Rydberg atom spectroscopy enabled by blackbody radiation ionization

We have excited helium atoms from their metastable 2 3S state to Rydberg states in the range 13 < n < 50 in a two-step process via the 3 3P state using light at λ = 389 nm and 785–815 nm. Atoms in a thermal beam (100 K) cross partially overlapping laser beams of the appropriate frequencies in the counterintuitive order to exploit the high efficiency of stimulated rapid adiabatic passage. The interaction region is between two plates that can be used for Stark tuning in a few V/cm field or for field ionization. At fields much too low for field ionization, we observe signals attributed to ionization by blackbody radiation. Multiple tests confirm this attribution as the cause of ionization. For example, by heating the plates we observe the expected signal increases. Our experiments reinforce previous work where the interaction between Rydberg atoms and room temperature blackbody radiation is important for experiments.