M. Sankari

Investigations on the 1 S 0 λ 1 → 1 P 0 1 λ 2 → 1 S 0 nonresonant——→ M+ photoionization pathway for selective ionization of rare calcium and strontium isotopes

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 calculation of calcium isotopes in a double-resonance ionization scheme

Optical selectivities of calcium isotopes have been calculated for the 4s2 1S0 4s4p 1P1 4s4d 1D2→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 2.2×105, which is adequate for selective excitation of the 41Ca isotope for certain applications.

Calculation of 91 Zr optical selectivities in two-color resonant three-photon ionization schemes

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%.

Study of line shapes in the selective ionization of ^176Yb isotope in a two-step resonance, three-step ionization scheme

Ytterbium enriched in 176Yb is used for the production of the 177Lu radioisotope, which has applications in cancer treatment. We have theoretically studied the two-step resonant, three-step photoionization of the 176Yb isotope using the density-matrix approach. To simulate experimentally realistic conditions, the Doppler averaging, magnetic sublevel degeneracy, time-varying Rabi frequencies, ionization rate, angular divergence, and laser bandwidth have all been incorporated into the theoretical model. Calculations have been carried out to identify the optimized Rabi frequencies for evaluating the separation factor. The effect of the laser line shape on the excitation profile has also been thoroughly studied. We could obtain large separation factors of ~5000 for the 176Yb isotope, and the excitation conditions identified in this work may be utilized in the separation of the desired isotope.

Isotope selective near-resonant two-photon ionization of calcium isotopes

A near resonant two-photonionization scheme , has been investigated for the isotope selective excitation of 41Ca isotope for its applications as a tracer in bio-medical studies. The ionization efficiency and optical selectivity have been calculated for various powers of the excitation and ionization laser. Under the optimized excitation/ionization laser powers the ionization efficiency and the optical selectivity for the studied scheme are found to be 1.3 × 10−4 and ~1.0 × 104, respectively. The ionization efficiency is two orders of magnitude higher than the step-wise excitation process. In combination with a mass spectrometer, an overall selectivity of ~1010 can be obtained which is adequate for bio-medical applications.

Isotope-selective excitation of 41Ca isotope in Doppler-free two-photon continuous-wave excitation: a case study

Seven schemes are studied theoretically for Doppler-free two-photon excitation of rare (41)Ca isotope using single-mode continuous-wave lasers. The ionization efficiencies and optical selectivities for all the schemes are calculated for various powers of the excitation and ionization lasers and for various focusing conditions of the two lasers. To maximize the ionization efficiencies and the optical selectivities, wavelength-dependent Stark compensation is used. Certain laser wavelengths of the ionization step termed as magic wavelengths are identified for compensating the Stark shift induced by the excitation laser. The effects of the Stark-shift-induced asymmetry and its reversal by selecting the appropriate magic wavelength for the ionization step for various excitation and ionization laser intensities are investigated. The ionization efficiency and optical selectivity for the best scheme after Stark compensation are found to be 8.4 x 10(-4) and approximately 9 x 10(3), respectively.

Optical selectivity calculations of 90Sr isotope in double optical resonance photoionization schemes

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.

Isotope selective near-resonant two-photon ionization of 41 Ca isotope via the 4s11 s 1 S 0 intermediate state

Isotope selective excitation of a 41Ca isotope using a near-resonant two-photon ionization scheme 4s2 1S0-->422.7924 nm,413.3685 nm4s11s 1S0-->514.5 nmCa+ has been proposed for using the 41Ca isotope in applications as a tracer in biomedical studies. The ionization efficiency and optical selectivity have been calculated for various powers of the excitation and ionization laser. Under the optimized excitation and ionization laser powers the ionization efficiency for the studied scheme is found to be 1.7x10(-4). The optical selectivity value is approximately 1.0x10(5) and both of these values are either comparable or slightly better than the earlier published work by our group. The overall ionization efficiency for the two-photon ionization scheme considering the throughput factor is 5x10(-3), which is 2 orders of magnitude higher than the stepwise excitation process. Therefore, the higher ionization efficiency of the process enables monitoring of the tracer isotope for longer durations. In combination with a mass spectrometer, an abundance sensitivity of approximately 10(10) can be obtained, which is adequate for biomedical applications.