Abstract
We calculate the diamagnetic spectrum of lithium at highly excited states up to the positive energy range using the exact quantum defect theory approach. The concerned excitation is one-photon transition from the ground state 2s to the highly excited states np with π and σ polarizations respectively. Lithium has a small quantum defect value 0.05 for the np states, and its diamagnetic spectrum is very similar to that of hydrogen in the energy range approaching the ionization limit. However, a careful calculation shows that the spectrum has a significant discrepancy with that of hydrogen when the energy is lower than −70 cm−1. The effect of the quantum defect is also discussed for the Stark spectrum. It is found that the σ transition to the np states in an electric field has a similar behavior to that of hydrogen due to zero interaction with channel ns.