Investigation of the glow and contracted discharge plasmas in nitrogen by coherent anti-Stokes Raman spectroscopy,optical interferometry,and numerical simulation

The translational temperature in the plasma of glow and contracted discharges is measured using the methods of coherent anti-Stokes Raman spectroscopy and optical interferometry. The current density in the discharge is determined by measuring the electron concentration with optical interferometry and emission spectroscopy. The distribution of nitrogen molecules over vibrational and rotational levels in the ground state, the electron energy distribution, and the time dependence of the gas temperature are numerically found based on a model including the homogeneous Boltzmann equation and balance equations for the concentrations of charged and excited particles and for the gas temperature. The dynamics of transition to the quasi-steady-state distribution of nitrogen molecules over vibrational levels is studied.     

Absorption of ultrashort electromagnetic pulses in plasma with inhomogeneous distribution of density and temperature

The analysis of influence of the inhomogeneous distribution of temperature and density of atoms on the probability of absorption of ultrashort electromagnetic pulses in plasma is carried out. A specific example of divertor plasma of tokamaks and absorption in the Lyman series (at the Ly-α line) is considered. It is shown that the absorption probability may exceed relative populations of excited levels in divertor plasma by several orders of magnitude. The influence of the pulse duration on the probability of excitation of an atomic level is considered. The results suggest a possibility of a sharp increase in a fluorescence signal under the action of an ultrashort pulse.     

Numerical and experimental study of excited light and auto-fluorescence diffusion in teeth

Excited light and corresponding intrinsic fluorescence diffusion inside teeth tissue are an essential problem for light-based carious lesion detection. Based on finite element numerical analysis of diffusion equation, the photon density distribution of both excited light and autofluorescence of 2D premolar teeth model is obtained. The dependence of excited light and autofluorescence density distribution inside the teeth model on the scattering coefficient of enamel (5–25 mm⁻¹) and dentine (100–140 mm⁻¹) is numerically simulated and analyzed. The fitted results reveal that fluorescence intensity decreases exponentially. Optical penetration depth and fluorescence relative depth declined with the increment of scattering coefficient of enamel. And the dentine had the opposite effect. Finally, the experiment of measurement of fluorescence intensity on the teeth surface is conducted and the result is compared with the numerical computation.     

Fluorescence quenching as a method of identification of the character of photoreactions in excited states of molecules

We consider the steady-state monochromatic excitation of a luminophore that has fluorescent products. We consider how the fluorescence of singlet states is affected by different means of quenching highlying excited states such as quenching with impurities, temperature quenching, and shortening the lifetime with induced transitions in the light field. We show that the use of different methods of fluorescence quenching opens new possibilities for studying photoreactions that proceed via excited singlet states. Our consideration concerns a wide range of primary photoprocesses, such as electron density redistribution (internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bond formation, intermolecular relaxation of molecules in solutions, and formation of excimers and exciplexes. The obtained relations have been used to analyze experimental fluorescence spectra of solutions of some organic compounds, including derivatives of 3-hydroxyflavone and 3-aminophthalimide, subjected to quenching by different methods.     

Polarization-induced control of two-photon excited fluorescence in a chiral polybinaphthyl

The fluorescence behavior of a chiral polybinaphthyl excited with 100 fs 800 nm laser pulses was investigated in tetrahydrofuran solution. The peak fluorescence intensity versus the input irradiance was measured to meet a square dependence, giving evidence for two-photon excited fluorescence (TPF). The variations of the TPF intensity were found to be strongly modulated by the different polarized incident lights and tightly depend on the linearly polarized component of the incident light. Furthermore, combining with the characteristics of chiral molecules, the two-photon polarization ratio was studied to reveal the symmetry of the involved excited states.     

Atomic populations and temperatures in mixed plasmas

Atomic populations and temperatures have been measured in a mixed plasma formed by a mercury-lead-iodide arc in a quartz lamp. The excited levels observed for the mercury atom show a Boltzmann distribution when the electron density is above 4 × 10¹⁵ cm^-3. Departures from this distribution occur when the temperature falls below 0.25 eV with an electron density around 2 × 10¹⁵ cm^-3.     

Vibrational excitation and negative ion production in radio frequency parallel plate H2 plasmas

A theoretical study of the vibrational kinetics and attachment in low pressure hydrogen plasmas produced by Radio Frequency (RF) discharges is performed. In particular we study the influence of gas/surface kinetic processes such as the vibrational deactivation and the atomic recombination of molecules. The production of vibrationally excited molecules by the surface recombination of atoms is also considered. The study is realized by means of a self-consistent one dimensional kinetic model, and a parallel plate RF discharge test case is implemented. Results show that surface processes are able to affect the vibrational distribution function (vdf) and the negative ion (H-) density. The effect of vibrational exothermicity of H atom recombination is also discussed as a way to reduce the gap between theory and experimental results. Moreover, it is shown that the H- ion heating by the electric field strongly affects the detachment rate: this effect is specially important for negative ions produced through Rydberg states in this kind of discharges.     

Formation of chemical compounds in a laser plasma

Using the methods of laser-induced fluorescence and emissive spectroscopy, we carried out investigations of the formation of TiO molecules in a laser plasma produced by focusing the radiation of an AYG:Nd³⁺ laser on the surface of a titanium target in air. The radiation flux density varied within the range 10⁸–10¹⁰ W/cm². We investigated the distribution of molecules over internal states and the space-time distributions of Ti atoms in the ground, metastable, and excited states, as well as of TiO molecules in the ground and excited states. We found that gas-phase reactions with participation of Ti atoms in the ground state provide the most probable channel for the formation of TiO molecules; the role of reagents in ionized, excited, and metastable states is of secondary importance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 109–115, January–February, 1999.     

Derivative fluorimetry analysis of new cluster structures formed by ethanol and Water molecules

The ultraviolet （UV） light excited fluorescence spectra of ethanol-water mixture with different concentrations are investigated by derivative fluorimetry. It is found that there are 8 types of luminescent cluster molecules, formed by ethanol and water molecules in different ways, existing in the solution. The peak wavelengths of all these clusters＇ fluorescence spectra are measured and their contents are obtained by measuring the peak values in the second derivative fluorescence spectra. The spectra corresponding to the 8 types of clusters are obtained by Gaussian decomposition. It is found that two kinds of cluster molecules whose peak wavelengths are 330 and 345 nm have an optimal excitation wavelength located at （236 4- 3） nm. This research contributes to the study of ethanol-water cluster structures and their physical and chemical characteristics.     

介质势垒放电(DBD)等离子体中NO荧光发射谱研究

利用介质阻挡放电 (DBD)等离子体技术对大气污染物NO分子进行了光谱研究 ,得到了低气压条件下放电等离子体在 2 1 0～ 2 80nm光谱范围内的荧光发射谱。该谱明显的表现为双峰结构 ,谱线均成对出现 ,强度分布符合Frank Condon原理 ,且最大峰值位置出现在 2 36nm处 ,将该组谱线归属为NO分子的A2 ΣA →X2 Π1 / 2 ,2 / 3 跃迁。荧光产生过程为 :基态NO分子与高能电子发生非弹性碰撞被激发至激发态A2 Σ 后自发跃迁回基态同时辐射出荧光。通过测量等离子体中NO分子和N2 分子 337nm谱线强度随时间的变化关系 ,初步证实了放电等离子体中存在的NO分子的分解机制为 :e NO→N O e,N NO→N2 O ,O NO→NO2 hν。     

Cs(6P)激发态的辐射及与N2碰撞的能量转移

应用激光吸收和荧光方法,测量了Cs(6P)态与N2碰撞的精细结构转移和碰撞猝灭截面。Cs原子被激光激发到6P3/2态,将与泵浦激光束反向平行的检测激光束调到6PJ→8S1/2的跃迁,测量了6PJ激发态的密度及空间分布,由此计算了6PJ→6S的有效辐射率。在T=337 K(蒸气压公式给出Cs密度N0=1.25×1012cm-3)和N2密度2×1016相似文献   

Impacts of air pressure on the evolution of nanosecond pulse discharge products

Based on the nonequilibrium plasma dynamics of air discharge, a dynamic model of zero-dimensional plasma is established by combining the component density equation, the Boltzmann equation, and the energy transfer equation. The evolution properties of nanosecond pulse discharge (NPD) plasma under different air pressures are calculated. The results show that the air pressure has significant impacts on the NPD products and the peak values of particle number density for particles such as O atoms, O3 molecules, N2(A3) molecules in excited states, and NO molecules. It increases at first and then decreases with the increase of air pressure. On the other hand, the peak values of particle number density for N2(B3) and N2(C3) molecules in excited states are only slightly affected by the air pressure.     

采用发射光谱和朗缪尔探针诊断低温低压氢等离子体

使用发射光谱诊断法和Langmuir探针诊断法,测量了螺旋波激发等离子体化学气相沉积装置中产生的氢等离子体的发射光谱和电流-电压曲线。运用日冕模型和Druyvestey方法,对不同放电参数条件下激发态氢原子密度﹑等离子体密度及电子能量分布的变化规律进行了研究。结果表明：激发态氢原子密度随射频功率增大而增大,随工作气压的增大先增大,后缓慢下降。等离子体密度随射频功率增大线性增大,随工作气压的增加也是先增大,出现峰值后缓慢下降。电子平均能量随射频功率的变化是先增大,后达到平衡;随着工作气压的增大逐渐减小。两种诊断方法得到的结果基本相符。在低温低压等离子诊断中,两种诊断方法结合使用,可以得到更准确和更多的等离子体信息。     

高振动激发LiCs与CO_2碰撞中的振动-转动能量转移

受激发射泵浦得到的LiCs高位振动态与CO_2碰撞,研究了LiCs不同能量对碰撞能量转移过程的影响.利用高分辨率瞬时激光感应荧光(LIF)测量得到CO_2(0000,J=2-74)原生转动态分布.转动布居数的半对数描绘给出了一个双指数结构,得到了一个T_a=660±73K[对LiCs(E=4300cm~(-1))]和550±61K[对LiCs(E=3700cm~(-1))]的低能分布,一个T_b=2380±261K[对LiCs(E=4300cm~(-1))]和1980±217K[对LiCs(E=3700cm~(-1))]的高能分布,低能分布属于弹性或弱非弹性碰撞,高能分布属于强非弹性碰撞.转动分布对LiCs能量是敏感的.但弹性与非弹性分支比基本是相同的.在一次碰撞的条件下,测量了各J态的出现和倒空速率系数,它们仅弱依赖于LiCs激发能.出现和倒空速率系数对不同LiCs能量基本一致,是振动-转动/平移弛豫轨道的重要证据.     

Surface-enhanced resonance Raman scattering spectroscopy of single R6G molecules

Surface-enhanced resonance Raman scattering (SERRS) of Rhodamine 6G (R6G) adsorbed on colloidal silver clusters has been studied. Based on the great enhancement of the Raman signal and the quench of the fluorescence, the SERRS spectra of R6G were recorded for the samples of dye colloidal solution with different concentrations. Spectral inhomogeneity behaviours from single molecules in the dried sample films were observed with complementary evidences, such as spectral polarization, spectral diffusion, intensity fluctuation of vibrational lines and even ``breathing' of the molecules. Sequential spectra observed from a liquid sample with an average of 0.3 dye molecules in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1 or 2 molecules. Difference between the SERRS spectra of R6G excited by linearly and circularly polarized light were experimentally measured.     

Heterodyne Detection of Pulsed Co2 Laser Light Scattered from Ion Acoustic Waves in a Plasma

The small angle scattering of light from a pulsed CO2 laser has been used to measure ion acoustic waves excited in a low density plasma. The laser is a hybrid laser consisting of a high pressure TEA discharge and a low pressure continuous discharge. The scattered light is detected by heterodyning, using laser light as the local oscillator. Two methods of providing the laser light local oscillator have been used. The dependence of the scattered light on the ion acoustic wavelength has been measured. The amplitude of the scattered light compared with the amplitude of the waves in the plasma indicates that the heterodyne mixing efficiency is better than 10 percent.     

一种共轭聚合物单分子发色团吸收和发射特性动态演变过程的实时测量

利用频域信息重构的散焦宽场成像测量了Poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT)共轭聚合物单分子发色团的吸收与发射特性及其动态演变过程.通过调制用于激发共轭聚合物单分子的超短脉冲对的相对相位,对单分子荧光进行傅里叶变换的频域测量,跟踪发色团吸收偶极取向变化;通过测量散焦荧光成像光斑探测发色团发射偶极取向变化.研究发现, PFO-DBT共轭聚合物单分子发色团存在吸收和发射偶极取向均保持不变、其中之一变化以及两者同时变化三种情况.这种对共轭聚合物单分子发色团吸收和发射偶极取向演化过程的实时测量可用于分析共轭聚合物构象变化及其对能量转移过程的影响.     

用偏振荧光光谱表征乙醇-水团簇分子的取向行为

采用波长为236nm紫外光激励乙醇水溶液获得荧光光谱并对其偏振特性进行了研究.实验中分别改变入射光的偏振度，在300—400nm波段内进行荧光光谱、水平和垂直偏振荧光光谱检测.实验结果表明，当采用线偏振光照射乙醇水溶液时，其发射的荧光具有一定的偏振性，并计算了其偏振度.经理论分析得到了描述荧光偏振态的四个斯托克斯矢量，进而对该荧光的偏振状态进行了描述，通过对乙醇-水溶液的米勒矩阵的计算，对溶液中分子的取向特性进行了分析.本结果能对乙醇-水团簇分子的结构提供理论参考. 关键词： 荧光光谱 偏振光谱 斯托克斯矢量 米勒矩阵     

Dispersion of phase response of dye solutions using picosecond excitation

Dispersion of phase response (frequency-dependent light-induced variations of the refractive index) of a solution of bleachable dye 3955 due to variation in the populations of the ground and excited states of molecules is investigated using wavelength-tuned picosecond light pulses. On the wavelengths of the dye absorption band the refractive index increases under bleaching, and in the fluorescence band it decreases, what is in qualitative agreement with the expected picture.     

Photon statistics in single molecule fluorescence with Rabi oscillations

An expression for the distribution function of photons detected in non-resonant single molecule fluorescence excited by CW-laser light is derived. Numerical calculations of the photon distribution for fluorescence whose autocorrelation function exhibits Rabi oscillations are carried out. It is found that phase memory does not influence noticeably on the photon distribution function in non-resonant fluorescence.