We report the measurement of the absolute photoionization cross section for the 5P 3/2 state of 87 Rb at wavelength of 473 nm, which results in the photoelectron energies of 33 meV above the ionization threshold, using cold atoms confined in a vapor-loaded magneto-optical trap. The 87 Rb 5P 3/2 photoionization cross section at 473 nm is determined to be 𝜎 PI = 10.5 ± 2.2 Mb. Considering the spatial distribution of the trapped atoms, the average intensity 𝐼 PI of the ionization laser seen by an atom in the MOT instead of ionizing laser intensity 𝐼 PI is used in our calculations for the photoionization cross sections. The excited state fraction is also accurately estimated using the latest experimental result.

The influence of an ionizing laser on the pump-probe spectra of 87 Rb over the transition 5 2 𝑆 1/2 , 𝐹 = 2 → 5 2 𝑃 3/2 , 𝐹 ′ = 3 is experimentally studied in an operating magneto-optical trap. These spectral features, including gain peak, a dispersion-like structure and absorption peak, become weak as the intensity of the ionizing laser increases. Moreover, the profiles of the absorption peak and gain peak vary as the ionizing laser intensity changes. Such results indicate that there is more than one component in the two features and that each component has different dependences on the number of 87 Rb atoms.

The temperature of the remaining cold 87 Rb atoms confined in a vapor cell magneto-optical trap after two-step photoionization has been measured. In the two-step photoionization process, the first excitation laser is served by the cooling laser and the second excitation laser is served by a continuous semiconductor laser with a wavelength of 450 nm. The results show that the temperature of the remaining cold atoms decreases as the intensity of the second excitation laser increases. Moreover, the relationship between the temperature 𝑇 and number 𝑁 of the remaining cold atoms generally follows a power law, while it deviates from the well-known 𝑇 ∝ 𝑁 1/3 and the power factor is smaller than 1/3. We propose that ion-atom collisions occurring during a photoionization process strongly influence the temperature scaling law in an optically dense magneto-optical trap in the presence of an ionization laser. In addition, the forced evaporative cooling due to the combined effect of the detuning of the first excitation laser and the two-step photoionization plays a role in cooling the remaining cold atoms and results in the dependence of the power factor on the detuning of the first excitation laser.

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