Dipolar resonant excitation

In Fig. 3.3 the extinction efficiency is reported for a prolate spheroid excited with an electric field E polarized at tt/4 respect to the y-axis. As we can observe, the dipolar resonance mode splits into one mode at small wavelengths (the transverse mode) and one mode at longer wavelengths (the longitudinal mode) this in agreement with Eq. (3.7) which for axially symmetric ellipsoidal particles provides two resonances. 

Figure 6.6 Near-field photopolymerization based on the resonant excitation of the dipolar LSPR mode of silver nanoparticles, (a) Topographic AFM image of silver nanoparticles before photoexcitation, (b) Close-up image of nanoparticle selected in panel a. (c) Close-up topographic image of the same silver nanoparticle after photopolymerization. (d) Differential image of panels c and d. (e) Spatial map of near-field intensity calculated around a 60-nm silver nanoparticle using the FDTD method. Reprinted with permission from Ref [56]. Copyright (2010) American Chemical Society.

Multiple-pulse sequences based on off-resonance excitation - where the effective interaction frame rotates at the magic angle and dipolar interactions can be made to vanish. Imaging in this interaction frame requires a combination of matched RF and d.c. gradients. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

The first two terms in (5) are called D-terms or dipolar terms, which are nonzero only if Ape =/= 0. The two-photon resonance denominator, ( leg — 2hco), indicates that an electron is excited into the lower excited state e. If we consider a near resonance condition hco = %imaginary part of the D-terms can be written in SI units as ... 

The results from EFISHG or HRS experiments can be corrected for resonance enhancement by using a simple theoretical two-state model (TSM, Equations (3) and (4)),35,36 which is reasonably valid for dipolar molecules in which f3 is primarily associated with a single ICT excitation ... 

The simplest description of compoimds of this type is obtained limiting the siun-over-state equations to terms depending only on the properties of the groimd state g and the first excited state e (two-state model) [93]. This is analogous to the two-level model introduced to describe other nonlinear optical properties, for example the nonlinear polarizability pS - co coi,co2) [ 104]. In the case of 2PA, this two-state, or dipolar, contribution to the cross section is, on resonance ... 

To help us understand how a polar solvent can help to stabilize an excited state, we will consider the ti tt transition of an alkene. We can represent the ground state and excited state species in a simple way with resonance structures. It is important to realize, though, that the dipolar structures in Scheme 2.1 are not the excited state but they do make a (minor) contribution to the excited state structure. 

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