Diatomic Molecular Ions Research Articles

Wall Induced-oscillations in Photo detachment of Homo Nuclear Diatomic Molecular Negative ion

The Pd (Photodetachment) of a hydrogen molecular negative ions near a soft reflecting wall is being investigated theoretically, focusing on the differential and total Pd.CS (Photodetachment Cross Section) when the laser polarisation is along the molecular axis of the ion. Quantum interference becomes evident on the observable plane when the two components of the electron wave superimpose at a considerable distance and propagate outward. The investigation of these interferences contributes to improving our understanding of the structure of diatomic molecular negative ions. Wall-induced fluctuations are observed in the detached electron spectra. As the distance between the ion and wall (R) increases, the wall effect diminishes, and the results approach those of the Ions in free space, the oscillation amplitude in the Pd.CS decreases, and their frequency increases as both Walls (R) and the distance between the two opposing ion centers (d) increase. The oscillations eventually cease to exist, and the CS (cross-section) exhibits uniform behavior at tremendous values of R and (R→∞,d→∞ ) resulting in the match to the Ions in free space.

Mass-ratio dependent strong-field dissociation of artificial helium hydride isotopologues

We study the effect of the nuclear-mass ratio in a diatomic molecular ion on the dissociation dynamics in strong infrared laser pulses. A molecular ion is a charged system, in which the dipole moment depends on the reference point and therefore on the position of the nuclear center of mass, so that the laser-induced dynamics is expected to depend on the mass asymmetry. Whereas usually both the reduced mass and the mass ratio are varied when different isotopologues are compared, we fix the reduced mass and artificially vary the mass ratio in a model system. This allows us to separate effects related to changes in the resonance frequency, which is determined by the reduced mass, from those that arise due to the mass asymmetry. Numerical solutions of the time-dependent Schrödinger equation are compared with classical trajectory simulations. We find that at a certain mass ratio, vibrational excitation is strongly suppressed, which decreases the dissociation probability by many orders of magnitude.

Nondipole effects on the double-slit interference in molecular ionization by xuv pulses.

The double-slit interference in single-photon ionization of the diatomic molecular ion H2 + is theoretically studied beyond the dipole approximation. Via simulating and comparing the interactions of the prealigned H2 + and the hydrogen atom with the xuv pulses propagating in different directions, we illustrate two kinds of effects that are encoded in the interference patterns of the photoelectrons from H2 +: the single-atom nondipole effect and the two-center-interference one, both associated with the finite speed of light. While the two effects could modify the maxima of the interference fringes, we show that the former one hardly affects the interference minima. Our results and analysis show that the interference minima rule out the influences of the photon-momentum transfer and, potentially, the multielectron effect, thus performing a better role in decoding the zeptosecond time delay for the pulse hitting one and the other atomic centers of the molecule.

Distinguishing two mechanisms for enhanced ionization of H2+ using orthogonal two-color laser fields

We theoretically study the ionization enhancement of the diatomic molecular ion ${\mathrm{H}}_{2}{}^{+}$ at two critical internuclear distances $R$, using orthogonal two-color laser fields. The polarization of the fundamental infrared laser field and a weak second-harmonic field is parallel and perpendicular to the molecular axis, respectively. It is observed that adding the second-harmonic field raises slightly the first ionization peak at the smaller critical $R$, whereas it enhances the second one at the larger critical $R$ significantly. We further analyze the observable evidence which distinguishes two underlying mechanisms responsible for the enhanced ionization of ${\mathrm{H}}_{2}{}^{+}$: (i) the resonant excitation along with the coherent interference of the ionizing wave packets from the $1s{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\sigma}}_{u}$ states and (ii) the easier ionization from the up-field site of the molecule.

Photodestruction of Diatomic Molecular Ions: Laboratory and Astrophysical Application.

In this work, the processes of photodissociation of some diatomic molecular ions are investigated. The partial photodissociation cross-sections for the individual rovibrational states of the diatomic molecular ions, which involves alkali metals, as well as corresponding data on molecular species and molecular state characterizations, are calculated. Also, the average cross-section and the corresponding spectral absorption rate coefficients for those small molecules are presented in tabulated form as a function of wavelengths and temperatures. The presented results can be of interest for laboratory plasmas as well as for the research of chemistry of different stellar objects with various astrophysical plasmas.

The Optical Spectrum of Au2+

AbstractThe electronic structure of the Au2+ cation is essential for understanding its catalytic activity. We present the optical spectrum of mass‐selected Au2+ measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290–450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin‐orbit coupling. The experimental spectra are compared to high‐level quantum‐chemical calculations at the CASSCF‐MRCI level including spin‐orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H2+‐like diatomic molecular ion strictly requires multireference and relativistic treatment including spin‐orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.

The Optical Spectrum of Au2.

The electronic structure of the Au2 + cation is essential for understanding its catalytic activity. We present the optical spectrum of mass‐selected Au2 + measured via photodissociation spectroscopy. Two vibrationally resolved band systems are observed in the 290–450 nm range (at ca. 440 and ca. 325 nm), which both exhibit rather irregular structure indicative of strong vibronic and spin‐orbit coupling. The experimental spectra are compared to high‐level quantum‐chemical calculations at the CASSCF‐MRCI level including spin‐orbit coupling. The results demonstrate that the understanding of the electronic structure of this simple, seemingly H2 +‐like diatomic molecular ion strictly requires multireference and relativistic treatment including spin‐orbit effects. The calculations reveal that multiple electronic states contribute to each respective band system. It is shown that popular DFT methods completely fail to describe the complex vibronic pattern of this fundamental diatomic cation.

Molecular strong-field approximation for photodetachment of electrons from homonuclear diatomic molecular anions

Molecular strong-field approximation is applied to above-threshold detachment of homonuclear diatomic molecular negative ions. Differences between the photodetachment amplitudes for neutral diatomic molecules and diatomic anions, for both direct and rescattered electrons, are examined. Numerical results for the photoelectron spectra of C 2 − molecular anions for different intensities and wavelengths of a linearly polarized laser field and different molecular anion orientations are shown and discussed. Two-center destructive interference minima (suppression regions) in the rescattering part of the photoelectron spectra are observed. For molecules with molecular orientation defined by the angle θ L with respect to the laser-field polarization axis, these minima manifest as two parallel straight lines in the distribution of the photoelectron yield presented in the photoelectron momentum plane. These lines make the angle 90 ∘ − θ L , with the momentum component parallel to the laser-field polarization axis. Focal-averaged photoelectron spectra are also presented and analyzed.

Electron-Induced Excitation, Recombination, and Dissociation of Molecular Ions Initiating the Formation of Complex Organic Molecules

We review the study of dissociative recombination and ro-vibrational excitation of diatomic and small polyatomic molecular ions initiating complex organic molecules formation. In particular, we show how Multichannel Quantum Defect Theory (MQDT) and R-matrix methods are used to compute cross sections and rate coefficients for cations in well defined ro-vibrational levels of the ground electronic state, from sub-meV up to few eV collision energies. The most recent MQDT results are compared with either other theoretical data, or with measured data obtained in storage-ring experiments.

BEAMDB and MOLD—Databases at the Serbian Virtual Observatory for Collisional and Radiative Processes

In this contribution we present a progress report on two atomic and molecular databases, BEAMDB and MolD, which are web services at the Serbian virtual observatory (SerVO) and nodes within the Virtual Atomic and Molecular Data Center (VAMDC). The Belgrade Electron/Atom (Molecule) DataBase (BEAMDB) provides collisional data for electron interactions with atoms and molecules. The Photodissociation (MolD) database contains photo-dissociation cross sections for individual rovibrational states of diatomic molecular ions and rate coefficients for the chemi-ionisation/recombination processes. We also present a progress report on the major upgrade of these databases and plans for the future. As an example of how the data from the BEAMDB may be used, a review of electron scattering from methane is described.

Photodetachment cross section of a negative molecular ion near a nanospherical surface

The photodetachment of a diatomic negative molecular ion (H) near a nanospherical surface is investigated. Analytical results for the flux of detached electron waves and photodetachment cross section are derived. It is demonstrated that the flux of the detached electron waves and photodetachment cross section of H near a nanospherical surface are directly related to the radius of curvature, inter-ion–surface distance, and bond length or separation of the atomic centers. These parameters strongly influence the oscillating structure of the detached electron spectra of the molecular ions. Fine control over these parameters can be used to determine structural information of negative ions, curvature and roughness in the surface at the nanoscale, and dissociation of molecules near surfaces and fields.

BEAMDB and MolD – databases for atomic and molecular collisional and radiative processes: Belgrade nodes of VAMDC

We present two atomic and molecular (A&M) databases, MolD and BEAMDB, hosted by the SerVO – the Serbian virtual observatory ( http://servo.aob.rs ). These databases and web applications have been implemented in accordance to the standards developed by Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu ). The MolD database contains photo-dissociation cross-sections for individual rovibrational states of the diatomic molecular ions and rate coefficients for the atom-Rydberg atom chemi-ionisation and inverse electron-ion-atom chemi-recombination processes. The Belgrade electron/atom(molecule) database (BEAMDB) provides collisional data for electron interactions with atoms and molecules. Differential cross sections (DCS) are presented for both elastic and inelastic (excitation) cross sections in tabulated data tables. These DCS data are integrated over a full range of scattering angles in order to achieve integral, momentum transfer and viscosity cross sections as functions of impact electron energy. Beside these tables, energy loss spectra are presented in the graphical form.

Single-laser-induced quantum interference in photofragmentation reaction of D+2

ABSTRACTWe theoretically study quantum interference phenomena in the photofragmentation reaction of breaking up the diatomic molecular ion D+2 into an atom D and an ionic D+ by a single ultrashort laser pulse. The numerical results show that the occurrence of quantum interference in kinetic energy release spectra is sensitive to the angle between the molecular axis and the laser field polarisation direction. Such sensitivity is shown to be dependent on the initial vibrational quantum state of the system and the pulse intensity. The underlying mechanism for the single-pulse-induced quantum interference is analysed by using a light-induced potential and light-induced conical intersection concept.

Alignment dependent ultrafast electron-nuclear dynamics in molecular high-order harmonic generation.

We investigated the high-order harmonic generation (HHG) process of diatomic molecular ion H2+ in non-Born-Oppenheimer approximations (NBOA). The corresponding three-dimensional time-dependent Schrödinger equation is solved with arbitrary alignment angles. It is found that the nuclear motion can lead to spectral modulation of HHG in both the tunneling and multiphoton ionization regimes. The universal redshifts of the whole spectrum are unique in molecular HHG. The spectral width of HHG increases in NBOA. We calculated possible influences on redshifts of HHG in real experimental conditions and found that redshifts decrease with the increase of alignment angles of the molecules and are sensitive to the initial vibrational states. It can be used to extract the ultrafast electron-nuclear dynamics and image molecular structure. It will be instructive to related experiments.

超伝導で質量分析の限界を越える試み

Superconducting tunnel junction (STJ) detectors for ions and neutral molecules have the ability to overcome the limits of conventional mass spectrometry, utilizing the performance of kinetic energy measurement of individual atoms and molecules in an energy range of 100 eV–100 keV. This unusual performance is realized by a small superconducting energy gap of ～meV, which is lower than the energies of phonons that are the quanta of crystal lattice vibration. The superconducting detectors are sensitive to molecule impact on the surface of superconductors through lattice vibration generation instead of using secondary electron emission that is used in conventional molecule detectors. An output pulse height is determined by the energy deposited to the detector surface, which is proportional to the product of charge number and acceleration voltage (zV [eV]). Consequently, it is possible to simultaneously determine m/z and z values of individual ions in vacuum, which is an advantage over ion mobility mass spectrometry (IMS) in a drift gas or charge detection mass spectrometry (CDMS). The STJ detectors can overcome the two fundamental MS limits of “m/z overlap” in different charge states and “neutral loss” in tandem mass spectrometry. For example, no one has identified doubly-charged homonuclear diatomic molecular ions of 14N22+ before us. In this paper, we report the experimental results obtained by using double-focused-MS, MALDI-TOF-MS, and ESI-TOF-MS equipped with a STJ detector. The superconducting detectors are expected to be employed in space and in electrostatic ion storage rings for planetary science, interstellar chemistry (origin of life), atomic and molecular sciences, and life sciences.

Photodissociation of Trapped Rb_{2}^{+}: Implications for Simultaneous Trapping of Atoms and Molecular Ions.

The direct photodissociation of trapped ^{85}Rb_{2}^{+} (rubidium) molecular ions by the cooling light for the ^{85}Rb magneto-optical trap (MOT) is studied, both experimentally and theoretically. Vibrationally excited Rb_{2}^{+} ions are created by photoionization of Rb_{2} molecules formed photoassociatively in the Rb MOT and are trapped in a modified spherical Paul trap. The decay rate of the trapped Rb_{2}^{+} ion signal in the presence of the MOT cooling light is measured and agreement with our calculated rates for molecular ion photodissociation is observed. The photodissociation mechanism due to the MOT light is expected to be active and therefore universal for all homonuclear diatomic alkali metal molecular ions.

Localization of laser-driven HD+ and fragments

Electron localization during the dissociation of HD+ and is investigated by solving the corresponding time-dependent Schrödinger equation. Two different dissociation channels HD+ + nγH + d or HD+ + nγp + D and + nγH2 + p or + nγ for HD+ and are investigated, respectively. The numerical simulation demonstrates that for HD+ is in a single ultraviolet laser pulse, the electron localization distribution is symmetric for the symmetric distribution of the Coulomb potential wells though HD+ is a heteronuclear diatomic molecular ion. While for 92.6% electrons of all the dissociation events are localized at the potential well with higher energies (H+) for the asymmetric distribution of the Coulomb potential of H3+. When a terahertz pulse is used to steer the electron motion after the excitation of the ultraviolet laser pulse, more than 96.4% electrons of the dissociation state move opposite to the electric field force and stabilize at the one of the nuclei.

Formation probability of metastable molecular hydrogen anions HmDn− (m, n = 0–3 and m + n = 2, 3) in sputtering

The formation of molecular hydrogen anions (H2−, D2−, HD−, H3−, HD2−, and D3−) was investigated. These anion species were produced by sputtering of TiD2 and HfH2 targets with Cs+ ions and were identified by secondary-ion mass spectrometry (SIMS), yielding the corresponding abundance distributions. The intensity ratios of both the diatomic (HD−/D2−) and triatomic (HD2−/D3−) molecular ions are (distinctly) lower than what would be expected from the respective statistical isotopic pattern. This observation indicates that the different lifetimes of these metastable anions are decisive for their formation probability in sputtering and their survival in the mass spectrometer. In addition to molecular ions, the isotopic fractionation of the atomic hydrogen ions, D−/H−, was determined. A strong depletion of the heavier D isotope in the sputtered ion flux by (47.0±3.1)% was found. This pronounced isotope fractionation could be due to a velocity-dependence of the ionization probability, amplified by the large mass difference of the hydrogen isotopes.

Enhanced high-order-harmonic generation and wave mixing via two-color multiphoton excitation of atoms and molecules

We consider harmonics generation and wave-mixing by two-color multi photon resonant excitation of three-level atoms/molecules in strong laser fields. The coherent part of the spectra corresponding to multicolor harmonics generation is investigated. The obtained analytical results on the basis of generalized rotating wave approximation are in a good agreement with numerical calculations. The results applied to the hydrogen atom and homonuclear diatomic molecular ion show that one can achieve efficient generation of moderately high multicolor harmonics via multiphoton resonant excitation by appropriate laser pulses.

MOL-D: A Collisional Database and Web Service within the Virtual Atomic and Molecular Data Center

MOL-D database is a collection of cross-sections and rate coefficients for specific collisional processes and a web service within the Serbian Virtual Observatory (SerVO) and the Virtual Atomic and Molecular Data Center (VAMDC). This database contains photo-dissociation cross-sections for the individual ro-vibrational states of the diatomic molecular ions and rate coefficients for the atom-Rydberg atom chemi-ionization and inverse electron-ion-atom chemi-recombination processes. At the moment it contains data for photodissociation cross-sections of hydrogen H2+ and helium He2+ molecular ions and the corresponding averaged thermal photodissociation cross-sections. The ro-vibrational energy states and the corresponding dipole matrix elements are provided as well. Hydrogen and helium molecular ion data are important for calculation of solar and stellar atmosphere models and for radiative transport, as well as for kinetics of other astrophysical and laboratory plasma (i.e. early Universe).