Experimental studies of three-photon ionization of Ba: Evidence of channel interference and Raman coupling

Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6$s^2$ ${\mathrm{}}^1$ $S_0$ ground state and the 6snd J=2,6s(n+1)s ${\mathrm{}}^1$ $S_0$ (16≤n<40) Rydberg states and 5d7d ${\mathrm{}}^1$ $D_2$,${\mathrm{}}^3$ $P_0$,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{-}\left(4\mathrm{}\mathrm{-}5\right)}$ a.u. was used to drive the excitation in a low-density atomic beam (n≲3×${10}^8$ ${\mathrm{cm}}^{\mathrm{-}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.