Atomic and molecular processes driven by ultrafast intense laser fields
编者按：
利用光与物质相互作用是人类探索微观物质结构及运动规律的重要手段. 飞秒强激光技术的出现与发展为揭示极端强场条件下的原子物理新现象、新效应, 认识分子、原子及其内部电子的超快运动规律提供了强有力的技术手段和研究条件. 超快强激光驱动原子分子过程的研究进展还直接推动了新兴学科领域——阿秒科学的发展, 强激光驱动原子分子的高次谐波辐射已成为产生极紫外阿秒光源的重要途径, 利用强场电离及高次谐波产生过程可实现对分子结构及动力学的超快成像, 在原子级时间与空间尺度上的电子调控已成为可能.
“超快强激光驱动的原子分子过程”专题结合对超快强激光驱动的原子分子电离、高次谐波产生、中性里德堡态原子产生及分子解离等基本物理过程的研究, 介绍该领域的一些基本理论方法、实验技术以及研究成果, 以帮助读者了解该研究领域的最新进展, 推动对超快强激光与原子分子相互作用相关研究的进一步深入.
2016, 65 (22): 223201.
doi: 10.7498/aps.65.223201
Abstract +
We experimentally investigate the Rydberg state excitations (RSEs) of noble gas atoms, He, Ar and Xe, in an 800-nm 50-fs strong laser field, by using the mass resolved pulsed electric field ionization method combined with the time-of-flight mass spectrometer. We measure the yields of the atomic RSE at different laser intensities and ellipticities, and compare the results with those of the nonsequential double ionization (NSDI) in strong laser fields. Our study shows that like that of NSDI, the yield of the atomic RSE increases as the atomic number increases, i.e., RSE yield trend is He Ar Xe. On the other hand, for any of the atoms, the probability of NSDI is lower than that of total RSE at the same laser intensity, which can be understood as that the yield of high energy electrons (for NSDI) is less than that of low energy electrons that can be captured into the Rydberg states. Additionally, our results show that the RSE yield strongly depends on the laser ellipticity, which is completely suppressed by a circularly polarized laser field. The dependence of RSE on laser ellipticity turns weaker as the atomic number increases, and is weaker than that of NSDI for any of the atoms. It is indicated that the atomic RSE in strong laser field can be attibuted to the capture of the low energy electrons after tunneling ionization into Rydberg states by the Coulomb potential at the end of the laser pulse.
2016, 65 (22): 220203.
doi: 10.7498/aps.65.220203
Abstract +
As the advances of laser technology, more and more nonlinear phenomena are observed in the atoms and molecules driven by strong laser pulses. Systematic investigations on these findings, such as above threshold ionization and high-order harmonic generation, will lead us to understanding the mechanisms in the microscopic world. The most exact way to simulate the experimental measurements is to solve the time-dependent Schrdinger equation (TDSE) numerically, in which the system is described by the wave function and thus one cannot have an intuitive insight into the underling process. Therefore, several semiclassical methods have been developed to understand the strong field ionization. In the classical point of view, the electrons tunnel out when the strong laser field suppresses the Coulomb potential. Then the electrons are driven by the laser electric field according to the Newtonian equations. Semiclassical methods take into account the tunnelling of the electron, the classical orbit of the electron, and the action as the phase of trajectory, which have successfully explained main structures in the ionization spectrum. Two of the most popular semiclassical methods are the quantum trajectory Monte Carlo method and the Coulomb-corrected strong field approximation method. In the present review, we will introduce these basic methods and show how they have been developed step by step, covering the most relevant and important works in the strong field physics. Finally we give two example of applications to show how these methods work. With the advantage of the classical picture, we can identify different kind of structures in the 2D photoelectron momentum distributions and tell how the structures are formed. Nonadiabatic effects can be studied by comparing the results of the two methods, together with accurate simulation from the numerical solution of TDSE. The current semiclassical methods can be further developed into advanced ones, which can be used in more complex molecular systems or multi-electron systems, and be widely used in the study of dynamics of molecule and atoms in strong laser fields.
2016, 65 (22): 223202.
doi: 10.7498/aps.65.223202
Abstract +
When an atom or a molecule interacts with an intense laser field, a coherent high-order harmonic emission is observed at a frequency that is an integer multiple magnitude of the initial frequency of the incident laser field. The harmonic emission has the characteristic of high emission efficiency at relatively high orders, and it also has a wide expansion in the frequency domain. Thus, the high-order harmonic generation can be utilized to generate coherent EUV or soft X-ray light sources as well as ultrashort at to second laser pulses. It is promising that the attosecond laser pulse will be an important tool for detecting and controlling the electron dynamics in atom and molecule systems.
The mechanisms of high-order harmonics especially the high energy part of the harmonic spectrum can be explained by the well-known three-step model. The three-step model assumes that the electron in the bound state firstly are ionized by the potential barrier formed by the laser electric field and the atomic potential, then the ionized electrons oscillate in the laser field, and finally the electron with high kinetic energy gained in the laser field has the possibility to return back to the parent ion and recombines with the ground state of the system with a high energy photon emitted. As for harmonics with low orders, especially those with single photon energy near the ionization threshold, the Coulomb potential of the atom has significant influences on them. However,the effect of the Coulomb potential of the atom are not included in the three-step model, so the mechanism of near-threshold harmonics (NTH) cannot be clearly interpreted with the three-step model alone. In this circumstance, the study of the mechanism of near-threshold harmonic emission attracted people's attention in general. One important application of NTH is that it can be utilized to generate optical comb with EUV frequencies.
Theoretically, Xiong et al. studied the mechanism of below-threshold harmonic (BTH) emission and found that the mechanism of this part of harmonics include the effect of the quantum-path interference and the Coulomb potential. He et al. analyzed the emission of BTH in various laser intensity regions and found that the harmonic spectrum exhibits a periodic structure as a function of the harmonic frequency when the incident laser intensity is about 1013 W/cm2. Utilizing the quantum-path and time-frequency analyses of the harmonic emission, He et al. indicated that this periodic structure can be attributed to the interference effect between two specific quantum paths. Li et al. adopted the synchrosqueezing scheme to study the near-and below-threshold harmonic emission of Cs atoms in an intense mid-infrared laser field and they showed that the multiphoton and the multiple rescattering trajectories have an effect on the NTH and BTH generation processes. Shafir et al. found that the ionic potential plays an critical role in NTH emission. Under the interaction between the atom and the intense laser field, electron in the ground state not only can be ionized but also be pumped into excited state, and these excitation processes also affect the harmonic emission.
We studied the harmonic emission process near the ionization threshold by solving the time-dependent Schrdinger equation of an atom interacting with a strong laser field. Utilizing the obtained wavefunction, we systematically studied the high-order harmonic emission with the variation of the incident laser intensity. Meanwhile, through solving the TDSE with the momentum-space method, the excited-state population is precisely calculated and achieved. We show that the ninth harmonic exhibits a periodic oscillation structure with the intensity of the incident laser field increasing, and we reveals that there is a synchronous variation between the harmonic intensity and the relatively high bound state population.Within a certain range of laser intensity, the increase of the total population of the excited states corresponds to the low efficiency of harmonic emission, and this competition relationship is quite clear. Therefore, when the wavelength of the driving laser pulse is fixed, we can optimize the driving laser intensity to achieve the near-threshold harmonic emission with high efficiency.
2016, 65 (22): 223203.
doi: 10.7498/aps.65.223203
Abstract +
A Wigner-distribution-like function is proposed to obtain various distributions of photoelectron emitted from H atoms in few-cycle laser pulses with different frequencies:time-energy distribution, time ionization distribution for linearly polarized laser field and time-emission angle distribution, angular distribution, and time ionization distribution for elliptically polarized laser field. With decreasing frequency, all the distributions clearly show a transition of ionization process from the multi-photon regime to the tunneling regime. For the case of linearly polarized laser pulse, accompanying this transition, the semiclassical relationship between the ionization moment and the final drift energy is becoming more and more close to the time-energy distribution. Meanwhile, the time-energy distribution clearly shows the interference structures in the tunneling regime, which can be attributed to the interference between the electrons with the same energy ejected at different times. For the case of elliptically polarized laser pulse, both the angular offset in the angular distribution and the time offset in the time ionization distribution are obtained by comparing the quantum calculation with the semi-classical result. The results show that the time offset is much smaller than the angular offset. This indicates that the attoclock technique which is based on the correspondence between two offsets is in principle inaccurate. Furthermore, the time offset can be both positive and negative. So this time offset cannot be interpreted as the tunneling time.
2016, 65 (22): 223204.
doi: 10.7498/aps.65.223204
Abstract +
The semi-classical method based on the recently developed analytical R-matrix theory is reviewed in this work. The method is described with the application to ultra-fast strong-field direct ionization of atoms with one active electron in a linearly polarized field[Torlina L, Smirnova O 2012 Phys. Rev. A 86 043408]. The analytical R-matrix theory separates the space into inner and outer regions, naturally allowing the possibility of an analytical or semi-analytical description of wave function in the outer region, which can be approximated by Eikonal-Volkov solutions while the inner region provides well-defined boundary conditions. Applying the stationary phase method, the calculation of the ionization amplitude is cast into a superposition of components from trajectories and their associated phase factors. The shape of the tunneling wave packets associated with different instants of ionization is presented. It shows the exponential cost of deviating from the optimal tunneling trajectory renders the tunneling wave packet a Gaussian shape surrounding the semi-classical trajectory. The intrinsically non-adiabatic corrections to the sub-cycle ionization amplitude in the presence of both the Coulomb potential and the laser field is shown to have different influences on the probability of ionization. As a specific study case, soft recollisions of the released electron near the ionic core is investigated by using pure light-driven trajectories with Coulomb-corrected phase factor[Pisanty E, Ivanov M 2016 Phys. Rev. A 93 043408]. Incorporating the Coulomb potential, it is found problematic to use the conventional integration contour as chosen in other methods with trajectory-based Coulomb corrections, because the integration contour may run into the Coulomb-induced branch cuts and hence the analyticity of the integrand fails. In order to overcome the problem, the evolution time of the post-tunneling electron is extended into the complex domain which allows a trajectory to have an imaginary component. As the soft recollision occurs, the calculation of the ionization amplitude requires navigating the branch cuts cautiously. The navigating scheme is found based on closest-approach times which are the roots of closest-approach times equations. The appropriately selected closest-approach times that always present in the middle of branch-cut gate may serve to circumvent these branch cuts. The distribution of the closest-approach times presents rich geometrical structures in both the classical and quantum domains, and intriguing features of complex trajectories emerge as the electron returns near the core. Soft recollisions responsible for the low-energy structures are embedded in the geometry, and the underlying emergence of near-zero energy structures is discussed with the prediction of possible observations in experiments.
2016, 65 (22): 223205.
doi: 10.7498/aps.65.223205
Abstract +
The time-dependent Schrodinger equation of alkali metal Na atom in an infrared laser field is solved numerically by using the pseudo-spectral method. In the calculation, an accurate model potential of Na atom is used. The bound state energy levels, which are consistent with experimental data, are obtained with the potential, so that we can study the characteristics of high-order harmonic generation for emission of the exited stated of Na atom. Our results show that the high-order generation spectrum of emission of 4s, and 5s excited states of Na atom is super-continuum in the over-barrier ionization regime. By superposed certain orders harmonics below threshold, a single pulse can be obtained with the central frequency from high frequency of visible light to the ultraviolet band. Through the calculated ionization probability of Na atom and the time-frequency analysis by wavelet transform of the superposed harmonics, it reveals that the emission process of low-order harmonic generation in over-barrier ionization regime is different from in the tunnel ionization regime.
2016, 65 (22): 223206.
doi: 10.7498/aps.65.223206
Abstract +
The advent of the ultrafast laser pulse provides the powerful and efficient tool for probing the ultrafast electron dynamics in atoms and molecules. The various nonlinear process induced by the laser-matter interaction allows one to obtain the electron motion information on the sub-femtosecond time scale. A series of the ultrafast spectroscopic technique, such as attosecond streak camera, attosecond transient absorption spectrum, and etc., have been successfully applied to the probe of electron dynamics in atoms, molecules, and solids. Using two-color field is one of the significant methods to achieve the coherent control and exploring of the electron motion. This paper summarizes recent research activities in the field of the atomic and molecular ultrafast process investigated in State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, including the detection of the electron dynamics of the multi-bound states, measurement of the carrier envelope phase (CEP) and the phase of the attosecond pulse, and the ultrafast electron control with the THz/UV and MIR/IR field. To measure the dynamics of the multi-bound states, a broadband attosecond pulse can be used to ionize the electrons after it is excited by the pump laser. By changing the delay between the pump laser and the attosecond pulse, the measured electrons ionized by the broadband xuv attosecond pulse can present the multi-bound states dynamics simultaneously. The XUV/IR scheme is popularly used in attosecond dynamics measurement. But usually, the IR field is not very strong. We find that, if the IR field is strong enough to induce the above threshold ionization (ATI), the interference between the ATI electron and the electron from XUV pulse can be used to measure the CEP of the attosecond XUV pulse. Besides, if the electron ionized by attosecond pulse can be pushed back to the nuclei, the emission from the recombination can be used to determine the spectral phase of the attosecond pulse, which is an all-optical measurement. We also investigate the two color scheme of THz/UV and MIR/IR fields. With THz/UV two color scheme, very high electron localization can be achieved duration molecular dissociation when we use the UV pulse to excite the electron and the THz pulse to control the following electron movement. When we use the MIR/IR field to control the electron motion during the high harmonic generation, the recollision can be greatly decreased and the single attosecond pulse can be produced with multi-cycle MIR laser field.
Strong field photoelectron holography studied by a generalized quantum-trajectory Monte Carlo method
2016, 65 (22): 223207.
doi: 10.7498/aps.65.223207
Abstract +
Strong-field photoelectron holography encodes detailed temporal and spatial information about both theelectron and ion dynamics. Here, we review a series of numerical studies of strong-field photoelectron holographyin atoms and molecules by a generalized quantum-trajectory Monte Carlo method. By comparingthe generalized quantum-trajectory Monte Carlo simulationwiththe numerical solution of thetime-dependent Schrdinger equation, we demonstrate that, in the nonadiabatic tunneling regime, pronounced nonadiabatic effects occur which manifest in the energy cutoff of the holographic interference structure. Moreover, we found that a profound ring-like pattern can be observed in the deep tunneling ionization regime. Theappearance of the ring-like interference pattern masks the holographic interference structure. In contrast to the tunneling regime, the long-range Coulomb potential is found to play an essential role in the formation of the photoelectron holography in the nonadiabatic tunneling regime.
2016, 65 (22): 224205.
doi: 10.7498/aps.65.224205
Abstract +
The research of laser-matter interaction has become a major direction in the field of laser physics since the invention of laser in 1960. Based on the development of the laser technique in the recent several decades, the ranges of the laser's frequency, intensity and pulse width have been explored widely. Therefore, the excitation, emission and ionization dynamic processes of a complex system in intense laser fields have been studied deeply. Especially, the nonsequential double ionization (NSDI) process has continuously attracted much attention from both experimental and theoretical sides. So far, the recollision picture is widely accepted as a dominating mechanism accounting for the NSDI process under an infrared (IR) laser field condition. This recollision picture can be classified into two mechanisms:the collision-ionization (CI) mechanism and the collision-excitation-ionization (CEI) mechanism. Recently, it is found that the NSDI process can take place in an extreme ultraviolet (XUV) laser field, and thus few-photon double ionization has been extensive studied by solving the full-dimensional time-dependent Schrdinger equation (TDSE) and the conventional nonstationary perturbation theory. This article reviews the frequency-domain theory of the NSDI processes of an atom in a monochromatic IR and IR+XUV two-color laser fields. In contrast with other approaches, such as the TDSE calculation and S-matrix method, the frequency-domain theory based on the nonperturbative quantum electrodynamics is involved in some advantages:(i) all the recollision processes, including high-order above-threshold ionization (HATI), high-order harmonic generation (HHG) and NSDI, can be dealt under the unified theoretical frame and can be decoupled into two processesa direct above-threshold ionization (ATI) followed by a laser-assisted collision (LAC) or by a laser-assisted recombination process, where these subprocesses can be investigated separately; (ii) the approach can save a lot of computation time because of its nature of time-independent. In this review, we show the different momentum spectral distributions under the CI and CEI mechanisms in the IR and IR+XUV laser fields. With the help of the channel analysis, we compare the contributions of the forward and backward collisions to the NSDI under two conditions of the monochromic IR and IR+XUV two-color laser fields. It is found that, in the CI mechanism, the backward collision makes major contribution to the NSDI in the IR laser field, while the forward collision plays a crucial role in the NSDI when the energy of the recolliding electron is very large in the IR+XUV two-color laser fields. Furthermore, by employing the saddle-point approximation, it is found that the momentum spectrum, whether in the monochromic IR or the IR+XUV two-color laser fields, is attributed to the interference between two trajectories at different saddle-point t0 and 2/1-t0 (1 is the frequency of an IR laser field) when the collision happens in each channel. On the other hand, in the CEI mechanism, the momentum spectra in the monochromic IR or the IR+XUV two-color laser fields present a distinct difference. It is further found that the momentum spectrum in the IR+XUV two-color laser fields is involved in the much more channels than that in the monochromic IR laser field, and thus the complex interference patterns in the momentum spectrum in the two-color laser fields are shown. Moreover, it is found that, in both the CI and CEI mechanisms, the XUV laser field in the NSDI not only can enhance the ionization probability of the first electron, but also can accelerate the first ionized electron so that the bound electron can gain much energy by collision, which is in favor of significant boost of the NSDI probability. This work can help people understand more deeply about the NSDI, and also may pave a way for us to continue investigating the NSDI process of complex system in intense laser fields.
2016, 65 (22): 224206.
doi: 10.7498/aps.65.224206
Abstract +
High harmonic generation (HHG) is one of the most fundamental processes in the interaction of strong laser fields with atoms and molecules. Because of wide applications of HHG, for example, imaging atomic or molecular orbitals, visualizing chemical reactions, synthesizing a single attosecond pulse, the HHG attracts huge attentions in both theories and experiments. The HHG can be explained by the famous three-step model:first, the laser field bends the Coulomb potential and the electron tunnels out; second, the electron is accelerated in the laser field and gains kinetic energy; Third, the energetic electron recombines with the parent ion and release its energy as high energetic photons. The HHG can be tailored by controlling the each step. In this paper, we conceive a strategy to control the third step. We simulate the HHG when He+ is exposed to the combined few-cycle Ti-Sapphire (800 nm) IR femtosecond laser pulse and XUV laser pulse by numerically solving the time dependent Schrdinger equation. The simulation shows that after the electron tunnels out and gains energies from the infrared laser field, extra XUV photons may be absorbed during the electron and parent ion recombination, contributing multiple cutoffs separated by XUV photon energies in the high harmonic spectrum. This scenario is confirmed by time-delay-dependent HHG in the time-frequency representation, and by the power scaling of the cutoffs' intensities as a function of the XUV intensity.
2016, 65 (22): 224207.
doi: 10.7498/aps.65.224207
Abstract +
When atoms and molecules are excited by ultrashort laser pulses, highly nonlinear strong-field processes like above-threshold ionization and high harmonic generation occur. By analyzing the emitted light and electron signals, the atomic and molecular structures and ultrafast dynamics can be detected with a combination of Angstrom spatial resolution and sub-femtosecond temporal resolution, which provides a powerful tool to study the basic structures and physical processes in the microscopic world. The molecular orbital tomography (MOT) developed since 2004 enables one to image the wavefunction of the molecular orbital itself, which will help people gain deeper insight into the chemical reactions. In this paper, the theory of MOT will be introduced, and the progresses of MOT in the past ten years will be reviewed.
2016, 65 (22): 224208.
doi: 10.7498/aps.65.224208
Abstract +
Recently, three major types of minima (i.e., Cooper-like minimum, two-center interference minimum and multi-channel interference minimum) have been observed in high-order harmonic generation (HHG) spectra. Identification of the origin of the minimum in a HHG spectrum is critical for self-probing of the molecular structures and dynamics, which has been an important subject in attosecond physics. In this paper, we report the investigation of the multi-electron dynamics in HHG from N2 molecules driven by intense mid-infrared laser pulses. Based on a pump-probe experimental setup, clear spectral minima in the cutoff region of high harmonic spectra from N2 molecules are observed in measurements with mid-infrared laser pulses at three wavelengths (i.e., 1300, 1400 and 1500 nm). A systematic investigation has been carried out for clarifying the origin of these minima. We carefully measured the spectral minima under three different experimental conditions:1) different alignment angles of molecules; 2) various peak laser intensities; 3) tunable driving laser wavelengths. Experimental results show that the positions of the spectral minima do not depend on the alignment angles of molecules. In addition, the measured spectral minima shift almost linearly with the laser intensity for all three wavelengths, and the positions of the spectral minima strongly depend on the wavelengths of the driven field. These findings are in conflict with the Cooper-like and two-center interference minima predictions, providing strong evidences on the dynamic multi-channel interference origin of these minima. Besides, we theoretically calculated the positions of multi-channel interference minima by using a classical three-step model and found out perfect agreements between the experimental results and theoretical calculations, which again strongly support the multi-channel interference picture. Moreover, the advantages of the observed dynamic multi-channel interference based on HHG driven by long wavelength lasers are discussed. The long wavelength driver lasers are attractive for not only generating coherent XUV radiation and attosecond pulses, but also investigating structures and dynamics of molecules in strong laser fields.
2016, 65 (22): 224209.
doi: 10.7498/aps.65.224209
Abstract +
We experimentally studied the dissociative single and double ionization of CO molecules by counter-rotating circularly polarized two-color (CRTC) laser fields. By coincidently measuring the electrons and the fragmented ions, trefoil asymmetric momentum distributions of C+ in the polarization plane were observed, which are mainly determined by the selective ionization of CO with asymmetric orbitals. The threefold pattern could rotate continuously in the two-dimensional space by finely tuning the relative phase of the CRTC fields, providing a new method to manipulate the directional bond breaking of molecules by strong laser fields.
