Sonoluminescence from nonaqueous liquids: emission from small molecules

Abstract

as a purely additive composite of single terminal oscillators, it is striking to see its good description of experimental values.Taking the empirical values kXY of the heavy-atom-containing oscillators C-Cl, C-Br, and C-I for aromatic and aliphatic solvent molecules and for the rest of oscillators kXY values resulting from the corresponding EXY and from the fitted curve of Figure 2, it is possible to calculate values using eq 1 and 8. The linear least-squares fit of a logarithmic plot of calculated vs experimental '02 lifetimes shown in Figure 4 results in slope 1.00, intercept 0.00, and a correlation coefficient of 0.99. The only value deviating considerably (by a factor of 3) is measured in D20. Thus the excellent quantitative description of the experimental *02 lifetimes by the collisional E -* V energy-transfer model is demonstrated.The fitted curve of Figure 2 and the above-mentioned empirical values of kXY can of course be used to calculate approximative O2 lifetimes for solvents not included in Figure 4. For that purpose it is only necessary to take from the fit In kXY values using literature data on EXY for the desired oscillators X-Y (e.g., £c=o = 1750 cm'1). Since values have been measured in highly purified solvents using low irradiation powers, the estimated lifetimes are only realistic in weakly deactivating solvents under corresponding conditions. ConclusionsThe radiationless deactivation of O2 by solvent molecules occurs under partial penetration of the electron clouds as a collisional E -V energy transfer from O2 to single oscillators X-Y (=terminal atom pairs) of the solvent molecule. Its rate constant kXY correlates exponentially with the energy of the maximum vibrational quantum, which X-Y can accept. Consequently, a mass dependence results and solvents with light-atom oscillators like C-H deactivate *02 much stronger than solvents with oscillators C-F. Since O2 deactivation includes an intercombinational transition, a strong internal heavy-atom effect takes place if a heavy atom is part of the deactivating oscillator. However, as heavy-atom substitution in X-Y results in a decrease of vibrational frequency of X-Y, the actual spin-orbit interaction dependent effect on & is weakened and operates only in the range of small rate constants kXY. That is the reason why a clear heavy-atom effect on is observed only in weakly deactivating solvents but not in hydrogen-containing solvents. The magnitude of the internal heavy-atom effect is described quantitatively by the theoretical model on spin-orbit interaction developed by McClure. The external heavy effect exerted from heavy-atomsubstituted solvents on radiationless deactivation of '02 is very weak.Acknowledgment I thank Prof. Dr. H.-D. Brauer for numerous fruitful discussions and generous support. I gratefully acknowledge financial assistance by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie.

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