Diagnostics of a Diluted Ultracold Plasma Using the Autoionization Effect of Rydberg States of 40Ca Atoms

A sensitive method to study a dilute ultracold plasma based on the autoionization resonances of Rydberg atoms is demonstrated. Ultracold 40Ca Rydberg atoms are prepared in a continuously operating magneto-optical trap. A method that allows the detection of the plasma with ion and electron densities below 109 m–3 is shown. Experimental studies of resonances at the two-photon $$4s3d{{\,}^{1}}{{D}_{2}}{-} 90{{\,}^{1}}{{D}_{2}}$$ Rydberg transition have been carried out using 672 and 798 nm resonant laser beams. The autoionization resonance is observed during fluorescence of single-charged 40Ca ions at a wavelength of 397 nm. The dependence of the amplitude of autoionization resonance on the density of ions in the continuously generated ultracold plasma is obtained.