分子核间距对非时序双电离的影响

利用半经典再散射模型研究了激光强度在一定范围内时分子核间距对非时序双电离过程的影响.分别计算了非时序双电离率、两电子的电离能、两电子的动量相关、Coulomb和激光场的复合势随分子核间距的变化关系.研究表明,分子核间距在1.0—6.0 a.u.范围内时,非时序双电离率和两电子动量和为零的双电离事件数随着分子核间距的增大而增大.当分子核间距继续增大(大于6.0 a.u.)时,非时序双电离率和两电子动量和为零的双电离事件数却减小.     

Effects of the internuclear vector on the photoelectron angular distributions of H<Subscript>2</Subscript><Superscript>+</Superscript>

We study the photoelectron angular distributions (PADs) of diatomic molecule H₂ ⁺ irradiated by intense laser fields using a nonperturbative scattering theory. We find that the internuclear vector may change the PADs. The PADs have qualitative changes with the increasing of the internuclear distance. The molecular orientation affect the symmetry of the PADs. When the internuclear vector is vertical or parallel to the laser polarization vector, the PADs are four-fold symmetric; for other case the PADs are two-fold symmetric. Due to the modulation effect resulting from the molecular multi-core nature, the size of the jet and the main lobe can be enlarged or reduced. The molecular modulation effect become obvious for large internuclear distance.     

Strong orientation effects in ionization of H+2 by short, intense, high-frequency light pulses

We present three-dimensional time-dependent calculations of ionization of arbitrarily spatially oriented H+2 by attosecond, intense, high-frequency laser fields. The ionization probability shows a strong dependence on both the internuclear distance and the relative orientation between the laser field and the internuclear axis. The physical features are explained in terms of two-center interference effects.     

从单个混合时间的2D NOESY峰强度矩阵求取核间距

提出了从单个混合时间的2D NOESY峰强度或混合系数矩阵直接求取核间距的新方法。该方法分成二步进行。第一步采用规范化2D NOESY峰强度或对称化混合系数矩阵来计算对称化核交叉弛豫速率矩阵。其优点是计算过程简单且普适。第二步直接用对称化核交叉弛豫速率来求取核间距。该过程考虑到分子内部运动的贡献。本文方法是自治的,只要能测出单个混合时间的2D NOESY峰强度或混合系数矩阵即可直接求出自旋系统的核间距了。     

HeH~(2+)分子谐波辐射的空间分布

理论研究了HeH~(2+)分子发射高次谐波的空间分布情况.结果表明:分子谐波发射主要集中在He核和H核附近.当核间距离较小时,He核辐射谐波强度要比H核辐射谐波强度高2~3个数量级.当核间距离较大时,He核辐射谐波强度要比H核辐射谐波强度高4~5个数量级.当空间非均匀激光场引入后,谐波截止能量得到延伸,并且,随着空间非均匀激光场位置由负向到正向移动(-100 a.u.到100a.u.),谐波截止得到进一步延伸.但是He核辐射谐波强度依然大于H核.最后,通过叠加谐波可获得脉宽在60 as的超短脉冲.     

HeH2+分子谐波辐射的空间分布

理论研究了HeH2+分子发射高次谐波的空间分布情况。结果表明：分子谐波发射主要集中在He核和H核附近。当核间距离较小时,He核辐射谐波强度要比H核辐射谐波强度高2~3个数量级。当核间距离较大时,He核辐射谐波强度要比H核辐射谐波强度高4~5个数量级。当空间非均匀激光场引入后,谐波截止能量得到延伸,并且,随着空间非均匀激光场位置由负向到正向移动(-100 a.u.到100 a.u.),谐波截止得到进一步延伸。但是He核辐射谐波强度依然大于H核。最后,通过叠加谐波可获得脉宽在60 as的超短脉冲。     

HeH2+分子谐波辐射的空间分布

理论研究了HeH2+分子发射高次谐波的空间分布情况。结果表明：分子谐波发射主要集中在He核和H核附近。当核间距离较小时,He核辐射谐波强度要比H核辐射谐波强度高2~3个数量级。当核间距离较大时,He核辐射谐波强度要比H核辐射谐波强度高4~5个数量级。当空间非均匀激光场引入后,谐波截止能量得到延伸,并且,随着空间非均匀激光场位置由负向到正向移动(-100 a.u.到100 a.u.),谐波截止得到进一步延伸。但是He核辐射谐波强度依然大于H核。最后,通过叠加谐波可获得脉宽在60 as的超短脉冲。     

How to measure the internuclear vector from photoelectron angular distributions

This paper uses a nonperturbative scattering theory to study photoelectron angular distributions of homonuclear diatomic molecules irradiated by circularly polarized laser fields. This study shows that the nonisotropic feature of photoelectron angular distributions is not due to the polarization of the laser field but the internuclear vector of the molecules. It suggests a method to measure the molecular orientation and the internuclear distance of molecules through the measurement of photoelectron angular distributions.     

空间非均匀场下核间距对不对称分子谐波发射的影响

数值研究了空间非均匀场下核间距离对于不对称分子HeH2+发射高次谐波以及阿秒脉冲的影响。计算结果表明,通过适当调节HeH2+分子离子的核间距离以及空间非均匀参数,不仅谐波发射的截止能量得到了延伸,而且单一的短量子路径也被选择出来对谐波发射起作用。随后适当引入第二束控制激光场,谐波截止能量得到了进一步扩展,形成了两个带宽分别在431eV和372eV的连续平台区。 最后,通过适当的叠加谐波,可获得脉宽为31as-65as的一系列阿秒X射线光源。     

Statistical complexity and Fisher-Shannon information measure of H2

Using the simple Coulson molecular wave function for the ground state H⁺₂ molecule as a function of internuclear distance, the first molecular calculations of (a) the statistical complexity measure due to López-Ruiz, Mancini, and Calbet and (b) the Fisher-Shannon information measure in the position and momentum spaces are reported. It is found that the two measures exhibit the molecular bonding in the form of a characteristic minimum around the equilibrium internuclear distance in the product space.     

The hydrogen molecule in an arbitrarily oriented magnetic field

Summary The modifications induced by a magnetic field of arbitrary direction and intermediate strength (i.e not larger than 2.35·10⁵ tesla, the ?atomic tesla?) on the lowest singlet and triplet energy states of the hydrogen molecule are studied. Using a linear combination of products of field-modified atomic orbitals, it is found that increasing the field strength the depth of the singlet energy well increases and the equilibrium internuclear distance decreases, yielding more rigid and localized nuclear vibrations. For sufficiently strong fields perpendicular to the internuclear axis, the triplet state exhibits a bonding behaviour. An explanation of the above results is given in terms of the field-modified electronic-charge distributions in the internuclear region. Based on the thesis submitted by S. Basile to the University of Palermo for graduation in Physics.     

Low energy atomic collisions

The Schrödinger equation for the system H⁺-H developed in a previous paper is considered using new expansion functions for electronic states obtained from H₂ ⁺ molecular ion electronic eigenfunctions by a unitary transformation. These new functions have the advantage of remaining orthonormal at all internuclear separations and asymptotically becoming symmetrized atomic hydrogen states. Although they are eigenfunctions of the H₂ ⁺ hamiltonian only in the limit of large internuclear distance, the effect of the H₂ ⁺ hamiltonian on these functions is readily found.

Due to coupling which remains non-zero in the limit of large interproton distance, each independent formal solution of the H⁺-H equations involves more than one expansion state in this limit. These solutions may be expressed asymptotically as column vectors multiplied by incoming or outgoing spherical waves.

The formal theory of scattering as developed by Gell-Mann and Goldberger has been utilized along with the projection formalism of Feshbach to obtain the correct asymptotic form of the scattering wave function. The procedure employed involves formulating the problem in terms of two-potential scattering and requires application of renormalization techniques for treating level shifts produced by the infinite-ranged coupling. This asymptotic form may be used in imposing scattering boundary conditions on numerical solutions of coupled equations for H⁺-H scattering.

Finally, it is shown that one cannot interpret coefficients of all outgoing spherical waves as scattering amplitudes. In addition, new interference phenomena are found to result from the presence of the infinite-ranged coupling. The present formalism is shown to reduce to the usual perturbed stationary-states method in the approximation that the infinite-ranged coupling is neglected.     

Practical Methods for Solid-State NMR Distance Measurements on Large Biomolecules: Constant-Time Rotational Resonance

Simple modifications of the rotational resonance experiment substantially reduce the total experimental time needed to measure weak homonuclear dipolar couplings, a critical factor for achieving routine internuclear distance measurements in large biomolecular systems. These modifications also address several problems cited in the literature. Here we introduce a constant-time rotational resonance experiment that eliminates the need for control spectra to correct for effects from variable RF heating, particularly critical for accurate long-distance measurements. This reduces the total number of experiments needed by as much as a factor of 2. Other improvements incorporated include achieving selective inversion with a delay rather than a weak pulse (P. R. Costa et al., J. Am. Chem. Soc. 119, 10487-10493, 1997), which we observe results in the elimination of oscillations in peak intensities for short mixing time points. This reduces the total experiment time in two ways. First, there is no longer a need to average different "zero"-time points (S. O. Smith et al., Biochemistry 33, 6334-6341, 1994) to correct for intensity variations. Second, short-mixing-time lineshape differences observed in large membrane-bound proteins only appear with the weak-pulse inversion and not when using the delay inversion. Consistent lineshapes between short and long mixing times permit the use of a single spectrum for subtraction of natural abundance background signals from all labeled-protein time points. Elimination of these effects improves the accuracy and efficiency of rotational resonance internuclear distance measurements.     

NOESY around the critical point ωτc = 1.12. Precise determination of proton-proton distances and reorientation correlation times with two-field NOESY

When nuclear Overhauser enhancement spectroscopy (NOESY) experiments are performed around the critical point ωτ_c = 1.12, internuclear cross-relaxation rates are dependent on the resonance frequency ω, where τ_c is the reorientation time of the distance vector between two magnetic dipoles. Therefore, the correlation times and, consequently, the internuclear distances can be determined precisely by measuring the cross-relaxation rates under two magnetic fields. An application with a cyclic pentapeptide is given as an example, demonstrating the use of the two-field NOESY method in molecular structure studies. NOESY experiments with mixing times beyond 500 ms are discussed.     

On the determination of scale and temperature factors in crystal structure analysis

The treatment is described for placing the relative intensities of X-ray diffractions on the absolute scale and for simultaneously determining the temperature factor. It is based on similar principles as the exact procedure given by Kartha for the determination of the scale factor but has the advantage that its use is not limited by the a priori knowledge of the temperature factor. The present relations can be transformed into Wilson's well-known statistical treatment by introducing certain approximations. This enables the reliability of Wilson's method to be discussed.     

Nuclear structure in the dinuclear model with rotating clusters

The dinuclear-system model can be applied to nuclear structure. Here, we study deformed clusters which rotate with respect to the internuclear distance and exchange nucleons. The model can be used to explain the band structure of nuclear spectra, especially the parity splitting observed in actinides, e.g., in ²³⁸U. The text was submitted by the authors in English.     

Efficient double-quantum excitation in rotational resonance NMR

We present a new technique for double-quantum excitation in magic-angle-spinning solid-state NMR. The method involves (i) preparation of nonequilibrium longitudinal magnetization; (ii) mechanical excitation of zero-quantum coherence by spinning the sample at rotational resonance, and (iii) phase-coherent conversion of the zero-quantum coherence into double-quantum coherence by frequency-selective spin inversion. The double-quantum coherence is converted into observable magnetization by reversing the excitation process, followed by a pi/2 pulse. The method is technically simple, does not require strong RF fields, and is feasible at high spinning frequencies. In [(13)C(2),(15)N]-glycine, with an internuclear (13)C-(13)C distance of 0.153 nm, we achieve a double-quantum filtering efficiency of approximately 56%. In [11, 20-(13)C(2)]-all-E-retinal, with an internuclear (13)C-(13)C distance of 0.296 nm, we obtain approximately 45% double-quantum filtering efficiency.     

Ab initio calculations of accurate dissociation energy and analytic potential energy function for the second excited state B1∏ of 7LiH

The reasonable dissociation limit of the second excited singlet state B¹∏ of ⁷LiH molecule is obtained. The accurate dissociation energy and equilibrium geometry of the B¹\Pi state are calculated using a symmetry-adapted-cluster configuration--interaction method in full active space. The whole potential energy curve for the B¹∏ state is obtained over the internuclear distance ranging from about 0.10nm to 0.54nm, and has a least-square fit to the analytic Murrell--Sorbie function form. The vertical excitation energy is calculated from the ground state to the B¹∏ state and compared with previous theoretical results. The equilibrium internuclear distance obtained by geometry optimization is found to be quite different from that obtained by single-point energy scanning under the same calculation condition. Based on the analytic potential energy function, the harmonic frequency value of the B¹∏ state is estimated. A comparison of the theoretical calculations of dissociation energies, equilibrium interatomic distances and the analytic potential energy function with those obtained by previous theoretical results clearly shows that the present work is more comprehensive and in better agreement with experiments than previous theories, thus it is an improvement on previous theories.     

Two-center problem for the dirac equation

The ground-state wave function and the energy term of a relativistic electron moving in the field of two fixed Coulomb centers are calculated analytically by the LCAO method. The resulting analytic formula is used to calculate the critical internuclear distance at which the energy term crosses the boundary of the lower continuum.