The absorption of X rays in liquids provides low-energy electrons in the energy range 2 to 20 keV when synchrotron radiation is used as the X ray source. Such low-energy electrons have short ranges and produce a dense track of ionization where <tex-math>$dE/dx=10^{7}$</tex-math> to <tex-math>$10^{8}\ {\rm eV}/{\rm cm}$</tex-math>. Fluorescent molecules provide a sensitive probe of the early-time structure of such tracks. From the extent of quenching of excited states and the consequent decrease in fluorescence lifetimes, the concentration of free radicals in the track can be inferred. Experiments done with the hydrocarbons cis-decalin and dodecane demonstrate this effect. In both hydrocarbons, the lifetimes (τ) are significantly smaller with excitation by X rays than with UV radiation, and τ-1 increases linearly with dE/dx.

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