利用液晶空间光调制器产生高阶拉盖尔高斯光束

本文采用反射式纯相位液晶空间光调制器产生了实验需要的高阶拉盖尔高斯光束。理论上详细计算了生成高阶拉盖尔高斯(Laguerre-Gaussian)光束的最优参数选择,用MATLAB编写程序,制作产生高阶LG光束的相息图。通过CCD采集图像数据,观测了以平面波入射生成拉盖尔高斯光束的强度分布,并利用干涉法验证了产生高阶拉盖尔高斯光束的螺旋相位分布。在此基础上,对实验测得一维强度轮廓进行拟合分析并估算了实验产生LG光束的纯度。     

部分还原二氧化钛的NMR和EPR研究

Partially reduced TiO₂ nanomaterials have attracted significant interest because of their visible-light activity for catalysis and photodegradation. Herein, we prepared a partially reduced anatase TiO₂ (Re-A-TiO₂) nanoparticle material using a fast combustion method, demonstrating good activity toward decomposing methyl orange under visible light irradiation. The surface structure of the prepared material, after being surface-selectively ¹⁷O-labeled with H₂¹⁷O (¹⁷O-enriched water), was studied via ¹⁷O and ¹H solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy, and the obtained results were compared to those of non-reduced anatase TiO₂ (A-TiO₂). The EPR results showed that the concentrations of paramagnetic species (i.e., oxygen vacancies (OV) and Ti³⁺) in Re-A-TiO₂ were much higher than that in A-TiO₂, while the former was associated with a higher OV/Ti³⁺ ratio. The intensities of the EPR signals were significantly affected by the adsorbed water, and this phenomenon was explored in combination with ¹H NMR spectroscopy. The ¹H species on Re-A-TiO₂ appeared at larger chemical shifts, denoting the increased acidity of the sample, and these ¹H species on Re-A-TiO₂ were more difficult to remove than those on A-TiO₂. On the other hand, different features were observed for the signals arising from the two-coordinated oxygen atoms (μ₂-O) in ¹⁷O NMR, suggesting a typical anatase TiO₂(101) surface on A-TiO₂, but a more complex surface environment for Re-A-TiO₂. Furthermore, a larger amount of hydroxyl groups (OH) were observed on Re-A-TiO₂ compared to that on A-TiO₂, indicating a larger proportion of exposed (001) facets on Re-A-TiO₂. However, the μ₂-O signals broadened and became similar when the drying temperature was increased to 100 ℃, indicating a non-faceted anatase TiO₂ surface in such conditions. Based on the EPR and NMR results, a significant fraction of the OH species is believed to be formed from the reaction of the paramagnetic centers and adsorbed water molecules. The ¹H→¹⁷O cross polarization (CP) MAS and two-dimensional heteronuclear correlation (2D HETCOR) NMR spectra were used to verify the spatial proximity of the hydrogen and oxygen species, confirming the spectral assignments of a strongly adsorbed water and one type of surface OH species. In particular, the ¹H NMR signals at approximately 11 ppm were ascribed to the hydrogen species in the intramolecular hydrogen bond. In summary, this study investigated the paramagnetic species and surface structure of anatase TiO₂ materials by combining EPR along with ¹H and ¹⁷O solid-state NMR spectroscopy. The differences in the surface structures of Re-A-TiO₂ and A-TiO₂ should be closely related to their different properties toward the photodegradation of methyl orange.     

GC–MS with photoionization of cold molecules in supersonic molecular beams—Approaching the softest ionization method

A new type of photoionization ion source was developed for the ionization of cold molecules in supersonic molecular beams (named Cold PI). The system was based on a GC–MS with supersonic molecular beams and its fly‐through EI of cold molecules ion source (Cold EI) plus quadrupole mass analyzer. A continuously operated deuterium VUV photoionization lamp was added and placed above and between the supersonic nozzle and skimmer whereas the Cold EI ion source served only as a portion of the ion transfer ion optics. The supersonic nozzle and skimmer were voltage biased and the VUV light crossed the supersonic expansion about 10 mm from the nozzle. We obtained over three orders of magnitude enhancement in the relative abundance of the molecular ion of squalane in Cold PI versus in photoionization of this compound as a thermal compound. Accordingly, we also proved that standard photoionization is not as soft ionization method as previously perceived for large compounds. We found that Cold PI is as soft as and possibly softer than field ionization; thus, it could be the softest known ionization method. The ionization yield was about 200–300 times weaker than with Cold EI yet our limit of detection was about 200 femtogram in SIM mode for cholesterol and pyrene which is reasonable. Practically, all hydrocarbons gave only molecular ions with rather uniform response whereas alcohols gave some molecular ions plus major fragment ions particularly with a loss of water (similarly to field ionization). We tested Cold PI in the GC–MS analysis of diesel fuels and analyzed the time averaged data for group type information. We also found that we can analyze the diesel fuels by fast under 20‐s flow injection analysis in which the generated averaged mass spectrum of molecular ions only could serve for the characterization of fuels.     

Analysis of impurities in pharmaceuticals by LC‐MS with cold electron ionization

Pharmaceuticals require careful and precise determination of their impurities that might harm the user upon consumption. Although today, the most common technique for impurities identification is liquid chromatography‐mass spectrometry (LC‐MS/MS), it has several downsides due to the nature of the ionization method. Also, the analyses in many cases are targeted thus despite being present, some of the compounds will not be revealed. In this paper, we propose and show a new method for untargeted analysis and identification of impurities in active pharmaceutical ingredients (APIs). The instrument used for these analyses is a novel electron ionization (EI) LC‐MS with supersonic molecular beams (SMB). The EI‐LC‐MS‐SMB was implemented for analyses of several drug samples spiked with an impurity. The instrument provides EI mass spectra with enhanced molecular ions, named Cold EI, which increases the identification probabilities when the compound is identified with the aid of an EI library like National Institute of Standards and Technology (NIST). We analyzed ibuprofen and its impurities, and both the API and the expected impurity were identified with names and structures by the NIST library. Moreover, other unexpected impurities were found and identified proving the ability of the EI‐LC‐MS‐SMB system for truly untargeted analysis. The results show a broad dynamic range of four orders of magnitude at the same run with a signal‐to‐noise ratio of over 10 000 for the API and almost uniform response.     

Propagation of Lorentz and Lorentz-Gauss beams through an apertured fractional Fourier transform optical system

By expanding the hard-aperture function into a finite sum of complex Gaussian functions, we derive approximate analytical formulae for Lorentz and Lorentz-Gauss beams propagating through an apertured fractional Fourier transform (FRT) optical system. As an application example, properties of a Lorentz-Gauss beam in the FRT plane after propagating through a squarely or annularly apertured FRT optical system are studied numerically. The results obtained using the approximate analytical formula are in good agreement with those obtained using numerical integral calculation. The FRT optical system provides a convenient way for laser beam shaping.     

Circular partially coherent flattened Gaussian beam

An alternative theoretical model called circular partially coherent flattened Gaussian beam (FGB) is developed to describe a circular partially coherent beam with a flat-topped spatial profile. Explicit expression for the propagation factor of a circular partially coherent FGB is derived. We drive the analytical formulae for the cross-spectral density and mean-squared beam width of a circular partially coherent FGB propagating through a paraxial ABCD optical system based on the generalized Collins formula. The intensity, spreading and directionality properties of a circular partially coherent FGB propagating in free space are studied as numerical examples. The propagation properties of a circular partially coherent FGB agree well with those of a partially coherent flat-topped beam reported in the literature. Thus, our model provides an alternative but reliable model for describing a circular partially coherent beam with flat-topped profile.     

Oriented O(3P2), Ne(3P2), and He(3S1) atoms emerging from a bent magnetic guide

We describe our observation of strongly oriented total electronic angular momentum J in O(³P₂), Ne(³P₂), and He(³S₁) atoms emerging from a bent magnetic multipole guide, as measured by resonant multiphoton ionisation. This was contrary to our expectation because no additional (uniform) magnetic fields were applied to orient the atoms behind the exit of the guide. Two- and three-photon ionisation techniques were employed to determine the degree of J polarisation, from which we infer that atoms become oriented as a result of a combination of weak fringe fields, possible stray fields, and the fact that molecular beam packets do not oscillate around the geometric center of the bent multipole guide. We conclude that similar effects may exist in other, related experiments and that a detailed characterisation of the degree of orientation is required prior to any study of chemical dynamics or spectroscopy. This paper should serve as a warning for anybody using similar devices not to assume isotropic angular momentum distributions of atoms and molecules emerging from a magnetic guide or a decelerator, particularly when it is bent; whenever possible, the possibility for a J anisotropy should be experimentally checked.     

Propagation of generalized Mathieu–Gauss beams through paraxial misaligned optical systems

Based on the generalized diffraction integral, we derive an analytical formula for generalized Mathieu–Gauss beams (gMGBs) passing through an apertured misaligned optical system. Furthermore, we use the fact that a hard aperture function can be expanded into a finite sum of complex Gaussian functions to establish an approximate propagation equation of gMGBs through paraxial circularly apertured optical system. As an example, the propagation of ordinary and modified zeroth order MGBs through a misaligned thin lens is studied numerically.     

A multi-beam ion/electron spectra-microscope design

This paper presents the design of a multi-beam charged particle instrument that simultaneously focuses electrons, gallium, oxygen and cesium ions onto the same sample. In addition, the instrument has provision to capture the spectra of both secondary electrons and ions in parallel. The mass spectrometer part of the instrument is expected to detect and identify secondary ion species across the entire range of the periodic table, and also record a portion of their emitted energy spectrum. The electron energy spectrometer part of the instrument is designed to acquire the entire range of scattered electrons, from the low-energy secondary electrons through to the elastic backscattered electrons.     

Fast Gaussian wavepacket transforms and Gaussian beams for the Schrödinger equation

This paper introduces a wavepacket-transform-based Gaussian beam method for solving the Schrödinger equation. We focus on addressing two computational issues of the Gaussian beam method: how to generate a Gaussian beam representation for general initial conditions and how to perform long time propagation for any finite period of time. To address the first question, we introduce fast Gaussian wavepacket transforms and develop on top of them an efficient initialization algorithm for general initial conditions. Based on this new initialization algorithm, we address the second question by reinitializing the beam representation when the beams become too wide. Numerical examples in one, two, and three dimensions demonstrate the efficiency and accuracy of the proposed algorithms. The methodology can be readily generalized to deal with other semi-classical quantum mechanical problems.