Studies of afterglows in noble-gas mixtures: A model for energy transfer in He/Xe+

Time-resolved spectroscopy of excited mediums following electrical discharges in helium/xenon mixtures, in which xenon is the minority constituent, show enhancement in the emission of all the ionic-xenon levels relative to that observed in pure-xenon discharges. The time characteristics depend on the helium pressure and the decay follows the molecular helium emission. The total pressure ranges between 20 and 300 Torr, of which the xenon pressure is up to 1 Torr. The discharge has a peak current density of 500 A/${\mathrm{cm}}^2$ and it flows for about 100 nsec. It is suggested that helium plays an active role in the generation of excited xenon ions by transferring excitation energy to xenon. The acceptors can be ${\mathrm{Xe}}^{+}$ in its ground state and neutral Xe in its metastable state. In the investigated discharges both, ${\mathrm{Xe}}^{+}$ and metastable Xe* exist in high densities for up to 5-10 μsec into the early afterglow. During this period, atomic helium metastables and molecular helium states such as $a{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, $A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$, and $B{}_{}{}^1{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ are also highly populated and in fact constitute an energy store. The molecular states of helium can resonantly transfer energy through the bound-free transitions to ground-state ${\mathrm{Xe}}^{+}$ to form excited xenon ions ${\mathrm{Xe}}^{+*}$ with an estimated cross section of 4 × ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$—a resonable order of magnitude considering the large enhancement observed in Xeii emission. Penning ionization and charge transfer have only little effect on observed enhancement.