Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast ele...Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the former two are usually employed to study photoionization processes, the latter is typically used to investigate tunneling ionization. In this review, we briefly overview these timing techniques towards an attosecond temporal resolution of ionization processes in atoms and molecules under intense laser fields. In particular, we review the backpropagation method, which is a novel hybrid quantum-classical approach towards the full characterization of tunneling ionization dynamics. Continued advances in the interrogation techniques promise to pave the pathway towards the exploration of ever faster dynamical processes on an ever shorter timescale.展开更多
The important features of the rescattering trajectories in strong field ionization process such as the cutoff of the return energy at 3.17Up and that of the final energy at 10Up are obtained, based on the adiabatic ap...The important features of the rescattering trajectories in strong field ionization process such as the cutoff of the return energy at 3.17Up and that of the final energy at 10Up are obtained, based on the adiabatic approximation in which the initial momentum of the electron is assumed to be zero. We theoretically study the nonadiabatic effect by assuming a nonzero initial momentum on the rescattering trajectories based on the semiclassical simpleman model. We show that the nonzero initial momentum will modify both the maximal return energy at collision and the final energy after backward scattering, but in different ways for odd and even number of return trajectories. The energies are increased for even number of returns but are decreased for odd number of returns when the nonzero （positive or negative） initial momentum is applied.展开更多
Coherent superposition of electronic states induces attosecond electron motion in molecules.We theoretically investigate the strong-field ionization of this superposition state by numerically solving the time-dependen...Coherent superposition of electronic states induces attosecond electron motion in molecules.We theoretically investigate the strong-field ionization of this superposition state by numerically solving the time-dependent Schrodinger equation.In the obtained photoelectron momentum distribution,an intriguing bifurcation structure appears in the strong-field holographic interference pattern.We demonstrate that this bifurcation structure directly provides complete information about the status of the transient wave function of the superposition state:the horizontal location of the bifurcation in the momentum distribution reveals the relative phase of the involved components of the superposition state and the vertical position indicates the relative coefficient.Thus,this bifurcation structure takes a snapshot of the transient electron wave packet of the superposition state and provides an intuitive way to monitor electron motion in molecules.展开更多
By numerically solving the time-dependent Schr¨odinger equation, we observe a remarkable strong-field interference pattern in the photoelectron momentum distribution of a hydrogen atom ionized by a few-cycles las...By numerically solving the time-dependent Schr¨odinger equation, we observe a remarkable strong-field interference pattern in the photoelectron momentum distribution of a hydrogen atom ionized by a few-cycles laser pulse. This interference pattern is joined together with the familiar near-forward strong-field photoelectron holographic interference. By applying the strong-field approximation theory, we investigate the formation of this interference pattern, which arises from the interference between the backward rescattered part and the direct part of the tunneling ionized electron wave packet. We demonstrate that this backward rescattered photoelectron holographic interference can also be observed in a more realistic parallel two-color laser field. These results pave a new way to look into the atomic and molecular structure with ultrafast timescale.展开更多
We develop a numerical scheme for solving the one-dimensional(1D)time-dependent Schrödinger equation(TDSE),and use it to study the strong-field photoionization of the atomic hydrogen.The photoelectron energy spec...We develop a numerical scheme for solving the one-dimensional(1D)time-dependent Schrödinger equation(TDSE),and use it to study the strong-field photoionization of the atomic hydrogen.The photoelectron energy spectra obtained for pulses ranging from XUV to near infrared are compared in detail to the spectra calculated with our well-developed code for accurately solving the three-dimensional(3D)TDSE.For XUV pulses,our discussions cover intensities at which the ionization is in the perturbative and nonperturbative regimes.For pulses of 400 nm or longer wavelengths,we distinguish the multiphoton and tunneling regimes.Similarities and discrepancies between the 1D and 3D calculations in each regime are discussed.The observed discrepancies mainly originate from the differences in the transition matrix elements and the energy level structures created in the 1D and 3D calculations.展开更多
We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses.The denitrogenation and deoxygenation channels are accessed via various ...We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses.The denitrogenation and deoxygenation channels are accessed via various pathways.It leads to distinct asymmetries in directional breaking of the doubly ionized N2O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons.Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N2O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.展开更多
We derive a general ionization rate formula for the system of diatomic molecules in the velocity gauge. A more concise expression of the photoionization rate in the tunnel region is obtained for the first time. Compar...We derive a general ionization rate formula for the system of diatomic molecules in the velocity gauge. A more concise expression of the photoionization rate in the tunnel region is obtained for the first time. Comparisons are made among the different versions of strong-field approximation. The numerical study shows that the ionization rate in the velocity gauge is underestimated by a few orders compared with that in the length gauge. Our simple formula of ionization rate may provide an insight into the ionization mechanism for the system of diatomic molecules.展开更多
圆偏振激光场中原子的非绝热强场电离为产生自旋极化电子提供了机会.我们应用这些解析电离速率公式[Ingo Barth and Olga Smirnova,Phys.Rev.A 88,013401(2013)]更系统地研究了通过Kr和Xe原子在右旋圆偏振激光脉冲中的强场电离产生自旋...圆偏振激光场中原子的非绝热强场电离为产生自旋极化电子提供了机会.我们应用这些解析电离速率公式[Ingo Barth and Olga Smirnova,Phys.Rev.A 88,013401(2013)]更系统地研究了通过Kr和Xe原子在右旋圆偏振激光脉冲中的强场电离产生自旋极化电子,并证实了不同自旋态的光电子能量分布有很大差异、电子的自旋极化敏感地依赖于光电子能量.另外,在光电子能谱的低能部分其自旋极化可以达到100%,并且通过调节激光强度和频率可以很好地控制能量积分的自旋极化.展开更多
To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phen...To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phenomena and processes and raises a series of questions of concepts, theories, and methods. Recent studies show that the Coulomb potential can cause the ionization time lag(about 100 attoseconds) between instants of the field maximum and the ionization-rate maximum. This lag can be understood as the response time of the electronic wave function to the strong-field-induced ionization event. It has a profound influence on the subsequent ultrafast dynamics of the ionized electron and can significantly change the time–frequency properties of electron trajectory(an important theoretical tool for attosecond measurement). Here, the research progress of response time and its implications on attosecond measurement are briefly introduced.展开更多
A Wigner-distribution-like(WDL)function based on the strong-field approximation(SFA)theory is used to investigate the ionization time of the photoelectron emitted from the initial states with different magnetic quantu...A Wigner-distribution-like(WDL)function based on the strong-field approximation(SFA)theory is used to investigate the ionization time of the photoelectron emitted from the initial states with different magnetic quantum number m in elliptically polarized electric fields.The saddle-point method is adopted for comparisons.For different m states,a discrepancy exists in the WDL distributions of the photoelectrons emitted in a direction close to the major axis of the laser field ellipse.Based on the saddle-point analysis,this discrepancy can be ascribed to the interference between electrons ionized from two tunneling instants.Our results show that the relationships between the tunneling instants and kinetic energy of photoelectrons are the same for different m initial states when the Coulomb potential is not considered.Our work sheds some light on the ionization-time information of electrons from different magnetic quantum states.展开更多
The interaction of an atom with an intense laser field provides an important approach to explore the ultrafast electron dynamics and extract the information of the atomic and molecular structures with unprecedented at...The interaction of an atom with an intense laser field provides an important approach to explore the ultrafast electron dynamics and extract the information of the atomic and molecular structures with unprecedented attosecond temporal and angstrom spatial resolution. To well understand the strong field atomic processes, numerous theoretical methods have been developed, including solving the time-dependent Schr ?dinger equation(TDSE), classical and semiclassical trajectory method, quantum S-matrix theory within the strong-field approximation, etc. Recently, an alternative and complementary quantum approach, called Bohmian trajectory theory, has been successfully used in the strong-field atomic physics and an exciting progress has been achieved in the study of strong-field phenomena. In this paper, we provide an overview of the Bohmian trajectory method and its perspective on two strong field atomic processes, i.e., atomic and molecular ionization and high-order harmonic generation, respectively.展开更多
Photoelectron momentum distribution of hydrogen molecular ion in a circularly polarized laser pulse is calculated by solving the three-dimensional time-dependent Schrodinger equation(3D-TDSE).At the intermediate inter...Photoelectron momentum distribution of hydrogen molecular ion in a circularly polarized laser pulse is calculated by solving the three-dimensional time-dependent Schrodinger equation(3D-TDSE).At the intermediate internuclear distance,an unusual multi-peak structure is observed in the angular distribution,which is proved to be a signature of the transient localization of the electron upon alternating nucleus.By tracing the time-dependent ionization rate and bound state populations,we provide a clear evidence that the transient electron localization still exists in circularly polarized pulse and the corresponding multiple ionization bursts are directly mapped onto observable angular distributions.In addition,we introduce an intuitive strong-field approximation model which incorporates laser-induced subcycle internal electron dynamics to isolate the effect of the Coulomb potential of the parent ions.In this way,the timing of each ionization burst can be directly read out from the angular distributions.Our results suggest that the ionization time serves as a sensitive tool encoding intramolecular electron dynamics and can be measured using attoclock technique.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant Nos.92150105,11834004,12227807,and 12241407)the Science and Technology Commission of Shanghai Municipality (Grant No.21ZR1420100)。
文摘Electronic processes within atoms and molecules reside on the timescale of attoseconds. Recent advances in the laserbased pump-probe interrogation techniques have made possible the temporal resolution of ultrafast electronic processes on the attosecond timescale, including photoionization and tunneling ionization. These interrogation techniques include the attosecond streak camera, the reconstruction of attosecond beating by interference of two-photon transitions, and the attoclock. While the former two are usually employed to study photoionization processes, the latter is typically used to investigate tunneling ionization. In this review, we briefly overview these timing techniques towards an attosecond temporal resolution of ionization processes in atoms and molecules under intense laser fields. In particular, we review the backpropagation method, which is a novel hybrid quantum-classical approach towards the full characterization of tunneling ionization dynamics. Continued advances in the interrogation techniques promise to pave the pathway towards the exploration of ever faster dynamical processes on an ever shorter timescale.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11425414 and 11504215the Scientific Research Training Program of Shanxi University
文摘The important features of the rescattering trajectories in strong field ionization process such as the cutoff of the return energy at 3.17Up and that of the final energy at 10Up are obtained, based on the adiabatic approximation in which the initial momentum of the electron is assumed to be zero. We theoretically study the nonadiabatic effect by assuming a nonzero initial momentum on the rescattering trajectories based on the semiclassical simpleman model. We show that the nonzero initial momentum will modify both the maximal return energy at collision and the final energy after backward scattering, but in different ways for odd and even number of return trajectories. The energies are increased for even number of returns but are decreased for odd number of returns when the nonzero （positive or negative） initial momentum is applied.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11874163,11604108,and 11604388)the Program for HUST Academic Frontier Youth Teamthe Fundamental Research Funds for the Central Universities,China(HUST No.2017KFXKJC002)。
文摘Coherent superposition of electronic states induces attosecond electron motion in molecules.We theoretically investigate the strong-field ionization of this superposition state by numerically solving the time-dependent Schrodinger equation.In the obtained photoelectron momentum distribution,an intriguing bifurcation structure appears in the strong-field holographic interference pattern.We demonstrate that this bifurcation structure directly provides complete information about the status of the transient wave function of the superposition state:the horizontal location of the bifurcation in the momentum distribution reveals the relative phase of the involved components of the superposition state and the vertical position indicates the relative coefficient.Thus,this bifurcation structure takes a snapshot of the transient electron wave packet of the superposition state and provides an intuitive way to monitor electron motion in molecules.
基金Project supported by the Key Science and Technology Research of Henan Province,China(Grant Nos.162102210111 and 172102310471)the National Key Research and Development Program of China(Grant No.2017YFB0403502)
文摘By numerically solving the time-dependent Schr¨odinger equation, we observe a remarkable strong-field interference pattern in the photoelectron momentum distribution of a hydrogen atom ionized by a few-cycles laser pulse. This interference pattern is joined together with the familiar near-forward strong-field photoelectron holographic interference. By applying the strong-field approximation theory, we investigate the formation of this interference pattern, which arises from the interference between the backward rescattered part and the direct part of the tunneling ionized electron wave packet. We demonstrate that this backward rescattered photoelectron holographic interference can also be observed in a more realistic parallel two-color laser field. These results pave a new way to look into the atomic and molecular structure with ultrafast timescale.
基金Project supported by the National Natural Science Foundation of China(Gant Nos.12074265,11804233,and 11575118)the National Key Research and Development Project of China(Grant No.2017YFF0106500)+1 种基金the Natural Science Foundation of Guangdong,China(Grant Nos.2018A0303130311 and 2021A1515010082)the Shenzhen Fundamental Research Program(Grant Nos.KQJSCX20180328093801773,JCYJ20180305124540632,and JCYJ20190808121405740).
文摘We develop a numerical scheme for solving the one-dimensional(1D)time-dependent Schrödinger equation(TDSE),and use it to study the strong-field photoionization of the atomic hydrogen.The photoelectron energy spectra obtained for pulses ranging from XUV to near infrared are compared in detail to the spectra calculated with our well-developed code for accurately solving the three-dimensional(3D)TDSE.For XUV pulses,our discussions cover intensities at which the ionization is in the perturbative and nonperturbative regimes.For pulses of 400 nm or longer wavelengths,we distinguish the multiphoton and tunneling regimes.Similarities and discrepancies between the 1D and 3D calculations in each regime are discussed.The observed discrepancies mainly originate from the differences in the transition matrix elements and the energy level structures created in the 1D and 3D calculations.
基金Project supported by the National Key R&D Program of China(Grant No.2018YFA0306303)the National Natural Science Fundation of China(Grant Nos.11425416,11834004,and 11761141004)the 111 Project of China(Grant No.B12024)
文摘We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses.The denitrogenation and deoxygenation channels are accessed via various pathways.It leads to distinct asymmetries in directional breaking of the doubly ionized N2O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons.Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N2O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.
基金supported by the National Natural Science Foundation of China(Grant No.11274149)the Natural Science Foundation of Liaoning Province,China(Grant No.20121032)
文摘We derive a general ionization rate formula for the system of diatomic molecules in the velocity gauge. A more concise expression of the photoionization rate in the tunnel region is obtained for the first time. Comparisons are made among the different versions of strong-field approximation. The numerical study shows that the ionization rate in the velocity gauge is underestimated by a few orders compared with that in the length gauge. Our simple formula of ionization rate may provide an insight into the ionization mechanism for the system of diatomic molecules.
文摘圆偏振激光场中原子的非绝热强场电离为产生自旋极化电子提供了机会.我们应用这些解析电离速率公式[Ingo Barth and Olga Smirnova,Phys.Rev.A 88,013401(2013)]更系统地研究了通过Kr和Xe原子在右旋圆偏振激光脉冲中的强场电离产生自旋极化电子,并证实了不同自旋态的光电子能量分布有很大差异、电子的自旋极化敏感地依赖于光电子能量.另外,在光电子能谱的低能部分其自旋极化可以达到100%,并且通过调节激光强度和频率可以很好地控制能量积分的自旋极化.
基金Project supported by the National Natural Science Foundation of China(Grant No.91750111)the National Key Research and Development Program of China(Grant No.2018YFB0504400)。
文摘To measure and control the electron motion in atoms and molecules by the strong laser field on the attosecond time scale is one of the research frontiers of atomic and molecular photophysics. It involves many new phenomena and processes and raises a series of questions of concepts, theories, and methods. Recent studies show that the Coulomb potential can cause the ionization time lag(about 100 attoseconds) between instants of the field maximum and the ionization-rate maximum. This lag can be understood as the response time of the electronic wave function to the strong-field-induced ionization event. It has a profound influence on the subsequent ultrafast dynamics of the ionized electron and can significantly change the time–frequency properties of electron trajectory(an important theoretical tool for attosecond measurement). Here, the research progress of response time and its implications on attosecond measurement are briefly introduced.
基金the National Key Research and Development Pro-gram of China(Grant No.2019YFA0307700)the National Natural Science Foundation of China(Grant Nos.12274300 and 12074261)。
文摘A Wigner-distribution-like(WDL)function based on the strong-field approximation(SFA)theory is used to investigate the ionization time of the photoelectron emitted from the initial states with different magnetic quantum number m in elliptically polarized electric fields.The saddle-point method is adopted for comparisons.For different m states,a discrepancy exists in the WDL distributions of the photoelectrons emitted in a direction close to the major axis of the laser field ellipse.Based on the saddle-point analysis,this discrepancy can be ascribed to the interference between electrons ionized from two tunneling instants.Our results show that the relationships between the tunneling instants and kinetic energy of photoelectrons are the same for different m initial states when the Coulomb potential is not considered.Our work sheds some light on the ionization-time information of electrons from different magnetic quantum states.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11922413,11834015,11874392,11804374,11847243,and 11774387)the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB21010400)
文摘The interaction of an atom with an intense laser field provides an important approach to explore the ultrafast electron dynamics and extract the information of the atomic and molecular structures with unprecedented attosecond temporal and angstrom spatial resolution. To well understand the strong field atomic processes, numerous theoretical methods have been developed, including solving the time-dependent Schr ?dinger equation(TDSE), classical and semiclassical trajectory method, quantum S-matrix theory within the strong-field approximation, etc. Recently, an alternative and complementary quantum approach, called Bohmian trajectory theory, has been successfully used in the strong-field atomic physics and an exciting progress has been achieved in the study of strong-field phenomena. In this paper, we provide an overview of the Bohmian trajectory method and its perspective on two strong field atomic processes, i.e., atomic and molecular ionization and high-order harmonic generation, respectively.
基金Project supported by the National Key Research and Development Program of China(Grant No.2018YFB1801904)the National Natural Science Foundation of China(Grant No.12075036)the Open Research Fund of National Mobile Communications Research Laboratory,Southeast University(Grant No.2019D14)。
文摘Photoelectron momentum distribution of hydrogen molecular ion in a circularly polarized laser pulse is calculated by solving the three-dimensional time-dependent Schrodinger equation(3D-TDSE).At the intermediate internuclear distance,an unusual multi-peak structure is observed in the angular distribution,which is proved to be a signature of the transient localization of the electron upon alternating nucleus.By tracing the time-dependent ionization rate and bound state populations,we provide a clear evidence that the transient electron localization still exists in circularly polarized pulse and the corresponding multiple ionization bursts are directly mapped onto observable angular distributions.In addition,we introduce an intuitive strong-field approximation model which incorporates laser-induced subcycle internal electron dynamics to isolate the effect of the Coulomb potential of the parent ions.In this way,the timing of each ionization burst can be directly read out from the angular distributions.Our results suggest that the ionization time serves as a sensitive tool encoding intramolecular electron dynamics and can be measured using attoclock technique.
