Precision measurements of quantum defects in the $n$P$_{3/2}$ Rydberg States of ${}^{85}$Rb

Rydberg States are used in our One Atom Maser experiment because they offer a large dipole moment and couple strongly to low numbers of microwave photons in a high Q cavity. Here we report the absolute frequencies of the P$_{3/2}$ states for principal quantum numbers $n=36$ to $n=63$. These measurements were made with a three step laser excitation scheme. A wavemeter was calibrated against a frequency comb to provide accurate absolute frequency measurements over the entire range, reducing the measurement uncertainty to 1MHz. We compare the spectroscopic results with known frequency measurements as a test of measurement accuracy.