Transitions excited state absorption

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

Figure 9.17 Simplified energy level diagram for Er33 showing the important up-conversion transitions (a) GSA and relaxation, (b) ESA and relaxation, (c) ESA and relaxation, and (d) emission of green and red. [GSA = ground-state absorption ESA = excited-state absorption. Spectroscopist s energy units (cm-1) are converted thus 25,000 cm-1 = 3.1 eV = 5.0 x 10-19J.]...

The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

The lowest-energy-resolved features in the absorption spectrum of M0S2-type semiconductors are the A and B excitons, shown in Fig. 3 for the case of M0S2 [30]. The dissociation limit of peaks built off of the A exciton correspond to the direct band edge. Calculations indicate that these transitions are at K and that the A and B excitons correspond to K4 K5 and K1 K5, respectively [25,26]. The lowest-energy direct-transition excited states have considerably different orbital character than the band-edge state. K1 and K4 correspond to 84% and 82% dyy, respectively corresponds to 77%... 

Aromatic polycarboxylates easily form 2D or 3D networks, for instance [Nd2(122)3(dmf)4]-H2O which present a 2D structure in which the 1,4-naphthalenedicarboxylate anions link Ndm ions of two adjacent double chains keeping them at a short distance of about 4.1 A (J. Yang et al., 2006). This allows up-conversion to take place, albeit with very low efficiency a blue emission is seen at 449.5 nm upon excitation at 580 nm (corresponding to the 4Gs/2 magnetic properties an energy-transfer up-conversion mechanism involving no excited state absorption is more likely. 

The population and reorientational dynamics are not indicated in the figure, but may be also derived from the pump-probe measurements. The population lifetime Ti in the first excited level can be inferred from the excited state absorption monitoring (lie v = I v = 2 transition. The molecular reorientation becomes experimentally accessible, introducing polarization resolution in the probing step. For known structural and population dynamics the temporal evolution of the width of the observed spectral hole also provides information on the dephasing time T2 of the vibrational transition (63). 

The stimulated-emission spectrum is calculated by the Fuchtbauer-Ladenburg equation. The excited-state absorption bands in the 1.3 jim region are obtained by subtracting the calculated stimulated emission profile from the recorded spectrum. The short-wavelength component, around 1230 nm, corresponds to the 4F3/2 -> 4Gg/2 transition. Despite its intensity, this band is not broad enough to alter the 4F3/2 4In/2 stimulated emission. Conversely, the long-wavelength... 

The excited state absorption starts on the lower 5d level and ends on a level in the conduction band. Therefore this two photon transition results in photoionization of the Ce ion. The conduction band minimum lies only 10,000 cm above the lowest (relaxed) 5d level. The higher energy part of the ground state absorption spectrum (X < 300 nm in Fig. 18) is due to direct (one-photon) photoionization of the Ce " ion. The excited state absorption of Ce is strongly host dependent. InCaF2 Ce andLiYF4 Ce it is situated in the near ultraviolet (46, 47). 

MgAl204 has been reported by Petermann et al. (50). Several types of excited state absorption transitions have been observed and can be understood in terms of a configurational coordinate mode. As a consequence the Mn ion cannot lase in the spinel lattice. 

X = 363 nm modulated at a frequency of 200 Hz with an optical blade chopper. The PA spectrum is dominated by a sharp transition at 1.43 eV which has been identified as excited-state absorption of triplet excitons.10 The PA spectrum of a doped PFO film containing 8 wt% PtOEP is shown in Fig. 10.41b. The PFO triplet peak at 1.43 eV is the strongest feature in the PA spectrum of the blend, although there is an additional PA band between 1.5 and 1.7 eV. This additional PA band is correlated with PtOEP, having the same relative weight to the in-phase and quadrature components as the phosphorescence. 

Ru(dmb)j(decb) ] , [Ru(decb) (dmb) ], and (Ru(decb) (dmb -4,4 -dimethyl-2,2 -pyridine, decb 4,4 -bis(ethylcarboxy)-2,2 -bipyridine). Emission properties of [Ru(bpy) nH O and the circular dichroism spectrum of the excited state absorption of (a)-[Ru(bpy)3] have both been reported. The luminescence of Ru(II) and Os(II) polypyridyls has been measured in MeCN as a function of pressure and temperature, and it has been found that at high pressures the radiative and non-radiative transition rates between the luminescent CT level and the ground state are generally increased by 5-10%. Temperature dependence luminescence studies on [Ru(bpy) L] ... 

Cascade lasing requires that the host be transparent and there be no deleterious ground or excited-state absorptions at either laser wavelength. Transitions L] and l 2 may be associated with two different ions. In this case the energy in the terminal level 3 of the first lasing ions must be quickly and efficiently transferred to the upper laser level 3 of the second ion. 

In Y3A1 0]2 the lowest 5d band is located below So and fluoresces with high quantum efficiency at temperatures 300 K (49). Several intense 4f5d- 4f2 transitions in the nearultraviolet could provide tunable laser action barring strong excited-state absorption. Tunable laser action in the 215-260 nm range in LiYF4 may also be possible (50). 

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