Anomalous linewidths and peak-height ratios in¹³⁷Ba hyperfine lines

Abstract

We have investigated several effects of optical pumping in the hyperfine spectrum of the6s6p ¹ P ₁⇔6s ^{2 1} S ₀ transition in¹³⁷Ba. Most of these effects are explained by absorption strength changes which occur because of redistribution of population among magnetic substates. At very low laser intensities, no redistribution effects are observed. At higher intensities, it is possible to either empty the magnetic substates that are accessible to optical excitation, or to redistribute the population among these states until a steady-state condition is achieved. The first case results in the familiar disappearance of a hyperfine line. The less familiar second case can result in peak-height ratios in the Ba¹ P ₁⇔¹ S ₀ hyperfine spectrum that differ by almost a factor of three from the low-intensity case. In this second case, the observed linewidth can either broaden or narrow, depending on whether redistribution decreases or increases absorption strength. At high intensities, saturation effects dominate and branching to intermediateD states becomes apparent. We report here the result of a numerically integrated rate equation model which shows good agreement with our experiments.