Spectral broadening

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

In the conclusion of the present chapter we show how comparison of NMR and Raman scattering data allows one to test formulae (3.23) and (3.24) and extract information about the relative effectiveness of dephasing and rotational relaxation. In particular, spectral broadening in nitrogen caused by dephasing is so small that it may be ignored in a relatively rarefied gas when spectrum collapse proceeds. This is just what we are going to do in the next sections devoted to the impact theory of the isotropic Raman spectrum transformation. 

In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kVcm . Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots. 

DHT may occur over different tryptophan forms in proteins as they quite often have inhomogeneously broadened electronic spectra [31]. A very interesting case of DHT is described between two indole rings in bichromophoric solutes tryptophan dipeptide [32]. Such directed transport allows to correctly interpret spectral properties of dipeptide and other multichromophoric solutes. The theory of inductive-RET in solutions with inhomogeneous spectral broadening is given in Ref. [33]. In more detail, DHT mechanism will be explained in Sect. 2.2 (vide infra). 

The choice of material for SG is very important and was first reported a decade ago by Brodeur and Chin [21, 75] using a femtosecond laser in condensed media. They observed that spectral broadening of the white light depends on the band-gap of the irradiated material [75]. Furthermore, they found the existence of a band gap threshold, 4.7eV, below which a medium... 

At lower power values, there is relatively less spectral broadening, and it appears to be mostly symmetrical around the incident wavelength of 801 nm,... 

Fig. 9.4. Inhomogeneous spectral broadening responsible for directed energy transfer. The spectral overlap between the emission spectrum Eo of an excited species (whose absorption spectrum is Ao) and the absorption spectrum A2 of a solvate absorbing at higher...

Figure B9.3.1 shows the parallelism between the increase in emission spectrum displacement and fluorescence anisotropy observed for the red-edge of most vibronic bands and especially for the 0-0 one. It can be interpreted in terms of inhomogeous spectral broadening due to solvation heterogeneity. The decrease in energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower energy chromophores, as in photosynthetic antennae.

Another explanation for their resonance Raman results could be a change in the zwitterionic nature of the merocyanine isomers in the different solvents which may result in changes in the Raman transition probabilities, or the spectral changes could be due to solvent shifts of the absorption spectrum, resulting in a change in the relative contribution of the different vibrational modes to each resonance Raman spectrum. We note that in the same article, the authors report the transient absorption spectra of the merocyanine forms, which clearly show that the BIPS spectrum in cyclohexane has more discrete vibrational modes than are observed in the polar solvents, which show more spectral broadening. Al-... 

Tamai and Masuhara [26] also worked on NOSH, but in 1-butanol. They could examine femtosecond dynamics for the C—O bond breaking and formation of a primary photo-product X, which formed within 1 psec and had a broad absorption with peaks at 450 and 700 nm. The spectrum of X then evolved, forming a broad merocyanine-type spectrum, which itself evolved with time to form the usual merocyanine spectrum in that solvent after less than 400 psec. The spectral broadening was said to be either due to the formation of a vibrationally hot ground state or to an equilibration between isomeric forms because the spectrum that formed at early times was similar to the spectrum usually obtained in cyclohexane. Tamai s spectra are shown in Fig. 3. 

Nuclear Overhauser enhancement spectroscopy ( H- H NOESY and NOE) experiments show only the TTC isomer in equilibrium with the TTT isomer for 6,8-dinitro-BIPS [36,55] and 6-nitro,8-bromo-BIPS. The TTC form dominates the equilibrium. Spectral broadening for several proton resonances in the spectra of 6,8-dinitro-BIPS and 6-nitro,8-bromo-BIPS indicate a rapid exchange between these two isomeric forms. The activation energy for this isomerisation is reported to be 43.6 kJ mol and the energy difference between the the TTC and TTT forms is 4.6 kJ mol [55]. 

Attempts to study or manipulate chemical processes in the condensed phases are inevitably complicated by the spectral broadening induced by the sur-... 

The definition of the spectral density [Eq. (15)] allows to connect the various correlation functions relevant to spectral broadening and spectral diffusion. For example, the fluorescence Stokes shift function S(t) can be written as... 

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