THE FLUORESCENCE ACTION SPECTRA OF SOME SATURATED-HYDROCARBON LIQUIDS FOR EXCITATION-ENERGIES ABOVE AND BELOW THEIR IONIZATION THRESHOLDS

Fluorescence action spectra have been obtained for the neat liquids, cis-decalin, trans-decalin, bicyclohexyl, cyclohexane, methylcyclohexane, isobutylcyclohexane, 2,3,4-trimethylpentane, 2,3-dimethylbutane, 3-methylhexane, 3-methylpentane, n-decane, n-dodecane, and n-pentadecane at excitation energies, epsilon, ranging from their absorption onsets (at ca. 7 eV) to 10.3 eV. For all compounds, with the exception of cis-decalin, the fluorescence quantum yield is observed to monotonically decline with increasing epsilon, reaching a minimum value at an energy, epsilon(m) (a few tenths of an eV above the liquid phase ionization threshold, epsilon(l)) followed by a slow increase. In the case of cis-decalin, the fluorescence quantum yield remains constant over the entire range of excitation energies studied, permitting its use as a quantum counter replacing the standard sodium salicylate, at least over a spectral range from 185 to 120 nm. The recovery of the fluorescence quantum yield for epsilon > epsilon(m) is attributed to an increasing probability for electron ejection followed by e- + RH+ geminate recombination, to produce an excited state of RH with energy less than epsilon(l). From a simple analysis of the action spectrum, a lower bound estimate of the electron ejection probability, phi+/-, is obtained as a function of epsilon. In the case of cyclohexane, where phi+/- has been obtained by other techniques at epsilon congruent-to 10 eV, the lower bound estimate agrees with the experimental value. From this agreement, arguments are presented to make plausible the conjecture that in all these liquids, the initially produced e- + RH+ geminate ion pair first rapidly internally converts to an ion-pair state ca. 1.6 eV below epsilon(l), which then collapses to generate the fluorescent state of RH with an energy close to its absorption threshold value (ca. 7 eV).