Resonantly enhanced three-photon excitation spectra of lithium

We present the first measurement on the resonantly enhanced three-photon excitation spectra of natural lithium using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. Exploiting the linear and circular polarizations, the np 2P3/2(8 ⩽ n ⩽ 11) and nf  2F7/2 (8 ⩽ n ⩽ 38) series have been observed via three-photon excitation from the ground state. The measured level energies reveal a dynamic shift from calculated values, which increases with an increase of the principal quantum number n. The ac stark shift and line broadening mechanisms are studied as a function of laser intensity. It is noted that the width increases and the line center shifts towards the higher energy side as the laser intensity is increased. The maximum observed shift for the 12f 2F7/2 line is 0.33 cm−1 corresponding to the laser intensity variation from 1.34 × 1012 W/m2 to 1.03 × 1013 W/m2, whereas its width increases from 0.36 cm−1 to 0.82 cm−1.