Zero-phonon peak

Nuclear absorption of incident X-rays (from the synchrotron beam) occurs elastically, provided their energy, y, coincides precisely with the energy of the nuclear transition, Eq, of the Mossbauer isotope (elastic or zero-phonon peak at = E m Fig. 9.34). Nuclear absorption may also proceed inelasticaUy, by creation or annihilation of a phonon. This process causes inelastic sidebands in the energy spectrum around the central elastic peak (Fig. 9.34) and is termed nuclear inelastic scattering (NIS). 

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

A host material is activated with a certain concentration of Ti + ions. The Huang-Rhys parameter for the absorption band of these ions is 5 = 3 and the electronic levels couple with phonons of 150 cm . (a) If the zero-phonon line is at 522 nm, display the 0 K absorption spectrum (optical density versus wavelength) for a sample with an optical density of 0.3 at this wavelength, (b) If this sample is illuminated with the 514 nm line of a 1 mW Ar+ CW laser, estimate the laser power after the beam has crossed the sample, (c) Determine the peak wavelength of the 0 K emission spectrum, (d) If the quantum efficiency is 0.8, determine the power emitted as spontaneons emission. 

Fig. 2. Temperature dependence of the homogeneous width (a) and the peak shift (b) of the 637 nm zero-phonon line in luminescence spectrum of N-V centers in diamond films points experiment the line theoretical approximations according to the laws y — y0 + aT3 + bT1 and 8 = fiT2 - vT4.

It is possible to model the vibronic bands in some detail. This has been done, for example, by Liu et al. (2004) forthe 6d-5f emission spectrum of Pa4+ in Cs2ZrCl6, which is analogous to the emission spectrum of Ce3+. However, most of the simulations discussed in this chapter approximate the vibronic band shape with Gaussian bands. The energy level calculations yield zero-phonon line positions, and Gaussian bands are superimposed on the zero-phonon fines in order to reproduce the observed spectra. Peaks of the Gaussian band are offset from the zero phonon fine by a constant. Peak offset and band widths, which are mostly host-dependent, may be determined from examination of the lowest 5d level of the Ce3+ spectrum, as they will not vary much for different ions in the same host. It is also common to make the standard... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

Fig. 12. Pait of the emission and excitation spectrum of Ba2CaU06 at 4.2 K. For the emission spectrum the excitation wavelength is 400 nm, for the excitation spectrum the 560 nm emission was recorded. Crosses are Xe-lamp peaks, a, b, and c are zero-phonon lines in emission, a in excitation. Note that a does not coincide with the emission zero-phonon lines (after Ref. 119)...

Figure 9 shows that the zero-phonon line peaking at 500.4 nm in the emission spectrum of Ba2MgW05-U is not single. Selective excitation experiments revealed that this splitting is due to the presence of several sUghtly different luminescent uranate centres. 

Since the zero-phonon lines of the emissions of several uranium-doped tungstates are broadened compared with the zero-phonon Une peaking at 500.4 nm in the emission spectrum of Ba2MgW06—U, we conclude that the broadening of the lines in the vibronic spectra might be due to the fact that the emission in these compounds originates from various slightly different uranate centres. 

For the general phosphor composition, (Srx Cay Baz )2P207 Sn, where x y z, there is a linear relation between peak emission energy (the zero-phonon line) and the cationic radius, namely-... 

Referred to the ground state 5f 1 FguC 19/2). Referred to 1 Tgg. Aproximate data extracted from prominent peaks in Fig. 1 of Ref [88] the real zero-phonon line could well correspond to weaker features peaking at 100-200 cm" lower energy. 

For this complex cation the zero-phonon emission is peaked at 519.9 nm. The ODMR data showed that the emission is due to an excited triplet state localized on the thpy anion [65]. This anion is positioned at the same crystallographic site as the energy trapping site of Rh(TTB)+ [64,65]. The lifetimes and emissive properties of the triplet sublevels of the Rh(TPB)+ species are very similar to... 

At low temperatures the width of the pure electronic transition is much narrower than that of the associated phonon transitions because electronic relaxation is much slower than vibronic relaxation. This is the reason why the zero-phonon line is so prominent in the spectrum. With increasing temperature the line shape will lose its characteristic features because (1) the Debye-Waller factor drops rapidly with increasing temperature, so that for many systems the intensity in the zero-phonon line is close to zero above 50 K and (2) the width y of the transition increases strongly with temperature. As a consequence of this thermal broadening, the peak... 

According to the vibronic interpretation of the absorption spectrum at 1.7 K of open R.vi ridis Res /ll/, the shoulder at 1015 nm reflects 0-0 transition, the peak at 1000 nm belongs to 0-1 transition, the shoulder at 985 nm to 0-2 transition, etc. Fig.4 shows that at 1.7 K in R.vi ridis Res with reduced HL the longest component in the absorption spectrum is located at 1011 nm and the shortest component in the fluorescence spectrum is at 1014 nm. It implies the Stokes shift of 30 cm"l for 0-0 transition. This is consistent with the phonon frequency of 30 cm and Pekar-Huang-Rhys factor Sail found from hole-burning experiments. The latter show that in the presence of reduced HL the narrow hole (zero-phonon line, ZPL) is bleached in the P band at wavelength of excitation (1014 nm) at 1.7 K (Fig.3 /lO/). 

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