Raman scattering for weakened bonds in the intermediate state: enhancement of low-frequency vibrations

A multimode theory of the Raman scattering in resonance with an electronic transition causing a strong weakening of atomic bonds is proposed. Simple analytical relations between the Fourier transforms of the first- and second-order Raman amplitudes and the absorption are derived. It is predicted that the Raman scattering on low-frequency vibrations will be strongly enhanced. Besides the second-order Raman scattering is also enhanced as compared to the first-order scattering. The Raman excitation profiles show a structure caused by the Airy oscillations. The shapes of the profiles of all vibrations contributing to the weakening bond are the same as a consequence of the strong mode mixing under the virtual vibronic transition.     

Second-order Raman scattering and infra-red absorption in SnS2

Summary The second-order Raman scattering and infra-red absorption spectra of 2H−SnS₂ are reported. The temperature, excitation wavelength and polarization dependences of Raman spectra are studied; resonance effects are put in evidence. The assignment of the second-order bands is discussed on the basis of the appliable selection rules. The role of phonons of high symmetry points different from Γ in second-order spectra is also taken into account. Work partially supported by M.P.I.     

Der Resonanz-Raman-Effekt bei Erregung im Absorptionskontinuum

When the frequency used for excitation of Raman spectra of gases is high enough to reach the region of continuous absorption typical resonance Raman spectra may be observed. It is shown theoretically that the scattering formula for this case leads to selection rules at variance with the normal Raman effect, allowing an enhanced appearance of overtones. Also this resonance Raman effect has to be understood as a single quantum process.     

Resonance and composition effects on the Raman scattering from silver-gold alloy clusters

Low-frequency Raman scattering experiments have been performed on thin films consisting of pure gold or gold-silver alloy clusters embedded in alumina matrix. It is clearly shown that the quadrupolar vibrational modes are observed by Raman scattering because of the effect of resonance with the excitation of the electronic surface dipolar plasmon. This is due to the strong coupling between the collective electronic dipolar excitation and the quadrupolar vibrational modes. This effect of resonance does not exist with the core electron excitations. The mixing of the conduction electron dipolar excitation (surface plasmon) with the core electrons leads to the quenching of the resonant Raman scattering. Received 16 November 2000     

Survival of molecular information under surfaced‐enhanced resonance Raman (SERRS) conditions

In the present paper, we discuss the molecular information that can be derived from surface‐enhanced resonance Raman Scattering (SERRS) experiments performed with different excitation wavenumbers, which are close to resonance with an excited electronic state of the molecule [surface‐enhanced Raman dispersion spectroscopy (SERADIS)]. We specifically consider the situation, where a molecule is physisorbed to a site characterized by a local electric field with a direction independent of the direction of the external, exciting field. The molecular information available in this experimental situation is compared with the information available in a corresponding Raman dispersion spectroscopy (RADIS) experiment performed on a free molecule or a molecule physisorbed to a site, where the local field is isotropic. The consequences for resonance Raman scattering (RRS) and RADIS, when the molecule is adsorbed in the highly anisotropic hot spot (HS), are discussed; here it is shown that only the molecular information originating from the symmetric part of the scattering tensor can survive in SERRS and in SERADIS. Besides, it is shown that the depolarization ratio can no longer be used to discriminate between totally and non‐totally symmetric modes in the polarized surface‐enhanced Raman scattering (SERS) spectra. These results have implications for the resonance Raman spectra, but even more important for the application of the resonance Raman effect in the investigation of excited vibronic molecular states, in general, and in the investigation of electronic states in larger bio‐molecules, such as the various metallo‐porphyrins. Copyright © 2009 John Wiley & Sons, Ltd.     

Nonlinearity in Intensity versus Concentration Dependence for the Deep UV Resonance Raman Spectra of Toluene and Heptane

Abstract: The relation between Raman scattering, resonance Raman scattering, and absorption is reviewed to determine to what extent quantitative analysis can be applied in resonance Raman spectroscopy. In addition, it is demonstrated experimentally that normal Raman spectra can be dramatically inhibited by absorption and resonance Raman effects. Raman spectra of toluene and heptane mixtures—with progressively increasing concentrations of heptane—were measured using 229-nm laser excitation. The results show that the characteristic band intensities are not directly proportional to the relative concentrations of the compounds and deviate due to absorption resonance effects. An approximated mathematical model is developed to demonstrate that the intensities of the normal Raman scattering bands are suppressed. An inhibition coefficient K_i is introduced to describe the situation and determine the penetration depth. Most remarkably, it is shown that the intensity of the resonance Raman scattering bands can be constant even when the concentration ratios differ substantially in the sampled mixtures.     

Double resonance Raman scattering of second-order Raman modes from an individual graphite whisker

Resonant Raman scattering of second-order Raman modes from an individual graphite whisker synthesized by a high-temperature heat-treatment method at a special pressure was discussed here. The dependence of phonon frequencies on the incoming laser light and the frequency difference between Stokes and anti-Stokes scattering show their origin from double resonance Raman scattering. Our results show that all the experimental results of second-order Raman modes in graphite whiskers, such as the excitation-energy dependence on the mode frequency, the frequency shift between a second-order Raman mode and its fundamentals, and the frequency discrepancy between Stokes and anti-Stokes components of a second-order Raman mode can be well understood by double resonance Raman scattering.     

光纤拉曼放大器中增益的偏振相关特性研究

提出了一种改进的、可用于计算偏振相关拉曼增益的光纤拉曼放大器的非线性耦合波方程。建立了单模双折射光纤拉曼增益的数学模型,分析了线偏振抽运光以与光纤快轴成45°激励的单模双折射光纤拉曼放大器模型与实际具有随机双折射的光纤拉曼放大器的拉曼增益偏振相关特性的等价性。基于上述模型,提出了一个可定量表征单模光纤偏振模色散统计特性的拉曼增益偏振相关因子,用以替代常规的光纤拉曼放大器非线性耦合波方程中的偏振相关因子。计算结果与已报道文献的实验数据非常吻合。同时对抽运增益在同向和反向抽运方式下截然不同的增益偏振相关特性给出了合理的解释。     

共振拉曼效应和电子-声子耦合对线性多烯分子共振拉曼光谱的影响

线性多烯分子具有高强度且信息丰富的共振拉曼光谱,在生物学、光电材料和医学等方面都有一定应用.而含有共轭双键的短链β胡萝卜素分子是多烯分子中极具有代表性的分子.在激发光作用下π电子与CC键振动相互作用影响着吸收光谱和拉曼光谱,而共振拉曼效应和电子-声子耦合影响着共振拉曼光谱的强度、频率和线型.测量了β胡罗卜素分子在二氯乙...     

CW measurements of resonance Raman profiles,line‐widths,and cross‐sections of fluorescent dyes: application to Nile Blue A in water and ethanol

The aim of this work is to illustrate the power of recently developed methods for measuring resonance Raman scattering (RRS) spectra of efficient fluorophores (using a standard continuous wave excitation and a charge‐coupled device (CCD)‐based Raman spectrometer), by applying them to a detailed study of a specific fluorophore: Nile Blue A. A combination of methods are used to measure the RRS properties of Nile Blue A in water (quantum yield (QY) of 4%) and ethanol (QY of 22%) at excitation wavelengths between 514 and 647 nm, thus covering both pre‐resonance and RRS conditions. Standard Raman measurements are used in situations where the fluorescence background is small enough to clearly observe the Raman peaks, while the recently introduced polarization‐difference RRS and continuously shifted Raman scattering are used closer to (or at) resonance. We show that these relatively straightforward methods allow us to determine the Raman cross‐sections of the most intense Raman peaks and provide an accurate measurement of their line‐width; even for broadenings as low as ∼ 4 cm ^{− 1}. Moreover, the obtained Raman excitation profiles agree well with those derived from the optical absorption by a simple optical transform model. This study demonstrates the possibility of routine RRS measurements using standard Raman spectrometers, as opposed to more complicated time‐resolved techniques. Copyright © 2013 John Wiley & Sons, Ltd.     

Deep UV resonance Raman spectroscopic study on electron‐phonon coupling in hexagonal III‐nitride wide bandgap semiconductors

Electron–phonon coupling (EPC) is an important issue in semiconductor physics because of its significant influence on the optical and electrical properties of semiconductors. In this work, the EPC in wide bandgap semiconductors including hexagonal BN and AlN was studied by deep UV resonance Raman spectroscopy. Up to fourth‐order LO phonons are observed in the resonance Raman spectrum of hexagonal AlN. By contrast, only the prominent emission band near the band‐edge and the Raman band attributed to E_2g mode are detected for hexagonal BN with deep UV resonance excitation. The different behavior in resonant Raman scattering between the III‐nitrides reflects their large difference in EPC. The mechanism for EPC in hexagonal BN is the short‐range deformation interaction, while that in hexagonal AlN is mainly associated with the weak long‐range Fröhlich interaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS₂, HfS₂, MoS₂ and WS₂ using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS₂ and WS₂ cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS₂ is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.     

Raman spectroscopy and Fermi resonance in Mn-doped ZnO bulk single crystal

For the first time the bulk oriented single crystals ZnO:Mn are obtained and the polarized Raman spectra are studied at excitation in the visible and near infrared regions. The resonance enhancing of the Raman scattering by Mn-related modes is found at the visible excitation due to the extra optical absorption in ZnO at the addition of Mn. It is shown that the resonance-enhanced overtone of Mn-related silent modes may be responsible for an appearance of anomalous modes of the A₁ symmetry at 500-600 cm⁻¹. A Fermi resonance between the overtone and one-phonon mode is analyzed.     

Polarization effects of picosecond CARS in liquids

Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal:

1. scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability
2. scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation
3. four-wave mixing via the non-resonant part χ_NR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C₆H₅Br the ratio of non-resonant to resonant contributions is measured to be χ_NR/χ_res=0.037±0.015.

     

Tunneling‐induced transparency in a chaotic microcavity

A new form of induced transparency enabled by dynamical tunneling coupling of continuous chaos and discrete regular modes in a slightly deformed optical microcavity is demonstrated experimentally. An optical beam is focused on the cavity boundary and tuned on resonance with a high‐Q mode, which leads to destructive interference for the excitation of chaotic field and induces a transparency in the transmission. The experimental results are in excellent agreement with a model based on quantum scattering theory. This tunneling‐induced transparency is accompanied by extremely steep normal dispersion, and holds great potential in slow light and enhanced nonlinear interactions.     

Analytical investigations on crosstalk in fiber Raman amplification for WDM systems

In this paper, the analytical investigations for the crosstalk in fiber amplification for WDM channels have been reported. The crosstalk between WDM channels with external Raman amplification including second-order dispersion terms has been investigated. It has been shown that the higher-order dispersion severely degrades the performance of optical communication systems. To ensure small crosstalk, the signal gain and the injected pump power should be limited to the value well below the threshold of Raman amplification. Analytical formula for signal interference ratio (SIR) and gain in fiber Raman amplifier including the impact of second-order dispersion terms for WDM systems has been derived at different wavelengths. It has also been shown that there is increase in crosstalk due to second-order dispersion.     

GaAs/AlAs超晶格的近共振喇曼散射研究

本文介绍GaAs/AlAs超晶格的室温近共振喇曼散射测量结果。由于超晶格中Fr?hlich相互作用的共振增强效应,GaAs LO声子偶模的散射得到了很大的增强。和前人的结果一样,在偏振谱我们观察到了偶模。但和前人的结果不同,在退偏振谱中我们观察到的是奇模,而不是偶模。从而证明了在近共振条件下LO声子限制模仍遵从与非共振时一样的选择定则。二级喇曼散射实验结果表明,在偏振谱中二级谱是由两个偶模组合而成,而在退偏振谱中的二级谱与前人的结果不同,由一个奇模与一个偶模组合而成。上述结果与最近提出的黄朱模型的预言是一 关键词：     

天线共振子理论对吡啶分子在铁钴镍衬底上SERS增强因子的计算

天线共振子模型是在研究表面增强拉曼散射增强机理过程中提出的理论.作为一项非常具有代表意义的工作,文章利用天线共振子模型计算了吡啶分子1010 cm-1特征振动模在过渡金属Fe,Co,Ni衬底上的表面增强拉曼散射增强因子:给出了共振时三种金属增强因子与激发光能量的依赖关系;研究了它们的表面粗糙度对增强因子的影响,计算了能量为0.89 eV的入射光在不同表面粗糙度的三种金属衬底上的增强因子.计算结果表明,它们在可见光范围增强因子不大,这与已报道的实验结果一致;但在红外波长却有大的增强因子,可以达到104～107,有待实验检验.     

Time—dependent Theory of Raman Scattering with Pulses—Application to Continuum Raman Spectroscopy

A theory of real-time dependence of Raman scattering for a pulse-mode laser is developed within second-order perturbation theory and using the wavepacket terminology.We apply the theory to continuum Raman scattering for short and long pulses and varying pulse carrier frequency,For an initial ground virational state,it is shown that the rate of Raman emission as a funcition of time and pulse carrier frequency is structureless for all pulses,and for pulses that are longer than the dissociation time the rate also decays with the pulses.This is contrary to recently reported resonance fluorescence type structures at long times (M.Shapiro,J.Chem.Phys.99,2453(1993),We explain why such structures are unphysical for continuum Raman scattering.