Dipole moment resonance-induced shifts

Equation (1) is, strictly speaking, not suitable for optical fields, which are rapidly varying in time. Even for linear polarization, the oscillation of the induced dipole moment may be damped (by material resonances) and thereby phase-shifted with respect to the oscillation of the external electric field. The usual way of expressing this phase shift is by considering the relationship between the Fourier components of the induced effect (oscillation of the induced dipole) and the stimulus (the electric field), with the damping and phase shift conveniently expressed by treating the terms involved as complex. Thus, the linear polarizability can be written as... 

The large resonance-induced dipole shifts found in 1/0-6.3 are precisely those due to NBO donor-acceptor delocalizations studied previously (cf. Figs. 5.2 and 5.4). Although the quantity of charge transferred in such delocalizations appears modest, the transfer distance is appreciable, and such distance-dependence is directly sampled by the dipole operator (6.17) and integral (6.16). Thus, some of the most striking consequences of nonclassical resonance-type delocalizations are to be found in dipole moments and transition values (spectral intensities) that are often portrayed superficially as classical electrostatic in nature. 

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