“…Since then, various transitions have been observed involving intercombinations of the high-l Rydberg states 3d, 4d, 4f , 5f , 5g, 6g, 6h, 7h, 7i, and 8i using Fouriertransform infrared ͑FTIR͒ spectroscopy 9,10 and diode laser spectroscopy. 3,11,12 Also, microwave spectroscopy ͑see, for example, Sturrus, Hessels, Arcuni, Lundeeen, 13 and refer-ences therein͒ has been used to study inter-Rydberg transitions with nϾ9 to very high precision. In the absence of autoionization or predissociation, these high-l states have lifetimes which are long enough to allow very precise determination of their energies.…”