Phonon modes excitation

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

Summary. Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation. 

As the lattice interacts with light only through electrons, both DECP and ISRS should rely on the electron-phonon coupling in the material. Distinction between the two models lies solely in the nature of the electronic transition. In this context, Merlin and coworkers proposed DECP to be a resonant case of ISRS with the excited state having an infinitely long lifetime [26,28]. This original resonant ISRS model failed to explain different initial phases for different coherent phonon modes in the same crystal [21,25]. Recently, the model was modified to include finite electronic lifetime [29] to have more flexibility to reproduce the experimental observations. 

Despite the difficulty cited, the study of the vibrational spectrum of a liquid is useful to the extent that it is possible to separate intramolecular and inter-molecular modes of motion. It is now well established that the presence of disorder in a system can lead to localization of vibrational modes 28-34>, and that this localization is more pronounced the higher the vibrational frequency. It is also well established that there are low frequency coherent (phonon-like) excitations in a disordered material 35,36) These excitations are, however, heavily damped by virtue of the structural irregularities and the coupling between single molecule diffusive motion and collective motion of groups of atoms. 

By scattering within molecular solids and at their surfaces, LEE can excite with considerable cross sections not only phonon modes of the lattice [35,36,83,84,87,90,98,99], but also individual vibrational levels of the molecular constituents [36,90,98-119] of the solid. These modes can be excited either by nonresonant or by resonant scattering prevailing at specific energies, but as will be seen, resonances can enhance this energy-loss process by orders of magnitude. We provide in the next two subsections specific examples of vibrational excitation induced by LEE in molecular solid films. The HREEL spectra of solid N2 illustrate well the enhancement of vibrational excitation due to a shape resonance. The other example with solid O2 and 02-doped Ar further shows the effect of the density of states on vibrational excitation. 

Ions in the lattice of a solid can also partake in a collective oscillation which, when quantized, is called a phonon. Again, as with plasmons, the presence of a boundary can modify the characteristics of such lattice vibrations. Thus, the infrared surface modes that we discussed previously are sometimes called surface phonons. Such surface phonons in ionic crystals have been clearly discussed in a landmark paper by Ruppin and Englman (1970), who distinguish between polariton and pure phonon modes. In the classical language of Chapter 4 a polariton mode is merely a normal mode where no restriction is made on the size of the sphere pure phonon modes come about when the sphere is sufficiently small that retardation effects can be neglected. In the language of elementary excitations a polariton is a kind of hybrid excitation that exhibits mixed photon and phonon behavior. 

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

The Hamiltonian Eq. (7) provides the basis for the quantum dynamical treatment to be detailed in the following sections, typically involving a parametrization for 20-30 phonon modes. Eq. (7) is formally equivalent to a class of linear vibronic coupling (LVC) Hamiltonians which have been used for the description of excited-state dynamics in molecular systems [66] as well as the Jahn-Teller effect in solid-state physics. In the following, we will elaborate on the general properties of the Hamiltonian Eq. (7) and on quantum dynamical calculations based on this Hamiltonian. 

The LVC model further allows one to introduce coordinate transformations by which a set of relevant effective, or collective modes are extracted that act as generalized reaction coordinates for the dynamics. As shown in Refs. [54, 55,72], neg = nei(nei + l)/2 such coordinates can be defined for an electronic nei-state system, in such a way that the short time dynamics is completely described in terms of these effective coordinates. Thus, three effective modes are introduced for an electronic two-level system, six effective modes for a three-level system etc., for an arbitrary number of phonon modes that couple to the electronic subsystem according to the LVC Hamiltonian Eq. (7). In order to capture the dynamics on longer time scales, chains of such effective modes can be introduced [50,51,73]. These transformations, which are briefly summarized below, will be shown to yield a unique perspective on the excited-state dynamics of the extended systems under study. 

The nanostructure dependence of the excited state dynamics can be derived from the interaction of the electronic excitation with the surrounding environment and its phonon modes. A variety of nanophenomena, particularly, the lifetime of excited states of lanthanide ions in nanostructures may exhibit strong size-dependence (Prasad, 2004). Energy transfer rate and luminescence efficiency in lanthanide activated phosphors are also sensitive to particle size and surrounding environment. 

Although no quantum confinement should occur in the electronic energy level structure of lanthanides in nanoparticles because of the localized 4f electronic states, the optical spectrum and luminescence dynamics of an impurity ion in dielectric nanoparticles can be significantly modified through electron-phonon interaction. Confinement effects on electron-phonon interaction are primarily due to the effect that the phonon density of states (PDOS) in a nanocrystal is discrete and therefore the low-energy acoustic phonon modes are cut off. As a consequence of the PDOS modification, luminescence dynamics of optical centers in nanoparticles, particularly, the nonradiative relaxation of ions from the electronically excited states, are expected to behave differently from that in bulk materials. 

Another effects of the interaction with the surrounding disordered matrix is the increase in the one-phonon direct relaxation of Ho3+ in oxyfluoride glass ceramics due to coupling to the higher density of low-frequency phonon modes in the glass compared with the crystal (Meltzer et al., 2002). The direct relaxation time between the two lowest 5Fs sublevels (a gap of 14.5 cm-1) of Ho3+ in nanocrystals of different sizes embedded in oxyfluoride glass ceramics was measured (see fig. 14), under excitation of the second low-... 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

Fig. 3.7. Polarized micro-Raman spectra of a (0001) ZnO bulk sample (a) and a (0001) ZnO thin film (d 1,970 nm) on (0001) sapphire (b). The vertical dotted and dashed lines mark ZnO and sapphire (S) phonon modes, respectively. MP denotes modes due to multi-phonon scattering processes in ZnO. Excitation with Ar+-laser line A = 514.5 nm and laser power P < 40 mW. Reprinted with permission from [38]...

ZnO bulk sample [43], Spectra are shifted for clarity, (b) Phonon-mode frequencies vs. temperature as determined from the Raman data in Fig. 3.11a. The solid lines are model approximations according to (3.22). Excitation with Nd YAG-laser line A = 532 nm and laser power P 60 mW... 

Thus, both the resonance mode at k = Q, and the excitations at incommensurate Q 2q, are double-svivon excitations, around its energy minimum at ko, shown in Fig. 3. These are excitations towards the destruction of the stripelike inhomogeneities their width determines the speed of the inhomogeneities dynamics, and they exist for stoichiometries where SC exists [21]. Because of the lattice dressing of svivons, these double-svivon excitations are expected to be lattice-dressed spin excitations, and indeed, Cu-0 optical phonon modes have been found to be involved in such excitations [25],... 

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