Fluorescence Studies of Melanin by Stepwise Two-Photon Femtosecond Laser Excitation

Fluorescence of synthetic melanin in the solvents H₂O, KOH, ethylene glycol monomethyl ether, and dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120-fs pulses with photon flux densities of 10²⁷ cm^–2.S^–1. Compared to the one-photon (400-nm)-induced fluorescence of melanin, the overall spectral shape is red-shifted and shows a strong environment sensitivity. The decay of the two-photon-induced fluorescence (TPF) of melanin is three-exponential, with a shortest main component of about 200 ps. The results of the TPF studies in line with the unique light absorption property of melanin of a monotonously decreasing absorption spectrum between the near UV-region and the near infrared region indicate that the TPF is realized via stepwise absorption of two 800-nm photons. In comparison to the simultaneous absorption of two photons, the stepwise process needs lower photon flux densities to get a sufficient population of the fluorescent level. This stepwise process offers new possibilities of selective excitation of melanin in skin tissue in a spectral region where there is no overlap with any absorption of another fluorescent tissue component. The first results with different samples of excised human skin tissue (healthy, nevus cell nevi, malignant melanoma) suggest that fluorescence excited in this way yields information on malignant transformation.     

Fluorescence Study of Excited-State Relaxation Processes of 2-Pyridyl-5-Aryloxazoles

Anomalously high fluorescence spectrum Stokes' shifts of 2(4-pyridyl)-5-(4-methylphenyl)oxazole, 2(4-pyridyl)-5-(4-methoxyphenyl)oxazole, 2(4-pyridyl)-5-(4-N,N-dimethylaminophenyl)oxazole have been found. The fluorescence spectra of the compounds studied are shifted to longer wavelengths as the solvent dielectric constant increases. The dipole moments of these compounds increase by 3–4 times upon excitation. The fluorescence spectra of the compounds investigated are shifted to shorter wavelengths, and the fluorescence quantum yields increase as the temperature decreases. The fluorescence rate constant of 2(4-pyridyl)-5-(4-N,N-dimethylaminophenyl)oxazole changes slightly as the temperature decreases from 293 K (relaxed state) to 114 K (mainly nonrelaxed state). It was concluded that the anomalously high fluorescence Stokes' shift of 2-pyridyl-5-aryloxazole derivatives is caused by solvent orientation relaxation.     

CdSe:Nd纳米晶及CdSe:Nd@SiO2核壳结构的合成及发光性能

利用有机相法合成Nd³⁺掺杂CdSe纳米晶（CdSe∶Nd）,通过X射线粉末衍射（XRD）、透射电镜（TEM）、紫外吸收光光谱及荧光光谱表征,证明Nd³⁺已经成功掺入到CdSe的晶格中。与纯CdSe纳米晶相比,CdSe∶Nd纳米晶的结构仍为立方晶型,且形貌近似球形,均匀分散,粒径约为2~4 nm。紫外吸收峰和荧光发射峰都发生红移,而且掺杂后的CdSe∶Nd纳米晶量子产率也提高,这可能是由于掺杂Nd³⁺引入了新的杂质能级,带隙减小。为了实现CdSe∶Nd纳米晶的可加工性和功能性,通过微乳法合成SiO₂壳包覆的CdSe∶Nd纳米球（CdSe∶Nd@SiO₂纳米球）,CdSe∶Nd@SiO₂纳米球呈均匀球形,直径约为100~115 nm,并且包壳后的CdSe∶Nd@SiO₂纳米球发射峰（581 nm）与CdSe∶Nd纳米晶（598 nm）相比,发光强度提高且发射峰蓝移,蓝移约为17 nm,可能是因为SiO₂壳可以减少纳米晶表面的非辐射跃迁以及改善表面缺陷导致的。     

Fluorescence and Electronic Structure of the Laser Dye DCM in Solutions and in Polymethylmethacrylate

The photophysical properties and polarization of the fluorescence of the laser dye 4-dicyanomethylene-2-methyl-6-(p(dimethylamino)styryl)-4H-pyran (DCM) in solutions of different polarities and in polymethylmethacrylate (PMMA) at 293 and 77 K were investigated by stationary and pulsed fluorescent spectroscopy. Based on the data of polarized fluorescence, an oscillator model has been suggested, according to which the electronic absorption spectrum of DCM in the 240–500-nm range is formed by at least three electron transitions. The quantum yield of fluorescence _f increases linearly with increase in the polarity of a solvent in a toluene dimethylsulfoxide (DMSO) mixture from 0.08 (toluene) to 0.80 (DMSO). An increase of _f to 0.90 was also observed on increase in the rigidity of the medium by freezing a solution of DCM in n-propanol at 77 K or introducing of a dye to the polymeric matrix of PMMA at room temperature ( _f = 0.76). The fluorescence-decay kinetics of DCM in toluene, n-propanol, and PMMA at 293 K follows a biexponential law, whereas in n-hexane and vaseline oil at 293 K and in n-propanol at 77 K it follows a monoexponential law. The mechanisms underlying radiationless deactivation of the electron-excitation energy in solutions and in polymeric media are discussed.     

Relation Between Fluorescence Decays and Temporal Evolution of Excited States

A differential equation system describing the temporal evolution of excited substates and fluorescence emission were tested using a DOPRI algorithm. The numerical solutions show that there is significant difference in the measurable parameters according to the type of connectivity among the excited substates. In the globally connected case, the fluorescence emission exhibits a double exponential behavior, and the first moment of the emitted spectrum decays with stretched exponential characterized by β < 1. In the diffusive case the fluorescence emission cannot be always fitted with double exponential, and the first moment of the emitted spectrum may decay with stretched exponential characterized by β > 1. Details of modeling and the possibilities of drawing conclusions are also presented.     

Using two-dimensional photon echo spectroscopy to probe the fine structure of the ground state biexciton of CdSe nanocrystals

The origin of the fine structure of the ground state single- and biexciton of CdSe nanocrystals is reviewed, along with the theoretical framework used to describe these states. Calculations were performed to determine the transition dipole moments of optically allowed transitions from the single- to biexciton fine structure states. Two-dimensional photon echo spectroscopy measurements for a sample of CdSe nanocrystals are reported. The two-dimensional electronic spectrum at a population time of 0 fs is analyzed using a simulation based on k.p theory predictions of the exciton and biexciton manifolds of states. The analysis suggests that a particular excited state absorption transition from the single- to biexciton fine structure dominates the 2D spectra. These excited state absorptions are clearly resolved in 2D spectra and the method therefore has promise for gaining clearer insights into quantum dot spectroscopy.     

太阳辐射对溶解有机质荧光光谱特征的影响

利用三维荧光光谱研究了太阳辐射对溶解有机质(DOM)荧光光谱特征的影响。实验结果表明,太阳辐射使DOM的荧光发射光谱强度减弱。在三维荧光光谱图中,红枫湖样品的荧光峰A,C,南明河样品的荧光峰A,B,C,尤其是荧光峰C的λ_Exmax／λ_Emmax位置随着太阳辐射时间的延长而逐渐向短波长移动。从腐殖酸样品的三维荧光光谱图中则发现,原始样品荧光峰位置出现在λ_Ex／λ_Em=275/500 nm处,光照作用后出现两个荧光峰：λ_Ex／λ_Em=245/450 nm和310/450 nm。太阳辐射对DOM中不同荧光组分的光降解速率存在差异,从荧光峰A和C的比值r(A, C)随太阳辐射时间的变化趋势得知,荧光峰C比荧光峰A的荧光丢失速率更大,即对于太阳辐射作用更敏感。在整个光降解过程中,红枫湖、南明河DOM的r(A, C)值比标准腐殖酸Fluka HA的r(A, C)值的变化更显著。通过本研究表明,可以利用三维荧光光谱所提供的完整的光谱信息来研究DOM在光降解过程中的光谱特征以及物理化学组成等变化。     

紫外光辐照对偶氮苯光谱特性的影响

采用紫外可见吸收、稳态荧光光谱和荧光衰减测量技术,观察了4-硝基-4'-氨基偶氮苯(NAA)在氯仿溶液中的光致变色现象及分子的跃迁能级和能级寿命。结果表明:NAA分子的顺反异构化反应并非一级反应;分子处于反式异构体和顺式异构体的跃迁能分别为255.6,240.2 kJ/mol;分子处于反式异构体的能级寿命为0.02 ns,处于顺式异构体的能级寿命为3.54 ns。顺反异构体的含量、NAA分子之间及NAA分子与溶剂分子之间的相互作用影响NAA分子激发态的能级寿命。     

紫外光区S_2分子激光诱导荧光光谱

利用真空脉冲放电超声射流气体束(H_2S/Ar~3%混合气体)的方法产生了气相S_2分子,并研究了30 400~34 400cm~(-1)范围内S_2分子的时间分辨和基态振动频率分辨的激光诱导荧光光谱,获得了184支谱带的高分辨率(0.1cm~(-1))和低分辨率(0.3cm~(-1))转动光谱。实验观测并归属了S_2分子B_u~(Σ-)-X~3Σ_g~-和B″~3Π_u-X~3Σ_g~-共84支振动跃迁,分析得到了激发态B~3Σ_u~-态ν=0~9和B″~3Π_u态ν=2~12的分子常数以及B~3Σ_u~-态的基态平衡分子构型。由于S_2分子B~3Σ_u~-与B″~3Π_u态之间存在微扰,这两个电子激发态的振动能级间隔、自旋分裂常数和自旋-轨道分裂常数变化不规律,转动跃迁强度和跃迁选择定则存在异常,利用~3Σ-~3Π的齐次微扰哈密顿量定性地对这些异常光谱进行了解释,进一步丰富了S_2分子紫外区低能电子激发态的信息。     

Hydrodynamics of Plasma Interaction with an Intense Laser Field

A set of moment relations, which can describe the charged fluids response to an intense pump laser, and a linearization substitution relation, which is more appropriate as compared with the past treatment, are given by theoretical analyses. The relevant equations of state (adiabatic and isothermal), momentum and energy equations are derived self-consistently.The dispersion relations of the electron plasma wave and the ion acoustic wave driven by an intense pump laser field are-obtained. The results show that the frequencies of both the excited electron plasma wave and the excited ion acoustic wave have a great modification in the case of strong pump. The former bears out the theoretical result obtained from Vlasov equation and the later is consistent with experimental observations. It is proved that the zero-frequency component of the laser light wave contribution to the plasma pressure tensor is un-neglected,which implies a greatly change to the wave excitation properties, particularly in the direction of parallel or approximately parallel to the laser field vector.     

Scanning two photon fluorescence microscopy with extended depth of field

We describe the design and implementation of a light efficient binary phase-only diffraction optical element which converts a laser beam into an annular ring with the loss of less than 25% of the overall power. The use of annular illumination permits us to obtain an increase in the axial extent of the point spread function of a microscope objective without compromising the lateral resolution. The light efficiency of the system permitted two photon fluorescence images with extended depth of field to be obtained. The combination of two such extended depth of field images obtained at different parallaxes allowed the generation of high resolution two photon stereo pairs.     

Fluorescence Properties of Paracetamol During Interactions with Mitochondria

Paracetamol interaction with rat liver mitochondria in respiration media in the presence of succinate was the focus of this experiment. Fluorescence of paracetamol in water was studied by three-dimensional synchronous fluorescence fingerprint (SFF) and by excitation emission matrix (EEM). The direct molecular interactions of paracetamol and mitochondria were studied by fluorescence polarization technique. The paracetamol fluorescence maximum of SFF was Δλ = 110/λ_ex = 320 nm, F_max = 508 nm, and EEM maximum was λ_ex = (320 nm)/λ_em = 425 nm, F_max = 508. The fluorescence polarization results showed nonsignificant elevation of fluorescence polarization after addition of paracetamol into mitochondria in comparison to the control mitochondria group without paracetamol at time point t = 0. Paracetamol probably covalently bound to the mitochondrial surface proteins at time point t = 0, but paracetamol also entered mitochondria, which was observed as nonsignificant decline of fluorescence polarization during 30 min in the paracetamol-treated group. The practical advantages of spectral techniques (EEM, SFF, fluorescence polarization) are high sensitivity, reproducibility, minimal quantity of material, and capability to measure the mitochondrial autofluorescence.     

Determination of the Dipole Moments of Excited Complexes of Polar Compounds in Proton-Donor Solvents by the Quenching of their Fluorescence

A method of determining the permanent dipole moment of polar compounds in the excited S₁ state by the dependence of the rate of radiationless conversion of the energy of this state on the polarity of the solvent (medium) is proposed. The method was used for determining the dipole moment µ_e of hydrogen-bonded complexes formed by 4-amino-, 4-methylamino-, and 4-dimethylamino-N-methylphthalimides (4AMP, 4MAMP, and 4DMAMP) in proton-donor solvents. It has been established that µ_e = 11.68 D for 4AMP in alcohols, µ_e = 11.84 D for 4MAMP in alcohols, and µ_e = 13.19 D for 4DMAMP in alcohols and water.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 186–191, March–April, 2005.     

Structure and Luminescent Properties of the 4-Arylidene-2-Aryl-5-Oxazolones(Azlactones)In Solution and Crystalline State

Experimental and theoretical evaluations have proven that very low fluorescence quantum yields of azlactones in solution are not caused by an efficient inter system crossing from S₁*(ππ) to T*(nπ) states, but rather from solvation and steric effects, that result in non-planarity of the molecular system. High fluorescence quantum yields in the solid state are attributed to the planarity of the azlactone molecule upon packing into the crystal lattice. Supporting evidence was found upon observation of the excited state proton transfer (ESIPT) bands of fluorescence emissions of o-hydroxyarylidene derivatives. The photoinstability of azlactones in liquid states are attributed to photochemical E-Z isomerization and cleavage of the hetero ring α to the carbonyl group.     

355 nm纳秒激光作用下荧蒽的时间分辨荧光光谱特征

多环芳烃由于具有三致(致癌、致畸、致突变)特性,其在环境中的检测受到人们广泛关注。利用时间分辨光谱技术,研究了荧蒽乙醇溶液的荧光光谱随延时时间和门宽改变的特性。研究了不同浓度荧蒽的时间分辨荧光光谱特性,以原始浓度的荧蒽为初始溶液,通过逐级稀释的方式,最终将原始溶液稀释16倍,拟合了不同稀释倍数下的荧蒽荧光强度衰减随延时时间变化的动力学曲线,得到了不同浓度荧蒽的拟合荧光寿命。研究结果表明,荧蒽的荧光光谱特性与光谱仪探测器延时时间和门宽宽度密切相关。固定延时时间,随着光谱仪门宽宽度的变化,荧蒽的荧光强度随着门宽的增大而逐渐增强。固定门宽,改变延时时间的过程中,荧蒽的荧光强度随延时时间呈现先增大,后减小的趋势。荧蒽的荧光强度随延时时间的衰减过程符合指数衰减过程,将荧蒽乙醇溶液进行逐级稀释,荧蒽荧光强度与延时时间的衰减进行指数拟合后,得到不同稀释倍数的荧蒽乙醇溶液的衰减动力学参数,随着稀释倍数的增大,拟合得到的荧蒽荧光寿命增大。多环芳烃时间分辨光谱特征的研究,可以为环境中多环芳烃的检测提供技术基础,由于不同荧光物质具有特征的荧光寿命,因此,可以利用多环芳烃与环境中其他荧光物质的不同荧光寿命特性,准确识别环境中的多环芳烃污染物。     

不同表面修饰和尺寸的纳米银的荧光增强效应的研究

分别以柠檬酸钠(CTS)和聚乙烯吡咯烷酮(PVP)为表面修饰剂,采用化学法制备纳米银(分别标记为Ag-CTS和Ag-PVP)。当甲基橙溶液中加入Ag-CTS或Ag-PVP时,S₂→S₀和S₁→S₀荧光强度分别明显减弱和增强。不同的是Ag-PVP引起S₂→S₀和S₁→S₀荧光强度减弱和增强的幅度更大,且S₂→S₀荧光峰出现红移。当甲基橙溶液中加入Ag-CTS时,S₁→S₀荧光强度随时间延长逐渐增强,而加入Ag-PVP时,S₁→S₀荧光强度则不随时间而变化。颗粒尺寸较大的片状结构纳米银引起S₁→S₀荧光强度增强比率较小。甲基橙溶液浓度越低,S₁→S₀荧光强度增强比率越大。研究结果表明,纳米银表面吸附的不同表面修饰剂通过影响纳米银与甲基橙分子间距离进而影响纳米银的荧光增强效应;纳米银尺寸大小则由于所产生的局域面表面等离子共振特点等差异影响其局域场增强效应。     

毛细管阵列电泳检测过程中的杂散光分析

建立了激光诱导荧光检测系统.采用光学仿真方法,建立与实际光学系统相同的模型.模拟表明:聚焦光束扫描毛细管阵列,轴上光束入射到毛细管内径中心时产生的杂散光最大,在两边逐渐减小.由毛细管产生的杂散光的平均光强是无毛细管时的2.725倍,说明由它产生的杂散光比较严重.对不同大小的内径产生的杂散光影响进行了分析比较,增大毛细管的内径,杂散光增大,但毛细管内径减小会使进样量少,检测困难,同时还会加大清洗与灌胶的难度.综合考虑,选取内径为50 μm的毛细管较为合适.利用自行设计的激光诱导荧光检测系统扫描毛细管阵列,进行了杂散光检测实验,光电倍增管记录所收集到的信号,作出了激光束扫描毛细管的不同位置时的杂散光信号强度分布图,实验与模拟结果相一致.     

磁力搅拌和激光辐照处理水溶液对ICP辐射的增强作用

在电感耦合等离子体原子发射光谱法中,仍然是以溶液方式把样品引入到光源。为了提高对水溶液的处理效果,改变其物理性质,采用磁力搅拌与激光辐照相结合的手段,测量了不同实验条件下水溶液的表面张力和粘度。将处理后的溶液引入到电感耦合等离子体(ICP)中,测量了样品元素的谱线强度和信背比以及等离子体的激发温度和电子密度。实验结果表明：在磁力搅拌器的转速为1 197 r·min-1、激光输出功率密度为0.227 6 W·cm-2和样品处理时间为15 min的优化条件下,溶液的表面张力和粘度比未经处理时的分别减小了27.85%和8.66%;样品元素谱线As 188.980 nm,Cd 214.439 nm,Cr 267.716 nm,Cu 324.754 nm,Hg 253.652 nm和Pb 220.353 nm的强度分别提高了32.07%,65.36%,18.27%,32.29%,19.38%和54.28%,信背比分别增大了25.13%,60.97%,18.18%,27.69%,21.11%和48.93%。通过测量等离子体的激发温度和电子密度两个主要参数,在一定程度上解释了水溶液被处理以后等离子体辐射增强的原因。这种预处理水溶液的方法能够明显提高ICP发射光谱强度,而且与单独利用激光辐照水溶液的方法比较,明显缩短了处理样品的时间,有利于提高工作效率。此方法操作简便,在处理样品溶液过程中不存在二次污染问题,便于推广使用。     

Wavepacket Propagation Algorithm with Wavefunction Splitting:Application to Calculate Harmonic Emission of Atom in Intense Laser Pulse

1.IntroductionInrecentyears,intenselaserphysicshasbecomeanactivefield[1'2].WiththelaserintensityincreasingmanynewPerformanceshaveemerged,suchasabovethresholdion-ization(ATI)[3J,high-orderopticalharmonicemission[4~6Jandstabilizationofatomsinintenselaserfield[7~9J.Uptodate,thelaserint6nsityoflO"W/cm'hasbeenobtained.ItismuchgreaterthantheIimitationoflO"W/cm'forwhichthePerturbativetheoryisva1id.Therefore,nonperturbativemethodshavebeenemployedtodealwiththeintensefieldphysicalproblems.Generall…     

微生物接种堆肥胡敏酸三维荧光特性研究

为提高堆肥效率,试验采用接种微生物堆肥技术,通过对堆肥样品中胡敏酸三维荧光光谱特性进行动态分析,探讨了微生物接种对堆肥腐殖化进程的影响。在堆肥初期,胡敏酸只产生一个类富里酸荧光峰(peak a,Ex/Em为330/440)。在堆肥的336 h,与对照(RUN_ck)相比,接种微生物堆肥(RUN_mic)胡敏酸三维荧光光谱中类富里酸特征峰(peak a,Ex/Em约为350/440)荧光强度明显减少,而类胡敏酸荧光峰(peak b,Ex/Em约为390/470)强度则略有增加;并且peak a和peak b对应的激发、发射波长均有一定程度的红移。胡敏酸元素分析表明,堆肥336 h,RUN_ck胡敏酸分子中C/H和O/C依次增加9.68%和10.91%;RUN_mic中胡敏酸C/H和O/C依次增加了11.11%和20.37%,表明RUN_mic中胡敏酸分子缩合度明显。以上结果证实,接种微生物可明加快堆肥腐殖化进程,进而提高堆肥效率。