Atomic and molecular ions in intense ultra-fast laser fields

The interaction of a 60 fs 790 nm laser pulse with beams of Ar⁺, C⁺, H₂ ⁺, HD⁺ and D₂ ⁺ are discussed. Intensities up to 10¹⁶ Wcm^-2 are employed. An experimental z-scanning technique is used to resolve the intensity dependent processes in the confocal volume.Received: 6 January 2003, Published online: 15 July 2003PACS: 32.80.Fb Photoionization of atoms and ions - 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift     

An expansion method for calculating molecular properties IV.

The energies of the 2pσ and 2sσ states of H₂ ⁺ and HeH²⁺ and those of the a ³, A ¹, b ³, B ¹Σ⁺ states of HeH⁺ are calculated up to second order in the heavy nuclear charge. For the one-electron systems the first-order equation is solved analytically employing a special separation technique for degeneracies. The consideration of screening enables a recently proposed interchange result for the degenerate case to be tested. The results for the 2sσ states are good while those for the 2pσ states are only good at small internuclear distances.

For the two-electron systems the method employed is a single perturbation approach for two perturbations in the presence of degeneracy. The results for HeH⁺ appear to be good only at small internuclear distances. The corresponding results for the excited states of H₂ are omitted since an interesting problem arises at first order.     

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.     

4HeH+核间距的实验测定

用MeV量级⁴HeH⁺在超薄无衬碳膜中的库仑爆炸,获得高频离子源产生⁴HeH⁺核间距的实测值为0.094±0.003nm.发现⁴HeH⁺与H⁺₂,H⁺₃不一样,其核间距大小和离子源种类及离子源参数有密切关系,并从⁴HeH⁺的形成机理与形成条件,解 关键词：     

Properties of the adiabatic energy curves for the ground states of H2+, HD+, and D2+

Equilibrium internuclear distances, energies, and force constants are reported for the diatomic molecules H₂⁺, HD⁺, and D₂⁺ at the adiabatic level of approximation. The differences between these values and those calculated previously at the Born-Oppenheimer level of approximation are analyzed.     

Morse Franck-Condon Factors and R-Centroids for Some Lyman Bands of H2, HD and D2

Using new rotational and vibrational constants, Morse Franck-Condon factors and r-centroids are computed for 108 Lyman bands of astrophysical importance H₂, HD and D₂ molecules. Experimental oscillator strengths conjoint with Morse Franck-Condon Factors for six Lyman bands of molecular hydrogen yield radiative lifetimes for v = 0–5 levels of the B 1σ_u ⁺ state that are inconsistent with the experimental lifetime measurements. The limitation of analytic Morse function in the region of small internuclear separation for the B 1σ_u ⁺ state of H₂ is discussed. The r-centroids (r_v′v″) are found to increase with frequencies of the Lyman bands of H₂, HD and D₂. For a sequence, Δr = r_{v′+1,v″+1} - r_v′v″ is constant. A comparison of r_v′v″ values of the Lyman bands shows that r_v′vv″ of the band (v′v″) increases in going from H₂ to D₂.     

Enhanced dissociation of H2+ into highly excited states by UV pulses

We theoretically study the dissociation of H₂⁺ by UV laser pulses as a function of the photon energy ω of the pulse. Our results show that pronounced enhancements of the dissociation into highly excited electronic states can be achieved at some critical ω. This is found to be attributed to a consecutively resonant excitation mechanism where the population is first transferred to the first excited state by absorbing one photon and then to higher states by absorbing another one or more photons at the same internuclear distance. This study indicates that the strong coupling between the lowest two states of H₂⁺ can significantly affect the dissociation through higher lying states.     

Rydberg states of the HeH+ ion calculated by ab initio methods: potential energies and effective principal quantum numbers

More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm^?1. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H⁺₂.     

Energies of the H+2 ion in strong magnetic fields

Variational calculations of the energy values and the equilibrium internuclear separations of the H⁺₂ ion in the 1σ_g and 1π_g states are carried out to confirm our previous results based on the Monte Carlo method; in the absence of an external magnetic field the antibonding 1π_g state changes to a bonding state with increasing magnetic field. As a byproduct, a simple variational form is proposed for the wave function of the H atom in strong magnetic fields.     

Application of CI expansion techniques to the computation of two-photon absorptivities

Summary We show how conventional linear expansion techniques for both electronic and vibrational wave functions can be used for the computation of matrix elements governing the two-photon absorption probability in molecules. In particular, the²Σ⁺ A←²∏X transition matrix elements of OH and the¹Σ _g ⁺ E,F←¹Σ _g ⁺ X of H₂are computed using 1) the sum over states in the Born-Oppenheimer approximation with inclusion of vibrational wave functions, 2) solution of perturbation theory equations by expansion in the CI basis at fixed nuclei, followed by a kind of vibrational averaging. The results are compared with experiment and discussed. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Semiempirical valence bond potential energy surfaces for simple chemical reactions: 1 A′ states of H2O and H2F+

A semiemprical valence bond method is employed to study the potential energy surfaces (PESs) of the lowest ¹ A′ states of H₂O and isoelectronic H₂F⁺. The calculation is based on the search for the electronic configurations which play the most important rôle in the formation of the stable electronic states of the molecules and the relevant diatomic fragments. Several approximations are used to reduce the number of permutations in the calculation of the energy matrix. The Moffit atoms-in-molecule approximation is used to correct for the atomic errors arising from the poor basis set. The computed data for the lowest electronic states of OH, HF, HF⁺, H₂O and H₂F⁺, as well as the general behaviour of the PESs for different molecular geometries, are in good agreement with the available ab initio and experimental data.     

Energy values for the H+2 ion in superstrong magnetic fields using the adiabatic approximation

Energy values, equilibrium internuclear separations, and zero-point energies of nuclear vibrations parallel to the magnetic field are calculated for the lowest states of the H⁺₂ ion in magnetic fields B?10⁶ T (characteristic of neutron stars) using numerical wave functions that are products of Landau states and arbitrary z-dependent functions, thus improving on previous estimates of variational or LCAO type, which have been obtained in the high-field regime.     

Calculation of the cross sections for the photoionization of H2 and D2 into different vibrational states of the ion

The dependence of the electronic transition moment on the internuclear distance is explicitly taken into account in the calculation of the photoionization cross section. Thus, the partial ionization cross sections for producing different vibrational states of the residual ion are obtained without invoking the Franck-Condon factor approximation. The exact electrostatic potential of H₂⁺, as well as the two-center Coulomb field, is used in the evaluation of the continuum wave function. The result can explain fairly well the ratios of the partial cross sections measured at 584 Å. The effect of the polarization of H₂⁺ due to the departing photoelectron is also studied and found to be small as far as the relative cross section is concerned. The total photoionization cross sections of H₂ and D₂ are compared with previous results obtained by other authors.     

Ion-pair states of I<Subscript>2</Subscript>, Br<Subscript>2</Subscript>, IBr,and ICl

The experimentally determined energies and rotational constants of the vibrational levels v = 0–20 of the Ion-Pair states Ω = 0⁺, Ω = 1 of the I₂, Br₂, IBr, and ICl molecules are modeled. The model used includes three diabatic states, which correlate to X⁺(3P, 1D) + Y^～(1S₀). These states are coupled by the spin-orbit interaction, which is assumed to be independent of the internuclear distance. For IBr and ICl, as well as for the ungerade states of I₂ and Br₂, satisfactory results are obtained. The model is less applicable to the gerade states of I₂ and Br₂, which is possibly results from the retainment of the asymptotic J _A J _B coupling of the angular momenta at equilibrium internuclear distances.     

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne₂ ⁺, Ar₂ ⁺ and Kr₂ ⁺. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar₂ ⁺, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne₂ ⁺ suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne₂ ⁺ and Ar₂ ⁺ to possess a double-well potential and that of Kr₂ ⁺ to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr₂ ⁺ and Xe₂ ⁺ dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar₂ ⁺ does not.     

稀土离子Ho+,Ce+,La+与H2碰撞的单电子俘获截面测量

在复旦大学加速器实验室的３０ｋｅＶ同位素分离器上，使用新近建立的一套用于单价离子与气体分子碰撞的单电子俘获截面测量装置，测量了５—３０ｋｅＶ Ho⁺,Ce⁺,La⁺与H₂碰撞的单电子俘获截面．实验发现中空型离子源的阳极电压的变化对于稀土离子的亚稳态比例有一定影响，同时造成了单电子俘获截面的很大变化，而阳极电压对Ａｒ⁺的影响很小．还对实验结果进行了分析 关键词：     

Perturbative upper bounds for the electronic energy of diatomic molecules from level-set boundaries of nuclear charge space

Simple bounds to the electronic energy of diatomics are obtained using curvature properties in the space of nuclear charges. The bounds present the same analytic structure in terms of the internuclear separation as the exact electronic energy. They are determined from a single-atom property and can be improved using perturbative expansions of the energy. The approximate continuation of the large-R perturbation series for the electronic energy of H₂⁺ is discussed as an illustrative example.     

An effective potential method for the calculation of hydrogen molecule ion energy in a strong magnetic field

Using the adiabatic approximation and adiabatic variational approximation with an effective potential, the total energies and the equilibrium internuclear separations of H₂⁺ ion in states σ_g, π_u, δ_g, φ_u, γ_g, η_u in strong magnetic fields have been calculated. Our results reproduced those obtained by Vincke and Baye (Vincke M and Baye D 1985 J. Phys. B: At. Mol. Phys. 18 167), which are considered the most reliable. We found a basic function with better convergence than the Landau functions.     

Optimizing the photoassociation of cold atoms by use of chirped laser pulses

Photoassociation of ultracold atoms induced by chirped picosecond pulses is analyzed in a non-perturbative treatment by following the wavepacket dynamics on the ground and excited surfaces. The initial state is described by a Boltzmann distribution of stationary continuum states. The chosen example is photoassociation of cesium atoms at temperature K from the continuum to bound levels in the external well of the 0 _g ^-(6s + 6p _3/2) potential. We study how the modification of the pulse characteristics (carrier frequency, duration, linear chirp rate and intensity) can enhance the number of photoassociated molecules and suggest ways of optimizing the production of stable molecules.Received: 30 June 2004, Published online: 23 November 2004PACS: 33.80.Ps Optical cooling of molecules; trapping - 33.80.-b Photon interactions with molecules - 33.90. + h Other topics in molecular properties and interactions with photons - 33.80.Gj Diffuse spectra; predissociation, photodissociation     

Formation of ultracold RbCs molecules by photoassociation

The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the ⁸⁵Rb(5S_1/2) + ¹³³Cs(6P_3/2) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0⁺ or the 4th 0^? electronic states correlated to the Rb(5P_{1/2, 3/2}) + Cs(6S_1/2) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.