J. Phys. Chem. 1982, 86. 2572-2586
2572
FEATURE ARTICLE Picosecond Spectroscopy and Solvation Clusters. The Dynamics of Localizing Electrons in Polar Fluids 0. A. Kenney-Wallace Departments of Chemistry and Physics, University of Toronto, Toronto, Canada M5S 1A 1
and C. D. Jonah Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: December 29, 198 1)
New spectroscopic evidence concerning the dynamics of electron-induced solvation clusters in polar liquids is presented and integrated with previous picosecond data, in order to outline the roles molecular dynamics and structure can play both in initiating electron localization at subpicosecond times and in governing the solvation dynamics to form e; in the picosecond domain. Particular emphasis is placed on the picosecond time-resolved absorption spectroscopy of electrons in a wide range of alcohols and alcohol-alkane systems at 300 K as a framework for the cluster model of electron solvation. While the configurationally relaxed final quantum state of e; appears identical for e; generated by different techniques, it is possible that the time evolution of the solvation cluster and the dynamics of electron populations between localized and continuum states could be influenced by the initial state of the system. Selected examples are discussed for alcohols, amines, and water, and comparisons are made for picosecond observationsfrom different visible and IR spectroscopic techniques, NMR, and complementary nanosecond electron mobility data to demonstrate the overall consistency of a model in which only the dynamical, microscopic properties of the liquid determine these solvation events.
Introduction Those questions which probe the quintessence of the dynamical and physical properties of electrons in fluids are also at the core of contemporary discussions on the nature of electronic states of disordered systems in atomic and molecular liquids and solids. If one reviews*-3 the theories and experiments on the dynamics of spectroscopic transitions of excess electrons (e;) in crystals> glasses and amorphous solids>6and liquids- it becomes evident that, despite increasing degrees of disorder in the matrix, there are many common molecular features that underlie the diverse macroscopic observations. For example, stimulated emission from FII color centers and ultrafast saturable absorption and nonradiative recovery of e; in liquids are the complementary photophysical pathways of the phonon-broadened, bound-bound transitions believed to be responsible for the unusually broad, optical spectra of excess-electron states in polar media. Similarly, experimental studies of electron scattering and mobility in solids, liquids, and dense gases have proven to be a rich source of data for theories of electron transport.1°-13 Furthermore, in studying the dynamics of electron localization and solvation in liquids, one inevitably is'drawn to a study of the dynamical molecular structure of the host fluid in which the electron is to be so1vateda2 In this article, we briefly review our earlier work on the picosecond dynamics of electron solvation which prompted this latter observation, and then present in some detail many new spectroscopic observations, which not only affirm this viewpoint, but also suggest many new avenues of inquiry for future picosecond and subpicosecond studies *Alfred P. Sloan Research Fellow, 1977-81. 0022-3654/82/2086-2572$01.25/0
of the mechanisms of electron trapping in dense media. In systems where the electron is ultimately stabilized over a timescale comparable to that of the local molecular motion, the electron can be perceived as a microscopic probe of its environment, and the problem becomes one of describing (a) how its electronic motion couples into the fluctuating dynamical structure, and (b) how the molecular motion and intermolecular forces are perturbed by this sudden, transient, strong local field. The long-standing issue of whether preexisting solvation sites exist or the (1) (a) Colloque Weyl V, J.Phys. Chem., 84 (1980). (b) J. Jortner and A. Gaathon, Can. J. Chem., 55, la01 (1977). (2) (a) G. A. Kenney-Wallace in "Photoselective Chemistry",J. Jortner, Ed.,part 2, Adu. Chem. Phys., 47,535 (1981). (b) "Electron-Solvent and Anion-Solvent Interactions", L. Kevan and B. Webster, Ed.,Elsevier Scientific, New York, 1976. (c) D. C. Walker in ref la, p 1140. (3) B. Webster, Ann. Rep. Phys. Chem., 76, 287 (1979), Roy. Soc.
Chem. (4) R. T. Williams, J. N. Bradford, and W. L. Faust, Phys. Reu. B , 18, 7038 (1978). (5) L. Kevan, J.Phys. Chem., 84,1232 (1980);Acc. Chem. Res., 14,138 (1980). (6) (a) J. H. Baxendale and P. Wardman, J. Chem. Soc., Faraday Trans. 1, 69, 5&4 (1973); (b) Can. J. Chem., 55, 1996 (1977). (7) (a) G. A. Kenney-Wallace and C. D.Jonah, Chem. Phys. Lett., 39, 596 (1976); (b) ibid., 47, 362 (1977). (8) W. J. Chase and J. W. Hunt, J.Phys. Chem., 79,2835 (19751, and references therein. (9) P. M. Rentzepis, R. P. Jones, and J. Jortner, J. Chem. Phys., 59, 766 (1973). (10) (a) "Electron and Ion Swarms", L. G. Christophorou, Ed., Pergamon, New York, 1981; (b) W. F. Schmidt, Can. J . Chem., 55, 2197 (1977). (11) J. P. Dodelet and G . Freeman, Can. J. Chem., 55, 2264 (1977). (12) P. Krebs and M. Wantachik, J.Phys. Chem., 84,1155 (1980); L. G. Christophorou, J. G. Carter, and D. V. Maxey, J.Chem. Phys., 76,2653 (1982). (13) H. T. Davis and R. G. Brown, Adu. Chen. Phys., 31,329 (1975).
@ 1982 American Chemical Society
The Journal of Physical Chemisfry, Vol. 86,No. 14, 1982 2573
Feature Article
I
e-,
vocuum
I
Flgure 1. Schematic of quasi-free-bound electron transition. See text for discussion.
electron induces its own potential well can really only be answered from the perspective of these two points. In order to address these points, we present the results from a comprehensive picosecond spectroscopic study of electron localization and solvation in liquid alcohols and alcoholalkane systems. These alcohols display a wide range of structural, dynamical, and dielectric properties which permit a detailed investigation of the role of the liquid in determining the sequence of events that occur following the sudden injection of the electron into the fluid. The general conclusions that emerge form the theme of our opening remarks. The specific results are of significance not only for the development of a quantitative theory of electron solvation but also for a range of phenomena in the liquid and dense gas phase such as nucleation, solvation, electron-molecule scattering, dielectric breakdown in gases and liquids, and electron transfer processes in chemical and biophysical systems.
Mechanisms of Electron Injection Following electron injection into a dense polar medium the quasi-free electron (e,
