A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li. After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon, the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations. In the latter case, these 2 electrons are in the superposition of the singlet and triplet states with different energies, forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds. The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.

本文以铜浆料用ZnO-B2O3-SiO2-BaO玻璃为对象, 通过X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)、拉曼光谱(Raman)、核磁共振铝谱(27Al NMR)、差示扫描量热(DSC)和扫描电子显微镜(SEM)等表征方法, 研究了ZnO含量对其结构和性能的影响。结果表明,随着ZnO含量的增加, 玻璃试样中BO/NBO(摩尔比)、玻璃试样的平均桥氧数和［ZnO4］/［ZnO6］摩尔比均先增大后减小, ［BO4］/［BO3］摩尔比增大, ［AlO6］和［AlO5］摩尔占比增大, ［AlO4］摩尔占比降低。玻璃试样网络结构连接度整体先致密后疏松, 且当ZnO的质量分数为36%时玻璃试样网络结构最为致密。当ZnO质量分数从32%增加到42% 时, 玻璃试样的DSC曲线中玻璃化转变温度Tg、起始烧结温度TS1和热膨胀系数(CTE)均先降低后升高, CTE数值范围为6.0×10-6~6.3×10-6 K-1, 符合铜浆料与Ca1-xSrxZrO3陶瓷基板封接时的热膨胀与烧结温度等要求。

利用DSC-TG、XRD、IR、Raman和NMR等测试手段, 研究了不同煅烧温度(650、700、750、800、850 ℃)下低品位黏土物相和结构的变化。结果表明: 低品位黏土主要由高结晶度的石英组成, 并含有少量晶化程度较低的多硅白云母、高岭石、伊利石、方解石、黄铁矿和微斜长石。650~850 ℃高温煅烧后, 高岭石和伊利石分解为非晶的SiO2和Al2O3, 部分石英转变为无定形的SiO2; 低品位黏土矿物中的吸附水和结构水分解, 铝氧八面体逐渐转变为铝氧四面体, 石英中Si-O-Si的近程有序结构也受到了一定程度的破坏。

为提高放疗的治疗精度，需要实时监测放疗中患者病灶区域的位移。针对放疗过程中患者体表被热塑膜遮挡的特点，通过标志物的材质、形状等设计，最大限度地减少热塑膜遮挡对标志物定位精度的影响。首先根据标志物的颜色特征分割出热塑膜遮挡下的标志物并用凸包算法计算标志物的外轮廓；其次采用改进的最小二乘椭圆拟合算法拟合标志物外轮廓上的点并定位标志物；最后，基于标志物的位置特征匹配标志物并计算患者的位移值。通过搭建模拟实验平台多次实验，验证了该方法测量患者位移的精度是0.2 mm，帧频可以达到30 Hz，可以满足放疗中位移监测的实时性和精度要求。

Terahertz radiation with a Bessel beam profile is demonstrated experimentally from a two-color laser filament in air, which is induced by tailored femtosecond laser pulses with an axicon. The temporal and spatial distributions of Bessel rings of the terahertz radiation are retrieved after being collected in the far field. A theoretical model is proposed, which suggests that such Bessel terahertz pulses are produced due to the combined effects of the inhomogeneous superluminal filament structure and the phase change of the two-color laser components inside the plasma channel. These two effects lead to wavefront crossover and constructive/destructive interference of terahertz radiation from different plasma sources along the laser filament, respectively. Compared with other methods, our technique can support the generation of Bessel pulses with broad spectral bandwidth. Such Bessel pulses can propagate to the far field without significant spatial spreading, which shall provide new opportunities for terahertz applications.

More than ten years ago, the observation of the low-energy structure in the photoelectron energy spectrum, regarded as an “ionization surprise,” has overthrown our understanding of strong-field physics. However, the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption, one of the well-observed phenomena in light-molecule interaction, still lacks an unambiguous mechanism and remains mysterious. Here, we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules. By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H₂⁺ driven by a polarization-skewed laser pulse, we reveal that the low-energy protons (below 0.7 eV) are produced via dipole-transitions at large bond lengths. The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H₂⁺ cation, which is distinct from the well-accepted bond-softening picture. Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field, and the energy gap between the field-dressed transient electronic states may favor the one- or three-photon transitions at the internuclear distance larger than 5 a.u. The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.

High-harmonic spectroscopy can access structural and dynamical information on molecular systems encoded in the amplitude and phase of high-harmonic generation (HHG) signals. However, measurement of the harmonic phase is a daunting task. Here, we present a precise measurement of HHG phase difference between two isotopes of molecular hydrogen using the advanced extreme-ultraviolet (XUV) Gouy phase interferometer. The measured phase difference is about 200 mrad, corresponding to ~3 attoseconds ($1\text{as}={10}^{-18}\text{s}$) time delay which is nearly independent of harmonic order. The measurements agree very well with numerical calculations of a four-dimensional time-dependent Schödinger equation. Numerical simulations also reveal the effects of molecular orientation and intramolecular two-center interference on the measured phase difference. This technique opens a new avenue for measuring the phase of harmonic emission for different atoms and molecules. Together with isomeric or isotopic comparisons, it also enables the observation of subtle effects of molecular structures and nuclear motion on electron dynamics in strong laser fields.