Ar2 Molecule Research Articles

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.

New results in above-threshold ionization and high-order harmonic generation of atomic and molecular systems

Numerical results obtained applying the strong-field approximation to atomic and molecular processes in intense laser fields are presented. In particular, the forward-backward asymmetry in high-order above-threshold ionization (HATI) of Ar atoms by a few-cycle laser pulse is considered. It is shown that the resonantlike enhancements observed in the focal-averaged spectra disappear with the decreasing laser pulse duration. For HATI and high-order harmonic generation (HHG) by molecular systems, we present new results for diatomic molecules. The choice of gauge and the problem of dressing of the initial bound state are discussed. The appearance of the interference minima in molecular HATI and HHG spectra and their role in the characterization of the molecular structure are considered using the example of the Ar2 molecule.

Potential energy curves and transition probabilities for the excimer states 0u+ (3P1) and 1u (3P2) of Ar2

Exactly solvable reference potentials of several smoothly joined Morse-type components were constructed for the lowest two excimer states of the Ar2 molecule. The parameters of the potentials have been ascertained by fitting to the experimental data, and they are reliable in a wide range of nuclear separations (r⩾1.9 Å). A large number of quantum mechanical Franck–Condon factors for the 0u+→ 0g+ and 1u→ 0g+ transitions have been calculated and compared with the observed spectroscopic features. The fitting procedure also involved dipole transition moments, which have been adjusted to the known radiative lifetimes of the vibrational levels. The resulting potential energy curves accurately reproduce the first and the second emission continua of Ar2* as well as the oscillatory spectrum related to their inner turning point region. The numbering and the positions of the vibrational levels reported by Herman et al (1988 J. Chem. Phys. 89 4535) have been confirmed. The calculated spectroscopic parameters, ωe=296.26 cm−1, De=6128.3 cm−1 for the 0u+ state and ωe=287.30 cm−1, De=5929.6 cm−1 for the 1u state, are consistent with previous estimations. Separation between the minima of the potential wells at Re=2.3893 Å was found to be 416.1 cm−1 (51.59 meV), compared with 606.8 cm−1 (75.24 meV) splitting in the separated-atom limit.

Wave packet study of the UV photodissociation of the Ar2HBr complex

The photodissociation dynamics of the Ar2HBr van der Waals molecule is studied using the multiconfiguration time-dependent Hartree method. Standard Jacobian coordinates are used to describe the molecule. Two four-dimensional calculations are carried out where the rotation of the Ar2 molecule and, in addition, either the vibration of Ar2-Br or that of Ar2 are frozen. The time-evolution of the probability density in the different modes and the calculation of the dissociative flux show that the dissociating hydrogen atom preferentially moves out of the plane defined by Ar2 and Br. A comprehensive study of the cage effect in the process is presented.

Numerical grids for density functional calculations of molecular properties

AbstractThere is an increasing number of applications using methods based on density functional theory (DFT) describing a large variety of systems of chemical interest. All these calculations are performed through evaluations of integrals of exchange and correlation contributions to the energy and the potential, which depend on the density and also on its first and second derivatives (nonlocal functionals). Within our DF program, the integrand is decomposed into single‐center components, through the introduction of weight functions, and its radial and angular dependence can be chosen. In this article, we give a brief description of these features, with compared performances of different grids for the evaluation of different integrals, such as the total number of electrons and the exchange and correlation energies. More complex properties, such as total energies and equilibrium geometries, are also studied with respect to the choice of the grid of points, in order to determine the most favorable radial and angular quadrature schemes. The incidence of this choice is analyzed in the case of the metal–metal bond of the Ru2, Rh2, and Pd2 dimers. Finally, the use of an extended grid of points is shown necessary for systems involving weak interactions, such as the Ar2 molecule. © John Wiley & Sons, Inc.

Rotational analysis of the 7pσ 3Σ+g←a3Σ+u system of the Ar2 molecule

Rotationally resolved transitions from the metastable 4sσ a3Σ+u state of Ar2 to the 7pσ 3Σ+g state have been observed by laser excitation spectroscopy in a pulsed corona discharge. Laser induced fluorescence spectra for the 7pσ 3Σ+g←a3Σ+u transition observed near 19 530 cm−1 have led to the first resolved triplet splittings and rotational analyses for Rydberg–Rydberg transitions in Ar2. Analyses for the (0–0), (0–1), (1–0), and (1–1) bands of 7pσ←a are presented here. Bands with v′=2 are observed for this system but exhibit no rotational structure and bands with v′≳2 are not observed, indicating that the predissociation lifetimes of these upper levels fall rapidly for v′≳1. Data presented lead to rotational energies and molecular constants for the a3Σ+u and 7pσ 3Σ+g states, including the effective rotational constant Be which is determined to be 0.1412 cm−1 and 0.1345 cm−1 for the 4sσ a3Σ+u and 7pσ 3Σ+g states, respectively. Results are compared to ab initio and other calculations, and combined with existing data to produce experimental estimates of D0 and De for the a3Σ+u state of 5700±200 cm−1 and 5850±200 cm−1, respectively.

Nonrigid large van der Waals molecules

We address the breakdown of the (nearly) rigid molecule description of molecular structure and spectra, which is realized when the amplitude of atomic motion becomes comparable with the average bond length. This situation prevails for van der Waals molecules and clusters at finite temperatures. Molecular nonrigidity was characterized in terms of an “intramolecular melting” Lindemann criterion, which reveals that aromatic molecule (M)-rare gas (A≡Ar, Kr, Xe) van der Waals molecules (clusters) are nonrigid atT≥10 K. A quantification of the concept of molecular nonrigidity in these systems was provided by constant energy molecular dynamics simulations for the smallest cluster of the M·A n family, i.e., the one-sided (2¦0) benzene · Ar2 molecule, which reveals a hierarchy of isomerization phenomena with increasing temperature.

Rydberg states of the Ar2 molecule

Extensive spectra attributable to transitions from the 4sσ a 3Σ+ metastable state of Ar2 to excited Rydberg states have been observed by intracavity absorption spectroscopy and by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Of these the most extensive and best resolved were the laser induced fluorescence spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ (3Πg,3Σ+g) and npλ (3Πg,3Σ+g)←a 3Σ+u series. The 5pπ3Πg←a 3Σ+u and 7pσ 3Σ+g←a 3Σ+u transitions observed near 19 823 and 19 529 cm−1, respectively, exhibit many bands with v′≠0 in their (v′–v″) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. Rotational structure is partially resolved in a few bands of the 7pσ 3Σ+g–a system, but most of the spectra observed appear to be either rotationally unresolved or made up of blended collections of rotational lines. Above the 7pσ, 5pπ pair, the (n+2)pσ 3Σ+g and npπ 3Πg members of the npλ series rapidly coalesce, indicating a rapid onset of decoupling of the electronic orbital angular momentum, L̂, from the internuclear axis. Such decoupling leads to np-complex formation at n values much lower than observed in the npλ series of the lighter dimers: He2 and Ne2. From the data for the observed series, the lowest ionization limit of Ar2 (relative to a 3Σ+u,v=0 ) was determined to be 29 373±3 cm−1. Vibrational intervals ΔG(v+1/2) for a 3Σ+u(v≤4) and 7pσ 3Σ+g(v≤2) yield the vibrational constants ωe=296 and 282 cm−1 and xωe=2.5 and 17 cm−1, respectively.

Possible artifacts occurring in the calculation of intermolecular energies from delocalized pictures

The intermolecular energy between two identical subsystems may be calculated from symmetrydelocalized MO's resulting for instance from a preliminary SCF calculation of the supersystem. Then each second-order energy correction mixes intramolecular correlation,R −6 intermolecular dispersion energy, andR −3 components. TheR −3 components disappear through subtle cancellations. The shifted Epstein-Nesbet energy denominators introduce an artificial second-order intermolecularR −1 component, which would be cancelled by off-diagonal third-order terms, as well as a bad asymptotic limit at infinite distances. TheR −1 artifact will also occur in strong symmetrical chemical bonds calculated in the Epstein-Nesbet perturbation scheme from delocalized MO's. These defects will occur in all variational approximate CI techniques which neglect off-diagonal elements between delocalized doubly excited determinants. These artifacts are avoided when using the Moller-Plesset definition of the zeroth order Hamiltonian or when starting from (SCF)localized MO's (even in the Epstein-Nesbet perturbation). The discussion is exemplified on an accurateab initio calculation of the Ar2 molecule.

Time-resolved spectroscopy of the Ar* 2-excimer emission

High-pressure argon was excited by a 2-ns 600-keV e-beam pulse, and time integrated as well as time resolved fluorescence spectra were measured. No significant dependence of line center (126.2±0.1 nm) or spectral width (80±10 nm) of the Ar2 continuum on gas pressure was found in the range 0.5–20 bar. From the buildup time constant of fluorescence we calculated the three-body rate constant for Ar2 formation to be 5×10−33 cm6/sec. The fluorescence decay was found to consist of two exponentials with time constants 8.6 and 39.2 ns at 20 bar and 12.6 and 73.9 ns at 10 bar, respectively. We attribute this behavior to the fluorescence originating from the singlet and triplet upper state. From the spectral shift between the two components we determined the singlet-triplet splitting in the excited Ar2 molecule to be 12 Å corresponding to 760 cm−1.

Finite Element Approach to the Vibrational Schrödinger Equation of Diatomic Species

Abstract The finite element approach was successfully applied to the vibrational Schrödinger equation of diatomic species. An O2 molecule with a harmonic potential and an Ar2 molecule with a Lennard-Jones potential were examined and their eigen values and the associated eigen function were obtained. The calculated eigen values of the O2 molecule agree with those expected. In the case of the Ar2 molecule the calculated eigen values agree with those experimentally obtained when Lennard-Jones parameters are ε=115.1 (cm−1) and σ=3.35 (Å). This verifies the validity of the present approach.

Absorption Spectra of Ne2 and HeNe Molecules in the Vacuum-uv Region

The absorption spectra of the Ne2 and HeNe molecules have been investigated in the 585–760 Å region with a 6.65 m vacuum spectrograph using the helium continuum as a background source. Twelve discrete band systems are identified in the Ne2 spectrum. The ground state is stable, with a potential depth De=30.2 cm−1, supporting two stable vibrational levels, v=0 and 1, and with B0=0.17 cm−1, giving r0=3.1 Å. A number of existing intermolecular potential calculations agree with these results. State designations, dissociation products, stabilities, and observed transitions are discussed, and analogies are drawn with a previous study of the Ar2 molecule. It is suggested that the observed band systems represent members of Rydberg series converging to some of the Ne2+ states derived from atom combinations Ne (2p6 1S0)+ Ne+ (2p5 2P3/2,1/20). In addition, six band groups are attributed to the HeNe molecule. Here the ground state potential supports only one vibrational level, v=0.