Charge transfer and radiative dissociation dynamics in fluorine-doped solid krypton and argon

Abstract

The photodissociation of F2 in crystalline Kr is subject to only a minor cage effect—quantum yield of 0.5 at 308 nm, at T=12 K. Two-photon-induced harpooning in the same system leads to dissociation with near unit quantum efficiency; it is shown that this absorption is coherent with a giant cross section of 3(±2)×10−45 cm4 s at 275 nm. Excitation and emission spectra of charge-transfer transitions in solid Kr doped with F atoms, and solid Ar multiply doped with F/Kr are reported. The vertical excitation in F/Kr leads to a state delocalized over immediate neighbors—assigned to Kr+6F−. This state localizes on a subpicosecond time scale to the triatomic Kr+2F− configuration. The latter relaxes radiatively (τr=140 ns), to the repulsive wall of the ground state (∼1 eV above ground). The excitation in F:Kr/Ar solids shows the diatomic KrF(B←X) resonances. From the analysis of the spectrum, a harmonic frequency ω=75(±3) cm−1 is extracted for F atoms in the ground state, consistent with molecular-dynamics simulations of an octahedrally trapped F atom [ω=70(±1) cm−1]. The diatomic KrF(B) state relaxes on a time scale of 20 fs<t<500 fs to the mixed triatomic configuration, (ArKr)+F−, which then relaxes radiatively (τr=60 ns). The kinetic energy released upon the radiative dissociation of triatomic exciplexes generates acoustic waves, and defects, and leads to mass transport. The modes of propagation of the acoustic waves, their subsequent thermalization, and self-annealing of the solid, are directly monitored by spatially and temporally resolved transient scattering experiments. A transient loss (σ≂10−17 cm2), the origin of which is not fully understood, is observed for Kr2F isolated in solid Kr. The mass transport, and long-range migration of F atoms (∼10 lattice sites), is directly monitored in multiply doped Ar solids in which the shuttle of F atoms between Xe and Kr centers can be demonstrated.