Raman scattering and the low-frequency vibrational spectrum of glasses

We present a theory of low-frequency Raman scattering in glasses, based on the concept that light couples to the elastic strains via spatially fluctuating elasto-optic (Pockels) constants. We show that the Raman intensity is not proportional to the vibrational density of states (as was widely believed), but to a convolution of Pockels constant correlation functions with the dynamic strain susceptibilities of the glass. Using the dynamic susceptibilities of a system with fluctuating elastic constants we are able for the first time to describe the Raman intensity and the anomalous vibration spectrum of a glass on the same footing. Good agreement between the theory and experiment for the Raman spectrum, the density of states, and the specific heat is demonstrated at the example of glassy As(2)S(3).     

Intensities of DNA ion-phosphate modes in the low-frequency Raman spectra

The Raman intensities of counterion vibrations with respect to the phosphate groups of the double-helix backbone (ion-phosphate modes) in the low-frequency spectra (< 200 cm^-1) of B -DNA with different alkali metal counterions have been calculated using the model for DNA conformational vibrations and the valence-optic approach. The results have showed that the spectra of DNA with heavy counterions ( Rb⁺ and Cs⁺ differ from the spectra of DNA with light counterions ( Na⁺ and K⁺ . The calculated spectra of DNA with heavy counterions are characterized by intensive modes of ion-phosphate vibrations that form one united band near 115cm^-1. Ion-phosphate modes in the spectra of DNA with light counterions are characterized by higher frequencies (near 180cm^-1) and much lower intensity. Our calculations explain why the modes of ion-phosphate vibrations are observed in Cs-DNA spectra rather than in Na-DNA. The determined sensitivity of the intensities of DNA low-frequency spectra to the counterion type proves the existence of the ion-phosphate modes.     

Resonant Raman scattering by N-related local modes in AlGaAs/InGaAsN multiquantum wells

We report resonant Raman scattering and secondary ion mass spectrometry measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. The appearance of a strong TO band at resonance with nitrogen (N)-related electronic levels has been observed. The N-induced vibration mode at 470 cm⁻¹ changes in intensity and shape with increasing N and In content. A new vibration mode has been observed at 320 cm⁻¹, whose intensity scales with the N concentration. This mode is not present in InGaAsN films, so it is linked to the presence of Al. Its frequency is close to the B₁ silent mode of wurtzite GaN. It is attributed to the formation of GaN pairs, near the MQW interfaces as a consequence of the preferential Al–N bonding.     

Raman scattering studies of the Ge-In sulfide glasses

Room temperature Raman scattering measurements of samples of the (1−x)GeS₂-xIn₂S₃ (x=0.00-0.35) system have been conducted together with the FTIR transmission spectra of partial samples. Based on the Raman scattering and FTIR transmission spectra of the prepared defect spinel polycrystalline In₂S₃, the additional peaks at 306 and 245 cm⁻¹ were ascribed to the local symmetric stretching vibration of InS₄ tetrahedra and InS₆ octahedra, respectively. According to the Raman scattering spectral evolution of the Ge-In sulfide glasses, the microstructure of the studied glasses was considered to be that S₃Ge-GeS₃ and S₃In-InS₃ ethane-like units originated from the sulfur deficient with the addition of In₂S₃ are homogeneously dispersed in a disordered polymer network formed by GeS₄, InS₄ tetrahedra and a small quantity of InS₆ octahedra interconnected by sulfur bridges.     

Raman scattering and optical susceptibilities of Nd-doped glasses

The absolute Raman scattering cross-sections of three Neodymium-doped glasses have been measured. These cross-sections have been used to determine the nonlinear optical susceptibility tensor of the three glasses. We have estimated the relative contributions of the “electronic” and “nuclear” nonlinearities to self-focusing index for linearly polarized light in these glasses. We have also performed numerical computations of the Hibert transform of our Raman scattering cross-sections, which are useful in the prediction of any other nonlinear effects. Work supported in part by the National Science Foundation, and in part by the U.S. Atomic Energy Commission under the University of California Lawrence Livermore Laboratory subcontract No. 2713005.     

Giant enhancement of the Raman scattering by local magnon modes in FeF2:Mn2+

A study of the local magnon mode in FeF₂ crystals with Mn²⁺ impurities by inelastic Raman light scattering is presented. Though the interaction between the radiation and the Mn spins is negligible the scattering by the s₀ local mode is very strong. The data of the frequency and the scattering intensity are analyzed in terms of a magnetization coupled mode approach.     

水的低频受激拉曼散射

利用532 nm的脉冲激光进行了水的受激拉曼散射研究. 水的低激发光能量下主要表现为受激Stocks和反Stocks 3426 cm^-1谱线; 在能量大于140 mJ时出现低频率313 cm^-1受激Stocks谱线, 同时出现后向3389和3268 cm^-1 的受激Stocks谱线. 实验结果表明, 在较强的激光作用下, 水的结构表现为冰的Ⅷ相.     

水的低频受激拉曼散射

利用532 nm的脉冲激光进行了水的受激拉曼散射研究. 水的低激发光能量下主要表现为受激Stocks和反Stocks 3426 cm^-1谱线; 在能量大于140 mJ时出现低频率313 cm^-1受激Stocks谱线, 同时出现后向3389和3268 cm^-1 的受激Stocks谱线. 实验结果表明, 在较强的激光作用下, 水的结构表现为冰的Ⅷ相.     

Far infrared transmission and Raman scattering in Pb-As-S semiconducting glasses

Various compositions (x = 0.1?0.625) in the pseudo-binary (As₂S₃)_1?x(PbS)_x have been prepared in the glassy form. Thermal analysis of the prepared glasses shows a multiphase non-crystalline structure. Their far-infrared transmission (50–450 cm^-1) and first order Raman scattering have been observed for the first time and the observations indicate a two mode behaviour of the glasses. The frequencies and symmetries of AsS₃ bond-stretching modes are maintained in this alloy system and are therefore determined by the local order, and not the dimensionality or longer range order of the network.     

Measurement of correlations between low-frequency vibrational modes and particle rearrangements in quasi-two-dimensional colloidal glasses

We investigate correlations between low-frequency vibrational modes and rearrangements in two-dimensional colloidal glasses composed of thermosensitive microgel particles, which readily permit variation of the sample packing fraction. At each packing fraction, the particle displacement covariance matrix is measured and used to extract the vibrational spectrum of the "shadow" colloidal glass (i.e., the particle network with the same geometry and interactions as the sample colloid but absent damping). Rearrangements are induced by successive, small reductions in the packing fraction. The experimental results suggest that low-frequency quasilocalized phonon modes in colloidal glasses, i.e., modes that present low energy barriers for system rearrangements, are spatially correlated with rearrangements in this thermal system.     

Origin of low-frequency local vibrational modes in high density amorphous ice

Incoherent-inelastic neutron scattering data are obtained from 5-80 K for high-density amorphous (hda) ice in the region 0-135 cm(-1). An excess contribution to the vibrational density of states is identified near 20 cm(-1). The origin of these vibrations has been identified by lattice dynamics calculations on an "experimental" structure derived from reverse Monte Carlo analysis of hda ice neutron diffraction data. An interpretation that localized oscillations of short chains and isolated water molecules are responsible for the excess low-frequency modes is consistent with our data.     

Inelastic ultraviolet scattering from high frequency acoustic modes in glasses

The dynamic structure factor of vitreous silica and glycerol has been measured as a function of temperature and of the momentum transfer up to Q=0.105 nm(-1) using a novel experimental technique, the inelastic ultraviolet scattering. As in the case of Brillouin light scattering and ultrasonic measurements, the temperature dependence of the acoustic attenuation shows a plateau below the glass transition whose amplitude scales as Q2. Moreover, a slight temperature dependence of attenuation has been found in vitreous silica at about 130 K, which seems to be reminiscent of the peak measured at lower Qs. These two findings strongly support the idea that anharmonicity is responsible for sound attenuation at ultrasonic and hypersonic frequencies. Finally, we demonstrate that the attenuation mechanism should show a change of regime between 0.105 and 1 nm(-1).     

Study of the initial stage of nucleation in minerals by low-frequency Raman scattering

It is shown that the low-frequency Raman scattering (RS) of light in the acoustic range is a sensitive method for studying the initial stage of nucleation, and is even more sensitive to the structural order than traditional RS on optical phonons.     

On the theory of Raman scattering in liquid helium

A modification of Stephen's theory is proposed which does not depend on any adjustable parameter. Numerical calculation of the Raman spectrum for free excitations shows that some interaction has to be taken into account in order to obtain detailed agreement with experiment.     

Dispersive phonon modes by resonant Raman scattering in AgBr

Dispersive Raman modes are observed in AgBr at low temperatures when the incident laser light is tuned across the indirect Γ-L^′₃ exciton absorption. These modes occur as sidebands of the resonantly enhanced 2 TO(L), TO+LA(L) and 2 LA(L) Raman peaks and are interpreted as being due to three- and four-phonon processes, respectively. They involve longitudinal acoustic phonons near the Brillouin zone center that exhibit strong dispersion. From analysing the spectra the relaxation of the intermediate exciton state can be studied in detail. The model applied enables us to obtain directly the average effective mass of the ls indirect exciton as 1.5 electron masses.     

Raman scattering from plasmon-LO phonon coupled modes in ZnTe

Raman scattering spectra of LO phonon-plasmon coupled modes were observed at various temperatures below 400 K. The contributions of plasmon damping and phonon damping were analyzed by the use of the theoretical expression of the cross section and of Hall data. The temperature dependence of the calculated LO-phonon damping was discussed.     

On the vibronic theory of resonance Raman scattering

Resonance Raman excitation profiles of totally symmetric vibrational modes are investigated using a model that analytically includes the complete subspace of Franck-Condon active vibrations associated with each intermediate electronic state. This model is used to fit data obtained in resonance with the Soret band of cytochrome c. The excitation profiles are asymmetric and peak distinctively to the blue of the Soret absorption maximum. Large damping factors and/or inhomogeneous site distributions by themselves cannot account for the observed data. The theoretical results imply that the excited state lifetime associated with the Soret band of ferrocytochrome c has a lower limit on the order of 50 fs and that, compared to the ferrous form, the ferric cytochrome has a larger x-y splitting and a shorter lifetime.In constrast to the large multidimensional heme system, we also present the results of a simple model calculation applicable to smaller molecules. A two dimensional subspace is explored, where Raman Franck-Condon (RFC) and Franck-Condon (FC) factors are calculated for different potential energy surface parameters. Under certain conditions the RFC based scattering profiles of one vibration are strongly coupled to the FC based behaviour of the other vibration. Rather complex profiles are then predicted even for the simple two- dimensional case.