J . Phys. Chem. 1992, 96, 2795-2800 seen between calculated and measured film thicknesses. Although our results, analyzed in accordance with eq 2d, are consistent with an exponential distance dependence of the electron-transfer rate with a very low attenuation coefficient, 0, for various arbitrary models containing differentially active components of the excited thiacyanine dye, such fits do not prove the mechanism. Thus, in the absence of more extensive fluorescence lifetime measurements we cannot determine the actual kinetics in this system. A more complete investigation of the timeresolved fluorescence kinetics is currently underway. This should establish whether there is more than one electron-transfer component or even inactive excited thiacyanine dye. In addition, a search for electron-transfer intermediates is also planned using transient absorption spectroscopy, both to codm the chargetransfer nature of the process and to complement the fluorescence kinetics. Even in the absence of a specific kinetic mechanism, we can say that the electron transfer has a lower attenuation with distance for stilbene-containing spacer molecules than for the saturated fatty acids. When treated by the exponential attenuation eq 2d, the fatty acid spacer case leads to a coefficient (3 consistent with previous LB studies if it is assumed that about half the fluorescing excited thiacyanine dye does not participate in the electron-transfer process. Using the same assumptions for the tram-stilbene series gives an attenuation coefficent about 50% smaller. The alternative approach, assuming two active components, gives even lower 0 values for both “spacers” but here again 0 for the tram-stilbenes is lower by a similar amount. Thus, the inclusion of a substantial aromatic functionality in the spacer decreases the attenuation of electron-transfer rate with distance significantly. Within the context of a tunneling model in which the spacer is viewed as a continuum barrier this would imply a lowering of the barrier potential with 0 decreasing as the
2795
square root of the barrier height. On the other hand, the smaller attenuation for the stilbenes is also consistent with a superexchange process, whereby the overlap of donor and acceptor wave functions is enhanced by mixing in energy states of the medium, although the electron does not directly reside in these medium state^.^^^^^ In its simplest form, the superexchange mechanism predicts that /3 will decrease logarithmically with a decrease in the energy difference between the nearest states of the medium and the donor or acceptor energy levels.38 Thus both mechanisms predict a decrease in /3 as a result of decreasing the difference in energy levels between the spacer and the electron donor or acceptor. Qualitatively it is expected that a stilbene-containing spacer will have its energy levels closer to those of the thiacyanine dye than a saturated hydrocarbon spacer will have.39 The application of these different electron-transfer mechanisms in LB films has been discussed in the literature for fatty acid spacers.40 But in the absence of more quantitative data on the energy levels of the present materials in the present system, particularly for the stilbenes, a more extensive treatment is not merited. Acknowledgment. The stilbenecarboxylic acids were synthesized by Cristina Geiger. Financial support was provided by the National Science Foundation in the form of a Science and Technology Center grant (CHE-8810024). We acknowledge the suggestions of two referees in the kinetic treatment of the quenching data in terms of multicomponent electron-transfer kinetics. (38) McConnell, H. J . Chem. Phys. 1961, 35, 508. (39) Lewis, F. D. Ado. Phorochem. 1986, 13, 165. (40) De Schryver, F. C.; Van der Auweraer, M.; Verschuere, B.; Willig, F. In Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Baston, 1987; p 385.
Probing Femtosecond Solvation Dynamics at the Condensed Phase-Vacuum Interface Mlungisi Kwini, Martin J. Iedema, James P. Cowin,* Pacific Northwest Laboratory,‘ Box 999, Richland, Washington 99352
and Terry L.Gilton Micron Technology, Inc., Boise, Idaho 83706 (Received: November 4, 1991; In Final Form: January 22, 1992)
The photoelectron-induced dissociation of CH3C1adsorbed on top of a multilayer deposit ejects methyl radicals into the gas phase. The kinetic energies of these methyls vary with the identity of the underlying multilayer (H20, hexane, CH3Cl!, from 0.44 to 0.7 eV at the peak, and are much higher than seen for the gas-phase dissociative electron attachment to this molecule. The additional energy is understood in terms of the effects of the “prompt” solvation (
