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 Select Universality of the Dynamic Characteristic Relationship of Electron Correlation in the Two-Photon Double Ionization Process of a Helium-Like System Fei Li, Yu-Jun Yang, Jing Chen, Xiao-Jun Liu, Zhi-Yi Wei, and Bing-Bing Wang Chin. Phys. Lett.    2020, 37 (11): 113201 .   DOI: 10.1088/0256-307X/37/11/113201 Abstract   HTML   PDF (2191KB) Universality of the dynamic characteristic relationship between the characteristic time $t_{\rm c}$ and the two-electron Coulomb interaction energy $\overline{V}_{12}$ of the ground state in the two-photon double ionization process is investigated via changing the parameters of the two-electron atomic system and the corresponding laser conditions. The numerical results show that the product $t_{\rm c}\overline{V}_{12}$ keeps constant around 4.1 in the cases of changing the nucleus charge, the electron charge, the electron mass, and changing simultaneously the nucleus charge and the electron charge. These results demonstrate that the dynamic characteristic relationship in the two-photon double ionization process is universal. This work sheds more light on the dynamic characteristic relationship in ultrafast processes and may find its application in measurements of attosecond pulses.
 Select Active Learning Approach to Optimization of Experimental Control Yadong Wu, Zengming Meng, Kai Wen, Chengdong Mi, Jing Zhang, and Hui Zhai Chin. Phys. Lett.    2020, 37 (10): 103201 .   DOI: 10.1088/0256-307X/37/10/103201 Abstract   HTML   PDF (1132KB) We present a general machine learning based scheme to optimize experimental control. The method utilizes the neural network to learn the relation between the control parameters and the control goal, with which the optimal control parameters can be obtained. The main challenge of this approach is that the labeled data obtained from experiments are not abundant. The central idea of our scheme is to use the active learning to overcome this difficulty. As a demonstration example, we apply our method to control evaporative cooling experiments in cold atoms. We have first tested our method with simulated data and then applied our method to real experiments. It is demonstrated that our method can successfully reach the best performance within hundreds of experimental runs. Our method does not require knowledge of the experimental system as a prior and is universal for experimental control in different systems.
 Select Significantly Improving the Escape Time of a Single $^{40}$Ca$^+$ Ion in a Linear Paul Trap by Fast Switching of the Endcap Voltage Peng-Peng Zhou, Shao-Long Chen, Shi-Yong Liang, Wei Sun, Huan-Yao Sun, Yao Huang, Hua Guan, and Ke-Lin Gao Chin. Phys. Lett.    2020, 37 (9): 093701 .   DOI: 10.1088/0256-307X/37/9/093701 Abstract   HTML   PDF (786KB) Sympathetic cooling is a method used to lower the kinetic energy of ions with complicated energy-level structures, via Coulomb interactions with laser-cooled ions in an ion trap. The ion to be sympathetically cooled is sometimes prepared outside of the trap, and it is critical to introduce this ion into the trap by temporarily lowering the potential of one endcap without allowing the coolant ion to escape. We study the time required for a laser-cooled ion to escape from a linear Paul trap when the voltage of one endcap is lowered. The escape time is on the order of a few microseconds, and varies significantly when the low-level voltage changes. A re-cooling time of a maximum of 13 s was measured, which can be reduced to approximately one hundred of milliseconds by decreasing the duration of the low-level voltage. The measurement of these critical values lays the foundation for the smooth injection and cooling of the ion to be sympathetically cooled.
 Select Deceleration of Metastable $\rm{Li}^{+}$ Beam by Combining Electrostatic Lens and Ion Trap Technique Shao-Long Chen, Peng-Peng Zhou, Shi-Yong Liang, Wei Sun, Huan-Yao Sun, Yao Huang, Hua Guan, Ke-Lin Gao Chin. Phys. Lett.    2020, 37 (7): 073201 .   DOI: 10.1088/0256-307X/37/7/073201 Abstract   HTML   PDF (853KB) Ion deceleration has played a critical role in ion-related research when the ions are produced in the form of a high-energy beam. We present a deceleration method combining electrostatic lens and ion trap technique, which can effectively decelerate ions to energy below the trapping potential of a typical ion trap. The experiments were performed on metastable $1s2s\,{}^{3}\!S_1\,{\rm Li}^{+}$ ions, and demonstrated that the kinetic energy could easily be reduced from $\sim$450 eV to a few eV, with the latter being confirmed using the Doppler-shifted fluorescence spectra.
 Select Momentum Spectroscopy for Multiple Ionization of Cold Rubidium in the Elliptically Polarized Laser Field Junyang Yuan, Yixuan Ma, Renyuan Li, Huanyu Ma, Yizhu Zhang, Difa Ye, Zhenjie Shen, Tianmin Yan, Xincheng Wang, Matthias Weidemüller, Yuhai Jiang Chin. Phys. Lett.    2020, 37 (5): 053201 .   DOI: 10.1088/0256-307X/37/5/053201 Abstract   HTML   PDF (643KB) Employing recently developed magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) combined with cold atoms, strong laser pulse, and ultrafast technologies, we study momentum distributions of the multiply ionized cold rubidium (Rb) induced by the elliptically polarized laser pulses (35 fs, $1.3\times 10^{15}$ W/cm$^{2}$). The complete vector momenta of Rb$^{n+}$ ions up to charge state $n = 4$ are recorded with extremely high resolution (0.12 a.u. for Rb$^{+}$). Variations of characteristic multi-bands are displayed in momentum distributions because the ellipticity varies from the linear to circular polarization, are interpreted qualitatively with the classical over-barrier ionization model. Present momentum spectroscopy of cold heavy alkali atoms presents novel strong-field phenomena beyond the noble gases.
 Select Three-Dimensional Compensation for Minimizing Heating of the Ion in Surface-Electrode Trap Ji Li, Liang Chen, Yi-He Chen, Zhi-Chao Liu, Hang Zhang, Mang Feng Chin. Phys. Lett.    2020, 37 (5): 053701 .   DOI: 10.1088/0256-307X/37/5/053701 Abstract   HTML   PDF (1084KB) The trapped ions confined in a surface-electrode trap (SET) could be free from rf heating if they stay at the rf potential null of the potential well. We report our effort to compensate three-dimensionally for the micromotion of a single $^{40}$Ca$^{+}$ ion near the rf potential null, which largely suppresses the ion's heating and thus helps to achieve the cooling of the ion down to $3.4$ mK, which is very close to the Doppler limit. This is the prerequisite of the sideband cooling in our SET.
 Select Unusual Destruction and Enhancement of Superfluidity of Atomic Fermi Gases by Population Imbalance in a One-Dimensional Optical Lattice Qijin Chen, Jibiao Wang, Lin Sun, Yi Yu Chin. Phys. Lett.    2020, 37 (5): 053702 .   DOI: 10.1088/0256-307X/37/5/053702 Abstract   HTML   PDF (865KB) We study the superfluid behavior of a population imbalanced ultracold atomic Fermi gases with a short range attractive interaction in a one-dimensional (1D) optical lattice, using a pairing fluctuation theory. We show that, besides widespread pseudogap phenomena and intermediate temperature superfluidity, the superfluid phase is readily destroyed except in a limited region of the parameter space. We find a new mechanism for pair hopping, assisted by the excessive majority fermions, in the presence of continuum-lattice mixing, which leads to an unusual constant Bose-Einstein condensate (BEC) asymptote for $T_{\rm c}$ that is independent of pairing strength. In result, on the BEC side of unitarity, superfluidity, when it exists, may be strongly enhanced by population imbalance.
 Select Discriminating High-Pressure Water Phases Using Rare-Event Determined Ionic Dynamical Properties Lin Zhuang, Qijun Ye, Ding Pan, Xin-Zheng Li Chin. Phys. Lett.    2020, 37 (4): 043101 .   DOI: 10.1088/0256-307X/37/4/043101 Abstract   HTML   PDF (20890KB) Recent discoveries of dynamic ice VII and superionic ice highlight the importance of ionic diffusions in discriminating high-pressure ($P$) water phases. The rare event nature and the chemical bond breaking associated with these diffusions, however, make extensive simulations of these processes unpractical to ab initio and inappropriate for force field based methods. Using a first-principles neural network potential, we performed a theoretical study of water at 5–70 GPa and 300–3000 K. Long-time dynamics of protons and oxygens were found indispensable in discriminating several subtle states of water, characterized by proton's and oxygen ion's diffusion coefficients and the distribution of proton's displacements. Within dynamic ice VII, two types of proton transfer mechanisms, i.e., translational and rotational transfers, were identified to discriminate this region further into dynamic ice VII T and dynamic ice VII R. The triple point between ice VII, superionic ice (SI), and liquid exists because the loosening of the bcc oxygen skeleton is prevented by the decrease of interatomic distances at high $P$'s. The melting of ice VII above $\sim$40 GPa can be understood as a process of two individual steps: the melting of protons and the retarded melting of oxygens, responsible for the forming of SI. The boundary of the dynamic ice VII and SI lies on the continuation line ice VII's melting curve at low $P$'s. Based on these, a detailed phase diagram is given, which may shed light on studies of water under $P$'s in a wide range of interdisciplinary sciences.
 Select Nonadiabatic and Multielectron Effects in the Attoclock Experimental Scheme Zhi-Lei Xiao, Wei Quan, Song-Po Xu, Shao-Gang Yu, Xuan-Yang Lai, Jing Chen, Xiao-Jun Liu Chin. Phys. Lett.    2020, 37 (4): 043201 .   DOI: 10.1088/0256-307X/37/4/043201 Abstract   HTML   PDF (1143KB) The problem of how long it takes for an electron to tunnel from one side of a barrier to the other has been debated for decades and the attoclock is a promising experimental procedure to address this problem. In the attoclock experiment, many physical effects will contribute to the experimental results and it is difficult to extract the tunneling time accurately. We numerically investigate a method of measuring the residual equivalent temporal offset (RETO) induced by the physical effects except for tunneling delay. The Coulomb potential effect, the nonadiabatic effect, the multielectron effect, and the Stark effect are considered in the theoretical model. It is shown that the ratio of the RETO of the target atoms to that of H is insensitive to the wavelength and is linearly proportional to (2$I_{\rm p}$)$^{-3/2}$. This work can help to improve the accuracy of the attoclock technique.
 Select Ionic Angular Distributions Induced by Strong-Field Ionization of Tri-Atomic Molecules Tian Sun, Shi-Wen Zhang, Rui Wang, Shuang Feng, Yang Liu, Hang Lv, Hai-Feng Xu Chin. Phys. Lett.    2020, 37 (4): 043301 .   DOI: 10.1088/0256-307X/37/4/043301 Abstract   HTML   PDF (801KB) Angular distributions of fragment ions from ionization of several tri-atomic molecules (CO$_{2}$, OCS, N$_{2}$O and NO$_{2}$) by strong 800-nm laser fields are investigated via a time-of-flight mass spectrometer. Anisotropic angular distributions of fragment ions, especially those of atomic ions, are observed for all of the molecules studied. These anisotropic angular distributions are mainly due to the geometric alignment of molecules in the strong field ionization. Distinct different patterns in ionic angular distributions for different molecules are observed. It is indicated that both molecular geometric structure and ionization channels have effects on the angular distributions of strong field ionization/fragmentation.
 Select Testing the Universality of Free Fall by Comparing the Atoms in Different Hyperfine States with Bragg Diffraction Ke Zhang, Min-Kang Zhou, Yuan Cheng, Le-Le Chen, Qin Luo, Wen-Jie Xu, Lu-Shuai Cao, Xiao-Chun Duan, Zhong-Kun Hu Chin. Phys. Lett.    2020, 37 (4): 043701 .   DOI: 10.1088/0256-307X/37/4/043701 Abstract   HTML   PDF (1237KB) We perform a precision atom interferometry experiment to test the universality of free fall. Our experiment employs the Bragg atom interferometer with $^{87}$Rb atoms either in hyperfine state $\left| {F = 1,{m_F} = 0} \right\rangle$ or $\left| {F = 2,{m_F} = 0} \right\rangle$, and the wave packets in these two states are diffracted by one pair of Bragg beams alternatively, which is helpful for suppressing common-mode systematic errors. We obtain an Eötvös ratio ${\eta_{1 - 2}} = \left({ 0.9 \pm 2.7} \right) \times {10^{- 10}}$, and set a new record on the precision with improvement of nearly 5 times. This measurement also provides constraint on the difference of the diagonal terms of the mass-energy operator.
 Select Generation of 88as Isolated Attosecond Pulses with Double Optical Gating Xiaowei Wang, Li Wang, Fan Xiao, Dongwen Zhang, Zhihui Lü, Jianmin Yuan, Zengxiu Zhao Chin. Phys. Lett.    2020, 37 (2): 023201 .   DOI: 10.1088/0256-307X/37/2/023201 Abstract   HTML   PDF (831KB) Isolated attosecond pulses with a duration of 88 as are generated in the spectral range of 29–72 eV using double optical gating technique. The gate width is set to be shorter than half the optical cycle to avoid carrier envelop phase stabilization of the 4.2 fs driving laser pulses centered at 800 nm. The attosecond pulse duration is measured with the technique of frequency resolved optical gating for complete reconstruction of attosecond bursts.
 Select Single- and Double-Electron Capture Processes in Low-Energy Collisions of N$^{4+}$ Ions with He Kun Wang, Xiao-Xia Wang, Yi-Zhi Qu, Chun-Hua Liu, Ling Liu, Yong Wu, Robert J. Buenker Chin. Phys. Lett.    2020, 37 (2): 023401 .   DOI: 10.1088/0256-307X/37/2/023401 Abstract   HTML   PDF (764KB) We investigate the electron capture processes of N$^{4+}$(1$s^{2}2s$) colliding with He(1$s^{2}$) in the energy range of 10–1700 eV/amu using the quantum-mechanical molecular-orbital close-coupling (QMOCC) method. Total and state-selective single-electron capture and double-electron capture (SEC and DEC) cross sections are obtained and compared with other available studies. The results agree better with the experimental data in both trend and magnitude when the electron translation factor (ETF) effects are included. Our results indicate that both the SEC and DEC processes play important roles in the considered energy region. For the SEC processes, the N$^{3+}$(1$s^{2}2p^{2}$) + He$^{+}$(1$s$) states are the dominant capture states, and the N$^{2+}$(1$s^{2}2s2p^{2}$) + He$^{2+}$ states are the main DEC states.
 Select Parameters of Isotope Shifts for $2s2p{}^{3,1}\!P_{1} \to 2s^{2}{}^{1}\!S_0$ Transitions in Heavy Be-Like Ions Xiang Zhang, Jian-Peng Liu, Ji-Guang Li, Hong-Xin Zou Chin. Phys. Lett.    2019, 36 (11): 113101 .   DOI: 10.1088/0256-307X/36/11/113101 Abstract   HTML   PDF (669KB) The field shift and mass shift parameters of the 2$s2p\,{}^{3,1}\!P_{1}\to 2s^{2}\,{}^{1}\!S_0$ transitions in Be-like ions ($70 \le Z \le 92$) are calculated using the multi-configuration Dirac–Hartree–Fock and the relativistic configuration interaction methods with the inclusion of the Breit interaction and the leading QED corrections. We find that the mass shift parameters of these two transitions do not change monotonously along the isoelectronic sequence in the high-$Z$ range due to the relativistic nuclear recoil effects. A minimum value exists for the specific mass shift parameters around $Z=80$, especially for the 2$s2p\,{}^{3}\!P_{1}\to 2s^{2}\,{}^{1}\!S_0$ transition. In addition, the field shifts and mass shifts of these two transitions are estimated using an empirical formula, and their contributions are compared along the isoelectronic sequence.
 Select Manipulating the Flipping of Water Dipoles in Carbon Nanotubes Dang-Xin Mao, Xiao-Gang Wang, Guo-Quan Zhou, Song-Wei Zeng, Liang Chen, Jun-Lang Chen, Chao-Qing Dai Chin. Phys. Lett.    2019, 36 (10): 103101 .   DOI: 10.1088/0256-307X/36/10/103101 Abstract   HTML   PDF (1208KB) Flipping of water dipoles in carbon nanotubes is of great importance in many physical and biological applications, such as signal amplification, molecular switches and nano-gates. Ahead of these applications, understanding and inhibiting the non-negligible thermal noise is essential. Here, we use molecular dynamics simulations to show that the flipping frequency of water dipoles increases with the rising temperature, and the thermal noise can be suppressed by imposed charges and external uniform electric fields. Furthermore, the water dipoles flip periodically between two equiprobable and stable states under alternating electric fields. These two stable states may be adopted to store 0 and 1 bits for memory storage or molecular computing.
 Select Towards the Same Line of Liquid–Liquid Phase Transition of Dense Hydrogen from Various Theoretical Predictions Binbin Lu, Dongdong Kang, Dan Wang, Tianyu Gao, Jiayu Dai Chin. Phys. Lett.    2019, 36 (10): 103102 .   DOI: 10.1088/0256-307X/36/10/103102 Abstract   HTML   PDF (1985KB) For a long time, there have been huge discrepancies between different models and experiments concerning the liquid–liquid phase transition (LLPT) in dense hydrogen. We present the results of extensive calculations of the LLPT in dense hydrogen using the most expensive first-principle path-integral molecular dynamics simulations available. The nonlocal density functional rVV10 and the hybrid functional PBE0 are used to improve the description of the electronic structure of hydrogen. Of all the density functional theory calculations available, we report the most consistent results through quantum Monte Carlo simulations and coupled electron-ion Monte Carlo simulations of the LLPT in dense hydrogen. The critical point of the first-order LLPT is estimated to be above 2000 K according to the equation of state. Moreover, the metallization pressure obtained from the jump of dc electrical conductivity almost coincides with the plateau of equation of state.
 Select Experimental Realization of Degenerate Fermi Gases of $^{87}$Sr Atoms with 10 or Two Spin Components Wei Qi, Ming-Cheng Liang, Han Zhang, Yu-Dong Wei, Wen-Wei Wang, Xu-Jie Wang, Xibo Zhang Chin. Phys. Lett.    2019, 36 (9): 093701 .   DOI: 10.1088/0256-307X/36/9/093701 Abstract   HTML   PDF (852KB) We report the experimental realization of quantum degenerate Fermi gases of $^{87}$Sr atoms under controlled 10- and dual-nuclear-spin configurations. Based on laser cooling and evaporative cooling, we achieve an ultracold Fermi gas of 10$^{5}$ atoms equally distributed over 10 spin states, with a temperature of $T/T_{\rm F}=0.21$. We further prepare a dual-spin gas by optically pumping atoms to the $m_{\rm F}=9/2$ and $m_{\rm F}=7/2$ states and observe a slightly lower $T/T_{\rm F}$ than that for a 10-spin gas under the same trapping condition, showing efficient evaporative cooling under a decreasing number ${\cal N}$ of spin states (${\cal N}\geqslant 2$) despite the increasing importance of Pauli exclusion. Given that rethermalization becomes less efficient with ${\cal N}$ approaching unity, we evaporatively cool an almost polarized gas to 130 nK. The simple and efficient preparation of ultracold Fermi gases of $^{87}$Sr with tunable spin configurations provides a first step towards engineering topological quantum systems.
 Select Influence of Debye Plasma on the KLL Dielectronic Recombination of H-Like Helium Ions Deng-Hong Zhang, Lu-You Xie, Jun Jiang, Chen-Zhong Dong Chin. Phys. Lett.    2019, 36 (8): 083401 .   DOI: 10.1088/0256-307X/36/8/083401 Abstract   HTML   PDF (619KB) Using the Debye shielding model, the effects of plasma shielding on the dielectronic recombination processes of the H-like helium ions are investigated. It is found that plasma shielding causes a remarkable change in the Auger decay rate of the doubly excited $2p^2$ $^3P_2$ state. As a result, the dielectronic recombination cross sections from the doubly excited $2p^2$ $^3P_2$ state increases with the decreasing Debye shielding length.
 Select Measurement of S-Wave Scattering Length between $^6$Li and $^{88}$Sr Atoms Using Interspecies Thermalization in an Optical Dipole Trap Xiao-Bin Ma, Zhu-Xiong Ye, Li-Yang Xie, Zhen Guo, Li You, Meng Khoon Tey Chin. Phys. Lett.    2019, 36 (7): 073401 .   DOI: 10.1088/0256-307X/36/7/073401 Abstract   HTML   PDF (708KB) We report the creation of the first mixture of $^6$Li and $^{88}$Sr atoms in an optical dipole trap. Using this mixture, a measurement of the interspecies thermalization process is carried out and the previously unknown interspecies s-wave scattering length between $^6$Li and $^{88}$Sr atoms is extracted to be $|a_{\rm ^6Li-^{88}Sr}|=(380^{+160}_{-100})a_0$ with $a_0$ being the Bohr radius from the rate of interspecies thermalization.
 Select Secular Motion Frequencies of $^{9}$Be$^{+}$ Ions and $^{40}$Ca$^{+}$ Ions in Bi-component Coulomb Crystals Hai-Xia Li, Min Li, Qian-Yu Zhang, Xin Tong Chin. Phys. Lett.    2019, 36 (7): 073701 .   DOI: 10.1088/0256-307X/36/7/073701 Abstract   HTML   PDF (659KB) We obtain bi-component Coulomb crystals using laser-cooled $^{40}$Ca$^{+}$ ions to sympathetically cool $^{9}$Be$^{+}$ ions in a linear Paul trap. The shell structures of the bi-component Coulomb crystals are investigated. The secular motion frequencies of the two different ions are determined and compared with those in the single-component Coulomb crystals. In the radial direction, the resonant motion frequencies of the two ionic species shift toward each other due to the strong motion coupling in the ion trap. In the axial direction, the motion frequency of the laser-cooled $^{40}$Ca$^{+}$ is impervious to the sympathetically cooled $^{9}$Be$^{+}$ ions because the spatially separation of the two different ionic species leads to the weak motion coupling in the axial direction.
 Select Carrier Envelope Phase Description for an Isolated Attosecond Pulse by Momentum Vortices Meng Li, Gui-zhong Zhang, Xin Ding, Jian-quan Yao Chin. Phys. Lett.    2019, 36 (6): 063201 .   DOI: 10.1088/0256-307X/36/6/063201 Abstract   HTML   PDF (1790KB) As a crucial parameter for a few-cycle laser pulse, the carrier envelope phase (CEP) substantially determines the laser waveform. We propose a method to directly describe the CEP of an isolated attosecond pulse (IAP) by the vortex-shaped momentum pattern, which is generated from the tunneling ionization of a hydrogen atom by a pair of time-delayed, oppositely and circularly polarized IAP-IR pulses. Superior to the angular streaking method that characterizes the CEP in terms of only one streak, our method describes the CEP of an IAP by the features of multiple streaks in the vortex pattern. The proposed method may open the possibility of capturing sub-cycle extreme ultraviolet dynamics.
 Select Experimental Investigation of the Electromagnetically Induced-Absorption-Like Effect for an N-Type Energy Level in a Rubidium BEC Khan Sadiq Nawaz, Cheng-Dong Mi, Liang-Chao Chen, Peng-Jun Wang, Jing Zhang Chin. Phys. Lett.    2019, 36 (4): 043201 .   DOI: 10.1088/0256-307X/36/4/043201 Abstract   HTML   PDF (672KB) We study the electromagnetically induced-absorption-like (EIA-like) effect for an n-type system in an $^{87}$Rb Bose–Einstein condensate (BEC) using the absorption imaging technique for coupling and driving lasers operating at the $D_{1}$ and $D_{2}$ lines of $^{87}$Rb. The coherent effect is probed by measuring the number of atoms remaining after the BEC is exposed to strong driving fields and a weak probe field. The absorption imaging technique accurately reveals the EIA-like effect of the n-type system. This coherent effect in an n-type system is useful for optical storage, tunable optical switching, and so on.
 Select Understanding Tunneling Ionization of Atoms in Laser Fields using the Principle of Multiphoton Absorption Long Xu, Li-Bin Fu Chin. Phys. Lett.    2019, 36 (4): 043202 .   DOI: 10.1088/0256-307X/36/4/043202 Abstract   HTML   PDF (2055KB) The elaborate energy and momentum spectra of ionized electrons from atoms in laser fields suggest that the ionization dynamics described by tunneling theory should be modified. Although great efforts have been carried out within semiclassical models, there are few discussions describing the multiphoton absorption process within a quantum framework. Comparing the results obtained with the time-dependent Schrödinger equation (TDSE) and the Keldysh–Faisal–Reiss (KFR) theory, we study the nonperturbative effects of ionization dynamics beyond the KFR theory. The difference in momentum spectra between multiphoton and tunneling regimes is understood in a unified picture with virtual multiphoton absorption processes. For the multiphoton regime, the momentum spectra can be obtained by coherent interference of each periodic contribution. However, the interference of multiphoton absorption peaks will result in a complex structure of virtual multiphoton bands in the tunneling regime. It is shown that the virtual spectra will be almost continuous in the tunneling regime instead of the discrete levels found in the multiphoton regime. Finally, with a model combining the TDSE and the KFR theory, we try to understand the different effects of virtual multiphoton processes on ionization dynamics.
 Select Analysis of Transition Mechanism of Cubic Boron Nitride Single Crystals under High Pressure-High Temperature with Valence Electron Structure Calculation Mei-Zhe Lv, Bin Xu, Li-Chao Cai, Feng Jia, Xing-Dong Yuan Chin. Phys. Lett.    2019, 36 (1): 013101 .   DOI: 10.1088/0256-307X/36/1/013101 Abstract   HTML   PDF (535KB) The possibilities of hexagonal boron nitride (hBN) and lithium boron nitride (Li$_{3}$BN$_{2}$) transition into cubic boron nitride (cBN) under synthetic pressure 5.0 GPa and synthetic temperature 1700 K are analyzed with the use of the empirical electron theory of solids and molecules. The relative differences in electron density are calculated for dozens of bi-phase interfaces hBN/cBN, Li$_{3}$BN$_{2}$/cBN. These relative differences of hBN/cBN are in good agreement with the first order of approximation ($<$10%), while those of Li$_{3}$BN$_{2}$/cBN are much greater than 10%. This analysis suggests that Li$_{3}$BN$_{2}$ is impossible to be intermediate phase but is a catalyst and cBN should be directly transformed by hBN.
 Select Sub-Doppler Laser Cooling of $^{23}$Na in Gray Molasses on the $D_{2}$ Line Zhenlian Shi, Ziliang Li, Pengjun Wang, Zengming Meng, Lianghui Huang, Jing Zhang Chin. Phys. Lett.    2018, 35 (12): 123701 .   DOI: 10.1088/0256-307X/35/12/123701 Abstract   HTML   PDF (730KB) We report on the efficient gray molasses cooling of sodium atoms using the $D_{2}$ optical transition at 589.1 nm. Thanks to the hyperfine split about 6${\it \Gamma}$ between $|F'=2\rangle$ and $|F'=3\rangle$ in the excited state 3$^{2}P_{3/2}$, this atomic transition is effective for the gray molasses cooling mechanism. Using this cooling technique, the atomic sample in $F=2$ ground manifold is cooled from 700 $\mu$K to 56 $\mu$K in 3.5 ms. We observe that the loading efficiency into magnetic trap is increased due to the lower temperature and high phase space density of atomic cloud after gray molasses. This technique offers a promising route for the fast cooling of the sodium atoms in the $F=2$ state.
 Select Simulation of Intermediate State Absorption Enhancement in Rare-Earth Ions by Polarization Modulated Femtosecond Laser Field Wen-Jing Cheng, Shi-Hua Zhao Chin. Phys. Lett.    2018, 35 (11): 113201 .   DOI: 10.1088/0256-307X/35/11/113201 Abstract   HTML   PDF (534KB) We extend the third perturbation theory to study the polarization control behavior of the intermediate state absorption in Nd$^{3+}$ ions. The results show that coherent interference can occur between the single-photon and three-photon excitation pathways, and depends on the central frequency of the femtosecond laser field. Moreover, single-photon and three-photon absorptions have different polarization control efficiencies, and the relative weight of three-photon absorption in the whole excitation processes can increase with increasing the laser intensity. Therefore, the enhancement or suppression of the intermediate state absorption can be realized and manipulated by properly designing the intensity and central frequency of the polarization modulated femtosecond laser field. This research can not only enrich theoretical research methods for the up-conversion luminescence manipulation of rare-earth ions, but also can provide a clear physical picture for understanding and controlling multi-photon absorption in a multiple energy level system.
 Select Theoretical Investigation on the Low-Lying States of LaP Molecule Nagat Elkahwagy, Atif Ismail, S. M. A. Maize, K. R. Mahmoud Chin. Phys. Lett.    2018, 35 (10): 103101 .   DOI: 10.1088/0256-307X/35/10/103101 Abstract   HTML   PDF (477KB) The completely unexplored LaP molecule is investigated by ab initio methods. Potential energy curves for the low-lying states of LaP are constructed by means of the diffusion Monte Carlo method combined with three different trial functions. Spectroscopic constants are also numerically derived and the ground state is assigned, looking forward to experimental comparisons. Moreover, variations of the permanent dipole moments as a function of the internuclear separation for the two lowest states of the diatomic LaP are studied and analyzed.
 Select Transition Dipole Moment Measurements of Ultracold Photoassociated $^{85}$Rb$^{133}$Cs Molecules by Depletion Spectroscopy Juan-Juan Cao, Ting Gong, Zhong-Hao Li, Zhong-Hua Ji, Yan-Ting Zhao, Lian-Tuan Xiao, Suo-Tang Jia Chin. Phys. Lett.    2018, 35 (10): 103301 .   DOI: 10.1088/0256-307X/35/10/103301 Abstract   HTML   PDF (762KB) The transition dipole moments (TDMs) of ultracold $^{85}$Rb$^{133}$Cs molecules between the lowest vibrational ground level, $X^{1}{\it \Sigma}^{+}$ ($v=0$, $J=1$), and the two excited rovibrational levels, $2^{3}{\it \Pi}_{0^{+}}$ ($v'=10$, $J'=2$) and $2^{1}{\it \Pi}_{1}$ ($v'=22$, $J'=2$), are measured using depletion spectroscopy. The ground-state $^{85}$Rb$^{133}$Cs molecules are formed from cold mixed component atoms via the $2^{3}{\it \Pi}_{0^{-}}$ ($v=11$, $J=0$) short-range level, then detected by time-of-flight mass spectrum. A home-made external-cavity diode laser is used as the depletion laser to couple the ground level and the two excited levels. Based on the depletion spectroscopy, the corresponding TDMs are then derived to be 3.5(2)$\times$$10^{-3}$$ea_{0}$ and 1.6(1)$\times$$10^{-2}$$ea_{0}$, respectively, where $ea_{0}$ represents the atomic unit of electric dipole moment. The enhance of TDM with nearly a factor of 5 for the $2^{1}{\it \Pi}_{1}$ ($v'=22$, $J'=2$) excited level means that it has stronger coupling with the ground level. It is meaningful to find more levels with much more strong coupling strength by the represented depletion spectroscopy to realize direct stimulated Raman adiabatic passage transfer from scattering atomic states to deeply molecular states.
 Select First-Principles Study of Magnetic Properties of TM$_{13}$ and TM$_{13}$@Au$_{32}$ Clusters (TM=Mn, Co) Yi-Bo Li, Li-Jin Zeng, Chun-Xiang Zhao, Chun-Yao Niu Chin. Phys. Lett.    2018, 35 (10): 103601 .   DOI: 10.1088/0256-307X/35/10/103601 Abstract   HTML   PDF (830KB) The structural and magnetic properties of TM$_{13}$ and TM$_{13}$@Au$_{32}$ clusters (TM=Mn, Co) are studied by first-principles calculations. We find that the Au$_{32}$ cluster can tune not only the magnetic moment but also the magnetic coupling properties between the TM atoms of the TM cluster. The Au$_{32}$ cluster can increase the net magnetic moment of Mn$_{13}$ clusters while reducing that of Co$_{13}$ clusters. The interaction between Au and Mn atoms induces more Mn atoms to form spin parallel coupling, resulting in an increase of the total magnetic moment of Mn$_{13}$ clusters, while for the Co$_{13}$ clusters, the interaction between Au and Co atoms does not change the magnetic coupling states between the Co atoms, but reduces the magnetic moment of the Co atoms, leading to a decrease of the total magnetic moment of this system. Our findings indicate that the encapsulation of Au$_{32}$ clusters can not only raise the chemical stability of TM clusters, but also can tune their magnetic coupling properties and magnetic moment, which enables such systems to be widely applied in fields of spintronics and medical science.
 Select Experimental Study on Double Resonance Optical Pumping Spectroscopy in a Ladder-Type System of $^{87}$Rb Atoms Yi-Hong Li, Shao-Hua Li, Jin-Peng Yuan, Li-Rong Wang, Lian-Tuan Xiao, Suo-Tang Jia Chin. Phys. Lett.    2018, 35 (9): 093201 .   DOI: 10.1088/0256-307X/35/9/093201 Abstract   HTML   PDF (734KB) Double resonance optical pumping spectroscopy has an outstanding advantage of high signal-to-noise ratio, thus having potential applications in precision measurement. With the counter propagated 780 nm and 776 nm laser beams acting on a rubidium vapor cell, the high resolution spectrum of $5S_{1/2}-5P_{3/2}-5D_{5/2}$ ladder-type transition of $^{87}$Rb atoms is obtained by monitoring the population of the $5S_{1/2}$ ground state. The dependence of the spectroscopy lineshape on the probe and coupling fields are comprehensively studied in theory and experiment. This research is helpful for measurement of fundamental physical constants by high resolution spectroscopy.
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