Characteristic energy losses, luminescence spectra, and their lifetimes for RbI and KI under low energy electron impact

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Abstract
Evaporated thin films of RbI and KI have been examined at 25–300°K under low energy electron impact. The emission of RbI at 470 nm was excited at 3.8 to 5.4 eV electron energy as well as at the first optical exciton (5.7 eV) and above. The 395 nm band was excited at ≥5.1 eV and possibly below this energy. The lifetimes τ470 and τ395 at 40°K are both 10−5 sec at ≥5.4 eV electron energy while τ395 decreases to 3.5 × 10−7 sec in the range 4.0 to 4.7 eV. τ470 decreases below 5.4 eV to 10−7 sec at 4.4 dV, and is too weak to measure at <4.4 eV. The 400 nm band of KI was excited at 30°K in the range 4.2–5.4 eV as well as at and above the first optical exciton, 5.8 eV. The 294 nm band of KI and the 315 nm band of RbI were excited principally at and above the region of the step, 6.2 and 6.0 eV, respectively. The lifetimes of the 376 nm band of KI in the range 25–80°K were the same for 4.7 and 6.2 eV electron energy, below and above the first optical exciton energy. For each system the evidence indicates that the low energy emitters can be excited directly. The small Stokes shifts approximate those expected for cubic relaxation. Population of lower energy emitting states from other states lying below the first optical exciton may be responsible for lifetimes anomalies.