Near-threshold photoionization spectra of strontium

Photoionization cross sections of ultracold 88Sr in 1P1 and 3S1 states at 390 nm and the resulting blue-detuned magic wavelength optical lattice clock constraints

We present the measurements of the photoionization cross sections of the excited ¹P₁ and ³S₁ states of ultracold ⁸⁸Sr atoms at 389.889 nm wavelength, which is the magic wavelength of the ¹S₀-³P₀ clock transition. The photoionization cross section of the ¹P₁ state is determined from the measured ionization rates of ⁸⁸Sr in the magneto-optical trap in the ¹P₁ state to be 2.20(50)×10^-20 m², while the photoionization cross section of ⁸⁸Sr in the ³S₁ state is inferred from the photoionization-induced reduction in the number of atoms transferred through the ³S₁ state in an operating optical lattice clock to be 1.38(66) ×10^-18 m². Furthermore, the resulting limitations of employing a blue-detuned magic wavelength optical lattice in strontium optical lattice clocks are evaluated. We estimated photoionization induced loss rates of atoms at 389.889 nm wavelength under typical experimental conditions and made several suggestions on how to mitigate these losses. In particular, the large photoionization induced losses for the ³S₁ state would make the use of the ³S₁ state in the optical cycle in a blue-detuned optical lattice unfeasible and would instead require the less commonly used ³D_1,2 states during the detection part of the optical clock cycle.

Measurement of Photoionization Cross-Section for the Excited States of Atoms: A Review

A review of experimental studies of the measurement of the photoionization cross-section for the excited states of the alkali atoms, alkaline earth atoms, and rare-gas atoms is presented, with emphasis on using multi-step laser excitation, ionization, and the saturation technique. The dependence of the photoionization cross-section from different intermediate states populated in the first step and ionized in the second step are discussed, including results on the photoionization cross-sections measured above the first ionization threshold. Results based on different polarizations of the exciting and the ionizing dye lasers are also discussed. Examples are provided, illustrating the photoionization cross-sections measured using thermionic diode ion detector, atomic beam apparatus in conjunction with a time-of-flight mass spectrometer and DC/RF glow discharge cell as an optogalvanic detection.