Dynamics of atoms and molecules at extremes
极端条件原子分子动力学涉及的强激光场、强电磁场和高温高压高密环境广泛地存在于核爆炸、聚变科学和天体物理等领域研究中, 由于涉及多通道、强关联、非微扰以及多体相互作用等难题, 如何建立极端条件下原子分子动力学先进的实验方法和准确的理论模型、获得高精度的结构和动力学过程数据, 是当前原子分子物理及其相关领域面临的巨大挑战. 超快超强激光和离子加速器等实验技术的发展, 极大地推动了极端条件原子分子动力学研究的开拓和深入. 开展极端条件原子分子动力学研究, 能够深入认识极端环境下原子分子过程的反应机制和动力学演化规律, 提升极端条件原子分子数据的精密研究能力, 这对于天体物理、等离子体物理、磁约束和惯性约束核聚变等多个领域以及超快物理等科学前沿, 具有重要的应用价值.
受《物理学报》编辑部委托, 我们策划组织了“极端条件原子分子动力学”专题, 邀请本领域中青年科学家撰稿, 涵盖双电子俘获过程, 离子的低能电子弹性散射, 强激光场下的 Rydberg态激发,超快强场调控分子电离、解离和准直, 等离子体动力学演化, 原子物理中的辐射过程, 电子碰撞激发过程, 高温非平衡气体分子态-态碰撞, 光场加速稠密物质中离子电荷转移等主题的多篇最新研究成果, 同时就强激光场中可能产生独特的光-核相互作用的前沿研究进行了展望, 综述了基于原子内壳层跃迁的 X射线腔量子光学、超快和高压结合的综合极端条件下分子动力学过程的研究进展等. 希望本专题能够为相关领域学者提供参考, 吸引更多青年学者进入本领域开展研究, 推动极端条件原子分子动力学领域的蓬勃发展.
2024, 73 (24): 244202.
doi: 10.7498/aps.73.20241456
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2024, 73 (24): 246101.
doi: 10.7498/aps.73.20241218
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2024, 73 (24): 240701.
doi: 10.7498/aps.73.20241290
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Electron capture in the collision of highly charged ions with atoms and molecules is a fundamental process related to the electron transition between bound states belonging to two atomic-centers. The X-ray emission after electron capture is important for X-ray astrophysical modeling, fusion plasma diagnostics, and ion irradiated biophysics. In the past few decades, momentum-imaging cold-target recoil ion momentum spectroscopy has been a significantly developed technique and widely used to measure the quantum state-selective population in electron capture processes. Based on the cold target recoil ion momentum spectroscopy installed on the 150 kV highly charged ion platform in Fudan University, Shanghai City, China, the state-selectivity of double electron capture in the bombardment of 1.4–20 keV/u Ar8+ on He is measured, and the relative cross sections of the 3l 3l' to 3l 7l' double excited states are obtained. It is found that with the increase of collision energy, more quantum state-selectivity channels are open in the double electron capture of Ar8+-He collision. It is also found that the relative cross section of the quantum state population is strongly dependent on the collision energy of the projectile ion. The present measurements not only enrich the state-selective cross-sectional library and collision dynamics of highly charged ion charge exchange processes, but also provide experimental benchmarks for existing theoretical calculations.
2024, 73 (24): 243201.
doi: 10.7498/aps.73.20241222
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Rydberg atoms are important building blocks for quantum technologies, and because of their unique tunable quantum properties, they possess new applications in quantum computing, quantum communication, and quantum sensing. Besides the widely-used few-photon resonant excitation for the specific Rydberg state, multiple Rydberg states can be populated coherently and efficiently through the frustrated tunneling ionization or the Coulomb potential recapture effect in a strong laser field. The excitation of Rydberg states in a strong field provides an opportunity for realizing the ultrafast quantum control on Rydberg atom and bridging the gap between strong field physics and quantum information technology. Using the classical trajectory Monte Carlo method and Qprop package to solve time-dependent Schrödinger equation, we calculate the population of Rydberg states. Our results show that the population increases with the increase of parameter of the asymmetric laser envelope. Based on the quantitative rescattering theory, the calculated time-dependent recapture rate is negatively related to the laser envelope and the residual laser interaction time, which is termed the envelope effect. Combined with the carrier-wave effect, an analytic formula can be used to calculate the Rydberg state population: $ Y(t) \propto $$ W_0\left(t\right) \dfrac{t-\tau+c}{f\left(t\right)} \cos \left(\omega t+\phi\right) . $ This result opens the way to enhancing the generation of Rydberg states by using the laser envelope control, which is beneficial to the future quantum technology based on the Rydberg states generated in the strong laser field.
2024, 73 (24): 243301.
doi: 10.7498/aps.73.20241401
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Coherent control of molecular dissociation in ultrafast strong fields has received considerable attention in various scientific disciplines, such as atomic and molecular physics, physical chemistry, and quantum control. Many fundamental issues still exist regarding the understanding of phenomena, exploration of mechanisms, and development of control strategies. Recent progress has shown that manipulating the spectral phase distribution of a single ultrafast strong ultraviolet laser pulse while maintaining the same spectral amplitude distribution can effectively control the total dissociation probability and branching ratio of molecules initially in the ground state. In this work, the spectral phase control of the photodissociation reaction of chlorobromomethane (CH2BrCl) is studied in depth by using a time-dependent quantum wave packet method, focusing on the influence of the initial vibrational state on the dissociation reaction. The results show that modifying the spectral phase of a single ultrafast pulse does not influence the total dissociation probability or branching ratio in the weak field limit. However, these factors exhibit significant dependence on the spectral phase of the single ultrafast pulse in the strong field limit. By analyzing the population distribution of vibrational states in the ground electronic state, we observe that chirped pulses can effectively control the resonance Raman scattering (RRS) phenomenon induced in strong fields, thereby selectively affecting the dissociation probability and branching ratio based on initial vibrational states. Furthermore, we demonstrate that by selecting an appropriate initial vibration state and controlling both the value and sign of the chirp rate, it is possible to achieve preferential cleavage of Cl+CH2Br bonds. This study provides new insights into understanding of ultrafast coherent control of photodissociation reactions in polyatomic molecules.
2024, 73 (24): 243401.
doi: 10.7498/aps.73.20241377
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This paper reports low-energy electron scattering with $ {{\mathrm{C}}}_{4}^{-} $ anions by using the ab initio R -matrix method in the single state close-coupling (CC) model and the fixed-nuclei approximation. We predict the elastic integral scattering cross sections (ICSs) of four conformers of $ {{\mathrm{C}}}_{4}^{-} $ ions in an energy range of 0 < E ≤12 eV and discuss the effects of configuration changes on resonance position and width. Additionally, the theoretical results and experimental data are compared and analyzed. The results indicate that the 8.8 eV resonance peak observed in experiment is mainly derived from the $ {{{\Sigma }}}_{{\mathrm{u}}}^{+} $ and $ {{{\Sigma }}}_{{\mathrm{u}}}^{-} $ resonances of the conformer A and the A2 resonance of the conformer C. The scattering cross-section reveals that the conformer A has five resonant states, and the conformer B has three resonances, while C and D each have four resonances. Finally, we use the Boltzmann distribution to calculate the populations of different conformers at different temperatures, and simulate the low-energy electron elastic integrated scattering cross-section at room temperature, which is in good agreement with available experimental results. We also find a shape resonance with a width of 0.20 eV at 3.3 eV in our total cross sections, which is not detected in the existing experimental results. This provides new opportunities for measurement.
2024, 73 (24): 244201.
doi: 10.7498/aps.73.20241378
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Owing to vortex light possessing the additional orbital angular momentum, its interaction with atoms and molecules can reveal in more depth insights into dynamics than the plane wave light. This paper aims to establish a theoretical framework for the photoionization of atoms and molecules by Bessel vortex light. In the case of macroscopic gas target, helium atoms are randomly dispersed around the entire region of the Bessel vortex beam. The final photoionization cross-section is not dependent on the angular momentum of the vortex light, but depends on the opening angle of the Bessel vortex light. This paper systematically computes the variation of photoionization cross-section with photon energy and the angular distributions of photoelectrons under different geometric conditions. The computation results demonstrate that there is a significant difference in the photo-ionization cross-section between vortex light and plane wave light. In order to further investigate the characteristics of the phase singularity of the vortex light (when the light intensity reaches zero), this paper further calculates the photo-ionization of the vortex light with opening angles of 5°, 30°, and 60° at the phase singularity, respectively. The results indicate that the angular distribution of photoelectrons at these three angles is significantly dependent on the orbital angular momentum and the opening angle of the vortex light, and the calculated absolute cross-section does not equate to zero. This represents an important distinguishing feature of the Bessel vortex light when interacting with atoms, distinguishing it from the plane wave. This work lays the foundation for further studying vortex light photo-ionization and their applications.
2024, 73 (24): 248201.
doi: 10.7498/aps.73.20241283
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In recent years, the rapid development of ultrashort pulse laser technology has made it possible to regulate the ionization and dissociation dynamics of atoms and molecules. Among them, the microscopic dynamics of molecular dissociation have always been a hot topic. The phenomenon of molecular dissociation, which is caused by the interaction between femtosecond intense laser fields and $\text{H}_2^+$ molecules, has attracted widespread attention. Previous theoretical studies on the dissociation of $\text{H}_2^+$ molecules mainly focused on studying its dissociation dynamics through numerical calculations, with relatively few theoretical models. This paper aims to establish a simple classical model to describe the dissociation dynamics. Firstly, this paper calculates the joint distribution of nuclear energy and electronic energy in the dissociation process of $\text{H}_2^+$ molecules under the action of pump lasers by numerically solving the Schrödinger equation. The results prove that $\text{H}_2^+$ molecules initially in the ground state are dissociated into ${\rm H}^+ + {\rm H}^*$ after absorbing a pump photon in the pump light field. Next, this paper studies the dissociation dynamics of $\text{H}_2^+$ molecules in time-delayed two-color femtosecond lasers. We find that it greatly depends on the specific forms of the pump light and the probe light. By utilizing the dependence of the dissociation kinetic energy release (KER) spectrum on the time delay of the two-color femtosecond lasers, we retrieve the sub-attosecond microscopic dynamic behaviors of electrons and atomic nuclei in the dissociation process. Furthermore, we establish a classical model based on the conservation of energy and momentum to describe the dissociation dynamics. This model can qualitatively predict the ion dissociation KER spectrum depending on the time delay of the two-color femtosecond lasers. The electronic resonant transition between the molecular ground state and the first excited state caused by the probe light will affect the ion kinetic energy spectrum in the dissociation process. Namely, the ion kinetic energy spectrum is dependent on the frequency of the probe laser. By taking advantage of this characteristic, we propose a scheme to reconstruct the evolution of the internuclear distance with time. Our reconstruction results can qualitatively predict the trend of the numerical simulation results, and this scheme may provide some theoretical guidance for experiments.
2025, 74 (3): 033401.
doi: 10.7498/aps.74.20241467
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Analysis of dynamic response and screening effects on electron-ion energy relaxation in dense plasma
2025, 74 (3): 035101.
doi: 10.7498/aps.74.20241588
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Accurate knowledge of electron-ion energy relaxation plays a vital role in non-equilibrium dense plasmas with widespread applications such as in inertial confinement fusion, in laboratory plasmas, and in astrophysics. We present a theoretical model for the energy transfer rate of electron-ion energy relaxation in dense plasmas, where the electron-ion coupled mode effect is taken into account. Based on the proposed model, other simplified models are also derived in the approximations of decoupling between electrons and ions, static limit, and long-wavelength limit. The influences of dynamic response and screening effects on electron-ion energy relaxation are analyzed in detail. Based on the models developed in the present work, the energy transfer rates are calculated under different plasma conditions and compared with each other. It is found that the behavior of electron screening in the random phase approximation is significantly different from the one in the long-wave approximation. This difference results in an important influence on the electron-ion energy relaxation and temperature equilibration in plasmas with temperature $T_{\rm{e}} < T_{\rm{i}}$. The comparison of different models shows that the effects of dynamic response, such as dynamic screening and coupled-mode effect, have stronger influence on the electron-ion energy relaxation and temperature equilibration. In the case of strong degeneracy, the influence of dynamic response will result in an order of magnitude difference in the electron-ion energy transfer rate. In conclusion, it is crucial to properly consider the finite-wavelength screening of electrons and the coupling between electron and ion plasmonic excitations in order to determine the energy transfer rate of electron-ion energy relaxation in dense plasma.
2025, 74 (3): 033402.
doi: 10.7498/aps.74.20241638
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Bremsstrahlung, as an important radiation process in atomic physics, has significant applications in the fields of astrophysics, plasma physics, magnetic and inertial confinement fusion. In this work, the relativistic partial-wave expansion method is used to investigate the bremsstrahlung of neutral carbon atoms and different charged carbon ions scattered from intermediate- and high-energy relativistic electrons, with special attention paid to the electronic screening effect produced by the target electrons. The target wave function is obtained from the Dirac-Hartree-Fock self-consistent calculations, and the electron-atom scattering interaction potential is constructed in the central-field approximation. By solving the partial-wave Dirac equation, the continuum wave functions of the relativistic electron are obtained, from which the bremsstrahlung single and double differential cross sections can be calculated via the multipole free-free transitions between the incident and exit free electrons. The target electronic screening effects on the bremsstrahlung single and double differential cross sections are analyzed under a variety of conditions of incident electron energy and emitted photon energy. It is shown that the target electronic screening effect will significantly suppress the cross sections both at low incident energy and in the soft-photon region. Such a suppressing effect decreases with the incident electron energy and the emitted photon energy gradually increasing. Overall, the electronic screening effect has no significant influence on the shape function of bremsstrahlung.
2025, 74 (3): 033301.
doi: 10.7498/aps.74.20241400
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Femtosecond laser-induced excitation of molecular rotational states can lead to phenomena such as alignment and orientation, which fundamentally stem from the coherence between the induced rotational states. In recent years, the quantitative study of coherence in the field of quantum information has received widespread attention. Different kinds of coherence measures have been proposed and investigated. In this work, the quantitative correlation is investigated in detail between the intrinsic coherence measurement and the degree of molecular alignment induced by femtosecond laser pulses at finite temperatures. By examining the molecular alignment induced by ultrafast non-resonant laser pulses, a quantitative relationship is established between the $l_1$ norm coherence measure $C_{l_1}(\rho)$ and the alignment amplitude ${\cal{D}}\langle \cos^2 \theta \rangle$. Here, $C_{l_1}(\rho)$ represents the sum of the absolute values of all off-diagonal elements of the density matrix ρ, ${\cal{D}}\langle \cos^2 \theta \rangle$ represents the difference between the maximum alignment and the minimum alignment. A quadratic relationship $ C_{l_1} = (a + b{\cal{E}}^2_0)\times $$ {\cal{D}}\langle \cos^2 \theta \rangle$ between the the $l_1$ norm coherence measure and ${\cal{D}}\langle \cos^2 \theta \rangle$ with respect to the electric field intensity ${\cal{E}}_0$ is obtained. This relationship is validated through numerical simulations of the CO molecule, and the ratio coefficients a and b for different temperatures are obtained. Furthermore, a mapping relationship between this ratio and the pulse intensity area is established. The findings of this study provide an alternative method for experimentally detecting the coherence measure within femtosecond laser-excited rotational systems, thereby extending the potential applicability of molecular rotational states to the study of the coherence measure in the field of quantum resources. This will facilitate the interdisciplinary integration of ultrafast strong-field physics and quantum information.
2025, 74 (3): 033201.
doi: 10.7498/aps.74.20241321
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The laser-produced Sn plasma light source is a critical component in advanced extreme ultraviolet (EUV) lithography. The power and stability of EUV radiation within a 2% bandwidth centered at 13.5 nm are key indicators that determine success of the entire lithography process .The plasma state parameter distributions and the EUV radiation spectrum for a laser-produced Sn plasma light source are numerically simulated in this work. The radiative opacity of Sn plasma within the 12–16 nm range is calculated using a detailed-level-accounting model in the local thermodynamic equilibrium approximation. Next, the temperature distribution and the electron density distribution of plasma generated by nanosecond laser pulses interacting with both a Sn planar solid target and a liquid droplet target are simulated using the radiation hydrodynamics code for laser-produced plasma, RHDLPP. By combining the radiative opacity data with the plasma state data, the spectral simulation subroutine SpeIma3D is employed to model the spatially resolved EUV spectra for the planar target plasma and the angle-resolved EUV spectra for the droplet target plasma at a 60-degree observation angle. The variation of in-band radiation intensity at 13.5 nm within the 2% bandwidth as a function of observation angle is also analyzed for the droplet-target plasma. The simulated plasma state parameter distributions and EUV spectral results closely match existing experimental data, demonstrating the ability of RHDLPP code to model laser-produced Sn plasma EUV light sources. These findings provide valuable support for the development of EUV lithography and EUV light sources.
