Time-dependence study of radiation trapping by time-delayed two-photon absorption

The transport of resonance radiation through an optically thick vapor of Sr atoms is studied. A pulsed dye laser tuned to the 461-nm resonance line excites a narrow (∼250-μm-diameter) column of Sr atoms along the axis of a cylindrical oven containing Sr vapor and Ar buffer gas. After a delay of ≤80 ns, a second dye laser excites the atom from the first excited state ($5s5p$) to a higher excited state ($5s7s$). The fluorescence from this latter transition is monitored as the second laser is translated parallel to the first. Since the excited-state-excited-state fluorescence is not trapped, the result is a plot of density of atoms in the $5s5p$ state as a function of position from the originally excited volume. The results are discussed qualitatively.