Condensation of He Gas on Optically Excited Ag Atoms Below the Critical Temperature (T<5.2K)

Laser induced fluorescence spectra of Ag atoms and their time profiles have been studied in cold He gas at temperatures 4.3–25 K and pressure $\lesssim 1\mathrm{atm}$. The emission spectra following the excitation of the atomic $D2\mathrm{}$ line consist of three narrow lines and three broadbands, which are assigned to transitions of Ag atoms and AgHe and ${\mathrm{AgHe}}_2$ exciplexes. Surprisingly long ( $\gtrsim 300\mathrm{ns}$) effective emission lifetimes are observed at $T>\mathrm{}{T}_c$ (5.2 K). The long lifetimes are the result of population trapping in the ${}^2{D}_{5/2}$ metastable state. At $T\lesssim T_c$, the effective lifetimes decrease to $\lesssim 20\mathrm{ns}$ due to the condensation of He gas on the Ag atom in its ${}^2{D}_{5/2}$ state.