和频与差频振动光谱实验构型的分析模拟

实验构型分析是定量分析和频振动光谱的基础.变换实验构型,不仅要考虑某一振动模式信号强度的大小,还要考虑不同构型下的信号检测效率.现有的和频振动光谱实验构型分析主要考虑前者.本文探讨实验构型分析中所涉及的信号检测效率问题,模拟在共向式和频(差频)及对射式和频(差频)振动光谱实验中选取何种实验构型对采集信号光更加合理有效.利用相干光学过程能量守恒和动量守恒原理,分析了入射角及入射光频率等因素对信号出射角的影响,并模拟了信号出射角与入射角及入射光频率的关系,得到了可选的入射角组合最多、出射角随入射光频率变化最小的实验构型.结果表明,和频振动光谱采取共向式实验构型,差频振动光谱采取对射式实验构型,有利于信号采集,进而有利于用实验构型分析方法对和频(差频)振动光谱进行定量研究.     

利用和频振动光谱研究空气/短链脂肪酸界面的分子取向

利用和频振动光谱(SFG-VS)方法检测了5种短链脂肪酸分子(乙酸、丙酸、正丁酸、正戊酸及正己酸)在空气/纯液体界面的结构, 得到了3种偏振组合(ssp, ppp, sps)下的和频振动光谱. 通过偏振选择定则对各个谱峰进行了指认和分析, 同时计算出空气/纯脂肪酸液体界面上脂肪酸分子的甲基取向角. 对比发现, 从丙酸到己酸, 分子甲基基团的界面取向角随碳链的增长略有增大. 并对其机理进行了分析.     

和频振动光谱研究多晶金电极/溶液界面乙腈分子取向的flip-flop行为

用和频振动光谱研究乙腈/金电极界面,观测到乙腈的甲基振动峰强度随施加的电极电势而变化.当电极电势越过零电荷电势（pzc）时,甲基振动峰符号发生反转,这意味着基团取向发生反转（flip-flop）.由此推断出乙腈分子在金电极界面的吸附构型.即在零电荷电势下,电极界面吸附的乙腈分子构型为甲基靠近电极表面而腈基远离电极表面;而高于零电荷电势则电极界面吸附的乙腈分子构型发生反转,变为腈基靠近电极表面而甲基远离电极表面的构型.     

二价阳离子诱导细胞膜融合的和频光谱研究（英文）

细胞膜融合是一种重要的基础生物学过程,细胞的很多生物学功能都涉及到细胞膜的融合.二价阳离子可以通过与带负电磷脂的结合诱导磷脂膜的融合,然而,其详细的分子学机制目前还不太清楚.本文应用表/界面敏感的和频振动光谱结合动态光散射实验研究了磷脂分子层对二价金属离子(如Ca²⁺和Mg²⁺)暴露的响应.动态光散射实验测量的粒度分布结果显示Ca²⁺可以诱导囊泡间融合,而Mg²⁺的介导却不能导致磷脂膜的融合.为了应用和频光谱研究磷脂分子不同基团对金属离子的响应,本文设计了十八烷基三氯硅烷自组装单分子层/磷脂单分子层组成的混合模型膜系统进行和频振动光谱实验.实验发现,相比于Mg²⁺,Ca²⁺与磷脂头部基团PO₂^-有更强烈地相互作用,会更容易诱导细胞膜融合.和频光谱实验还显示,虽然两种金属阳离子没有与磷脂中C=O基团直接连接.但是Ca²⁺/Mg²⁺-PO₂^-...     

和频振动光谱(SFG-VS)研究超铺展三甲基硅烷表面活性剂在空气/水界面的振动光谱和吸附

The C-H stretch vibrational spectra of the trisiloxane superspreading surfactant Silwet L-77((CH3)3Si-O-Si(CH3)(C3H6)(OCH2CH2)7-8OCH3)-O-Si(CH3)3) at the air/water interface are measured with the surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). The spectra are dominated with the features from the –Si-CH3 groups around 2905 cm-1 (symmetric stretch or SS mode) and 2957 cm-1 (mostly the asymmetric stretch or AS mode), and with the weak but apparent contribution from the -O-CH2- groups around 2880 cm-1 (symmetric stretch or SS mode). Comparison of the polarization dependent SFG spectra below and above the critical aggregate or micelle concentration (CAC) indicates that the molecular orientation of the C¡H related molecular groups remained unchanged at different surface densities of the Silwet L-77 surfactant. The SFG-VS adsorption isotherm suggested that there was no sign of Silwet L-77 bilayer structure formation at the air/water interface. The Gibbs adsorption free energy of the Silwet surfactant to the air/water interface is -42.2±0.8kJ/mol, indicating the unusually strong adsorption ability of the Silwet L-77 superspreading surfactant     

和频光谱研究油酸包覆的Fe3 O4 纳米颗粒

采用红外-可见和频振动光谱研究了表面包覆油酸分子的Fe3O4纳米颗粒, 得到了2种实验构型(构型1: 可见光入射角63°, 红外光入射角55°; 构型2: 可见光入射角45°, 红外光入射角55°)和3种偏振组合(ssp, ppp, sps)下的和频振动光谱, 比较了2种实验构型下和频光谱的特征, 通过偏振分析方法对各个光谱峰进行了归属.     

非线性光学研究水及电解质水溶液界面——空气/电解质水溶液界面特定阴阳离子效应

水及电解质溶液界面在物理、化学、环境及生物等各种过程中扮演着至关重要的角色。百年来科学家用各种实验及理论方法研究水及电解质溶液界面,试图理解其界面的结构及动力学。最近的实验和理论研究表明离子能够影响电解质水溶液界面结构,可极化性大的阴离子甚至会在界面富集。本文综述了我们研究组近年来利用二阶非线性光学方法--非共振的二次谐波与和频振动光谱研究水及电解质溶液界面的进展。首先我们研究了空气/纯水界面非共振二次谐波信号的来源,研究结果证明了空气/纯水界面非共振二次谐波信号完全可以归结为电偶极贡献,为此方法在电解质溶液界面的研究奠定了基础。同时,用偏振及对称性分析对空气/纯水界面和频振动光谱峰进行了归属,提供了纯水/空气界面结构新的物理图像。在此基础上,我们对几种电解质水溶液界面进行了深入研究,结果证明不仅可极化性大的离子会在水界面富集,并使界面层增厚,可极化性小的阴离子对界面层厚度也有影响,甚至阳离子也会在一定程度上影响界面水分子结构。     

六方稀土氧化水化和碳酸化产物的振动光谱研究

六方稀土氧化物在空气中极易水化和碳酸化。测定并讨论了光谱纯粉晶ＲＥ２Ｏ３（ＲＥ＝Ｌａ,Ｐｒ,Ｎｄ）样品的Ｒａｍａｎ光谱、红外光谱及粉晶Ｘ射线衍射谱。结果表明,其水化产物为ＲＥ（ＯＨ）３,碳酸化产物主要为六方双氧单碳酸盐ＲＥ２Ｏ３ＣＯ３以及少量的单氧双碳酸盐ＲＥ２Ｏ（ＣＯ３）２和四方ＲＥ２Ｏ２ＣＯ３。     

异丙醇的O-H伸缩振动泛频光谱和分子构像

使用高灵敏的光腔衰荡光谱(CavityRingDownSpectroscopy)技术测出了异丙醇的O-H伸缩v=4、5振动泛频光谱,每个振动能级都有三个吸收峰,被归属为分子构像的O-H伸缩泛频吸收.给出了光腔衰荡光谱的振动泛频吸收的谱带强度公式,并求得分子不同构像在不同振动能级的O-H伸缩泛频吸收的谱带强度;同时利用局域模理论,求得分子各O-H伸缩局域模振子的机械频率(X1)、非谐性(X2)以及解离能(D).用密度泛函(DFT)B3LYP/6-31+G*理论方法优化了分子的各种可能构像,验证了分子存在反式(trans)和偏转(gauche)两种稳定构像,计算的分子的O-H伸缩频率及构像稳定性同实验结果是一致的.     

Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

The constructive or destructive spectral interference between the molecular groups oriented up and down at the interface in the sum-frequency generation （SFG） spectra provides a direct measurement of the absolute orientation of these molecular groups. This simple approach can be employed to interrogate absolute molecular orientations other than using the complex absolute phase measurement in the SFG studies. We used the -CN group in the p-cyanophenol （PCP） molecule as the internal phase standard, and we measured the phases of the SFG fields of the -CN groups in the 3,5-dimethyl-4-hydroxy-benzonitrile （35DMHBN） and 2,6-dimethyl-4-hydroxy-benzonitrile （26DMHBN） at the air/water interface by measuring the SFG spectra of the aqueous surfaces of the mixtures of the PCP, 35DMHBN, and 26DMHBN solutions. The results showed that the 35DMHBN had its -CN group pointing into the aqueous phase; while the 26DMHBN, similar to the PCP, had its -CN group pointing away from the aqueous phase. The tilt angles of the -CN group for both the 35DMHBN and 26DMHBN molecules at the air/water interface were around 25°-45° from the interface normal. These results provided insights on the understanding of the detailed balance of the competing factors, such as solvation of the polar head groups, hydrogen bonding and hydrophobic effects, etc., on influencing the absolute molecular orientation at the air/water interface.     

Vibrational Spectroscopy of Surface Adsorbates by Sum Frequency Generation (SFG)

The vibrational spectroscopy by way of vibrational sum-frequency generation (VSFG) has been made of formic acid adsorbed on MgO(001) surface and of n-alkyltrichlorosilanes chemisorbed on quartz plates. It was revealed that the species on the MgO surface was formate ion (HCOO⁺) with CH bonds standing vertical to the surface and that the different adsorption sites showed up on repeated cycles of adsorption-desorption processes. The results of chemisorbed films indicated that the g-t and/or g-t-g′ conformational scquences of n-alkyl chains occurred more frequently by the length of alkyl chain and that the surface hydroxyls, which are the sites of chemisorption, are ripped off by pre-exposure of substrate to oxygen plasma.     

Sum Frequency Generation Vibrational Spectra of Perovskite Nanocrystals at the Single-Nanocrystal and Ensemble Levels

Determination of molecular structures of organic-inorganic hybrid perovskite (OIHP) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenging. In this study, we demonstrate that femtosecond sum frequency generation (SFG) vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (~10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)PbBr₃ and (R-MBA)₂PbI₄ nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067±3 cm^-1 (ν₂ mode) and a weak shoulder peak at 3045±4 cm^-1 (ν_7a mode) at the ensemble level, whereas a peak of the ν₂ mode and a peak at 3025±3 cm^-1 (ν_20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface, but stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.     

Methanol Perturbing Modeling Cell Membranes Investigated using Linear and Nonlinear Vibrational Spectroscopy (cited: 3)

Cell membranes play a crucial role in many biological functions of cells. A small change in the composition of cell membranes can strongly influence the functions of membrane-associated proteins, such as ion and water channels, and thus mediate the chemical andphysical balance in cells. Such composition change could originate from the introduction of short-chain alcohols, or other anesthetics into membranes. In this work, we have applied sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to investigate interaction between methanol and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) lipid bilayers. Lipid's hydrocarbon interior is deuterated while its head group is hydrogenated. At the same time, CH₃ symmetric stretch from methanol and lipid head amine group has different frequency, thus we can distinguish the behaviors of methanol, lipid head amine group, and lipid hydrocarbon interior. Based on the spectral feature of the bending mode of the water molecules replaced by methanol, we determined that the methanol molecules are intercalated into the region between amine and phosphate groups at the lipid hydrophilic head. The dipole of CH₃ groups of methanol and lipid head, and the water O-H all adopt the same orientation directions. The introduction of methanol into the lipid hydrophilic head group can strongly perturb the entire length of the alkyl chains, resulting that the signals of CD₃ and CD₃ groups from both leaflets can not cancel each other.     

Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy

Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm^?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak.     

A Highly Sensitive Femtosecond Time-Resolved Sum Frequency Generation Vibrational Spectroscopy System with Simultaneous Measurement of Multiple Polarization Combinations

Characterization of real-time and ultrafast motions of the complex molecules at surface and interface is critical to understand how interfacial molecules function. It requires to develop surface-sensitive, fast-identification, and time-resolved techniques. In this study, we employ several key technical procedures and successfully develop a highly sensitive femtosecond time-resolved sum frequency generation vibrational spectroscopy (SFG-VS) system. This system is able to measure the spectra with two polarization combinations (ssp and ppp, or psp and ssp) simultaneously. It takes less than several seconds to collect one spectrum. To the best of our knowledge, it is the fastest speed of collecting SFG spectra reported by now. Using the time-resolved measurement, ultrafast vibrational dynamics of the N-H mode of α-helical peptide at water interface is determined. It is found that the membrane environment does not affect the N-H vibrational relaxation dynamics. It is expected that the time-resolved SFG system will play a vital role in the deep understanding of the dynamics and interaction of the complex molecules at surface and interface. Our method may also provide an important technical proposal for the people who plan to develop time-resolved SFG systems with simultaneous measurement of multiple polarization combinations.     

Orientation and Structure of Ionic Liquid Cation at Air/[bmim][BF4] Aqueous Solution Interface

The watermiscible room temperature ionic liquid 1butyl3methylimidazolium tetrafluorob orate （[bmim] [BF4]） is a model system for studying the interactions between ionic liquid and water molecules. In this work the orientational structure of the low concentrated aqueous solution of [bmim] [BF4] at the air/liquid interface was investigated by sum frequency gener ation vibrational spectroscopy. It has been found that at very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented with a fairly small tilting angle at the surface. When the concentration increased, the cation ring tended to lie flat at the surface, and the gauche defects of the butyl chain decreased due to the intermolecular chainchain interactions and the consequent more ordered interfacial molecular arrangement. Additionally, the antisymmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, showing that there exists more than one kind of orientation or chemical environment for the butyl CH3 group. These results may shed new light on understanding the surface behavior of watermiscible ionic liquids as well as the imidazolium based surfactants.