Laser-induced interference, focusing, and diffraction of rescattering molecular photoelectrons

We solve the time-dependent Schr?dinger equation in three dimensions for H+2 in a one-cycle laser pulse of moderate intensity. We consider fixed nuclear positions and Coulomb electron-nuclear interaction potentials. We analyze the field-induced electron interference and diffraction patterns. To extract the ionization dynamics we subtract the excitations to low-lying bound states explicitly. We follow the time evolution of a well-defined wave packet that is formed near the first peak of the laser field. We observe the fragmentation of the wave packet due to molecular focusing. We show how to retrieve a diffraction molecular image by taking the ratio of the momentum distributions in the two lateral directions. The positions of the diffraction peaks are well described by the classical double slit diffraction rule.     

Clocking ultrafast wave packet dynamics in molecules through UV-induced symmetry breaking

We investigate the use of UV-pump-UV-probe schemes to trace the evolution of nuclear wave packets in excited molecular states by analyzing the asymmetry of the electron angular distributions resulting from dissociative ionization. The asymmetry results from the coherent superposition of gerade and ungerade states of the remaining molecular ion in the region where the nuclear wave packet launched by the pump pulse in the neutral molecule is located. Hence, the variation of this asymmetry with the time delay between the pump and the probe pulses parallels that of the moving wave packet and, consequently, can be used to clock its field-free evolution. The performance of this method is illustrated for the H(2) molecule.     

Evidence of single-photon two-site core double ionization of C2H2 molecules

We observe the formation in a single-photon transition of two core holes, each at a different carbon atom of the C2H2 molecule. At a photon energy of 770.5 eV, the probability of this 2-site core double ionization amounts to 1.6 ± 0.4% of the 1-site core double ionization. A simple theoretical model based on the knockout mechanism gives reasonable agreement with experiment. Spectroscopy and Auger decays of the associated double core hole states are also investigated.     

Channel mixing effects in the dissociative recombination of H+3 with slow electrons

We discuss the low-energy dissociative recombination of H+3, which strongly influences the abundance of this ion in diffuse interstellar molecular clouds. The kinetic couplings between the ionization continuum and the dissociative ground state of H3 have been used as input to a two-dimensional wave packet calculation of dissociation dynamics. The cross section obtained for direct dissociative recombination is much smaller than the latest experimental results. However, a multichannel quantum defect treatment shows that an indirect mechanism via bound Rydberg states of H3 prevails for this process.     

Resonant inelastic contact scattering of X-ray photons on atoms and ions

The existence of an extended resonance structure outside the X-ray emission regions is theoretically predicted in the total double differential cross section for the scattering of linearly polarized photons on free atoms (ions). This structure is almost entirely determined by inelastic photon scattering of the contact type. The amplitude of the inelastic contact scattering probability is described using an analytical expression for a non-relativistic transition operator, which was previously obtained by the author outside the dipole and momentum approximations. The resonant inelastic contact scattering of X-ray photons on a neon atom and neonlike ions of argon, titanium, and iron has been studied. Calculations were performed in a nonrelativistic approximation for the wave functions of the scattering states, with allowance for many-body effects of the radial relaxation of one-electron orbitals in the Hartree-Fock field of a deep 1s vacancy and (for neon atom) the double excitation/ionization of the ground atomic state.     

Kinematically complete study of dissociative ionization of by ion impact

We present a kinematically complete study of dissociative ionization of D(2) by 13.6 MeV/u S(15+) ions. The experiment allows us to unravel the competing mechanisms, namely, direct single ionization, autoionization of doubly excited states, ionization excitation, and double ionization, and to analyze the corresponding electron angular distribution from fixed-in-space molecules. The conclusions are supported by theoretical calculations in which the correlated motion of all electrons and nuclei and the interferences between them are described from first principles.     

Transition from strong-field sequential to nonsequential double ionization at near-infrared wavelengths and low intensities

Within a quantum-mechanical model, we investigate strong-field double ionization of a model helium atom by near-infrared, linearly polarized laser pulses at intensities far below the recollision threshold. The quantum simulations show a clear mechanism change from sequential to nonsequential double ionization (NSDI) as the laser intensity increases. For NSDI, the two-electron correlated momentum distribution exhibits a strong final-state Coulomb repulsion effect for high-energy photoelectrons, but absent for low-energy photoelectrons. This repulsion effect is ascribed to field double ionization from doubly-excited states populated by recollision of the first ionized electron when it returns to the parent ion. Such recollision-induced excited states are absent at ultraviolet wavelengths due to the very low returning kinetic energies, resulting to the absence of final-state repulsion effect in NSDI.     

High-Energy Two-Electron Photoabsorption as a Three-Body Problem

We study the high-energy behaviour of double photoionization and of the ionization accompanied by excitation of the states n¹S and n³S of the H^– ion, helium atom, and light helium-like ions. We show that the high-energy nonrelativistic limit of the ionization+excitation to the single-ionization cross section ratio is still valid for the photon energies, corresponding to the relativistic energies of the outgoing electron. The same applies to the double photoionization of the triplet states. The situation for the double photoionization of singlet states differs due to the quasifree mechanism. All these ionization ratios are calculated by using the high-precision locally correct wave functions for the bound electrons. The importance of using locally correct functions is emphasized.     

Ionization of lithium in a strong laser field

In this work, we present ab initio computations of the interaction of helium and lithium with a strong laser field. By computing the two-and three-particle wave functions, we retain the full electron correlation and numerically integrate the time-dependent Schrödinger equation (TDSE) restricted to one dimension. The correlated nonsequential double ionization (NSDI) and the sequential double ionization (SDI) is studied. Our results show a clear transition from NSDI to SDI for increasing intensities. The correlated double ionization is found to be sensitive to the spin configuration of the ionized pair. Our model allows us to “measure” two different channels of double ionization (one with both spins aligned and one with spins antialigned).     

电子碰撞激发氢原子和氦离子散射微分截面的计算

本文详细介绍计算电子碰撞激发散射截面的扭曲波玻恩近似（DWBA）理论模型,并对低能DWBA模型进行修正。利用修正的DWBA模型计算了电子碰撞激发氢原子和氦离子1s-2s 和1s-2p的散射微分截面。将关于氢原子由基态到n=2态的电子碰撞激发散射微分截面与绝对实验测量数据比较,发现二者符合得很好,这验证了我们对DWBA修正的正确性。本文工作为拟合强场诱导的氦原子非序列双电离关联电子动量谱提供了有效的理论方法。     

Calculation of the cross section of helium ionization by an electron impact with the formation of a helium ion in an excited state

The total cross sections of He and He⁺ ionization by an electron impact are calculated in the first Born approximation. Calculations of the matrix elements are carried out by the Fock-Dirac multiconfiguration relativistic method using an intermediate type of coupling with orthogonal functions of the initial and final states. A single-electron wave function of the continuous spectrum for an Auger electron is obtained using the Fock-Dirac single-configuration method. The results of the calculations performed with orthogonal and nonorthogonal wave functions of the initial and final states are compared. The ionization cross sections are calculated for cases in which a knock-on electron of the continuous spectrum is described by both the orthogonal and nonorthogonal wave functions with respect to the wave functions of the core electrons.     

初始振动态对氢分子离子电离通道的影响

通过数值求解非Born-Oppenheimer近似条件下的一维含时薛定谔方程,理论模拟了H2+在不同核初始振动态条件下(=0到=7)的库仑爆炸核初始动能释放谱。模拟结果表明：强激光场中氢分子离子的电离通道主要包括直接多光子电离、电荷共振增强电离及有中间过程的电荷共振增强电离,并且可以通过选取不同的初始振动态实现对氢分子离子电离通道的控制。     

Probing electron-electron correlation with attosecond pulses

We study two-photon double ionization of helium in its ground state at sufficiently low laser intensities so that three and more photon absorptions are negligible. In the regime where sequential ionization dominates, the two-photon double ionization one-electron energy spectrum exhibits a well defined double peak structure directly related to the electron-electron correlation in the ground state. We demonstrate that when helium is exposed to subfemtosecond or attosecond pulses, both peaks move and their displacement is a signature of the time needed by the He⁺ orbital to relax after the ejection of the first electron. This result rests on the numerical solution of the corresponding non-relativistic time-dependent Schrödinger equation.Received: 17 January 2003, Published online: 18 March 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

初始振动态对氢分子离子电离通道的影响

通过数值求解非Born-Oppenheimer近似条件下的一维含时薛定谔方程,理论模拟了H2+在不同核初始振动态条件下(=0到=7)的库仑爆炸核初始动能释放谱。模拟结果表明：强激光场中氢分子离子的电离通道主要包括直接多光子电离、电荷共振增强电离及有中间过程的电荷共振增强电离,并且可以通过选取不同的初始振动态实现对氢分子离子电离通道的控制。     

Rescattering double ionization of D2 and H2 by intense laser pulses

We have measured momentum spectra and branching ratios of charged ionic fragments emitted in the double ionization of D2 (and H2) molecules by short intense laser pulses. We find high-energy coincident D+ (and H+) ion pairs with kinetic energy releases between 8 and 19 eV which appear for linearly polarized light but are absent for circularly polarized light. The dependence on the polarization, the energy distributions of the ions, and the dependence on laser intensity of yield ratios lead us to interpret these ion pairs as due to a rescattering mechanism for the double ionization. A quantitative model is presented which accounts for the major features of the data.     

Resonant inelastic scattering of an X-ray photon by a linear molecule

The double differential cross section of resonant inelastic scattering of a linearly polarized X-ray photon by a spatially oriented HF molecule in a gas phase is theoretically described in the energy range of the ionization threshold of the deep molecular orbital 1σ. The effects of radial relaxation of the wave functions of the core and excited scattering states in fields of the formed core vacancies, the vibronic effects, and the effects of Auger and radiative decays of the vacancies are taken into account in the nonrelativistic Hartree-Fock approximation. The results of the calculations are predictive and agree well with the results of the experimental measurements of the Kα emission spectrum of the HF molecule at an incident photon energy of 2 keV, which considerably exceeds the energy of the 1σ ionization threshold.     

Statistical Ionization Equilibrium in Partially Degenerate Plasmas

The thermal ionization equilibrium in plasmas is considered at pressures for which the electron gas is partially to considerably degenerate. An ionization equation is derived which takes into account that i) the electron energies are distributed according to Fermi statistics and ii) the (heavy) ions and atoms obey Boltzmann statistics which is valid up to pressures at which the wave functions of the atoms begin to overlap. A comparison of the quantum statistical and Saha ionization equations indicates that the degeneracy effects in the electron gas suppress somewhat the ionization. It is remarkable that the Saha equation describes, approximately, the thermal ionization equilibrium up to the critical pressure at which the wave functions of the atoms begin to overlap (e.g., up to P ~ 103 Bar and P ~ 106 Bar in the cases of Cs and H plasmas, respectively), although the electrons are noticeably degenerate.     

Inelastic scattering of broadband electron wave packets driven by an intense midinfrared laser field

Intense, 100 fs laser pulses at 3.2 and 3.6 μm are used to generate, by multiphoton ionization, broadband wave packets with up to 400 eV of kinetic energy and charge states up to Xe(+6). The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distribution for (e, ne) electron impact ionization. The analysis also suggests a contribution from a 4d core excitation, or giant resonance, in xenon.     

Electric field controlled, pulsed autoionization in two electron wave packets

In this paper, control of the evolution of a two electron wave packet through the application of a static electric field is demonstrated. Specifically, application of a small electric field is used to produce pulsed autoionization events, the timing of which can be controlled on a picosecond time scale. The technique is demonstrated by exciting calcium atoms using a short-pulsed laser to the 4p(3/2)19d doubly excited state, which is energy degenerate with the 4p(1/2)nk stark states. Evolution of the resultant wave packet is monitored through the application of a second short laser pulse, which stimulates the atoms to emit a photon producing singly excited Rydberg states which are detected using field ionization.     

Strong field ionization to multiple electronic states in water

High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of H(2)O and D(2)O high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by solving the time-dependent Schr?dinger equation to simulate strong field ionization and high harmonic generation from the water isotopes. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.