Precision measurement of the ionization energy of Cs I

We present absolute-frequency measurements for the transitions from the 6s$_{1/2}$ ground state of $^{133}$Cs to $n$p$_{1/2}$ and $n$p$_{3/2}$ Rydberg states. The transition frequencies are determined by one-photon UV spectroscopy in ultracold samples of Cs atoms using a narrowband laser system referenced to a frequency comb. From a global fit of the ionization energy $E_\mathrm{I}$ and the quantum defects of the two series we determine an improved value of $E_\mathrm{I} = h c \cdot 31 406.467 732 5(14)$ cm$^{-1}$ for the ionization energy of Cs with a relative uncertainty of $5\times10^{-11}$. We also report improved values for the quantum defects of the $n$p$_{1/2}$, $n$p$_{3/2}$, $n$s$_{1/2}$, and $n$d$_{5/2}$ series.