Diatomic Molecular Ions Research Articles

Ultrahigh harmonic generation from diatomic molecular ions in highly excited vibrational states

Numerical calculations of the harmonic yield from diatomic molecular ions initially prepared in a highly excited vibrational state show a remarkable enhancement of the harmonic energies in the plateau in terms of the ponderomotive energy. The two-step scheme that explains the mechanism of harmonic generation by atoms is extended to our double potential-well structure, providing an explanation for the high photon energies produced.

Phase control of molecular ionization: H2 + and H3 2+ in intense two-color laser fields.

Charge resonance-enhanced ionization is examined for combinations of intense, linearly polarized laser fields, and their second harmonic (1\ensuremath{\omega}-2\ensuremath{\omega}) for both a diatomic molecular ion (${\mathrm{H}}_{2}$${\mathrm{}}^{+}$) and an asymmetric linear triatomic molecular ion (${\mathrm{H}}_{3}$${\mathrm{}}^{2+}$). Ionization is found to be strongly dependent on the relative phase between the two colors in both cases. In particular for ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ one can achieve nearly complete ionization of electrons at one of the nuclei only. For the asymmetric linear triatomic molecular ion (${\mathrm{H}}_{3}$${\mathrm{}}^{2+}$) enhanced ionization is shown also to occur at large critical distances. In the presence of two color fields, the ionization rate of the asymmetric ${\mathrm{H}}_{3}$${\mathrm{}}^{2+}$ can be controlled again by tuning the relative phase between the driving fields. The phase control of the enhanced ionization can be rationalized in terms of field-induced over-the-barrier ionization and laser-induced electron localization or equivalently electron tunneling suppression. \textcopyright{} 1996 The American Physical Society.

Limits on stability of positive molecular ions in a homogeneous magnetic field

The problem of stability of positive diatomic molecular ions with the nuclear chargesZ1 andZ2 andN electrons in a homogeneous magnetic fieldB is studied forZ1,Z2,N,B→∞. The conditions of instability are obtained for different relations amongZ1,Z2,N andB. A new version of the HVZ theorem for systems in a homogeneous magnetic field is proved.

Analysis of Collision Induced Dissociation of Na2+ Molecular Ions.

A full analysis of the collision induced dissociation of a diatomic molecular ion is presented for the first time. The experiment is essentially based on the simultaneous measurement of the momenta of the two fragments. This technique applied to the collision of Na{sub 2}{sup +} with a helium target reveals, at keV collision energies, the competition between two dissociation mechanisms: an impulsive mechanism when the target hits one Na{sup +} core in a close encounter and an electronic mechanism when dissociation occurs after excitation of repulsive or weakly bound states. {copyright} {ital 1996 The American Physical Society.}

Ar +2 molecules in intense laser fields

We have studied the interaction between intense laser fields (⩽ 7 × 10 12 W/cm 2) and molecules. For the diatomic molecular ion Ar + 2 the number of photofragments as a function of laser intensity can be quantitatively explained in terms of light-induced adiabatic molecular potentials. The effects predicted by this model tend to cancel under realistic experimental conditions with a thermal distribution of the rovibrational populations. This molecular beam experiment allows for a fully quantitative comparison with theory, since only one dissociation channel is operative and all parameters relevant for a computational simulation have been determined.

Reply to "Comment on 'Determination of highly excited states of diatomic-molecular ions using exact H2 +-like orbitals' "

Reply to "Comment on 'Determination of highly excited states of diatomic-molecular ions using exact H2 +-like orbitals' "

Charge-resonance-enhanced ionization of diatomic molecular ions by intense lasers.

We study the ionization of the ${\mathrm{H}}_{2}^{+}$ molecular ion in intense, short-pulse laser fields by numerically solving the three-dimensional time-dependent Schr\odinger equation as a function of internuclear distance $R$. Anomalously high ionization for the molecular ion at large internuclear separations is observed for orientations parallel to the linearly polarized laser field. The ionization rate is found to exhibit maxima at large $R$, exceeding the atom limit by an order of magnitude. This is attributed to transitions between pairs of chargeresonant states which are strongly coupled by the field in diatomic molecular ions. The effect is shown to also occur in higher odd-charge diatomic molecular ions and can be attributed to field-induced nonadiabatic transitions between the charge-resonant states and electron tunneling suppression by the instantaneous Stark field of the laser.

Long-lived, doubly charged diatomic and triatomic molecular ions

Lifetimes of doubly charged diatomic and triatomic molecules have been measured by monitoring the decay curves of such ions in a heavy-ion storage ring. CO2+, N22+, CO22+, CS22+ and SH2+ are all found to possess long-lived components which survive for time periods greater than a few seconds. All these dications are found to be essentially stable and their ultimate destruction is due to interactions with residual gases in the ring. CO2+ possesses many more lifetime components in the millisecond range than the isoelectronic N22+ ion. Translational energy spectrometry experiments on the latter species also fail to reveal any short-lived (microsecond) components. Ab initio configuration interaction calculations have been carried out and the potential energy curve for the lowest-energy metastable state of N22+ (1 Sigma g) has been determined, along with Franck-Condon factors for vertical transitions to different vibrational levels from the ground state of neutral N2; tunnelling times of each vibrational level have been computed.

Collision-induced interaction cross sections of 1–7 MeV B 2 ions incident on an N 2 gas target

Collision-induced interaction cross sections were measured for 1–7 MeV diatomic boron molecular ions incident on a nitrogen gas target by using a differentially pumped gas cell. The dissociation cross section of B 2 q+ ( q = 1–3) were measured to be on the order of 10 −15 cm 2. The dissociation cross section was found to decrease with increasing ion charge state, and with increasing collision energy. The single-electron-loss cross section of B 2 1+ was measured to be on the order of 10 −16 cm 2, and the double-electron-loss cross section of B 2 1+ was measured to be on the order of 10 −17 cm 2 for 3 MeV ions. The single-electron-capture cross section of B 2 2+ was measured to be on the order of 10 −20 cm 2.

Rotational and vibrational lifetime of isotopically asymmetrized homonuclear diatomic molecular ions.

Radiative relaxation times of rovibrational transitions for the electronic ground state of ${\mathrm{HD}}^{+}$, $^{16}\mathrm{O}^{18}$${\mathrm{O}}^{+}$, and $^{12}\mathrm{C}^{13}$${\mathrm{C}}^{+}$ are calculated. These lifetimes are compared with the storage time in a heavy-ion storage ring where molecular ions can be stored, in order to provide vibrationally and rotationally cold ions. The relevance of the results to the measurement of dissociative recombination of cold molecular ions is discussed.

Anapole moment of a diatomic polar molecule.

We discuss the magnetic toroidal dipole moments (anapole moments) of diatomic molecules by analyzing a calculable ``toy model'' for a heteronuclear diatomic molecular ion. The model consists of a single unpaired electron with spin \ensuremath{\sigma}, in the electrostatic potential of two different point charges; all spin-dependent interactions are neglected. We show that this model predicts a toroidal dipole moment of order \ensuremath{\alpha}${\mathit{ea}}_{0}^{2}$ oriented in the direction n^\ifmmode\times\else\texttimes\fi{}\ensuremath{\sigma}, where n^ is the molecular axis. A simple relation is derived between the three static dipole moments of this model: electric, magnetic, and toroidal. Numerical results are given for a particular choice of model parameters. In addition to the usual magnetic dipole field, there is a toroidal magnetic field inside the molecule, which we evaluate for this model. The two fields are similar in strength but have opposite behavior under spatial inversions, so that the magnetic field, as well as the electric field, fails to have a center of inversion.

Scattering of swift diatomic molecular ions from planar surfaces at grazing incidence

Grazing-incidence scattering of Coulomb-exploding fragments of a diatomic molecule from a surface has been analysed by molecular-dynamics simulation. The influence of a number of input parameters on the computed emergence pattern has been studied. Trajectories are strongly affected by Coulomb explosion, and the dominating parameter is the initial orientation of the impinging molecule. Other parameters analysed, in the order of decreasing significance, include position-dependent electronic stopping, electrostatic image force, internuclear distance, surface potential, the distance at which Coulomb explosion sets in, and electronic or nuclear multiple scattering. Only two-dimensional trajectories have been studied. Computed peak energies versus scattering angle agree well with experimental results for 400 keV H 2 + on W surfaces, while computed spectra are much narrower than found experimentally.

Barrier for the reaction X20++X20+-->X402+ in alkali-metal clusters related to electron density at the bond midpoint of the supermolecule (X20+)2.

Using the extended Thomas-Fermi version of density-functional theory (DFT), calculations are presented for the barrier for the reaction ${\mathrm{Na}}_{20}^{+}$+${\mathrm{Na}}_{20}^{+}$\ensuremath{\rightarrow}${\mathrm{Na}}_{40}^{2+}$. The deviation from the simple Coulomb barrier is shown to be proportional to the electron density at the bond midpoint of the supermolecule (${\mathrm{Na}}_{20}^{+}$${)}_{2}$. An extension of conventional quantum-chemical studies of homonuclear diatomic molecular ions is then effected to apply to the supermolecular ions of the alkali metals. This then allows the Na results to be utilized to make semiquantitative predictions of position and height of the maximum of the fusion barrier for other alkali clusters. These predictions are confirmed by means of similar DFT calculations for the K clusters.

SCATTERING AND ENERGY LOSS FOR SLOW MOLECU-LAR IONS IN ELECTRON GASES IN SOLIDS

With the theory of electron-molecule scattering, we have investigated the scattering and energy loss for slow diatomic molecular ions in electron gases in solids and discussed the influence of the coherence effects between two ions to the energy loss. The scattering phase shifts are calculated using atomic IPM potential and the variable phase method. The stopping powers for atomic ions are compared with the results given by the nonlinear density-functional theory.

On the kinetic energy released upon collision‐induced dissociation of oriented diatomic ions

AbstractThe kinetmatics of collision‐induced dissociation of oriented singly and doubly charged diatomic molecular ions have been investigated using computer simulation techniques. Asymmetries are predicted in the translational‐energy spectra of forward‐scattered and backward‐scattered fragment ions when dissociation occurs, in those cases where the internuclear axis of the diatomic molecules is oriented along the direction of the incident ion beam. Simulated translational energy spectra are compared with experimental data obtained for dissociation of oriented CO2+ ions in collision with helium atoms.

Generation of high-order harmonics from inertially confined molecular ions.

We consider the generation of high-order harmonics from inertially confined diatomic molecular ions driven by strong short pulses of laser radiation. Using a simple analytically solvable model, we show that diatomic molecular ions can be a very efficient source of high-order harmonics. The sources of the strong nonlinear response are charge-resonant states of odd-charge molecular ions. We also discuss possibilities for quasi-phase-matching, which are given by the molecular ions and which have no analogies in atomic systems.

Vicinage effects in the stopping power for diatomic molecular ions in solids

The stopping power for diatomic molecular ions is investigated within the framework of the linear-response dielectric theory and the effective charge model of Brandt and Kitagawa for atomic ions is extended to molecular ions. In the limits of low and high velocity, some analytical formulae are obtained. The vicinage effects in the stopping power are discussed and a comparison with the experimental data is made.

On the semiclassical localization of the quantum probability

The localization behavior of one-dimensional quantum systems for ℏ→0 is investigated by semiclassical methods. In particular the localization of the quantum probability around turning points of arbitrary even order associated to classical hyperbolic orbits is considered and a relation of the localization speed in ℏ with the classical motion is established. Our analysis is based on local norm comparisons of solutions to Schrödinger type equations; it relies mainly on a combination of scaling and asymptotic arguments and thus evades the use of special functions. Applications of the results to separable multidimensional Schrödinger equations are indicated by a brief discussion of the one-electron diatomic molecular ion.

Non-adiabatic ionization of hydrogen atoms by highly charged ions

Based on the concepts of the classical barrier model and the analytical properties of the potential energy curves of the one-electron diatomic molecular ion, a direct physical description of slow ionizing collisions between highly charged ions and atoms is given. New expressions for the energies of the quasi-stationary states localized on the top of the potential barriers are used to derive simple expressions for the ionization cross sections in the limit of high charge states. A simple scaling in reduced variables is possible for the low energy ionization cross section in this limit.

Vibration-translation exchange in diatom-diatom collisions

A general and computationally efficient (analytical) approach evaluating the transition probabilities for vibration-translation (VT) exchange, involving higher vibrational states, in diatomic gases is presented. The role of the slope of the repulsive part of the intermolecular potential on the transition probability is also investigated. Examples of numerical results are given for a number of transitions in nitrogen and oxygen molecules in the ground electronic states. The approach can be used for VT exchange involving diatomic molecular ions.