Measurement of Ionization Threshold of Ultracold Cesium Rydberg Atoms in Static Electric Field

We investigate the field ionization spectra of ultracold cesium Rydberg atoms in dc electric field. The ionization thresholds of different electric fields are measured and shift of the ionization threshold relative to field-free ionization threshold is accurately described by (6.06±0.14)F^1/2, which is in good agreement with the classical saddle-point model for field ionization. We obtain the field-free ionization threshold of cesium (6P_3/2) as 19674.89±2.99cm^-1 by fitting experimental data.