Low field linear Stark effect on quasi-hydrogenic caesium series

We present the experimental evidence of the linear Stark behaviour in an atomic spectrum at low (absolute) electric field strengths on high lying quasi-hydrogenic series of atomic caesium. This is the behaviour which would be observed on hydrogen atoms for any level. It is exhibited here in the 1-15 V/cm range on quasi-hydrogenic atoms, excited, in vapour phase, by a c.w. dye laser to n = 60 Rydberg states. Under these conditions, the core effects are almost negligible compared with the first linear-in-field term in the Stark expansion of the energy.