Spectrally dispersed femtosecond CARS investigation of vibrational characteristics in ethanol

Spectrally dispersed femtosecond time‐resolved coherent anti‐Stokes Raman spectroscopy is applied to study the ultrafast vibrational dynamics in ethanol at room temperature. The anti‐Stokes intensities were monitored as a function of delay time and wavenumber. By simply changing the timing of the laser pulses, the vibrational dynamics between the excited Raman transitions in ethanol molecules can be tracked and detected. Copyright © 2014 John Wiley & Sons, Ltd.     

硅甲烷射频放电分解动力学的CARS光谱研究

应用相干反斯托克斯喇曼光谱(CARS)技术对硅甲烷(SiH_4)的射频放电分解进行在位诊断.基于硅甲烷v_1振动模CARS信号强度的变化,研究了SiH_4-H_2及SiH_4-N_2体系中射频功率及总气压对硅甲烷分解速率的影响.此外,还用CARS方法检测硅甲烷分解的中间产物,得到信噪比较好的光谱,并试作了初步归属.     

Analyzing ultrafast multiplex coherent anti‐Stokes Raman spectra with picosecond probing

This article reports an efficient method to simulate time and frequency resolved coherent anti‐Stokes Raman scattering spectra measured with picosecond pump and probe fields and ultrashort Stokes pulses. A systematic comparison of measured and simulated time and frequency dependent data is presented for carbon tetrachloride, chloroform, cyclohexane, octane, and poly(methyl methacrylate). While the first compound exhibits no Raman active modes in the considered spectral region of the CH‐stretch vibrations, the other ones show Raman spectra of increasing complexity. Vibrational frequencies and homogeneous dephasing rates are extracted by fitting explicit analytical formulas to the recorded data. Interference between nonresonant and resonant contributions to the nonlinear polarization is taken fully into account. The ability to measure the influence of inhomogeneous broadening is discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

超快电子衍射技术及其应用

时间和空间上实时观测原子运动对于自然科学研究有着非常重大的意义, 而超快电子衍射(UED)技术同时具备飞秒激光脉冲的高时间分辨特性和电子衍射技术的高空间特性, 可以为实时观测原子级分辨尺度物质的结构变化提供一种有效工具. 本文综述了超快电子衍射技术的发展历史、实验方法以及相关应用, 并且展望了超快电子衍射技术未来的发展.     

Vibrational energy redistribution and vibrational dynamics of methanol mixed with Rhodamine 101 dye

Vibrational energy transfer that occurs after photoexcitation can be tracked in the first several femtoseconds by ultrafast time- and frequency-resolved CARS (coherent anti-Stokes Raman scattering) spectroscopy. Vibrational energy transfer from high-frequency modes to lower ones through chemical bond of pure methanol and methanol/Rhodamine 101 (Rh101) solution is detected. Through comparison and analysis of the experimental results, it is found that surrounding molecules have a significant influence on vibrational energy transfer and vibrational couplings among relevant modes.     

Effect of size polydispersity on the structural and vibrational characteristics of two-dimensional granular assemblies

Two-dimensional disordered granular assemblies composed of 2048 polydispersed frictionless disks are simulated using the discrete element method. The height of the first peak of the pair correlation function, g1, the local and global gbond orientational parameters lψ6and ψ6, and the fluctuations of these parameters decrease with increasing polydispersity s,implying the transition from a polycrystalline state to an amorphous state in the system. As s increases, the peak position of the boson peak ωBP shifts towards a lower frequency and the intensity of the boson peak D(ωBP)/ωBP increases, indicating that the position and the strength of the boson peak are controlled by the polydispersity of the system. Moreover, the inverse of the boson peak intensity ωBP/D(ωBP), the shear modulus G, and the basin curvature SIS all have a similar dependence on s, implying that the s dependence of the vibrational density of states at low frequencies likely originates from the s dependence of the basin curvature.     

Single‐beam coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO2 via phase and polarization shaping of a broadband continuum

Coherent anti‐Stokes Raman scattering (CARS) spectroscopy of gas‐phase CO₂ is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform‐limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm⁻¹) is employed for simultaneous excitation of the CO₂ Fermi dyads at ∼1285 and ∼1388 cm⁻¹. CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal‐to‐noise ratio of the single beam–generated CO₂ CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO₂ concentrations and rapid pressure fluctuations in hypersonic and detonation‐based chemically reacting flows are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of vibrational characteristics in BBO crystals by femtosecond CARS

Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) is utilized to study the ultrafast vibrational dynamics in BBO crystals at room temperature. Time-resolved two-beam and three-beam CARS are detected. The vibrational dephasing time is analyzed and the changes of vibrational mode intensities with the polarization of pump pulses are observed.     

飞秒激光对高反膜的破坏及其超快动力学过程

研究了800nm飞秒激光照射下45°高反膜ZrO2-Si O2的破坏及其超快动力学过程。利用原子力显微镜和扫描电镜观察了材料的烧蚀形貌,测量了破坏阈值与脉冲宽度、烧蚀深度与脉冲能量的依赖关系。随着脉冲宽度从50fs增加到900fs,其烧蚀阈值从0.35J/cm2增加到1.78J/cm2。烧蚀深度与激光能流密度近似成对数关系。当激光强度略高于烧蚀阈值时,材料很快被烧蚀到几百纳米,烧蚀深度表现出明显的层状特性。同时,利用建立的抽运探针实验系统,测量了高强度抽运脉冲作用下材料对探针光的反射率随延迟时间的变化,揭示了薄膜烧蚀的超快动力学过程。实验结果表明高反膜表层的材料对烧蚀特性有重要影响。     

Four-wave mixing in OH: comparison between CARS and DFWM

Summary A comparison between CARS and Degenerate Four-Wave Mixing experiments on OH is performed in a premixed CH₄-air flame. Both optical phase conjugation and forward BOXCARS geometries are studied in DFWM. The relative properties of these two configurations are experimentally investigated from the study of theP andQ branches of the (O−O) band. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

New opportunities offered by compact sub‐nanosecond supercontinuum sources in ultra‐broadband multiplex CARS microspectroscopy

We report, to the best of our knowledge, the first turn‐key and compact dual‐output sub‐nanosecond supercontinuum source applicable to ultrabroadband multiplex coherent anti‐Stokes Raman scattering microspectroscopy of biological samples. The light source design and the microspectroscopy setup are described in details. Easy multicolour imaging of Caenorhabditis elegans nematode is operated successfully and discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Diagrammatic representation of the third-order polarization in transient CARS

Summary A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization,P ⁽³⁾ which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions toP ⁽³⁾ are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency ω₁ are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.     

Time‐resolved investigation of the ν1 ro‐vibrational Raman band of H2CO with fs‐CARS

The technique of femtosecond time‐resolved coherent anti‐Stokes scattering (fs‐CARS) is used to investigate the strongly perturbed ν₁ ro‐vibrational Raman band of formaldehyde (H₂CO). The time‐dependent signal is simulated using a ‘Watson‐’Hamiltonian in A‐type reduction and Raman theory for asymmetric rotors. The results are compared with the experimental data. The fs‐CARS method measures the evolution of the polarization in a molecular ensemble via superposition of many states and is sensitive to spectral irregularities or line shifts of the involved transitions. ‘Coriolis’ interactions play a major role in the analysis of the ν₁ band of formaldehyde. We successfully simulate the fs‐CARS transient signal from the ν₁ band of formaldehyde including a model for multiple ‘Coriolis’ interactions, without the necessity of describing the complete interaction between all the vibrational levels. ‘Coriolis’ coupling coefficients and energy shifts are derived from the experiment by a least‐square fit. The results are discussed and compared to literature values. Copyright © 2006 John Wiley & Sons, Ltd.     

Ultrafast Raman loss spectroscopy (URLS): instrumentation and principle

In this paper, we report on the concept and the design principle of ultrafast Raman loss spectroscopy (URLS) as a structure‐elucidating tool. URLS is an analogue of stimulated Raman scattering (SRS) but more sensitive than SRS with better signal‐to‐noise ratio. It involves the interaction of two laser sources, namely, a picosecond (ps) Raman pump pulse and a white‐light (WL) continuum, with a sample, leading to the generation of loss signals on the higher energy (blue) side with respect to the wavelength of the Raman pump unlike the gain signal observed on the lower energy (red) side in SRS. These loss signals are at least 1.5 times more intense than the SRS signals. An experimental study providing an insight into the origin of this extra intensity in URLS as compared to SRS is reported. Furthermore, the very requirement of the experimental protocol for the signal detection to be on the higher energy side by design eliminates the interference from fluorescence, which appears on the red side. Unlike CARS, URLS signals are not precluded by the non‐resonant background and, being a self‐phase‐matched process, URLS is experimentally easier. Copyright © 2011 John Wiley & Sons, Ltd.     

纳秒激光单脉冲烧蚀硅表面的超快成像研究

利用ICCD可以在纳秒时间尺度下成像的特点,以飞秒准连续激光产生的超短脉冲光为探测光,对纳秒激光单脉冲烧蚀硅靶表面的演化过程进行动态监测。在能量密度为50J／cm^2时,捕获了纳秒单脉冲激光烧蚀硅靶面过程中等离子体演化的时间分辨图像。图像表明,纳秒激光烧蚀硅靶产生的等离子体开始时密度大,膨胀速度快,当纳秒激光脉冲过后,等离子体不再产生,并且其膨胀速度不再增加,直至完全消失。     

Ultrafast proton transfer dynamics of 2-(2'-hydroxyphenyl)benzoxazole dye in different solvents

The excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzoxazole dye in different solvents is investigated using ultrafast femtosecond transient absorption spectroscopy combined with quantum chemical calculations.Conformational conversion from the syn-enol configuration to the keto configuration is proposed as the mechanism of excited-state intramolecular proton transfer.The duration of excited-state intramolecular proton transfer is measured to range from 50 fs to 200 fs in different solvents.This time is strongly dependent on the calculated energy gap between the N-S;and T-S;structures in the S;state.Along the proton transfer reaction coordinate,the vibrational relaxation process on the S;state potential surface is observed.The duration of the vibrational relaxation process is determined to be from8.7 ps to 35 ps dependent on the excess vibrational energy.     

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

Summary We have carried out parallel studies of the quenching process in Na(3p)+H₂ collisions and the possible reactive process in Na(3p)+H₂ (v ^″=1,2,3) collisions. Rich CARS spectra which were obtained at H₂ pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H₂ and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.     

Comparison of different CARS variants for concentration measurements in combustion research

Summary Different strategies are discussed to use CARS spectroscopy for measuring mole fractions of combustion species. The influence of various physical parameters on both shape and total intensity of spectral bands is studied. Two variants for concentration measurements are compared based either on spectralshape analysis or frequency-integrated intensity ratios. The analysis leads to the conclusion that concentrations from a spectral-shape fit are more sensitive to inaccuracies of input parameters than band intensities. Paper presented at the ?XI European CARS Workshop?, Florence, Italy, 23–25 March, 1992.