Quantum defects of $n$F${}_J$ levels of Cs Rydberg atoms

We present precise measurements of the quantum defects of cesium $n$F${}_J$ Rydberg levels. We employ high-precision microwave spectroscopy of $(n+2){\mathrm{D}}_{5\mathrm{/}2}\to n{\mathrm{F}}_{5\mathrm{/}2,7\mathrm{/}2}$ transitions for $n=45$ to 50 in a cold-atom setup. Cold cesium $(n+2)$D${}_{5\mathrm{/}2}$ atoms, prepared via two-photon laser excitation, are probed by scanning weak microwave fields interacting with the atoms across the $n{\mathrm{F}}_{5\mathrm{/}2,7\mathrm{/}2}$ resonances. Transition spectra are acquired using state-selective electric-field ionization and time-gated ion detection. Transition-frequency intervals are obtained by Lorentzian fits to the measured spectral lines, which have linewidths ranging between 70~kHz and 190~kHz, corresponding to about one to three times the Fourier limit. A comprehensive analysis of relevant line-shift uncertainties and line-broadening effects is conducted. We find quantum defect parameters ${\delta }_0({\mathrm{F}}_{5\mathrm{/}2})=0.03341537(70)$ and ${\delta }_2({\mathrm{F}}_{5\mathrm{/}2})=-0.2014(16)$, as well as ${\delta }_0({\mathrm{F}}_{7\mathrm{/}2})=0.0335646(13)$ and ${\delta }_2({\mathrm{F}}_{7\mathrm{/}2})=-0.2052(29)$, for $J=5\mathrm{/}2$ and $J=7\mathrm{/}2$, respectively. Fine structure parameters $A_{FS}$ and $B_{FS}$ for Cs $n{\mathrm{F}}_J$ are also obtained. Results are discussed in context with previous works, and the significance of the results is discussed.