Lifetimes of excited states of barium and lanthanum and the modification of the Hanle effect by strong optical pumping

An atomic beam apparatus was built to undertake a study of the hyperfine structure and lifetimes of excited states of barium and lanthanum. The dependence of optical pumping signals, as a function of such parameters as light polarization, residual magnetic field, interaction time, mean pumping rate, and laser spectral profile asymmetry, was studied theoretically. The predicted lineshapes were compared to those observed experimentally, and shown to be in good agreement. The theoretical formulation was used to compute lineshapes for excited-state Hanle effect signals modified by strong optical pumping of the initial state. These computed curves were shown to be in good agreement with those observed experimentally. It was found that for a particular choice of parameters the optical pumping signal was minimized; measurement of the excited state lifetime by the Hanle effect in the usual manner was thus possible without the problem of deconvoluting the optical pumping contributions from the answer. The lifetime of the excited (5d6p) 3D3 state of barium was measured to be (10.2 +- 1.5) nanoseconds. A pulsed dye laser system was constructed, and the lifetimes of the 5d6s(a1D)6p z2F/sub 5/2/, 5d6s(a1D)6p z2D/sub 3/2/, 5d2(a3F)6p z4G/sub 5/2, and 5d6s(a3D)6p y2D/sub 3/2/ states of lanthanum I were measured by time-resolved spectroscopy. The measured values for the lifetimes are 120 +- 6, 165 +- 8, 51 +- 4 and 18 +- 3 nanoseconds, respectively