Abstract

Laser absorption and fluorescence spectroscopy were used to study the dipole-forbidden 2s²S to 3d² D transition in lithium at 319.6 nm and the two-photon allowed transition between the same two levels at a wavelength of 639.2 nm. Transmission of the incident pump laser radiation and the intensity of the single-photon fluorescence signal from the 3d² D to 2p²P transition at 610.4 nm were measured as a function of lithium vapor density (10¹³−10¹⁶ cm−³) and incident laser intensity (10⁴−10⁸W/cm²). Absorption line shapes and fluorescence lifetimes were also measured. The single-photon absorption cross section calculated from our 319.6-nm absorption data agrees to within a factor of 2 with theoretical calculations. Estimates of the two-photon absorption cross section are complicated by an anomalous increase in the610.4-nm fluorescence intensity when the 639.2-nm pump wavelength is shifted ~0.035 nm to the red and by saturation effects. The general dependence of the fluorescence intensity on lithium vapor density and pump laser intensity is explained by saturation of the incident pump laser radiation. The results of this work were used to establish single-photon and two-photon laser diagnostic measurements of absolute lithium vapor number densities for ion-diode experiments.

© 1986 Optical Society of America