Fluorescence studies of multiple-photon ionization processes: Four- and five-photon ionization of Sr at wavelengths of 558–590 nm

An ‘‘all-optical’’ technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (∼${10}^{11}$–${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558–590 nm at laser intensities ∼1–50 GW/${\mathrm{cm}}^2$. Fine-structure-resolved emission of the ${\mathrm{Sr}}^{+}$ (5$p^2$ $P_{3/2/,1/d}^0$) ions which are formed is observed at λ=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity ∼${1:10}^3$ with respect to the ionic-resonance emission.