Calculated photon echo signals for the aqueous solvated electron. The origin of ultrafast electronic dephasing

Photon echo and resonant transient grating signals are calculated for the hydrated electron based on the results of quantum molecular dynamics simulations. The echo decays in < 15 fs. Comparison to echo calculations for organic probes and different solvent response functions reveals the origin of this ultrafast dephasing. For the strongly coupled hydrated electron, small changes in solvent configuration lead to large fluctuations in the transition frequency causing a rapid loss of electronic coherence, even in the absence of fast inertial solvent motions. In such strongly coupled systems where the echo decay is insensitive to solvation dynamics, the resonant transient grating experiment can provide a measure of the solvent response. Photon echo measurements of organic dye molecules in liquids have been used to study both slow (static) and fast (dynamic) solvent-solute interactions [l-5]. In the limit of large inhomogeneous broadening ’ the photon echo technique eliminates contributions from different ‘static’ solvent configurations, allowing the characterization of fast solvent fluctuations and the degree of homogeneous line broadening in the linear absorption spectrum [ 7 1. A theoretical formalism describing a variety of nonlinear spectroscopies including the photon echo has been developed by Loring, Yan, and Mukamel [ S12 1. The time dependence of the fluctuations in the transition frequency of the chromophore arising from solvent motion, Ao( t), is characterized by the solvent correlation function, (1) In the limit of linear response the correlation function M( t ) is equivalent to the nonequilibrium solvent response function, C(t), which can be determined experimentally by time-resolved fluorescence measurements [ 13,14 1, c(t)= w(t) --o(m) o(O)-w(m). ’ Cho and Fleming have shown that when the broadening is not dominated by inhomogeneity a fifth-order echo experiment is required to disentangle the homogeneous dynamics [ 6 ] Here w(t) is a characteristic frequency of the timeresolved emission spectrum. Shemetulskis and Loring [ 15 ] have calculated two-pulse photon echo signals from both nonequilibrium and equilibrium molecular dynamic simulations and found the results to be identical. This demonstrates that C(t) (the nonequilibrium response) describes the solvent fluctuations for both the time period when the system evolves during an electronic coherence (the time between the first and second pulse) and during a population state 0009-2614/94/$07.00