分子双电离对激光偏振性的依赖关系

利用经典系综模型研究了氢分子双电离对激光场椭圆率的依赖性.研究发现,氢分子双电离机制对激光的椭偏性有强烈的依赖关系.随着激光场椭圆率的增加,双电离机制由非次序双电离转变为次序双电离.在大椭圆率情况的次序双电离中,电子关联动量分布随椭圆率的灵敏变化显示了电子出射时间对椭圆率的强烈依赖关系.     

Phase dependence of cross-phase modulation in asymmetric quantum wells

We study the Kerr nonlinearity associated with cross-phase modulation based on electromagnetically induced transparency in asymmetric double quantum wells. It is shown that, different from atomic system, not only the nonlinear dispersion and absorption but also the linear absorption depends on the relative phase of the laser fields because of the Fano interference. By choosing the parameters appropriately, large cross-phase modulation with nearly vanishing two-photon absorption, even π phase shift with single photon level, could be achieved in the asymmetric quantum wells.     

Ellipticity dependence of high-order above-threshold ionization from aligned diatomic molecules

We investigate high-order above-threshold ionization of the Ar₂ molecule by an elliptically polarized laser field. In order to describe this process we use the modified molecular strong-field approximation. We have found that the differential ionization rate is a very sensitive function of the ellipticity of the laser field and of the molecular orientation. For parallel orientation the rate has the maximum value for linear polarization, independently of the laser intensity. On the other hand, for perpendicular orientation the rate is maximal for a particular value of the ellipticity which is different from zero. Such a behavior can be related to the molecular symmetry. We consider the differential ionization rate of high-energy electrons as a function of the ellipticity for different internuclear distances. We also present, in the electron energy-emission angle plane, our new results for the focal-averaged spectra of electrons ionized by an elliptically polarized laser field. We have found that, in this case, the two-source two-rescattering-centers interference minima are blurred.     

Selective ionization of oriented nonpolar molecules with asymmetric structure by phase-controlled two-color laser fields

We report on the selective ionization of oriented nonpolar molecules with asymmetric structure by using phase-controlled two-color omega + 2omega laser pulses with an intensity of 1.0 x 10(13) W/cm(2) (tunneling ionization regime) and a pulse duration of 130 fs. The orientation of 1-bromo-2-chloroethane was monitored by the directional asymmetries of the forward-backward emission in dissociative ionization. The observed direction of orientation clearly confirms that molecular orientation is induced not by dynamic orientation but by selective ionization of oriented molecules, which reflects the structure of the highest occupied molecular orbital. This method can be applied for the vast majority of molecules.     

Tunneling ionization of Rydberg molecules

The theory of tunneling ionization of atoms is generalized to ionization of symmetric top molecules, either polar or nonpolar. Low-lying excited states of molecules, for which the ordinary Born-Oppenheimer approximation holds, and high-lying excited states, for which the inverse Born-Oppenheimer approximation holds, are discussed. Ionization in a constant external field is analyzed, as is ionization in an alternating field. It is shown that the orientation of the molecule’s axis along the field does not lead to any appreciable increase in the ionization probability as compared to other orientations. Zh. éksp. Teor. Fiz. 112, 115–127 (July 1997)     

Alignment-dependent strong field ionization of molecules

We demonstrate a method to measure strong field laser ionization of aligned molecules. The method employs a macroscopic field-free dynamic alignment, which occurs during revivals of rotational wave packets produced by a femtosecond laser pulse. We investigate the dependence of strong field ionization of N2 on molecular orientation. We determine that N2 molecules are four times more likely to ionize when aligned parallel to the field than when aligned perpendicular to it.     

Electron impact ionization of diatomic molecules

Cross sections for the ionization of N₂, CO and O₂ diatomic molecules by electron impact are calculated. The applied distorted wave model is based on our previous studies for positron impact, the molecular orbitals being described by Gaussian wavefunctions. Our study emphasizes the importance of electron exchange and of using correct distorted waves for the ejected electron.     

Raman spectra of asymmetric top molecules

A detailed analysis of the Raman-active v ₁, v ₃ (both A-type) and v ₅ (B_c -type) vibration-rotation bands of ethylene is presented. In addition to structural data simulation of the spectra yields values for the polarizability ratios of the totally symmetric bands, giving information on the polarizability ellipsoid as the molecule vibrates. v ₃ is shown to be perturbed by a c-type Coriolis interaction which complicates the interpretation of the negative value of the polarizability ratio required to match the observed spectrum.

The effects of asymmetry are pronounced; one result being the formation of heads in the ^R,PR _{K p} and ^R,P P _{K p} branches of low K _p in the v ₅ band.     

Dissociative ionization of molecules in intense laser fields

Accurate and efficient grid based techniques for the solution of the time-dependent Schrödinger equation for few-electron diatomic molecules irradiated by intense, ultrashort laser pulses are described. These are based on hybrid finite-difference, Lagrange mesh techniques. The methods are applied in three scenarios, namely H₂ ⁺ with fixed internuclear separation, H₂ ⁺ with vibrating nuclei and H₂ with fixed internuclear separation and illustrative results presented.Received: 19 November 2002, Published online: 24 April 2003PACS: 02.60.Cb Numerical simulation; solution of equations - 02.70.Bf Finite-difference methods - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 33.80.-b Photon interactions with molecules     

Phase transitions in isolated molecules

It is shown that an isolated polyatomic molecule can undergo a structural phase transition when optically pumped.     

Electron transport through asymmetric DNA molecules

We investigate quantum mechanical electron transport along the long axis of the DNA molecule using an effective tight-binding model. The overall contour plot of transmission, the current-voltage characteristics, and the differential conductance are examined for the variation of backbone onsite energy, the energy-dependent hopping strength, and the contact coupling between the leads and the DNA molecule. It is shown that as backbone asymmetry increases, the merging and collapse of the two mini-bands take place and an extra resonance peak in the transmission appears. In addition, we present the modulation of voltage threshold in the current-voltage curves and a double-peak structure in the differential conductance due to the disappearance of the merged mini-band. Finally, in the Coulomb blockade regime of asymmetric contact coupling, a distinct and under-unity resonance in the transmission appears due to the interference effects between the DNA molecular bands and the electronic structure of the leads at the DNA-lead interface.     

Ionization and dissociative ionization of methane molecules

A technique for mass-spectrometric investigation of the yield of positive ions produced by direct and electron-impact dissociative ionization of methane molecules is described, and respective experimental data are presented. Doubly charged C ₂ ⁺ , CH ₃ ²⁺ , and CH ₄ ²⁺ ions, as well as singly charged D ₂ ⁺ , CD ₃ ⁺ , and CD ₄ ⁺ ions, are detected in the mass spectrum of a methane molecule at electron energy U _e = 90 eV for the first time. From ionization efficiency curves, the ionization energy of the parent molecule and the appearance energy of fragment ions are determined. The ionization energy of the CH₄ molecule is found to be 12.62 ± 0.20 eV. Electron-molecular reactions that may take place when a low-energy electron beam interacts with a methane molecule are analyzed. The ionization process and the formation of methane molecule fragments are studied.     

Tunnel ionization of molecules and orbital imaging

We study whether tunnel ionization of aligned molecules can be used to map out the electronic structure of the ionizing orbitals. We show that the common view, which associates tunnel ionization rates with the electronic density profile of the ionizing orbital, is not always correct. Using the example of tunnel ionization from the CO(2) molecule, we show how and why the angular structure of the alignment-dependent ionization rate moves with increasing the strength of the electric field. These modifications reflect a general trend for molecules.     

Dispersion energies of interaction between asymmetric molecules

A formula is derived that expresses the contribution of the electric dipole-quadrupole forces to the dispersion energy between two unexcited molecules possessing axial symmetry. The relative orientation of the symmetry axes is arbitrary and the molecules need not be identical. Our formula holds for the case that each molecule belongs to a substance whose optical dispersion curve is characterized by a pair of resonance frequencies. A few orientations of special interest are subjected to numerical analysis based on the experimental data for polarizabilities of some diatomic molecules. Thus estimates are gained of the magnitude of errors inherent in energy values computed as if spherical symmetry obtained.     

Dispersion energies of interaction between asymmetric molecules

A derivation is given of a formula that describes the contribution of electric dipole-dipole forces to the dispersion energy between two unexcited molecules possessing axial symmetry. The symmetry axes may be oriented in any way relative to one another, and the molecules need not be identical. Our result is valid for the case that each molecule belongs to a substance whose optical dispersion curve can be characterized adequately by a pair of resonance frequencies. The method developed in the present paper, forms the basis for an extension of our theory that includes the dipole-quadrupole forces, and which will be expounded in a subsequent paper. A few special orientations of the symmetry axes are treated numerically with the aid of experimental data relating to some diatomic molecules. In this way estimates are made of the sizes of the errors committed in calculations carried out on the assumption that spherical symmetry obtains.     

Initial state dependence in multielectron threshold ionization of atoms

It is shown that the geometry of multielectron threshold ionization in atoms depends on the initial configuration of bound electrons. The reason for this behavior is found in the stability properties of the classical fixed point of the equations of motion for multiple threshold fragmentation. Specifically for three-electron breakup, apart from the symmetric triangular configuration also a breakup of lower symmetry in the form of a T shape can occur, as we demonstrate by calculating triple photoionization for the lithium ground and first excited states. We predict the electron breakup geometry for threshold fragmentation experiments.