Exact nondipole Kramers-Henneberger form of the light-atom hamiltonian: an application to atomic stabilization and photoelectron energy spectra

The exact nondipole minimal-coupling Hamiltonian for an atom interacting with an explicitly time- and space-dependent laser field is transformed into the rest frame of a classical free electron in the laser field, i.e., into the Kramers-Henneberger frame. The new form of the Hamiltonian is used to study nondipole effects in the high-intensity, high-frequency regime. Fully three-dimensional nondipole ab initio wave packet calculations show that the ionization probability may decrease for increasing field strength. We identify a unique signature for the onset of this dynamical stabilization effect in the photoelectron spectrum.     

极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外(XUV)阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展.讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应.采用H⁺₂作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰.在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍.并且这种非偶极效应随着红外基频光光强的增大而增强，随阿秒脉冲波长的增大而减弱. 关键词： 非偶极效应 光场空间不均匀性 阿秒脉冲 高次谐波产生     

Nondipole Effects in Atomic Ionization Induced by an Intense Counter-Rotating Bicircular Laser Field

Nondipole effects occurring in the process of atomic ionization by an intense, mid-infrared, counter-rotating bicircular laser field are investigated using the strong-field approximation with leading-order nondipole corrections. The time integrals appearing in the expression for the differential ionization rate are computed in two ways: numerically, and by applying the saddle-point approximation. The nondipole corrections introduce an asymmetry in the photoelectron momentum distribution along the field propagation direction. The asymmetry is quantified by the partial average value of the propagation-direction momentum component of the photoelectrons and by the normalized difference of the differential ionization rates computed including and excluding the nondipole corrections. Using the saddle-point approximation, it is investigated how the nondipole corrections change the solutions for direct photoelectrons and how this affects the momentum spectra. The impact of nondipole corrections increases with increasing photoelectron energy. Analysis of the complete photoelectron spectra including both direct and rescattered photoelectrons shows that, in the low-energy region, a shift against the propagation direction occurs. The partial average of the propagation–direction momentum component in the rescattering region exhibits a plateau structure and also a local minimum structure that was recently observed in an experiment with a linearly polarized laser field (Lin et al., Phys Rev. Lett. 128, 023201 (2022)).     

Ionization of a single hydrogen-like atom by laser pulse of near-atomic strength

The dynamics of high-harmonic generation and atom ionization by a strong and superstrong laser field are studied. In contrast to many earlier works, the present theory does not impose limitations on the laser field’s strength. We solve the nonrelativistic problem of a single hydrogen-like atom’s ionization from the ground state by a short laser pulse of subatomic, atomic, and superatomic field strength. Within the framework of the proposed method, we investigated the matrix elements of the ionization transition and revealed its substantially nonlinear dependence on the laser field strength. Both ionization and recombination processes are taken into account. The proposed method enables us to take into account the arbitrary order multiphoton ionization processes.     

类氢离子在强激光场中产生的阿秒脉冲

利用数值方法求解一维模型氢原子和类氢离子在强激光场中的含时薛定谔方程,研究了由它们产生的高次谐波谱构造的阿秒脉冲规律。研究结果表明,对于高电荷离子而言,由于高次谐波平台的显著展宽,使得构造的相应阿秒脉冲的宽度大幅度缩短,从而可以得到脉宽低于 阿秒的超短脉冲。     

强光场下类氢原子的电离能谱

用Ｆｌｏｑｕｅｔ方法迭代求解强光场下类氢原子的Ｓｃｈｒｏｅｄｉｎｇｅｒ方程,较严格地计算线极化光场下的波函数,并用电离边界条件,求出复本征值,最后得到多光子共振电离公式,并与实验比较。     

Numerical modeling of the photoionization of Rydberg atoms by the field of an electromagnetic wave

The ionization of excited hydrogen-like atoms in a femtosecond laser pulse is studied by direct numerical integration of the time-dependent Schrödinger equation for a quantum system in the field of an electromagnetic wave. Expressions are obtained for the ionization probability of the system as a function of the parameters of the laser pulse and the initial state of the atom. Ionization suppression is found, confirming the basic premises of the theory of interference stabilization of Rydberg atoms.     

The positronium atom as a benchmark for Rydberg excitation experiments in atomic physics

Positronium is a hydrogen-like pure leptonic atom that has gained great attention in basic physics for its role in antimatter studies, in quantum electrodynamics tests and in material science. Positronium spectroscopy is also an interesting research field, especially in the again unexplored region of Rydberg states, where motional effects turns out of overwhelming importance in determining the level structure, at variance with the usual Rydberg atomic spectroscopy. In this paper we present a simple theory of optical excitation of positronium high-n levels in strong magnetic fields, and determine the conditions for obtaining saturation of the transitions. It is shown that positronium atom can be an atomic physics benchmark for laser excitation experiments on Rydberg states in magnetic environments.     

Atom in electromagnetic field of near-atomic strength

Ionization and high-harmonics generation in a single hydrogen-like atom driven by a laser pulse of near-atomic field strength is the subject of this paper. We use exact solutions of the eigenvalue problem on particle motion in the cylindrically symmetric field (CSF) as a basis for the wave-function expansion. The superposition of the spherically symmetric intra-atomic field and linearly polarized laser field has the cylindrical symmetry. Hence, the use of the free-atom eigenfunctions as a basis for the wave-function expansion requires an infinitely increasing number of spherical harmonics (i.e., free-atom eigenfunctions) when the laser field strength approaches the intra-atomic field value. The eigenfunctions of the CSF problem depend on the laser field strength; therefore, the appropriate matrix elements show how the spectral width of atomic response and angular selection rules vary with increase in the laser field strength. The introduction gives a phenomenological semiclassical illustration of the problem. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

Autler-Townes triplet spectroscopy

We study the effects of quantum interference in the spontaneous emission spectrum of a four-level driven atomic system. We use three strong laser fields to drive the atom and a weak laser field to prepare the initial state of the atom. The atomic system exhibits Autler-Townes triplet in the spectrum. The single Lorentzian peak splits into triplet and their widths are controlled by the relative strengths of the laser fields.     

Hydrogen-like atom in the gravitational field of the universe

Exact solutions are obtained for a hydrogen-like atom in an external cosmological field of the closed and open universe. It is found that in the closed model, in addition to a gravitational analog of the Stark effect, there is a new effect-atom ionization by the gravitational field. In the open model, discrete and continuous spectra are obtained; furthermore, discrete spectrum consists of a finite number of states, and in the continuous spectrum the effect of the proton field is preserved despite absence of a bound state. It is shown that transition frequency depends on the radius (age) of the universe in both models, and, consequently, a hydrogen-like atom, in principle, cannot serve as a reference of time-frequency-length.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 55–59, January, 1985.     

Electromagnetic radiation of quantum systems under conditions of Lorentz-invariance and <Emphasis Type="Italic">CPT</Emphasis> violation

Solutions to the Dirac equation for an electron moving in a central electrostatic and a constant uniform magnetic field are considered within extended electrodynamics (in the presence of Lorentz-invariance violation of the minimal CPT-odd form). On the basis of these solutions, the features of electromagnetic radiation from a hydrogen-like atom and the features of electron synchrotron radiation are calculated, and possible observable effects caused by the assumed Lorentz-invariance violation are described.     

Magnetic dichroism in atomic core level photoemission

The overview covers selected topics on magnetic dichroism in atomic core level photoemission: the geometrical model in photoionization of laser polarized atoms and its generalization, contributions to the complete experiment from magnetic dichroism studies and nondipole effects in magnetic dichroism.     

Internal conversion in hydrogen-like ions

The probability of internal gamma-ray conversion is theoretically investigated for hydrogen-like ions versus the corresponding neutral atoms. The relevant calculations are performed by the relativistic Dirac-Fock method. The results reveal that the effect of multiple ionization on the coefficients of internal conversion in the K shell is maximal near the ionization threshold and for transitions of high multipole order, where this effect can be as great as a few orders of magnitude. The distinction between the coefficient of internal conversion in the K shell of a neutral atom and that in the respective hydrogen-like ion decreases with increasing transition energy, but it remains sizable for transitions of practical importance. It is found that the ionization of an atom to a hydrogen-like ion with allowance for conversion in external atomic shells may change significantly (by up to eight orders of magnitude) the lifetime of the nucleus being considered. The predicted effects can be observed in experiments with beams of relativistic heavy ions.     

Sideband structure spontaneous spectrum without the rotating wave approximation

The effects of the non-rotating wave approximation (non-RWA) on the spontaneous emission of a V-type three-level atom are studied, where the excited states are coupled to a common ground state by a weak laser field and the upper-level doublet is driven by a strong microwave field. When the non-RWA is applied to the interaction of the atom with the microwave field, for some values of the parameters involved, the spontaneous emission spectrum is comprised of a central peak and a series of sidebands with a constant spacing of the microwave frequency, and the central peak and/or sidebands can be split into two components. The physical interpretation of the spectral characteristics is given in light of the dressed states.     

Interaction of superintense laser pulses with relativistic ions

At high intensities, three-step recollision processes driven by low frequency laser pulses, such as high-order harmonic generation and high-order above-threshold ionization, are normally severely suppressed by the magnetic-field component of the laser field. It is shown that this suppression is not severe, even for ponderomotive energies well above 10 keV, for multicharged ions moving at a sufficiently high relativistic velocity against a counterpropagating infrared laser pulse. Numerical results are presented for high-order harmonic emission by a single Ne9+ ion moving with a Lorentz factor gamma=15 against a Nd:glass laser beam. The calculations are done within a Coulomb-corrected nondipole strong field approximation. The approximation is tested by comparing with accurate results.     

The Lamb shift in finite electrodynamics

Using the finite-electrodynamics model, in which the electromagnetic field is considered as classical but the nature, classical or quantum, of the source of the field is unspecified, we establish, in a very simple way, a general formula associated with the self-energy of a time-periodic system of charges. By applying this formula to the currents associated with an electron bound in a hydrogen-like atom, one obtains immediately the Lamb shift standard formulas.     

Interaction of a Hydrogen-Like Atom with an Ultrashort Pulse of Electromagnetic Waves

In the approximation of sudden perturbations, a solution of the Dirac equation describing the behavior of a hydrogen-like atom interacting with a spatially inhomogeneous ultrashort pulse of electromagnetic waves is derived. As an example, the single-electron inelastic processes accompanying the interaction of ultrashort pulses with hydrogen-like atoms are considered. The method developed here allows the spatial inhomogeneity of the ultrashort pulse of electromagnetic waves to be taken into account.     

Scattering of an ultrashort electromagnetic radiation pulse by an atom in a broad spectral range

The scattering of an ultrashort electromagnetic pulse by atomic particles is described using a consistent quantum-mechanical approach taking into account excitation of a target and nondipole electromagnetic interaction, which is valid in a broad spectral range. This approach is applied to the scattering of single- and few-cycle pulses by a multielectron atom and a hydrogen atom. Scattering spectra are obtained for ultrashort pulses of different durations. The relative contribution of “elastic” scattering of a single-cycle pulse by a hydrogen atom is studied in the high-frequency limit as a function of the carrier frequency and scattering angle.     

Hyperfine interactions of muonium and hydrogen in silicon and diamond: Quantum chemical calculations

The electronic structure of a hydrogen-like atom located at interstitial sites of the silicon and diamond crystals is calculated by the intermediate neglect of differential overlap (INDO) method. Calculations of the electronic g- and hyperfine interaction tensors of the impurity atom are performed. The results obtained are compared with the experimental properties of both “anomalous” muonium and hydrogen centers. It is shown that the most likely model for these centers in silicon and diamond is that in which interstitial neutral hydrogen-like atom locates at the bond-center site.