Phonon side bands

Figure 5.9 Two examples of dynamic induced band-shape effects, (a) Weak coupling an absorption single line of the Yb + ion in LiNbOs (denoted by an arrow) is accompanied by the appearance of phonon side bands (reproduced with permission from Montoya et al., 2001) (b) Strong coupling the broadband luminescence of the Cr + ion in LiNbOs (reproduced with permission from Camarillo et al., 1992).

Raman spectroscopy or far-IR spectroscopy can determine the fundamental vibration frequencies of the host. However, these methods give information about the whole glass matrix and do not account for the local nature of electron-phonon interactions. So, the fundamental frequencies are preferably determined by recording the phonon-side bands (PSB) of rare-earth transitions or by studying the temperature-dependence of multiphonon relaxations [42,43]. The phonon energies determined by PSB spectroscopy, which is the most direct method, are usually lower (400 cm-1 in ZBLAN) than those measured by other methods ( 500 cm-1) suggesting that weak M—F bonds are coupled to the rare-earth [43]. 

To obtain information on the role of dynamics of molecular motions in the reactive systems, the approach of phonon spectroscopy is used. Phonons are low-frequency cooperative lattice vibrations of a solid and, therefore, probe the lattice interactions and dynamics directly. Phonons can be observed as optical transitions in the Raman spectra and in the electronic spectra (in the latter as a phonon side band). Some information regarding averaged librational and translational phonon motions can also be obtained from the rigid-motion analysis of the thermal parameters of x-ray diffraction studies. 

The information on the formation of a polaron or an excimer is derived from the low temperature electronic absorption and emission spectra of the reactive crystals. The strong electron-phonon coupling in the reactive state manifests itself as a very strong phonon-side band In the liquid helium temperature spectra. 

There are three main EA spectral features in the energy range of band I a derivative-like feature with zero-crossing at 2.29 eV, followed by vibrational features, and an induced absorption band between 2.9 and 3.2 eV. The features below 2.5 eV are the results of a redshifted 1B exciton energy, and its phonon side-bands (Stark shift). These features are more easily observed in EA than in... 

In the case of phonon-assisted energy transfer the basic equation of the resonance transfer of Dexter (Eq. (14)) applies, however there is a need of modification. The interaction Hamiltonian must contain an electron phonon part. The initial and final states must include the initial and final phonon states which will differ by a number of phonons whose total energy is AE. The line-shape factors must include the phonon side-bands. If one phonon of energy hw = AE is created in the process of energy transfer the transfer rate is... 

A. Electron-Phonon Interaction Parameterization Scheme. In observing the fluorescence decay rate from a given J-manifold, it is generally found that the decay rate is independent of both the crystal-field level used to excite the system and the level used to monitor the fluorescence decay. This observation indicates that the crystal-field levels within a manifold attain thermal equilibrium within a time short compared to the fluorescence decay time. To obtain this equilibrium, the electronic states must interact with the host lattice which induces transitions between the various crystal-field levels. The interaction responsible for such transitions is the electron-phonon interaction. This interaction produces phonon-induced electric-dipole transitions, phonon side-band structure, and temperature-dependent line widths and fluorescence decay rates. It is also responsible for non-resonant, or more specifically, phonon-assisted energy transfer between both similar and different ions. Studies of these and other dynamic processes have been the focus of most of the spectroscopic studies of the transition metal and lanthanide ions over the past decade. An introduction to the lanthanide work is given by Hiifner (39). 

The absorption spectrum of atoms and molecules in low temperature solids is composed of a sharp zero-phonon line and a phonon side band (Table 2.12 ). The phonon side band corresponds to light absorption accompanied by phonon absorption or emission. The absorption shown in bold in Table 2.12 is a zero-phonon line. The sum of the absorption, drawn in a finer line, yields the phonon side band. The phonon side band appears on the higher energy side of the zero-phonon line at low temperatures. A measure of the interaction between guest molecule and host matrix is given by the Debye-Waller factor, DW(T), defined as a function of temperature, T, in Eq. (2.3), using the areas of the zero-phonon line, S0(T), and the phonon side band, SP(T). 

An experimental PHB hole profile generally consists of three parts a sharp zero-phonon hole, a small, broad hole of the phonon side band on the higher energy side, and a broad hole, called pseudo-phonon side hole, on the lower energy side of the laser frequency (Table 2.12 ). The pseudo-phonon side hole results from the overlap of the zero-phonon holes which exhibit phonon side bands at the laser frequency. 

Table 2.12 Zero-Phonon Line and Phonon Side Band...

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

Photoluminescence spectra of copolymer of pyrrole and bithiophene films whose thiophene content is higher than 50% consist of three peaks around 2.0, 1.8, 1.7eV corresponding to phonon side bands, at lOK. These peaks have been considered to be radiative relaxation from self-trapped exciton levels. The peak at the highest energy reflects band gap. 

Estimation of Low-Energy Excitation Modes. An absorption line profile of each dye molecule at low temperature consists of two components a sharp zero-phonon line and a broad phonon side band. Tbe energy difference between the zero-phonon line and the phonon side band coincides with the low-energy excitation mode or phonon frequency of the matrix when multi-phonon processes can be ignored. 

Figure 11 shows a saturated hole proffle burned to estimate the low-energy excitation mode, E of TPP/PnPMA. A deep zero-phonon hole is created at 644.8 nm. In addition, there are two broad holes at bodi sides of the zero-phonon hole. The broad hole at the shorter wavelength is called a phonon side hole and the one at the longer wavelength is called a pseudo-phonon side hole . The phonon side hole consists of a phonon side band of the zero-phonon hole, and the p udo-phonon side hole is made of the zero-phonon line of the reacted molecules which have been excited via phonon side band. 

Figure 1. Zero-phonon line (ZPL) and phonon-side band (PSB) of the electronic excitation of a dopant molecule in a solid host at low temperatures.

Although the involvement of a phonon side band in the laser effect is usually connected with the transition-metal tunable laser (Kaminskii 1981) because of the stronger S value... 

At room temperature, non-photochemical spectral holes usually are filled in by flucmations of the surroundings on the picosecond time scale. This process, termed spectral dijfusion, can be studied by picosecond pump-probe techniques. At temperatures below 4 K, non-photochemical spectral holes can persist almost indefinitely and can be measured with a conventional spectrophotometer. The shape of the hole depends on the lifetime of the excited state and the coupling of the electronic excitation to vibrational modes of the solvent, both of which depend in turn on the excitation wavelength. Excitation on the far-red edge of the absorption band populates mainly the lowest vibrational level of the excited state, which has a relatively long lifetime, and the resulting zero-phonon hole is correspondingly sharp (Fig. 4.22A). The zero-phonon hole typically is accompanied by one or more phonon side bands that reflect vibrational excitation of the solvent in concert with electronic excitation of the chromophore. The side bands are broader than the zero-... 

C Trallero-Giner, M Cardona, F likawa. Phonon side bands in the optical-emission of zincblende-type semiconductors. Phys Rev B 48 5187-5196, 1993. 

The fine structure observed in both the solid and solution phases is ascribed to phonon side bands," i.e., to the "vibronic structure." The electronic spectra of several alkyl-substituted PTh s have been studied [191,192], showing that the separation of the phonon side... 

Zero-Phonon Lines (ZPL) and Phonon Side-Bands (PSB). . . . 133... 

Consider an optical transition of a dye probe mole-cule doped a low concentration into a perfect crystal lattice (Figure lA). Since all the probes have the same local environment, their absorption frequencies coincide and the line shape of the transition considered is representative of the line shape of a single probe molecule. Excitation of the probe molecule is accompanied by a charge redistribution in the excited state. This leads to a different equilibrium configuration of the lattice molecules in the excited state. As a consequence, there is a certain probability that the optical excitation will be accompanied by excitation of lattice motions that give rise to so-called phonon side bands. The intensity distribution is determined by the Franck-Condon principle. The relative intensity of the transition with no lattice phonon excitation, the so-called zero-phonon line, is given by the Debye-Waller factor a ... 

In organic crystals, the inhomogenous broadening is of the order of a wavenumber in glasses it is of the order of several hundred wavenumbers. In this latter case, even the phonon side bands are largely buried beneath the inhomogeneous envelope (Figure 2B). 

The zero phonon lines of transition metal ions behave similarly to those of lanthanides but phonon side bands are more evident. Such is the extent of broadening due to electron-phonon coupling that the influence of environment is proportionally much smaller. It is unusual to see disorder-induced broad ening of phonon-assisted bands by more than a factor of two. 

Another choice of the interpretation of this IR band could be a phonon-side band of the trapped charge based on a polaron model (16, 17). However, we suppose that the ISCR model is more appropriate to explain our present results in which the spectrum indicates a variety of transition energies including rela-... 

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