Charge Asymmetric Dissociation Research Articles

Selective charge asymmetric distribution in heteronuclear diatomic molecules in strong laser fields

In this paper we study double-ionization-induced charge asymmetric dissociation (CAD) in heteronuclear diatomic molecules. In CO we find a selective charge distribution in two CAD channels, i.e., ${\mathrm{C}}^{2+}+\phantom{\rule{1.0pt}{0ex}}\mathrm{O}$ is abundantly produced but $\mathrm{C}+{\mathrm{O}}^{2+}$ is nearly nonexistent. This cannot be explained by the ionization energy difference between the two channels alone. Our study shows that the ${\mathrm{C}}^{2+}+\mathrm{O}$ channel is sequentially formed through an intermediate state ${\mathrm{C}}^{+}+\mathrm{O}$ and the selective charge distribution is the result of electron distribution in CO when exposed to intense laser fields.

Atomic site-sensitive processes in low energy ion-dimer collisions.

Electron capture processes for low energy Ar(9+) ions colliding with Ar(2) dimer targets are investigated, focusing attention on charge sharing between the two Ar atoms as a function of the molecular orientation and the impact parameter. A preference for charge-asymmetric dissociation channels is observed, with a strong correlation between the projectile scattering angle and the molecular ion orientation. The measurements here provide clear evidence that projectiles distinguish each atom in the target and that electron capture from near-site atoms is favored. Monte Carlo calculations based on the classical over-the-barrier model, with dimer targets represented as two independent atoms, are compared to the data. They give new insight into the dynamics of the collision by providing, for the different electron capture channels, the two-dimensional probability maps p(b), where b is the impact parameter vector in the molecular frame.

A comparative study of Dissociative Ionization of N2 and CO

A comparative study on the properties of charge symmetric dissociation (CSD) and charge asymmetric dissociation (CAD) of doubly ionized N2 and CO is performed. Kinetic energy release (KER) distributions resulting from the dissociation of doubly charged molecular ions are explained on the basis of calculated potential energy curves.

Multielectron effects in charge asymmetric molecules induced by asymmetric laser fields.

Using a 45 fs pump pulse at 800 nm, a wave packet is created in a charge asymmetric dissociation channel of iodine, I(2)(2+)→I(2+)+I(0+) (2,0). As the molecule dissociates, a two-color (1ω2ω) probe pulse is used to study the dynamics as a function of internuclear separation R. We find a critical region of R in which there is spatially asymmetric enhanced ionization of the (2,0) channel to a counterintuitive (1,2) channel. In this region the I(0+) is ionized such that one electron is released to the continuum and another is transferred to the I(2+) resulting in I(0+)→I(2+) and I(2+)→I(1+). At larger R, the ionization is consistent with simple one-electron ionization in a double well where I(0+)→I(1+). We find qualitative agreement between simulations and experiment further highlighting the importance of multielectron effects in the strong-field ionization of molecules.

Charge asymmetric dissociation of a CO+molecular-ion beam induced by strong laser fields

Charge asymmetric dissociation into ${\mathrm{C}}^{2+}+\mathrm{O}$ following ionization of a CO${}^{+}$ molecular-ion beam with intense ultrashort (7--40-fs) laser pulses is studied using a coincidence three-dimensional momentum imaging method. The data suggest that allowing the molecule enough time to stretch within a laser pulse such that it ionizes beyond the curve crossings of the ${\mathrm{C}}^{2+}+\mathrm{O}$ and ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ potentials enhances the otherwise hard-to-reach charge asymmetric dissociation channel.

Charge-asymmetric dissociation of nitrogen molecules colliding with highly charged ions

Ion pair fragmentation of nitrogen molecules following Ar8+ ion collision was observed by using a position-sensitive time-of-flight method. Kinetic energy release (KER) distributions were derived from the double hit data. Higher-asymmetric fragmentation channels have peak positions of the KER higher than the Columbic value.

Double Ionization of Nitrogen from Multiple Orbitals

In intense femtosecond laser fields, molecules could be tunnel ionized from multiple valence orbitals, which produces molecular ions in different electronic states. In this article, we have used a reaction microscope to study double ionization of nitrogen by intense femtosecond laser pulses. It is found that some doubly charged molecular ions N(2)(2+) are metastable while others dissociate through charge symmetric dissociation (N(2)(2+) --> N(+) + N(+)) or charge asymmetric dissociation (N(2)(2+) --> N(2+) + N). The kinetic energy releases and angular distributions of atomic ions are measured for the dissociation channels. With the aid of the CASSCF and MRCI calculations, the electronic states are identified and the contributions of the valence orbitals are discussed for these dissociated molecular dications.

Molecular dynamics of CO in few-cycle laser fields

We experimentally study the ionization, fragmentation and Coulomb explosion of CO using 6 fs laser pulses. Different from previous observations in tens or hundreds of femtoseconds laser pulses, strong charge asymmetric dissociation and CO 2+ are observed in the current intense few-cycle laser field.

Stability, spectroscopic constants, and dissociation of CO2+: A theoretical study

AbstractStability, spectroscopic constants, and dissociation of CO2+ have been studied in detail using ab initio MP2, CCSD and CCSD(T) methods, and density functional B3LYP method. The stability and the ambiguity between the ground and metastable state of the molecular dication have been discussed. The spectroscopic constants of the molecular dication have been compared with the experimental and theoretical values wherever available. Various charge symmetric and charge asymmetric dissociation pathways of CO2+ have been investigated. After dissociation, the fragmented atoms and ions are considered to be either in their ground or in their metastable state. Interesting results have been obtained for the charge symmetric and charge asymmetric dissociation of the diatomic dication. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Dissociative double photoionization of N2 using synchrotron radiation: Appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N2+ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2+/-0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N2+(2P)+N(4S) at 53.9 eV. The onset of a second threshold at 59.9+/-0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N2+ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N(2)2+ dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies.

Observation of enhanced excitation ofI22+by strong laser fields

Using pump-probe spectroscopy with ultrashort laser pulses, we see an enhancement of the charge-asymmetric dissociation (CAD) channel, ${({{\mathrm{I}}_{2}}^{2+})}^{*}\ensuremath{\rightarrow}{\mathrm{I}}^{2+}+\mathrm{I}$, over a narrow range of internuclear separation. The enhancement of the CAD channel appears to come from the excitation of the symmetric ground state dissociation channel $({\mathrm{I}}^{+}+{\mathrm{I}}^{+})$ for two reasons. First, there is a depletion in the symmetric channel at approximately the same pump-probe delay as the asymmetric enhancement. Second, for a fixed delay, the asymmetric channel increases as a function of probe intensity while the symmetric channel decreases. In addition, we find that the kinetic energy of the extra ${\mathrm{I}}^{2+}+\mathrm{I}$ ions decreases for increasing delay. To explain this dependence of the kinetic energy release on delay, we introduce model potential energy curves. Based on these curves, we conclude that the excitation is produced by a resonant three-photon transition within ${{\mathrm{I}}_{2}}^{2+}$ rather than by ionization of ${{I}_{2}}^{+}$.

Intense laser-field ionization ofH2enhanced by two-electron dynamics

Intramolecular electronic dynamics and tunnel ionization of ${\mathrm{H}}_{2}$ in an intense laser field $(I\ensuremath{\approx}{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ and $\ensuremath{\lambda}=760\mathrm{nm})$ are examined with accurate evaluation of three-dimensional two-electron wave-packet dynamics by the dual transformation method. We estimated the ionization probabilities at different values of the internuclear distance R and found that tunnel ionization of ${\mathrm{H}}_{2}$ is enhanced by field-induced two-electron dynamics. An ionic component characterized by the electronic structure ${\mathrm{H}}^{+}{\mathrm{H}}^{\ensuremath{-}}$ or ${\mathrm{H}}^{\ensuremath{-}}{\mathrm{H}}^{+}$ is created near the descending well, where the dipole interaction energy with the laser electric field $\ensuremath{\varepsilon}(t)$ becomes lower. Ionization proceeds via the formation of a localized ionic component in the descending well, in contrast to the ${\mathrm{H}}_{2}^{+}$ case, in which the electron is ejected most easily from the ascending well. As R increases, while the population of ${\mathrm{H}}^{\ensuremath{-}}{\mathrm{H}}^{+}$ (or ${\mathrm{H}}^{+}{\mathrm{H}}^{\ensuremath{-}})$ decreases, a pure ionic state ${\mathrm{H}}^{\ensuremath{-}}{\mathrm{H}}^{+}$ becomes easier to ionize in an intense field because of the smallar attractive force of the distant nucleus. As a result, ionization is enhanced at the critical distance ${R}_{c}=(4\char21{}{6)a}_{0}$ ${(a}_{0}$ is the Bohr radius). Although the rate of direct ionization from a covalent state is much smaller than that from an ionic state, the ionization at large R $(g~{8a}_{0})$ mainly proceeds from the remaining covalent component, which outmeasures the created ionic component. Thus, the field-induced intramolecular electron transfer between nuclei, which triggers strong electron-electron correlation, is governed by the molecular structure as well as the field intensity. The mechanism of the ionization enhanced by field-induced intramolecular electron transfer is consistent with the observation of charge-asymmetric dissociation channels of diatomic molecules such as ${\mathrm{N}}^{+}+{\mathrm{N}}^{3+}.$ We also investigated the intramolecular electronic dynamics by analyzing the populations of field-following adiabatic states defined as eigenfunctions of the instantaneous electronic Hamiltonian. An effective instantaneous Hamiltonian for ${\mathrm{H}}_{2}$ was constructed of three main electronic states, X, $B{}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ and $\mathrm{EF}.$ We found that the difference in electronic and ionization dynamics between the small R and large R cases originates in the character of the level crossing of the lowest two adiabatic states.

Direct Observation of Excited State Fragments Following Molecular Ionization and Dissociation in Strong Fields

Using a new double pulse technique, we have observed for the first time that charge asymmetric dissociation in diatomic molecules leaves one of the fragments in an electronically excited state. For example, we observed the reaction ${\mathrm{I}}_{2}+(\mathrm{pulse}1)\ensuremath{\rightarrow}({\mathrm{I}}_{2}^{2+}{)}^{**}\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+({\mathrm{I}}^{2+}{)}^{*}+(\mathrm{pulse}2)\ensuremath{\rightarrow}{\mathrm{I}}^{0+}+{\mathrm{I}}^{3+}$, demonstrating that the ${\mathrm{I}}^{2+}$ fragment must have been in an excited state. More generally, just as asymmetric dissociation implies that the initial molecular ion is in an excited electronic state, the observation of asymmetric channels in the postdissociation ionization shows that the ionic fragments are themselves electronically excited.

Charge Asymmetric Dissociation Induced by Sequential and Nonsequential Strong Field Ionization

Charge asymmetric dissociation (CAD) is observed, for the first time, with near-infrared 30 fs laser pulses, in small molecules, ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$. ${\mathrm{N}}_{2}^{2+}$ is created by a nonsequential (vertical) transition and the dissociation fragments appear to be in excited states. Unlike ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}^{2+}$ involves a sequential (nonvertical) transition. The charge asymmetric dipole coupling to the external field is insufficient to populate the charge transfer states leading to CAD. Rather, CAD is a natural result of strong field excitation and ionization. These results in the tunneling regime are also compared with previous experiments performed with UV radiation in the multiphoton regime.

Direct evidence of the generality of charge-asymmetric dissociation of molecular iodine ionized by strong laser fields

We present ion time-of-flight data showing strong-field ionization of iodine molecules up to ${\mathrm{I}}_{2}^{13+}$ using 33-fsec laser pulses. Every even molecular charge state shows an charge asymmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{(n+1)+}+{\mathrm{I}}^{(n\ensuremath{-}1)+})$ with a significant branching ratio (15--30 %) in addition to the symmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{n+}+{\mathrm{I}}^{n+})$ showing the general nature of this process. Furthermore, all charge states are formed at internuclear separations less than the critical separation for electron localization and enhanced ionization ${(R}_{c}).$ One channel, in particular, ${\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{6+}+{\mathrm{I}}^{4+},$ represents a direct electronic excitation by nonresonant strong-field interaction to a state 20.9 eV above the ground state $({\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{5+}+{\mathrm{I}}^{5+}).$ We show that the interaction of the dipole moment of the asymmetric charge distribution with the external field can account for this energy and the excitation mostly likely involves charge-transfer states. These results establish strong-field ionization of molecules as an efficient method for populating highly excited electronic states.

Role of non-Coulombic potential curves in intense field dissociative ionization of diatomic molecules.

Dissociative ionization of ${\mathrm{N}}_{2}$ induced by intense picosecond radiation has been studied. A two-step multiphonon model, based on ionization of ${\mathrm{N}}_{2}$ through vertical transitions and spontaneous dissociation of ${\mathrm{N}}_{2}^{\mathit{z}+}$ (Z\ensuremath{\ge}2) via Coulomb and non-Coulombic potentials curves, is presented to explain the origin of the kinetic energies of atomic ions observed. The model predicts that dissociation occurs near the internuclear separation of the neutral, charge-asymmetric dissociation channels are important, and dissociation via Coulomb curves will only be observed for pulses with extremely short rise times.

Kinetic energy distributions of ionic fragments produced by subpicosecond multiphoton ionization of N2.

A study of the kinetic energy distributions of ionic fragments produced by subpicosecond irradiation of ${\mathrm{N}}_{2}$ with 248-nm radiation at an intensity of \ensuremath{\sim}${10}^{16}$ W/${\mathrm{cm}}^{2}$ is reported. These measurements, in comparison to other findings involving molecular excitation with charged particles and soft x rays, reveal several important features of the nonlinear coupling. Four ionic dissociative channels are identified from the data on the multiphoton process. They are ${\mathrm{N}}_{2}^{2+}$\ensuremath{\rightarrow}${\mathrm{N}}^{+}$+${\mathrm{N}}^{+}$, ${\mathrm{N}}_{2}^{2+}$\ensuremath{\rightarrow}N+${\mathrm{N}}^{2+}$, ${\mathrm{N}}_{2}^{3+}$\ensuremath{\rightarrow}${\mathrm{N}}^{+}$+${\mathrm{N}}^{2+}$, and ${\mathrm{N}}_{2}^{4+}$\ensuremath{\rightarrow}${\mathrm{N}}^{+}$+${\mathrm{N}}^{3+}$, three of which are charge asymmetric. The data for the energy distributions are found to be in approximate conformance with a simple picture involving ionizing transitions occurring within a time of a few cycles of the ultraviolet wave at an internuclear separation close to that of the ground-state (X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$) molecule. The implication follows that a strong nonlinear mode of coupling is present which causes a high rate of energy transfer. A simple hypothesis is presented which unites the ability for rapid energy transfer with the observed tendency to produce charge-asymmetric dissociation.