由激光诱导荧光方法探测光碎片分子取向的理论研究

采用密度矩阵方法 ,推导了从激光诱导荧光 (LIF)强度中抽出光碎片取向参数的表达式 .光碎片的取向由分子态多极矩描述 .用于解离母分子和激发碎片分子的激光均为线偏振光 ,而探测荧光为非偏振光 .激光诱导荧光强度是光碎片分子初始态多极矩、线强度因子和解离—激发几何因子的函数 .光碎片的取向参数可以由测量荧光偏振比和计算动力学因子而获得     

Attenuation corrections for in-cylinder NO LIF measurements in a heavy-duty Diesel engine

Quantification of the nitric oxide (NO) concentration inside the cylinder of a Diesel engine by means of laser-induced fluorescence (LIF) measurements requires, amongst others, knowledge of the attenuation of the ultraviolet radiation involved. We present a number of laser diagnostic techniques to assess this attenuation, enabling a correction for laser intensity and detection efficiency of the raw NO LIF data. Methods discussed include overall laser beam transmission, bidirectional laser scattering (bidirectional LIF), spectrally resolved fluorescence imaging, and Raman scattering by N₂. A combination of techniques is necessary to obtain the complete attenuation of laser beam and NO fluorescence. The overall laser beam transmission measurements and bidirectional LIF measurements (the latter yielding spatially resolved transmission) provide evidence of a non-uniform attenuation distribution, with predominant attenuation within or near the piston bowl. Fluorescence imaging of multiple vibrational bands through a spectrograph is shown to be a powerful method for obtaining spatially resolved data on the transmission losses of fluorescence. Special attention is paid to the role of CO₂ and O₂ as UV light absorbers, and the consequences to different excitation-detection schemes for NO. PACS 82.33.Vx; 42.62.Fi; 33.20.t     

Integrated state multipoles and plane asymmetry

The relationship between the integral alignment and orientation coefficients and the polarizations of the decay photons measured in an experiment where the scattered electrons are not observed is expounded. The determination of some integrated state multipoles is shown to provide also a simple checking of the plane symmetry invariance of an atomic collision.     

Role of rotational state-selected for nonadiabatic alignment: OCS molecules in femtosecond laser fields

Nonadiabatic alignment by intense nonresonant laser fields is a versatile technique to manipulate the spatial direction of molecules. By solving the time-dependent Schrödinger equation numerically the degree of alignment of the molecules initially in different rotational state are calculated and the results show that the degree of alignment strongly depends on the initial rotational state. Thus, the present study indicates that, for obtaining a high degree of alignment for molecules, appropriate selection of molecular rotational states is necessary.     

Angular momentum polarization in molecular collisions : Classical and quantum theory for measurements using resonance fluorescence

Inelastic or reactive collisions typically produce an anisotropic distribution of rotational angular momentum. An explicit and general treatment is given for the intensity and polarization of resonance fluorescence from molecules produced in such processes. Both classical and quantum results are expressed in terms of bipolar harmonics and state multipoles formed from linear combinations of density matrix elements. The treatment provides an inversion procedure for determining moments of the rotational angular momentum distribution ; twelve independent moments can be obtained. The combinations of angular momentum operators involved are even in eight of these moments and odd in four, with respect to reflection in a plane containing the initial and final relative velocity vectors. Measurements of the even moments require linearly polarized excitation and fluorescence, whereas measurements of the odd moments require circularly polarized excitation. The requisite experimental geometry and other practical aspects are discussed. In the three appendices are discussed the classical limits of transition intensities, a density matrix treatment of atom-rigid-rotor collisions, including analysis of state multipole symmetries ; and the coupling coefficients for parallel angular momenta.     

Optical pumping with laser-induced-fluorescence

We describe a new procedure for optical pumping that is based on laser-induced fluorescence (LIF). The procedure is demonstrated by optically exciting a sample of Rb⁸⁵ atoms, which then creates a population imbalance between the ground state hyperfine levels of Rb⁸⁷ by “LIF depopulation pumping”. Though optical pumping with this technique increases the intensity dependent light-shift coefficient (i.e., ac Stark shift) of the Rb⁸⁷ 0-0 hyperfine transition, it reduces the frequency dependent light-shift coefficient by at least an order of magnitude. Since the stabilization of the diode laser wavelength is a significant challenge in the development of laser-pumped gas-cell atomic clocks, it is anticipated that optical pumping with LIF will be of benefit to atomic clock technology.     

Effect of tunneling ionization on dynamic alignment and post-ionization alignment of nitrogen molecules irradiated by different laser intensities

The dynamic alignment and post-ionization alignment of nitrogen molecules are investigated while considering the effect of tunneling ionization. The effects of tunneling ionization on the angular distribution are calculated when the molecules are irradiated by different laser intensities. The results show that laser intensity directly affects the time and extent of dynamic alignment. Furthermore, the extent of post-ionization alignment is not only determined by laser intensity but also affected by the final extent of dynamic alignment. The post-ionization alignment will dominate during the process of molecular (or molecular ion) rotational alignment for femtosecond laser pulse. The time of tunneling ionization is a significant factor to the final ensemble angular distribution of molecular ions when laser intensity is low.     

Laser coupling to nuclei via collective electronic oscillations—a simple heuristic model study

The induction field coupling of laser-driven collective electronic oscillations has been proposed as an interlevel transfer mechanism for a gamma-ray laser. Using a heuristic classical model for the electron oscillations, we verify in the model six general features which we conjecture are valid for laser-induced collective oscillations: (1) enhancement of the electric field seen by the nucleus, (2) harmonic generation, (3) generation of higher multipoles, (4) odd (even) order multipoles couple to odd (even) harmonics, (5) there exists an optimum laser intensity for each multipole order and harmonic, and (6) for a given multipole order, the optimum intensity increases with harmonic number. We briefly discuss the relevance of these results to a proposed interlevel transfer experiment in ²³⁵U.     

Experimental investigation of the effect of collisions and temperature on degenerate four-wave mixing

An experimental investigation (including a comparison with a simple theoretical model) of the effect of buffer-gas composition, pressure and temperature on resonant Degenerate Four-Wave Mixing (DFWM) has been performed. The DFWM signal from NO in a quartz cell was measured and the effect of quenching as well as elastic (phase-changing) collisions was studied by varying the total and partial pressures of N₂ and CO₂ as buffer gases. It was found that the DFWM signal first slowly increased with buffer-gas pressure (up to 10 mbar) and then rapidly decreased. It was furthermore found that the DFWM signal was considerably less sensitive to quenching collisions as compared to Laser-Induced Fluorescence (LIF) (for laser intensities approximately equal to half the DFWM saturation intensity of the transition). On the other hand, while LIF is virtually insensitive to elastic collisions, DFWM displays a larger sensitivity to elastic collisions than to quenching collisions. The DFWM saturation intensity was found to increase with buffer-gas pressure (although slower than expected). When varying the temperature of the gas composition, it was found that the DFWM signal decreased markedly with increasing temperature. This decrease is too fast to be explained solely by a change in the population of the molecular state probed by the laser.     

Molecular multipoles and (hyper) polarizabilities from the Buckingham expansion:revisited

The Buckingham expansion is important for understanding molecular multipoles and (hyper) polarizabilities. In this study, we give a complete derivation of the Buckingham expansion in the traced form using successive Taylor series. Based on the derivation results, a general Buckingham expansion in the traced form is proposed, from which highly accurate numerical calculations using the finite field method can be achieved. The transformations from the traced multipoles and multipole-multipole polarizabilities to the corresponding traceless counterparts are realized with an auxiliary traced electric field gradient. The applications of thefinite field method in this study show good agreements with previous theoretical calculations and experimental measurements.     

高能同步辐射光源验证装置振动线准直系统结构设计

振动线准直技术是为了满足新一代同步辐射光源对超低发射度的要求而正在研究的准直技术。新光源要求磁铁具有极高的准直精度,同一支架上的磁铁间准直精度要求小于±30μm。介绍了高能同步辐射光源验证装置振动线准直系统的设计,描述了振动线准直系统的具体机械结构方案、电路结构方案和运动控制与数据采集方案等。     

Is cosmic parity violation responsible for the anomalies in the WMAP data?

In this Letter, I argue that a parity violating extension to general relativity can simultaneously explain the observed loss in power and provides a first step for explaining the alignment at a preferred axis (‘Axis of Evil’) in the low multipole moments of the WMAP data. This observational possibility also provides an experimental window for an inflationary leptogenesis mechanism arising from large-scale parity violation. Similar to the arguments of Contaldi, Peloso, Kofman and Linde, large scale power is suppressed from the backreaction of the parity violating term by inducing a velocity dependent potential. We also argue that this modification can supress power of odd parity multipoles on large scales, a pattern that is observed in the WMAP alignment anomaly.     

Single-pulse collision-insensitive picosecond planar laser-induced fluorescence of OHA 2 Σ + (v′ = 2) in atmospheric-pressure flames

Single-pulse two-dimensional picosecond Laser-Induced Fluorescence (LIF) imaging of the OH density in a single quantum state was performed for the first time, using a premixed methane-oxygen flame at atmospheric pressure. A picosecond, excimer-Raman-laser system (268 nm, 470 ps FWHM) was used for excitation of OH. The fluorescence from the laser sheet was imaged onto a fast gated intensified camera with a 400 ps gate width. The short laser pulse minimizes the collisional redistribution of population in the ground state during excitation, while the short camera gate avoids significant quenching of the excited-state fluorescence. The fluorescence signal obtained in this way is a direct measure of the population in a selected quantum state. In contrast to common nanosecond LIF signals no corrections on variations of the collisional environment are necessary. This collision-insensitive approach to two-dimensional LIF yields an OH detection limit of 10 ppm in a cube of 330 µm per side with a single 1 mJ laser pulse. A rate-equation model is used to estimate the effects on the observed signal of fluctuations in pulse energy and duration, laser-camera timing jitter, and spatial variations in the collisional environment.     

Effect of laser intensity and of lower-state rotational energy transfer upon temperature measurements made with laser-induced fluorescence

4 –air flame, with OH at 2000 K. We calculate the ratio of LIF intensities that would be induced by doubled dye-laser light near 283 nm, by means of the A←X, 1←0, P₁(7), and Q₂(11) transitions in OH. Here we show that the ratio of LIF signals from those two transitions, and thus the deduced temperature, is sensitive to laser intensity. That is caused mainly by the competition between laser-pumping of molecules out of the lower rotational state and of rotational energy transfer (RET) collisions into that state. A-state collisional effects are normally important, but are minimized here by assuming that they are the same for both transitions. The laser spectral intensity dependence of the fluorescence ratio depends heavily upon the value of the RET coefficients within the X-state. While RET reduces the sensitivity of the observed signal to the laser spectral intensity, the conversion of a measured fluorescence ratio to temperature is particularly difficult. That is because RET rates, and quenching rates, can be a function of local conditions and of the rotational state being populated. Two different models are used to demonstrate these effects, and both predict large effects upon temperature. Received: 19 February 1998/Revised version: 16 June 1998     

Two Dimensional Rotational Temperature Measurement by Multiline Laser Induced Fluorescence of Nitric Oxide in Combustion Flame

Two-dimensional rotational temperature measurement was performed in a stable combustion flame of premixed butane and oxygen using multiline laser induced fluorescence (LIF) of nitric oxide molecules. Multiple rotational absorption lines of A²∑⁺Π;X²II(0,0) Q₁ and Q₂ lines were excited by laser light around 226 nm, and the LIF signal was observed by an image-intensified digital camera. Temperature was determined through least squares fitting correlation between LIF intensity and excitation rotational quantum number for the Boltzmann distribution function. The measured LIF intensity was approximated by the Boltzmann distribution with good accuracy, and the temperature obtained was between 500 K and 1800 K for the test flame. The measuring error of the temperature was evaluated and found to be 80 K, which corresponded to 8% of the measured fluorescence intensity. The two-line LIF scheme was evaluated by different pairs of excitation lines (Q₁(31.5)/Q₁(16.5) and Q₁(18.5)/Q₁(16.5)) for comparison with the multiline LIF approach. Temperature which was obtained by two-line LIF scheme corresponded well with multiline LIF results for Q₁(31.5)/Q₁(16.5) excitation. However, for Q₁(18.5)/Q₁(16.5) excitation, the obtained temperature did not agree with the multiline LIF result because the population of rotational states J=18.5 and J=16.5 is similar at high temperatures. We found that two-line LIF temperature measurement was reliable when excitation lines were suitably selected.     

Local Stability of Perfect Alignment for a Spatially Homogeneous Kinetic Model

We prove the nonlinear local stability of Dirac masses for a kinetic model of alignment of particles on the unit sphere, each point of the unit sphere representing a direction. A population concentrated in a Dirac mass then corresponds to the global alignment of all individuals. The main difficulty of this model is the lack of conserved quantities and the absence of an energy that would decrease for any initial condition. We overcome this difficulty thanks to a functional which is decreasing in time in a neighborhood of any Dirac mass (in the sense of the Wasserstein distance). The results are then extended to the case where the unit sphere is replaced by a general Riemannian manifold.     

Orbital and spin photon angular momentum transfer in liquid crystals

All-optical angular control of the molecular alignment in liquid-crystal films is demonstrated using a laser beam having an elliptically shaped intensity profile. The material birefringence is unimportant, as proven by the fact that good alignment is obtained with unpolarized light. This raises the possibility of achieving optical angular control of transparent isotropic bodies. A general theoretical approach, based on light and matter angular momentum conservation, shows that the optical alignment is due to the internal compensation between the transfer of the orbital and the spin part of angular momentum of the incident photons to the material.     

Nonadiabatic effects in the molecular high-order harmonic generation from two-center molecular ions

The nonadiabatic effects in the molecular high-order harmonic generation (MHHG) from H₂⁺ and its isotope have been theoretically investigated beyond the Born-Oppenheimer approximations. It is found that (i) due to the asymmetric contributions of the harmonic emission on the rising and the falling parts of the laser field, the frequency red-shifts in the MHHG can be found. As the initial vibrational state or the pulse intensity increases, the red-shifts of the harmonics are decreased and even the blue-shifts of the harmonics can be found in the higher pulse intensity with the higher vibrational state. With the increase of the pulse duration, the frequency red-shifts and the blue-shifts in the MHHG are decreased and enhanced, respectively. The shifts of the harmonics from the heavy nuclei are decreased compared with those from the light nuclei. (ii) Due to the coupling of the electron-nuclear dynamics, the non-odd harmonics can be produced at the larger internuclear distance. With the increase of the initial vibrational state or the pulse duration, the generations of the non-odd harmonics can be enhanced. As the pulse intensity increases, the intensities of the non-odd harmonics are first increased and then decreased at the much larger pulse intensity. Due to the slower nuclear motion, the generations of the non-odd harmonics are decreased as the nuclear mass increases. (iii) With the increase of the alignment angle of the molecule ions, the nonadiabatic effects in MHHG (including the shifts of the harmonics and the generations of the non-odd harmonics) are decreased.     

~（23）Na~（39）K分子D~1∏→X~1∑~＋激光感生荧光光谱及强度研究

实验获得了Ａｒ＋激光５１４．５ｎｍ线诱导２３Ｎａ３９Ｋ分子产生的Ｄ１∏→Ｘ１∑＋跃迁荧光谱．通过测量激光感生荧光光谱强度随泵浦功率、热管炉温度及缓冲气压变化规律，详细研究了其跃迁机制．用最小二乘法拟合获得２３Ｎａ３９Ｋ分于Ｘ１∑＋态振动常数．理论计算了各跃迁谱支波长值及Ｆｒａｎｃｋ－Ｃｏｎｄｏｎ因子和光谱强度值，与实验观测值符合得相当好，充分表明本文对激光感生荧光光谱的归属以及对各支谱线振、转量子数的赋值是合理的．     

Laser-induced fluorescence of tracers dissolved in evaporating droplets

Laser-induced fluorescence (LIF)-based spray volume and droplet-size measurements rely on assumptions about the evaporation or accumulation of fluorescent tracers during the evaporation of the droplets. We investigate the time-dependent variation of droplet-size and LIF signal intensity of CO₂-laser-heated evaporating water droplets doped with rhodamine 6G. After an initial decrease of fluorescence intensity by 30% due to temperature-dependent diffusion of oxygen into the droplets, the LIF signal remains constant, indicating that the tracers have fully accumulated in the droplet. This evaporation-independent signal can be used as a reference for Mie-scattering-based droplet surface-area measurements that will allow the sensitive observation of spray evaporation and droplet breakup. PACS 42.62.Fi; 32.50.+d; 42.62.Cf