Microwave spectroscopy of Al I Rydberg states: F terms

Energy level separations in high-lying Rydberg states of neutral aluminium have been measured by a two-photon millimetre-wave resonance method to obtain n 2F–(n + 1) 2F intervals for n = 29–37 with an accuracy ±1 MHz. An Al atomic beam was excited stepwise by pulsed lasers, and the Rydberg atoms were detected by state-selective field ionization. From these data and previous optical data for the 3s2n configuration of Al I new Ritz-expansion coefficients, which express the weakly n-dependent quantum defects for the nF Rydberg series, have been obtained. A previous laser-spectroscopic study of 2F Rydberg levels in Al I is shown to be in error, and quantum defects based on the classical spectroscopic work of Eriksson and Isberg are confirmed. The nF series is completely regular and shows no sign of perturbations by other electronic configurations.