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Journal de Physique IV (Proceedings)

Resonance enhanced multiphoton ionization within the ion trap

Alexei I. Suslov, V. A. Pushkin

Published Mar 1, 2000• 0 citations
Publisher

Abstract

Possible schemes of resonance-enhanced two-photon selective ionization of indium atoms by using a dye laser were theoretically investigated. Ionization and excitation were performed by a dye laser [BPO, (395-415)x10-9 m] which was pumped with a pulse nitrogen laser. An optimal value of the focusing spot diameter within the interaction region that provides the maximal ions to be created for a given radiant power. We registered of about 100 ions for one laser pulse and a concentration of indium atoms of about 1014 m-3 for a trapping time of 100 periods of a rf field. Ion-atom scattering and electrode system imperfection cause the number of trapped ions to decrease by a factor of 2.71 for 10-2 c and a pressure of 4x10-6 Torr.

DOI
10.1051/jp4:2000539

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Possible schemes of resonance-enhanced two-photon selective ionization of indium atoms by using a dye laser were theoretically investigated. Ionization and excitation were performed by a dye laser [BPO, (395-415)x10-9 m] which was pumped with a pulse nitrogen laser. An optimal value of the focusing spot diameter within the interaction region that provides the maximal ions to be created for a given radiant power. We registered of about 100 ions for one laser pulse and a concentration of indium atoms of about 1014 m-3 for a trapping time of 100 periods of a rf field. Ion-atom scattering and electrode system imperfection cause the number of trapped ions to decrease by a factor of 2.71 for 10-2 c and a pressure of 4x10-6 Torr.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Abstract We report a method of creating pure 9Be+ ions by a 2 + 1 resonance-enhanced multiphoton ionization for the ionization potential of 75 192.64(6) cm−1 (133 nm). The efficient generation of 9Be+ ions has been realized in a segmented linear ion trap. The average loading rates with a 10 ns, 1 mJ laser at 310 nm and 306 nm are 3.8 and 1.3 ions per pulse, respectively. This method has the advantage of reducing the electron contamination to the ion trap electrodes greatly. It also reduces the requirement of single-photon energy while satisfying the need for ionization probability and can be applied to other atoms with high ionization thresholds.
Two investigations are reported on the application of resonantly enhanced multiphoton ionization (RMPI) to gaseous mixtures of an atomic vapor and a few Torr of argon. Photon fluxes large enough to ionize atoms by RMPI were produced by focussing down the light from a tunable pulsed dye laser. The irradiances generated were of the order of 500MWcm⁻². Ionization was detected by a voltage biased wire electrode that simply collected the photoelectrons either directly or after some gas multiplication. One investigation was the measurement of the dependence of RMPI processes in mercury on the polarization of the incident light for comparisons with theoretical calculations. The processes were four-photon resonant absorptions to either a ¹S₀ or ¹’³D₂ level followed by single photon ionization. Complete photoionization of all atoms excited to the resonant levels is established and the measured polarization dependences are found to agree with the calculated polarization dependence for the resonant excitation step of the RMPI process. Unexplained observations of the distortions in the polarization dependence of the ion yield and the absorption linewidth of the 6d ¹D₂ resonance are discussed. The second investigation was a study of the density dependent electric dipole forbidden two-photon resonant transition ²S → ²P in three-photon RMPI spectra of the Rydberg states of potassium. Stark interactions are shown to be unlikely and too weak. From the characteristics of the spectra, the excitation process is identified as a laser-assisted collision interaction.
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We have developed a photoionization method for complete conversion of a quantum-selected population to ionization, making possible sensitive and absolute measurement of the selected populations in a gas. Each photoionization involves the absorption of two photons (from a pulsed dye laser), one of which is resonant with an intermediate state. In this demonstration we measured the absolute number of He(21S)\mathrm{He}(2^{1}S)He(21S) states per ion pair following interaction of pulses of 2-MeV protons with He.