Role of many-electron dynamics in high harmonic generation

High harmonic generation (HHG) in many-electron atoms is studied theoretically. The breakdown of the frozen-core single active electron approximation is demonstrated, as it predicts roughly the same radiation amplitude in all noble gases. This is in contradiction with experiments, where heavier noble gases are known to emit much stronger HHG radiation than lighter ones. This experimental behavior of the noble gases can be qualitatively reproduced when many-electron dynamics, within a simple approximation, is taken into account.     

氩气中高次谐波辐射的研究

报道了在４５ｆｓ－２ＴＷ激光装置上以Ａｒ惰性气体为介质产生高次谐波的实验结果,实验中通过研究气体密度和激光能量对谐波辐射的影响,找到了适当的气体密度和激光能量范围,并在Ａｒ气中观察到８１次（９．７ｎｍ）的皆波辐射,这时迄今为止,人们在Ａｒ气中所能观测到的最短波长的谐波辐射,经分析表明,高于５７次的谐波是由Ａｒ离子产生的。     

Studies on threshold pressures of sonoluminescence for bubbles with different noble gases

Large oscillations of gas and vapor filled bubbles in liquid during acoustic cavitation. This highly nonlinear bubble motion is accompanied by the emission of light-sonoluminescence (SL)[1, 2]. The noble gases inside the bubble can influence the SL[3—5]. At an acoustic pressure, the intensity of SL increases with the molecular mass of noble gas inside the bubbles[6]. There are several kinds of theories about SL mechanism. At present, the bremsstrahlung mechanism is widely admitted. The b…     

高阶阈值上电离的量子电动力学理论

用非微扰量子电动力学理论研究一个由再散射产生的高阶阈值上电离（ATI）.利用形式散射理论给出了高阶ATI谱的解析形式，并且讨论了在高阶ATI中不同ATI通道的干涉效应和光电子以不同角度入射时的高阶ATI谱. 关键词： 阈值上电离 再散射 跃迁概率     

强光场中氦原子的ATI谱

提出了一个新的处理强光中基态氦原子阈上电离谱的解析模型。由这个模型算得的基态氦原子阈上电离谱同最近的实验结果作了比较。同时简单地讨论了现在的模型对先前的模型的改进。     

惰性气体原子间相互作用势比较研究

原子间相互作用势是预测惰性气体输运性质的必要输入条件. 文章对描述惰性气体原子间相互作用的Lennard-Jones势、指数排斥势、Hartree-Fock-Dispersion-B (HFD-B)势和唯象势的形式和特点进行了分析. 基于Chapman-Enskog方法, 计算得到了惰性气体在300–5000 K温度区间内基于四种原子相互作用势的黏性和热导率, 并与文献报道的实验和理论计算结果进行了比较. 研究结果表明, 基于Hartree-Fock排斥理论与色散理论发展起来的HFD-B势能够合理反映惰性气体原子相互作用的趋势与特征, 因而可以较好地预测惰性气体的宏观输运性质.     

Role of thermal conduction in single-bubble cavitation

Effect of thermal conduction on radiation from a single cavitating bubble has been studied in a hydrochemical framework including variation of heat conductivity of noble gases up to 2500 K. Results of numerical simulation show that thermal conductivity plays an important role in determining ultimate cavitation temperature. Higher thermal conductivity of lighter noble gases causes to more thermal dissipation during the bubble collapse, leading to a lower peak temperature. Moreover, at the same driving conditions, radius of light emitting region is greater for heavier noble bubbles. Therefore, sonoluminescence radiation is more intensive from heavier noble gases. Phase diagrams of single-bubble sonoluminescence have also been calculated and in comparison with available experimental data, there is a relatively good agreement between the theory and experiment for noble gases.     

Experimental evaluation of the gas-phase collision cross-section of effusive Au beam with noble gases: The effect of size and flux

We find that the effusive atomic beam of Au atoms is deflected away by collision with noble gas atoms crossing in a perpendicular geometry with a beam flux of >1 × 10¹⁶/cm²s. The ratio of defected Au atoms is found to increase proportional to the flux of noble gases. In addition, the effective cross-section for the collision between Au and noble gases (Ne, Ar, Xe) is measured to increase in an order of Ne < Ar < Xe. As a result of the increased collision probability, the deflection ratio of Au beam in the noble gases is measured to be enhanced for the Au flux in the range of 1 × 10¹¹–10¹³ Au/cm²s. Our results show that the gas-phase collision can be reliably determined by measuring the deflection ratio. The experimentally determined collision cross-section also explains the variation in the deflection ratio among various noble gases and the importance of a long-range van der Waals interaction between Au and noble gases in the deflection efficiency.     

Nuclear magnetic resonance of laser-polarized noble gases in molecules,materials, and organisms

The sensitivity of conventional nuclear magnetic resonance (NMR) techniques is fundamentally limited by the ordinarily low spin polarization achievable in even the strongest NMR magnets. However, by transferring angular momentum from laser light to electronic and nuclear spins, optical pumping methods can increase the nuclear spin polarization of noble gases by several orders of magnitude, thereby greatly enhancing their NMR sensitivity. This review describes the principles and magnetic resonance applications of laser-polarized noble gases. The enormous sensitivity enhancement afforded by optical pumping can be exploited to permit a variety of novel NMR experiments across numerous disciplines. Many such experiments are reviewed, including the void-space imaging of organisms and materials, NMR and MRI of living tissues, probing structure and dynamics of molecules in solution and on surfaces, NMR sensitivity enhancement via polarization transfer, and low-field NMR and MRI.     

Multibubble sonoluminescence spectra of water which resemble single-bubble sonoluminescence

Multibubble sonoluminescence (MBSL) spectra of water from cavitation clouds were collected in the presence of different noble gases and at different acoustic intensities. Results show that at high acoustic intensity and with xenon as a dissolved gas the emission of the OH* radical becomes indiscernible from the continuum. These spectra resemble single-bubble sonoluminescence (SBSL) spectra. It is concluded that the source of emission in MBSL and SBSL can be the same, the difference in spectra is due to the higher temperature inside the bubble during SBSL.     

An Experimental Study of the Fluorescence Spectrum of Cesium Atoms in the Presence of a Buffer Gas

A direct experiment is performed to determine the quantum efficiency of a cesium fluorescence filter. The fluorescence spectra of cesium atoms are recorded under excitation of the upper states of the second resonance doublet with a Bell–Bloom cesium lamp. Introduction of different noble gases into the cell with cesium leads to the appearance of additional fluorescence photons. It is found that a fluorescence filter based on atomic cesium vapor with addition of helium in the working cell has the highest efficiency and response rate of all known fluorescence filters based on alkali-metal atomic vapors.     

超极化气体肺部磁共振成像

磁共振成像(MRI)技术具有非侵入、无放射性的特点,在临床疾病诊断中具有独特的优势,但是肺部空腔的特殊结构使传统质子MRI无法对其直接成像.自旋交换光抽运(SEOP)方法可以使惰性气体原子的极化度增强4个量级以上,从而使肺部的气体MRI成为可能.该文介绍了超极化惰性气体肺部MRI的最新研究进展,包括超极化气体磁共振相关参数的测量方法、肺部通气结构成像、肺部气体交换功能成像,同时比较了常用于肺部MRI气体的优点和缺点.     

利用双色激光场下阈上电离谱鉴别H3^2+两种不同分子构型

我们最近证明,利用红外和深紫外双色激光场,SF6分子的结构信息可以通过其电离谱上的相干条纹获得[arXiv,1912.08499(2019)].在本文中,我们利用该方法考察了两种不同几何结构的分子离子H32+ 在激光场中的直接阈上电离(ATI)过程.通过与单色激光场中电离谱的比较发现,双色激光场的电离谱可以分辨分子的不同几何结构.由相干条件导出的公式可以很好地解释直接ATI动能谱和动量谱中的干涉条纹.此外,还发现通过改变分子核间距或改变激光强度可以改变电离谱的形状.由此可以推断,双色激光场诱导的ATI谱具有鉴别分子不同构型的能力,对复杂分子的几何结构成像具有一定的参考意义.     

Van der Waals molecules and collision pairs of noble-gas atoms ahead of a shock wave front

The effect of creation of an excess concentration of free electrons in an anomalously thick layer (≈5 cm) ahead of an explosively driven shock wave in noble gases is discussed and interpreted. This effect is the ionization of excited 1_u-state molecules produced due to the absorption of a small intensity flux (as compared to the ionization one) of photons (with energies substantially lower than the atom ionization threshold) by unexcited colliding complexes and van der Waals molecules. A model is proposed which explains the excitation of xenon molecules ahead of the radiationless shock wave of an open discharge. The absorption spectra of colliding complexes and van der Waals molecules adjacent to each other near the atomic absorption lines can be resolved into two spectra, and these spectra can be changed by an increase in gas temperature. As a result, radiation capable of exciting van der Waals molecules penetrates through the shock wave of an open discharge and excites xenon molecules there. The present work develops further the knowledge concerning the radiation energy transport in the shock wave front. It also proves that in front of an explosively driven shock wave a great number of excited molecules of noble gases are actually formed, and this means considerable progress toward a VUV laser with optical pumping. Translated from Preprint No. 56 of the P. N. Lebedev Physical Institute, Moscow, 1993.     

Segregation of vapor and gas in a sonoluminescing bubble

Computer simulations of bubble oscillations in water are performed for various noble gases taking into account the segregation of water vapor and noble gas inside a collapsing bubble, which was predicted by Storey and Szeri [J. Fluid Mech. 396 (1999) 203]. It is clarified that the number of water vapor molecules dissociated inside a collapsing bubble is larger for heavier noble gases because of the lower thermal conductivity and the segregation of vapor and noble gas. It is also clarified that the temperature inside a helium bubble at the collapse increases considerably by the mixture segregation because a lesser amount of vapor is trapped inside a collapsing bubble. It is also clarified that multibubble sonoluminescence (MBSL) from heavier noble gases is brighter because of the lower ionization potential which results in the higher electron density and stronger plasma emissions.     

Optical spectra of water sonoluminescence

The sonolumiescence spectra of water saturated with noble gases were studied in a 220–500 nm interval. Total light power of sonoluminescence and the rate of hydrogen peroxide formation were measured. The number of photons emitted by OH^*-radicals was compared with the yield of OH-radicals. The intensity of the shortwave part of the spectra and the OH^*/OH ratio increased while the atomic weight of the gas increased.     

Femtosecond growth dynamics of an underdense ionization frontmeasured by spectral blueshifting

Time-resolved spectral blue shifts of 100-fs laser pulses caused by ionization of atmospheric density N₂ and noble gases subjected to high (10¹⁴ W/cm² to 10¹⁶ W/cm²) light intensities are examined. Included are data for two experiments: self-shifting of the ionizing laser pulses for varying peak intensities, pressures (1-5 atm), and gas species; and time-resolved blue shifts of a weak copropagating probe pulse for the same range of ionization conditions. The self-shift data reveal a universal, reproducible pattern in the shape of the blueshifted spectra: as laser intensity, gas pressure, or atomic number increase, the self-blueshifted spectra develop from a near replica of the incident pulse spectrum into a complex structure consisting of two spectral peaks. The time-resolved data reveal different temporal dependence for each of these two features. A quantitative model for a simplified cylindrical focal geometry is presented     

Hot oxygen atom ballistics on Pt(111): collisional desorption of noble gases

Hot oxygen atoms formed by ultraviolet photofragmentation of chemisorbed O₂ on Pt(111) at T_s=20 K induced collisional desorption of coadsorbed noble gases. Angular distributions of desorbing Ar, Kr, and Xe were sharply peaked at 35° from the surface normal. Mean translational energies of the noble gases were in the 0.12 eV (1400 K) range. The O atom photofragments were found to chatter between the surface and the noble gases during the desorption process. The fastest O atom photofragments produced at 250 nm had an energy of at least 0.73 eV.     

The Frequency Shifts of the Nuclear Magnetic Momenta Larmor Precession in the Mixture of Two Noble Gases

The dynamics of nuclear magnetization in the mixture of two noble gases with different gyromagnetic ratios of the nuclei is studied theoretically. The nuclear magnetization is induced by the radiofrequency electromagnetic radiation, which causes the nuclear magnetic resonance in both types of noble gases in the mixture. Frequency shifts of the nuclear magnetic resonance appeared due to an interaction between different types of the noble gases is analytically predicted. The specifics of these shifts are such that they cannot be compensated by means of the external magnetic field. The nature of the magnetic field distortion in the cell caused by the nuclear magnetization is also discussed.