One- and two-photon production of very highly excited states of caesium

Very highly excited p states of Cs with principal quantum numbers 30<or=n<or=85 were produced by one-step photoexcitation with frequency-doubled light from a flash-lamp-pumped dye laser. Field ionization was employed for selective detection. Ionization occurs at field strengths E>or=E min with a sharp threshold. The relation E min (n*) varies as (n*) -4 was verified over a wide range. Two-photon excitation to a very high state via a virtual intermediate state leads to immediate ionization of most of the excited atoms; in this case the ion signal as a function of the light intensity I is not proportional to I k , k>or=2, whereas a direct two-photon transition into the continuum yields proportionality with I 2 .