Time-resolved fluorescence studies of ${\mathrm{Ar}}_2$ formed by supersonic jet expansion and excited by monochromatic, tunable, vacuum-ultraviolet radiation have been used to measure the radiative lifetimes of selected high vibronic levels of the $A{1}_{\mathrm{u}}$ state. Lifetimes of levels $v^{\prime }=24\mathrm{} \mathrm{to} 30$ were found to have an average value of 160 ± 10 ns. This differs by a factor of ∼ 20 from the lifetime of 2.88 ± 0.06 μs obtained earlier for the $v^{\prime }\sim 0$ level with high gas pressures and excitation by charged particles. It is shown that this large difference in lifetimes arises from a sixfold increase in the electronic transition moment for a change of internuclear distance from 2.5 to 3.5 Å.

• Received 25 July 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.1574