Microwave Multiphoton Transitions between Rydberg States of Potassium

We report the observation of sequences of from 1- to as many as 28-photon transitions between potassium $\mathrm{}(n+2\mathrm{})s$ states and the lowest-energy Stark states of the $n$ manifolds for $n=15\mathrm{}-18$. The sequences are observed by the scanning of a static electric field from the field at which these pairs of levels nearly cross, to zero field where they are well separated. The maximum number of photons absorbed is observed to be proportional to the microwave field.