We report the measurement of the photoionization cross sections of the
5S1/2 and
5P3/2 states of
87Rb in a two-species Hg and Rb magneto-optical trap (MOT) by the cooling laser for Hg. The photoionization cross sections of Rb in the
5S1/2 and
5P3/2 states at 253.7 nm are determined to be ${1}_{\ensuremath{-}1}^{+4.3}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$ and $4.63(30)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$, respectively. To measure the
5S1/2 and
5P3/2 state fractions in the MOT we detected the photoionization rate of the
5P3/2 state by an additional 401.5 nm laser. The photoionization cross section of Rb in the
5P3/2 state at 401.5 nm is determined to be $\text{1.18(10)}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$.
We measure the total collisional cross-sections of caesium Rydberg nD ( n =37–40) states by measuring the corresponding effective decay rates versus Rydberg atoms density in a conventional magneto-optical trap using the state-selective pulse field ionization technique. The results indicate that the large collisional cross-sections mainly come from the contribution of Rydberg atoms interactions. We also obtain the spontaneous radiative lifetimes of different Rydberg states by fitting experimental data, which are in agreement with the existing theoretical values within experimental error.