Femtosecond laser pulse train effect on Doppler profile of cesium resonance lines

We present direct observation of the velocity-selective optical pumping of the Cs ground state hyperfine levels induced by the femtosecond (fs) laser oscillator centered at either D2 (6 ²S_1/2↦6 ²P_3/2, 852 nm) or D1 (6 P_1/2, 894 nm) cesium line. We utilized previously developed modified direct frequency comb spectroscopy (DFCS) which uses a fixed frequency comb for the excitation and a weak cw scanning probe laser centered at the ¹³³Cs 6 ²S_1/2↦6 ²P_3/2 transition (D2 line) for ground levels population monitoring. The frequency comb excitation changes the usual Doppler absorption profile into a specific periodic, comblike structure. The mechanism of the velocity selective population transfer between the Cs ground state hyperfine levels induced by fs pulse train excitation is verified in a theoretical treatment of the multilevel atomic system subjected to a pulse train resonant field interaction.     

溶液中金纳米棒形成过程的光谱动力学分析

通过动态光谱跟踪溶液中金纳米棒的尺度和长径比(AR)的变化,成功地获取粒子的生长过程的动态数据. 该过程分为两步：种子快速形成棒状粒子;棒状粒子在一定AR下的生长. 通过分析体系中的一价金、金粒子和抗坏血酸之间的电荷转移过程,建立了金纳米棒生长过程的电荷转移模型,并很好地解释了动态光谱的实验数据.     

Localized emitting state and energy transfer properties of quadrupolar chromophores and (multi)branched derivatives

In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor-acceptor-donor (D-A-D) electronic push-pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (-C═C- or -C≡C-) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.     

Absorption spectrum of rubidium and cesium dimers by compact computer operated spectrometer

In this paper we are presenting the visible absorption spectrum of both rubidium and cesium vapor in the 570-870 K temperature range. We used a classical absorption spectroscopy experimental scheme with several new features. The first concerns the use of modern, compact, computer operated spectrometer (Ocean Optics HR4000CG-UV-NIR), which allowed us to record spectra instantaneously resulting in higher signal-to-noise ratio. The second improvement is connected with the use of the all-sapphire cells (ASC) enabling work with a high density of alkali atoms within precisely defined vapor column. In the superheated regime (above 700 K) thermal destruction of dimer molecules clearly distinguishes triplet from singlet transitions.     

具有电子推拉结构的多支化分子的光物理特性的研究

利用稳态光谱和飞秒时间分辨荧光亏蚀的技术,研究了不同溶剂中一系列有分子内电荷转移特性的分子的结构与光物理性质的关系,研究体系为三苯胺作为电子给体,2,1,3-苯并噻二唑作为受体的单支分子及其对应的两支和三支分子. 并结合TD-DFT计算进一步解释了实验中所观察到的现象. 三个分子相似的吸收和荧光光谱以及强的溶剂依赖光谱特性表明两支与三支分子激发态与单支分子相似,表明激发态都定域在其中一支上. 激发时多支分子内发生多维电荷转移,然后快速地定域到某一支上发射. 另一方面多支分子相对于单支分子吸收和发射光谱的红     

Vacuum fluctuations of the supersymmetric field in curved background

We study a supersymmetric model in curved background spacetime. We calculate the effective action and the vacuum expectation value of the energy momentum tensor using a covariant regularization procedure. A soft supersymmetry breaking induces a nonzero contribution to the vacuum energy density and pressure. Assuming the presence of a cosmic fluid in addition to the vacuum fluctuations of the supersymmetric field an effective equation of state is derived in a self-consistent approach at one loop order. The net effect of the vacuum fluctuations of the supersymmetric fields in the leading adiabatic order is a renormalization of the Newton and cosmological constants.     

The Effects of γ-Irradiation on the Electrokinetic and Thermal Behaviour of Zirconium Hydroxide

Zirconium hydroxide particles produced by rapid precipitation at pH 10.4, 7 or 2 were subjected toγ-irradiation up to a final dose of 20 MGy. The effects of the γ-irradiation were examined by X-ray powder diffraction, laser Raman spectroscopy, differential scanning calorimetry and microelectrophoretic measurements. It was found that γ-irradiation had no influence on the behaviour of zirconium hydroxide during calcination and subsequent cooling. The results of microelectrophoretic measurements showed that γ-irradiation influences the surface properties of zirconium hydroxide as a function of the precipitation pH. Zirconium hydroxide precipitated at pH 2 proved to be the most susceptible to γ-irradiation, while the sameγ-irradiation had very little (if any) effect on the surface properties of zirconium hydroxide precipitated at pH 10.5. After γ-irradiation, the electrophoretic mobility of zirconium hydroxide precipitated at pH 2 was increased at both low and high pH, thereby indicating an increase in its adsorption capacity. The analogy observed between the pH-dependence of the effects of γ-irradiation on the electrokinetic behaviour of zirconium hydroxide and the influence of ball-milling on the thermal behaviour of zirconium hydroxide [8] suggested that the susceptibility of amorphous zirconium hydroxide increases with decrease of the precipitation pH. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of an optical frequency comb using modified direct frequency comb spectroscopy

We introduce a method for determination of the absolute frequencies of comb lines within an optical frequency comb spectrum. The method utilizes the experimental and theoretical approach of the velocity-selective optical pumping of the atomic ground state hyperfine levels induced by resonant pulse-train excitation. The information on the laser pulse repetition frequency and carrier–envelope offset are physically mapped onto the ⁸⁷Rb ground state hyperfine level population velocity distributions. Theoretical spectra are calculated using an iterative analytic solution of the optical Bloch equations describing the resonant pulse-train excitation of four-level ⁸⁷Rb atoms. They are employed to fit the measured spectra and obtain the parameters of the frequency comb, thus providing a practical algorithm which can be used in real-time measurements.