Fine Structure Measurements in High-L Rydberg States of Argon.

A Doppler-tuned CO$_{2}$ laser has been used to study n=9 to n=17 excitation transitions in high-L Rydberg states of Argon, revealing fully-resolved fine structure patterns in states with 5 $\le $ L $\le $ 8. The experimental method is similar to that used previously to study Ba Rydberg levels[1]. A fast beam of Argon Rydberg states with n $\sim $ 9 is produced by charge exchange between a 9.5 keV Ar$^{+}$ beam and a 9F Rb Rydberg target. Argon atoms excited to n=17 by the CO$_{2}$ laser are detected by Stark ionization. The measured fine structure patterns are used to deduce the dipole polarizability and quadrupole moment of the $^{2}$P$_{3/2}$ ground state of Ar$^{+}$. [1] E.L. Snow, et. al., Phys. Rev. A 71, 022510 (2005)