Formation of AgHe2Exciplex in Liquid Helium

We report the first experimental confirmation of a metal-atom–He exciplex formation in liquid helium. Following the excitation of the D2 line of an Ag atom, we have observed the atomic D1 line emission and a broadband emission to the red of the D1 line. The band has been assigned to the $\tilde{A}{}_{}{}^2{}_{}{}^{}\Pi _{3/2}^{}{}_{}{}^{}-\tilde{X}{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ bound-free transition of the He-Ag-He linear exciplex trapped in a microcavity in liquid helium. The assignment has been confirmed by an ab initio calculation on the ${\mathrm{AgHe}}_2$ molecule. The absence of the atomic D2 emission as well as an emission from AgHe indicates that the ${}_{}{}^2{}_{}{}^{}P_{3/2}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}P_{1/2}^{}{}_{}{}^{}$ relaxation and the formation of ${\mathrm{AgHe}}_2$ in the $\tilde{A}{}_{}{}^2{}_{}{}^{}\Pi _{3/2}^{}{}_{}{}^{}{,v}^{\prime }=0\mathrm{}$ state occur on a subnanosecond time scale.