  We demonstrate the single-photon Rydberg excitation of cesium atoms in a
magneto-optical trap (MOT). We excite atoms directly from ${{6S}_{1/2}}$ ground
state to ${{nP}_{3/2}}(n=70-100)$ Rydberg state with a narrow-linewidth 319 nm
ultra-violet laser. The detection of Rydberg states is performed by monitoring
the reduction of fluorescence signal of the MOT as partial population on
${{6S}_{1/2}} (F = 4)$ ground state are transferred to Rydberg state. We
clearly observe Autler-Townes doublet in the trap-loss spectra due to the
cooling lights. Utilizing the large electric polarizibility of Rydberg atoms,
we observe Stark splitting in the Autler-Townes doublet induced by background
DC electric fields. We investigate the dependence of Stark shift on electric
fields by theoretical analysis, and then infer the DC electric field from the
measured Stark splitting. We find that there is a 44.8(4) mV/cm DC electric
field in the vicinity of the cold ensemble. It indicates that high-lying
Rydberg atoms can be used as sensors for DC electric fields.
