Strong selective excitation of Raman-active vibrations and energy transfer between low-lying vibrational states of a CO2 molecule studied by PARS and CARS

We experimentally demonstrate the novel technique of inducing the highly nonequilibrium distribution of molecules over vibrational states by two-frequency coherent Raman excitation. The technique can be used for selective excitation of totally symmetric and other Raman-active molecular transitions. Two counter propagating focused laser beams (second harmonic of Nd: YAG and dye laser) were used to induce population difference changes at the 00⁰0–10⁰0 transition in CO₂ molecules. The excitation and relaxation kinetics of this and neighbours vibrational modes of CO₂ were studied both by CARS and PARS. Vibrational excitation of up to 20% of the total number of irradiated molecules is measured; previously unknown desactivation constant of CO₂ (10⁰0) and CO₂ (02⁰0) states via CO₂ (01¹0) state is estimated to be K = (3±1) × 1 ⁵s^-1torr^-1.     

Vibrational Spectra and Assignments for 3,4-Dibromothiophene

Abstract

The infrared spectrum of 3,4-dibromothiophene has been studied from 4000 to 200 cm^?1. The Laser Raman spectrum has also been recorded and depolarization values have been measured. An assignment of the 21 fundamental vibrations is proposed based on group frequency correlations, Raman polarization data and comparison with the spectra of parent and some halogeno-substituted molecules.     

Optical nutation at a Raman-active transition

Optical nutation at the Raman-active transition 6P _1/2−6P _3/2 of thallium atoms (ω _R /2πc=7793 cm ⁻¹) under resonant Raman excitation by a biharmonic picosecond pulsed field, giving rise to substantial motion of the population, is detected. Optical nutation appears as an oscillatory behavior of the energy of the anti-Stokes scattering of probe pulses, which follow with a fixed delay, as a function of the product of the energies of the excitation pulses. As a result of the dynamic Stark effect, which decreases the frequency of the transition under study, resonance excitation conditions are satisfied for negative initial detunings of the Raman excitation frequency from resonance. The Raman scattering cross section for the transition under study is estimated by comparing the experimental data with the calculations. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 1, 7–12 (10 July 1999)     

Vibrational Band Intensities of Hydrocarbons

Abstract

The experimental observables of a vibrational spectrum are depicted either in the form of their positions, i.e., frequency relating the energy required in a given quantum transition, or as the intensities of absorption and scattering related to their transition probabilities. Expressed in terms of molecular parameters, the frequencies depend on the geometry, atomic masses, and intramolecular forces [11 while the band intensities [2] reflect changes in dipole moment (infrared) or polarizabilities (Raman) which are caused during a vibrational displacement and are related to movement of electronic charges within the individual bonds. The mathematical basis for determining vibrational frequencies was well established as early as 1940 by Wilson [3] and others [4, 51. Applying the interpretation of vibrational spectra has become routine in the multitudinous disciplines of chemical literature [6–26]. Accounts of infrared and Raman spectra [27–29], collection of literature [30], and reasonable sets of intramolecular forces [31, 321 are now available for the prediction of normal frequencies of hydrocarbons.     

Inversion relations,phase transitions and transfer matrix excitations for special spin models in two dimensions

The infrared and Raman active phonons (k=0) of orthorhombic TbF₃ and of isomorphous HoF₃ have been observed and attributed to the different irreducible representations of the factor groupD _2h. Moreover we investigated crystal-field excitations of the 4f-configuration of the Rare-Earth-ions and effects of their interaction with optical phonons, especially as a function of an external magnetic field. The infrared reflection spectra of these materials not only display reststrahlen bands of electric dipolar but also of magnetic dipolar type with a characteristic shape of the latter bands.On entering the ferromagnetic ordered state pertinent symmetry reductions (D _{2hC 2h}) become evident in the system of electronic transitions and optical phonons in TbF₃ by a change of selection rules and a magnetization-induced transfer of transition intensities to new components of the transition dipole or of the Raman polarizability tensor. These observations are in accordance with general symmetry considerations.The delocalization of the crystal-field excitations results in a Davydov splitting of the single-ion transitions.     

Pressure-induced phase transitions for goethite investigated by Raman spectroscopy and electrical conductivity

We reported two pressure-induced phase transitions of goethite up to ～35?GPa using a diamond anvil cell in conjunction with ac impendence spectroscopy, Raman spectra at room temperature. The first pressure-induced phase transition at ～7.0?GPa is manifested in noticeable changes in six Raman-active modes, two obvious splitting phenomena for the modes and the variations in the slope of conductivity. The second phase transition at ～20?GPa was characterized by an obviously drop in electrical conductivity and the noticeable changes in the Raman-active modes. The variations in activation energy with increasing pressure were also discussed to reveal the electrical properties of goethite at high pressure.     

高压液态重水的拉曼光谱研究

应用金刚石压腔结合拉曼光谱技术研究了重水在291 K,0.1～800 MPa条件下的拉曼谱图。结果表明：压力增大的过程中,重水的拉曼伸缩振动光谱向低频方向移动,并且频移和压力基本呈线性相关。频移没有突变,没有发生相的转变。将重水的拉曼谱峰分解为代表分子内O—D振动的高频峰和代表分子间氢键振动的低频峰。研究这两种不同类型谱峰的性质,发现代表分子间氢键的低频峰峰面积在不同的压力范围内呈现出不同的变化特征,压力对分子间氢键的影响并不是持续不变的。拉曼峰的峰面积反映的是产生这种拉曼峰的振动的数目,峰面积的变化反映了特征振动数目的变化。由于分子间氢键的强相互作用,水分子总是倾向于形成对称的空间五分子四面体结构,因此最大峰面积代表了最稳定的五分子团簇结构。     

环丁醇温致相变的原位拉曼光谱研究

采用拉曼光谱和红外光谱解析了常温条件下环丁醇的各个振动模式及其与分子构象间的关联,结果表明液态环丁醇以赤道–反式构象为主,并含有少量的赤道间扭式构象。在此基础上结合差示扫描量热技术和变温拉曼光谱,原位研究了环丁醇的温致相变过程和分子构象随温度的变化。结果表明,冷却至140 K的过程中,环丁醇并未结晶固化,而是保持亚稳定的无序液体状态,即出现过冷现象,继续降温至138 K出现玻璃化转变。升温过程中,在170 K时出现放热峰,同时有新的拉曼峰出现,并且拉曼峰的半高宽和强度发生明显的突变,表明环丁醇由无序结构转变为有序的结晶相。因此,我们获得了环丁醇的温致相变序列： 液态→过冷液体→玻璃态→结晶态→液态。通过对环丁醇不同分子构象的拉曼特征峰的定量分析,证实环丁醇在降温过程中,反式构象和间扭式构象的比例未发生明显改变,即没有发生构象变化。然而升温时,伴随170 K时结晶态的转变,反式构象特征峰的相对强度减小,表明部分分子由赤道–反式构象转变为赤道–间扭式构象。该研究结果对进一步理解和研究其他有机小分子的温致相变和构象变化具有指导意义。     

CH4分子ν1模拉曼诱导克尔效应谱与受激拉曼光声光谱峰形的比较

以气相甲烷分子ν₁模Q支的拉曼光谱为例，采用拉曼诱导克尔效应谱(RIKES)进行峰形测量，并将其与同时测量的受激拉曼光声光谱(PARS)的峰形进行了比较.结果表明，在pump光和Stokes光均为线偏振的情况下，两者存在着差异；在拉曼共振峰的低频端，RIKES谱强度略高；而高频端则恰好相反.从信号产生机制出发，对此进行了合理解释. 关键词： 拉曼诱导克尔效应谱 受激拉曼光声光谱 峰形     

Broadband coherent light generation in a Raman-active crystal driven by two-color femtosecond laser pulses

We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO(4)). As many as 20 anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from the infrared, through the visible region, to the ultraviolet, and it consists of discrete spatially separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields and open possibilities for synthesis of subfemtosecond light waveforms.     

A lattice dynamical investigation for the Raman and the infrared frequencies of Bi2W2O9

Studies on infrared and Raman-active phonons in the Aurivillius ferroelectric Bi₂W₂O₉ were performed theoretically. The Raman and infrared phonons were calculated by applying a short-range force constant model using normal coordinates having space group Pna2₁. Sixteen stretching, two repulsive and seventeen bending force constants were used for the calculations of the zone center phonons. The calculated Raman and infrared wavenumbers are in very good agreement with the experimental ones in the literature. Potential energy distribution for this compound has also been investigated for the contribution of each force constant towards the Raman and the infrared wavenumbers.     

High-Pressure Vibrational Studies of HCP Metals by Raman Spectroscopy

The pressure response of Raman phonons, determined for several hcp metals, includes positive pressure shifts as well as anomalies like mode softening in connection with phase transitions. It is shown that the phonon frequencies and their pressure dependences are related to macroscopic elastic parameters. More general, these results show that the measurement of Raman-active phonons in metals provides a direct probe of bonding, and agreement with theoretical models gives additional confidence in ab initio techniques.     

高压下吡啶的Raman散射谱

在高静水压下研究了吡啶的Raman散射谱,研究了液—固相变前后振动模的高压行为。在60kbar附近,观察到晶态—塑料态的相变。在全呼吸模992cm^-1的高频边,观测到随压力移动较快的伴峰,它被认为是红宝石颗粒上吸附分子(界面分子)的全呼吸振动模。这一伴峰可能因Mie氏结构共振散射而受到极大的增强。 关键词：     

A simple and accurate approach for calculating the vibration spectra of molecules on surfaces: Comparisons to high resolution electron energy loss data for ethylene on silicon

Peak assignment is a complex but important task for analyzing the vibration spectra of surface-bound molecules. Here we describe a simple approach for calculating infrared and Raman spectra for surface-bound molecules using a cluster model approach with quantum capping potentials (QCPs). The utility of the approach is demonstrated by comparisons to the measured high resolution electron energy loss spectra for ethylene on clean silicon. By capping the silicon cluster with QCPs we computed spectra that agree very well with the HREEL spectrum, allowing us to easily assign the experimental peaks. QCPs are similar to effective core potentials, can be used with any ab initio technique and most computational chemistry packages, and their use requires no special expertise.     

The Raman spectra and cross-sections of H2O, D2O, and HDO in the OH/OD stretching regions

We report the OH and OD stretching regions of the vapor phase Raman spectra of H₂O, and of a D₂O/HDO mixture, at room temperature. Also, the corresponding spectrum of H₂O at ∼2000 K in a methane/air flame is reported. These spectra are interpreted in terms of transition moments of the molecular polarizability, based on high-level ab initio calculations of the polarizability surface, and on variational wavefunctions considering the rotational-vibrational coupling in full. As a byproduct of this analysis several tables have been compiled including scattering strengths and assignments for individual rotational transitions of the three species. From these tables the Raman spectra in the OH/OD stretching regions can be simulated over the range of temperatures up to 2000 K for H₂O, and up to 300 K for D₂O and HDO.     

Pressure and temperature dependences of the dielectric constant,Raman spectra and lattice constant of SnI42

The pressure and temperature dependences of the Raman frequencies, static dielectric constant and lattice constant of the molecular crystal SnI₄, were investigated. These results are combined to evaluate the pure volume and pure temperature (i.e. volume-independent) dependences of the molecular polarizability and of the Raman-active phonon self-energies. The pressure results also allow the separation of the Raman modes into external and internal modes of the crystal; the external modes exhibit a much stronger pressure dependence, and thereby larger Grüneisen parameters, than do the internal modes. The Raman frequencies increase (decrease) with increasing pressure (temperature) whereas the dielectric constant increases with both increasing pressure and temperature, emphasizing the importance of non-volume effects. In fact, it is found that the isobaric temperature dependence of the dielectric constant is dominated by the pure temperature dependence of the molecular polarizability. The pure volume dependence of the polarizability, on the other hand, is relatively small as is perhaps typical of most molecular crystals.     

RAMAN AND INFRARED SPECTRA OF IODOFORM AT HIGH PRESSURES

ABSTRACT

Raman and infrared spectra are reported for iodoform samples in diamond anvil cells at ambient temperature and at pressures up to 5 GPa (Raman) and 10 GPa (infrared). The spectra appear to evolve smoothly and no evidence of any structural phase transitions is found. The dependence on pressure of 7 Raman and 13 infrared peak wavenumbers is presented. A large increase in intermolecular bonding strengths is confirmed, together with a moderate increase in intramolecular I-C-I bending forces. Color changes in the samples at high pressures are found to be mostly reversible, but long exposure to high fluxes of visible photons causes some molecular dissociation, with the release of iodine.     

Temperature and pressure dependences of the properties and phase transition in paratellurite (TeO2: Ultrasonic,dielectric and Raman and Brillouin scattering results

The effects of temperature and pressure on the ultrasonic propagation properties, dielectric constants and the Raman and Brillouin spectra in paratellurite (TeO₂) were investigated with emphasis on the behavior in the vicinity of the newly-discovered, pressure-induced phase transition. The transition is found to be second-order and purely strain-induced, driven by a soft shear acoustic mode propagating along a <110〉 and polarized along a (110) crystal direction. Such pure-strain transitions were previously discussed by Anderson and Blount and the transition in paratellurite is the first observation of this kind of transition. No evidence was found for any coupling of the soft mode to any other acoustic or optic mode, although small anomalies associated with lattice strains accompanying the transition were observed in some of the elastic and dielectric constants. Analysis of the effective elastic constant C, governing the soft mode velocity indicates that, within experimental uncertainty, the transition can be described by mean-field theory. Although the apparent attenuation of the soft mode increased significantly near the transition, it is concluded that this effect is probably due to the fact that the phase and group velocities are not parallel rather than to intrinsic dissipative processes in the crystal. With the exception of C₄₄, the remaining elastic constants and Raman-active phonon frequencies displayed normal increases with pressure. No soft Raman-active modes were observed in either phase. The static dielectric constants ?₁ are large, due to the large electronic polarizability of TeO₂, and the anisotropy in ? results almost entirely from the anisotropy in the optical dielectric constants ?_∞. In the low pressure tetragonal phase both ?_a and ?_c exhibit normal temperature dependences and ?_c decreases with pressure; however, ?_a exhibits an anomalous increase with pressure. Temperature, pressure and uniaxial stress measurements are combined to evaluate the various contributions to the temperature and pressure dependences of ?. Combining the ? data with available i.r. measurements demonstrated that the generalized Lyddane-Sachs-Teller relation is well obeyed for TeO₂. Finally, the Szigetti effective charge ratios were determined for the lowest frequency IR-active modes. These ratios were found to be quite low, being 0.27 and 0.18 for the a-axis responses, respectively, indicating that the bonding is highly covalent.     

Polarized Raman scattering and low frequency infrared study of hydroxyapatite

A detailed investigation of the polarized Raman spectrum of monoclinic hydroxyapatite (OHAp) and a careful study of the low frequency infrared spectrum of polycrystalline OHAp, is reported. Most of the spectral details of Raman scattering are consistent with the local pseudo-hexagonal C₆ factor group symmetry of OHAp. However, some features, for example the absence of TO-LO splitting for particular E₁ phonon geometries and the splitting of the lowest frequency Raman line, are a reflection of the centrosymmetric monoclinic lattice of OHAp. Between 0 and 50cm^?1, a temperature-dependent background, having E₁ symmetry, and probably associated with OH^? disordering flucuations, was observed in OHAp. Both OHAp and FAp displayed low frequency Raman lines which softened with decreasing temperature. However, no evidence for a phase transition was found in either OHAp or FAp in the 12–295 K temperature range.     

Nonlinear Raman scattering of photons by a channeled particle

Channeled particles are characterized by the discrete spectrum of bound transverse motion. The interaction of photons with channeled particles in single crystals can be accompanied by energy transitions between the levels of transverse motion of the channeled particle. The Raman scattering of photons at a quasibound channeled particle leads to the appearance of a combination of frequencies: the incident radiation frequency ω₀ and the frequency Δω_{m, n}, i.e., ω = ω₀ ± Δω_m,n where Δω_m,n = 2Δε_m,nγ²; Δε_{m, n} is the energy of the transition between quantum states (m and n) of the transverse motion of the channeled particle; and γ = E/mc² is the Lorentz factor of the channeled particle. The appearance of a violet satellite (the anti-Stokes component) in the Raman scattering spectrum is analyzed. The three-photon Raman-type transition, which is the process of the simultaneous absorption of two photons with the frequency ω₀ with the emission of a photon with the frequency ωs = 2ω₀ ± 2Δε_m,nγ², is considered. The conditions for resonance observation during the formation of the second harmonic (ω = 2ω₀) are discussed.