately reproduce isotopic selectivities achieved in the experiment. Moreover, these measurements have been carried out with dynamic range of >106, revealing many subtle effects of simultaneously present coherent and incoherent excitation processes.
Optical selectivities have been calculated by use of the density matrix approach for ns2 1S0−nsnp 1P10− nsms (or np2) 1S0 double-resonance photoionization pathways to establish the possibility of selective ionization of rare calcium and strontium isotopes for continuous-wave laser excitation. Numerical integration of the density matrix equations for double-resonance ionization has been carried out by incorporation of the effects of Doppler broadening, velocity-dependent interaction times, time-varying Rabi frequencies, and laser bandwidths. The conditions for obtaining optimum selectivities have been evaluated. This study results in five new photoionization pathways (two for calcium and three for strontium) whose optical selectivities were found to be a few orders higher than the previously studied photoionization schemes. The effect of laser linewidth of the excitation lasers and Doppler width have also been investigated.
Optical selectivity calculations have been carried out for the 91Zr isotope for two-color resonant three-photon photoionization schemes. The density-matrix equations of motion were solved numerically for collinear, counterpropagating, linearly polarized laser beams having Gaussian temporal profile. Calculations were carried out incorporating the effects of Doppler broadening, magnetic sublevel degeneracy, and laser bandwidths. The selectivities and ion yields were determined for various Rabi frequencies and ionization rates. Thereby, the optimal Rabi frequencies for the first and second excitation steps and ionizaton rate for the ionizaton step were determined. The ionization efficiency as a function of detuning of the excitation lasers was also investigated. Two of the photoionization schemes investigated yielded a selectivity of ∼10, with an ion yield of ∼23%.
Seven new schemes have been theoretically studied using the density-matrix approach for the ns2(1S0)→nsnp(3P01 or 1P01)→nsms(1S0) and the ns2(1S0)→nsnp(3P01 or 1P01)→np2(1S0 or 1D2) double optical resonance photoionization pathways to establish the possibility of selective ionization of the rare strontium 90Sr isotope for continuous wave laser excitation. The double-resonance excitation is followed by photoionization using a 488 nm photon. Density-matrix equations for the double-resonance ionization have been numerically integrated by incorporating the effects of Doppler broadening, velocity-dependent interaction times, time varying Rabi frequencies, and laser bandwidths. All the optical selectivity calculations were carried out on a high-speed parallel processing machine. Theoretical computations on five previously studied photoionization pathways have also been performed and tabulated for the sake of completeness. The conditions for obtaining optimum optical selectivities have been evaluated. This study results in seven new photoionization pathways. The optical selectivities are a few orders higher compared to the previously studied photoionization schemes for five out of the seven schemes studied. The remaining two schemes are slightly better than the previously studied schemes.
Rare calcium isotope 43Ca+ ion (0.135%) is the only naturally abundant isotope with hyperfine structure. This isotope is considered an attractive candidate both for an optical frequency standard and for quantum information processing. We report a method to load 43Ca+ ions selectively into a linear Paul trap using a two-step, resonant photo-ionization process. Isotope selectivity is achieved by utilizing the isotope shifts for the resonant 4s2 1S0-4s4p 1P1 transition of neutral calcium. The second non-resonant excitation is achieved by an ultraviolet light emitting diode (LED). The use of an LED is simpler and more economical than using a laser for the second transition. Additionally the laser cooling of 43Ca+ is demonstrated using two ultraviolet-diode lasers and two infrared-diode lasers. Due to the hyperfine structure, the laser requirements for Doppler cooling 43Ca+ are more demanding than for the other isotopes.