Surface emission vibronic excited states

This subsection is devoted to the description of the upper excited states appearing in the excitation spectrum of the surface emission. The way this excitation is performed will be examined in Section III.B.3 below, in connection with the theory of Section III.B.l.b. The upper states examined are associated with the first singlet transition, b- and a-polarized, and are of two types purely electronic and vibronic states in an extended sense, as resulting from the coupling of electronic excitations to vibrations or to lattice phonons. 

Photophysics of Pyrene on Various Silica Gel Surfaces. Pyrene gives information about its environment via changes in its fluorescence fine structure (14). Typically, five vibronic bands are identified the ratio of the HI band at 392 nm to the I band at 372 nm, the 111/1 ratio, increases in noninteracting (nonpolar) media with a concomitant increase in fluorescence lifetime (15). The versatility of the pyrene probe arises from the forbiddenness of the So —> Si transition any intensity for the transition comes from vibronic coupling with higher excited states (10). Interactive or polar solvents, via their interaction with the arene, increase the intensity of the 0 —> 0 transition in both the absorption and the emission. 

Due to their direct relation to the spectral overlap integral, see Eq. (9), the emission and absorption spectra of the dye molecules are of interest in the context of EET processes. The simplest way to model excitation spectra employs the calculation of vertical energy separations, i.e., the separation of the Bom-Oppenheimer potential energy surfaces of the initial state and the final state at the equilibrium structure of the initial state. This energy separation is expected to coincide with the absorption maximum, as rationalized by the Franck-Condon principle (see for example [135]). This assumption is not always appropriate, rylene dyes being a prominent example. These dyes feature a strong 0-0 transition and a pronounced vibronic progression that is even visible in solution at room temperature (see for example [137]). A detailed simulation of the vibrational substructure of the absorption and emission bands is necessary to understand the details of the spectram. 

The most complex situation is sketched in Figure 7(b) for intermediate separation distances the chromophores excited either by plane waves from the dielectric side or by a surface plasmon mode excited from the prism side relaxes vibronically to the bottom of the excited state level of the chromophore but then can back-couple to the metal, thereby exciting a red-shifted siuface plasmon mode. This mode in turn can re-radiate via the prism (or the grating) and lea to an enhanced fluorescence emission. The optimum dye-metal separation for this decay mechanism has been reported to be in the range of d = 20 nm . 

Fig. 18.1 A dressed-state model that is used in the text to describe absorption, emission, and elastic (Rayleigh) and inelastic (Raman) light scattering. g) and. v> represent particular vibronic levels associated with the lower (1) and upper (2) electronic states, respectively. These are levels associated with the nuclear potential surfaces of electronic states 1 and 2 (schematically represented hy the parabolas). Rj are radiative continua— 1 -photon-dressed vibronic levels of the lower electronic states. The quasi-continuum L represents a nonradiative channel—the high-energy regime of the vibronic manifold of electronic state 1. Note that the molecular dipole operator /t couples ground (g) and excited (s) molecular states, but the ensuing process occurs between quasi-degenerate dressed states g,k and 5,0).

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