Electron-impact excitation of the rare-gas atoms to high-Rydberg states

High-Rydberg atoms are produced by an electron beam passing through each of the rare gases at ∼${10}^{-6\mathrm{}}$ Torr and are detected by electric field ionization after traveling 20.1 cm at thermal velocity. Their principal quantum number ($n$) distributions, time-of-flight distributions, absolute excitation cross sections ${\sigma }_T^{\mathrm{ex}}$, and excitation functions are measured. The atoms detected have $n$ ranging from 20-80 with the most probable $n$ varying with the gas from 28-46. From the measured ${\sigma }_T^{\mathrm{ex}}$ an apparatus-independent absolute cross section ${\sigma }^{\mathrm{ex}}\mathrm{}(n,E)\mathrm{} =\frac{{\sigma }^{\mathrm{ex}}\mathrm{}(n=1,E)\mathrm{}}{n^3}$, is determined. Values of ${\sigma }^{\mathrm{ex}}\mathrm{}(n=1,E)\mathrm{}$ range from 0.5-5 ${\mathrm{\AA }}^2$ at an incident electron energy $E=100\mathrm{}$ eV. Ne and Ar measurements taken for different values of electron current and gas density demonstrate that low-angular-momentum ($l$) high-Rydberg atoms collide with electrons and/or ground-state atoms, causing higher-$l$ states to be populated. The $l$-changing cross section for collisions with electrons is estimated to be ∼ ${10}^6$ ${\mathrm{\AA }}^2$ for Ne and Ar.