Transient absorption and luminescence in MgF2following electron pulse excitation

Abstract

We report time-resolved measurements of optical absorption and emission in pure Mg${\mathrm{F}}_2$ following excitation by an energetic electron pulse. At low temperature a single asymmetric absorption band having its peak at 275 nm is formed with very high efficiency; approximately one absorber of unit oscillator strength is formed for each electron-hole pair created in the crystal. The absorption band decays with time constants of 6.4 and 0.75 msec at 10 K. A single luminescence band with peak at 385 nm and decay time constants corresponding to those for the absorption is found at low temperature. Thermal quenching of both the absorption and luminescence sets in above 60 K. A second component of transient absorption with a spectrum more nearly resembling the $F$ band and with a decay time on the order of milliseconds appears at temperatures above about 170 K. This component grows stronger and decays faster as temperature is raised, but there is no corresponding component of luminescence. At 300 K approximately one absorber of unit oscillator strength is produced per 110 eV deposited by ionizing radiation, and the absorption decays with principal time constants of about 40 and 370 μsec. The low-temperature transient absorption and luminescence is attributed to self-trapped excitons. The high-temperature transient absorption is attributed to the formation of $F$ centers and $H$ centers which are initially separated by at least several lattice spacings, but which are unstable against nonradiative vacancy-interstitial recombination.