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, ε, 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 ε, reaching a minimum value at an energy, εm (a few tenths of an eV above the liquid phase ionization threshold, εl) 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 ε>εm 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 εl. From a simple analysis of the action spectrum, a lower bound estimate of the electron ejection probability, φ±, is obtained as a function of ε. In the case of cyclohexane, where φ± has been obtained by other techniques at ε congruent 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