Ionization Step Research Articles

Strong-field ionization of the triplet ground state of O2

Using strong-field ionization as a probe, we observe highly nonperiodic evolution of the spin-rotation wave packet launched by a nonionizing femtosecond pulse in oxygen. The nonperiodicity is readily apparent only in rotationally cold molecules that are pumped with a weak alignment pulse. We show that this behavior is a consequence of the spin-rotation and the spin-spin couplings in the triplet ground state of the neutral molecule. A model that includes these couplings in the field-free Hamiltonian but in neither the alignment nor the ionization step explains most of the observed dynamics, suggesting that neither process depends explicitly on the electronic spin. We also show that the angle dependence of strong-field ionization can be retrieved from the delay-dependent signal even when coupling to spin complicates the rotational dynamics.

Laser ionisation detection of O(3 P j ) atoms in the VUV; application to photodissociation of O2

Detection of O() atoms using one-photon resonant excitation to the state at nm followed by near-threshold ionisation, i.e. 1 + 1' resonance-enhanced multi-photon ionisation, has been investigated. The aim was to achieve low ion recoil, improved sensitivity, and reliable angular momentum polarisation information, with an as simple as possible laser setup. An efficient 1 + 1' scheme has been found where the VUV light for the first step is generated by difference frequency () VUV generation, and the ionisation step 1' uses around 289 nm. The presented scheme induces 9 m/s recoil of the O ion using a two-dye laser system, and zero recoil should be possible by generating 302 nm radiation with a third dye laser. While this approach is much more sensitive than a previous 1 + 1' scheme using 212.6 nm for the 1' step, we found that the relatively intense radiation of the beam does not saturate the 1' step. To test the ability of this scheme to accurately determine branching ratios, fine structure yields, and angular distributions including polarisation information, it has been applied to O photodissociation around 130 nm with subsequent O() fragment detection.

Pulse Energy and Pulse Duration Effects in the Ionization and Fragmentation of Iodomethane by Ultraintense Hard X Rays

The interaction of intense femtosecond x-ray pulses with molecules sensitively depends on the interplay between multiple photoabsorptions, Auger decay, charge rearrangement, and nuclear motion. Here, we report on a combined experimental and theoretical study of the ionization and fragmentation of iodomethane (CH_{3}I) by ultraintense (∼10^{19} W/cm^{2}) x-ray pulses at 8.3keV, demonstrating how these dynamics depend on the x-ray pulse energy and duration. We show that the timing of multiple ionization steps leading to a particular reaction product and, thus, the product's final kinetic energy, is determined by the pulse duration rather than the pulse energy or intensity. While the overall degree of ionization is mainly defined by the pulse energy, our measurement reveals that the yield of the fragments with the highest charge states is enhanced for short pulse durations, in contrast to earlier observations for atoms and small molecules in the soft x-ray domain. We attribute this effect to a decreased charge transfer efficiency at larger internuclear separations, which are reached during longer pulses.

Model Kinetic Equations for Multiply Ionized Gas Mixtures

Model kinetic equations are proposed for the description of ionized monoatomic gas mixture flows. The mixtures are assumed enough rarefied to be treated as ideal gases after multiple ionization steps. The model equations contain the equilibrium distribution functions for the components of the gas mixtures under consideration like it was done in BGK equations and their well-known generalizations. However, in this paper the new forms of the equilibrium distribution functions are used which correspond to the entropy maximum under the constraints of momentum, total energy, nuclei and electrons (both bound and free) conservation. It is shown that the derived model equations allow us to study the local equilibrium flows of the ionized gases and the transport processes of energy, nuclei and electrons in the non-equilibrium conditions.

High performance functionalized UiO metal organic frameworks for the efficient and selective adsorption of Pb (II) ions in concentrated multi-ion systems

In this work, we report novel amine-functionalized zirconium-based metal organic frameworks (MOFs) for the efficient and selective removal of Pb (II) in concentrated multi-component heavy metal ions systems. Pristine UiO-66 and UiO-67 MOFs were grafted with thiourea and amidinothiourea via a facile one-step post synthetic modification. This is the first report on grafting of amidinothiourea onto MOFs. Successful grafting was confirmed by Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. A mechanism involving dehydration, ionization and coordination steps was proposed for grafting and subsequent Pb (II) binding. The microporous amidinothiourea modified UiO-66 and UiO-67 MOFs demonstrated excellent performance in their high maximum adsorption capacities of 246 and 367 mg.g−1, respectively, as well as their remarkable selectivity for Pb (II) ions in highly concentrated multi-ion solutions. This is, in addition, to their effective removal efficiencies which reached up to more than 95% at a high range of Pb (II) concentrations (25–250 ppm). Furthermore, the structural stability of the MOF crystals was maintained after adsorption and the MOF was completely regenerated for up to four cycles. The developed MOFs can thus be utilized as reliable adsorbents for Pb (II) removal from industrial wastewater effluents.

Quantitative retrieval of the angular dependence of laser-induced electron rescattering in molecules

We demonstrate a time-domain method to quantitatively retrieve the angular dependence of laser-induced electron rescattering from the measured rotational half-revival signals of molecules with a reaction microscope. The method is based on the measured channel-resolved ion yields of single ionization and electron-rescattering-induced double ionization and the in situ measured alignment distribution of molecules. From the measured data for ${\mathrm{CO}}_{2}$, we retrieve the angular dependence of electron rescattering in nonsequential double ionization for both nondissociative and dissociative cases. The results imply that the angular dependence of electron rescattering is affected by the state of the parent ion populated during the first ionization step. The method demonstrated here opens up the possibility to quantitatively retrieve from measured data which molecular orbitals are involved in the interaction of strong laser fields with molecules.

Evaluation of ionic equilibria in mixed-buffer isothermal titration calorimetry and continuously stirred tank reactors

Determination of an equilibrium pH value in complex aqueous solution and deconvolution of this equilibrium to evaluate phenomena related to mixing, dilution, or progress of reaction is increasingly important in areas ranging from water quality to pharmaceutical formulations and manufacturing. Linearization of pH problems by simple algebraic substitution enables equilibria within complex buffered aqueous solutions to be modeled as an eigenvalue problem. This formulation approach makes rigorous determination of equilibrium pH values and reactor dynamics more accessible than with previous calculation methods, even when activity coefficients and non-ideality are considered. This work demonstrates how such calculations can enable detailed modeling of enthalpic changes in an isothermal titration calorimeter. In support of this work, the acid dissociation constants for three furancarboxylic acids (2-furancarboxylic acid, FA; 5-formyl-2-furancarboxylic acid, FFA; and 2,5-furandicarboxylic acid, FDCA), two of them novel, were determined and compared with multi-wavelength ultraviolet–visible spectrophotometry. The thermodynamic pKa values were determined to be 3.1 for FA, 2.2 for FFA, and 2.1 and 3.4 for the first and second ionization steps of FDCA, respectively.

Exploring photoelectron angular distributions emitted from molecular dimers by two delayed intense laser pulses

We describe the results of experiments and simulations performed with the aim of extending photoelectron spectroscopy with intense laser pulses to the case of molecular compounds. Dimer frame photoelectron angular distributions generated by double ionization of N$_2$-N$_2$ and N$_2$-O$_2$ van der Waals dimers with ultrashort, intense laser pulses are measured using four-body coincidence imaging with a reaction microscope. To study the influence of the first-generated molecular ion on the ionization behavior of the remaining neutral molecule we employ a two-pulse sequence comprising of a linearly polarized and a delayed elliptically polarized laser pulse that allows distinguishing the two ionization steps. By analysis of the obtained electron momentum distributions we show that scattering of the photoelectron on the neighbouring molecular potential leads to a deformation and rotation of the photoelectron angular distribution as compared to that measured for an isolated molecule. Based on this result we demonstrate that the electron momentum space in the dimer case can be separated, allowing to extract information about the ionization pathway from the photoelectron angular distributions. Our work, when implemented with variable pulse delay, opens up the possibility of investigating light-induced electronic dynamics in molecular dimers using angularly resolved photoelectron spectroscopy with intense laser pulses.

UV post-ionization laser ablation ionization mass spectrometry for improved nm-depth profiling resolution on Cr/Ni reference standard.

Laser ablation combined with mass spectrometry forms a promising tool for chemical depth profiling of solids. At irradiations near the ablation threshold, high depth resolutions are achieved. However, at these conditions, a large fraction of ablated species is neutral and therefore invisible to the instrument. To compensate for this effect, an additional ionization step can be introduced. Double-pulse laser ablation is frequently used in material sciences to produce shallow craters. We apply double-pulse UV femtosecond (fs) Laser Ablation Ionization Mass Spectrometry to investigate the depth profiling performance. The first pulse energy is set to gentle ablation conditions, whereas the second pulse is applied at a delay and a pulse energy promoting the highest possible ion yield. The experiments were performed on a Cr/Ni multi-layered standard. For a mean ablation rate of ~3 nm/pulse (~72 nJ/pulse), a delay of ~73 ps provided optimal results. By further increasing the energy of the second pulse (5-30% higher with respect to the first pulse) an enhancement of up to 15 times the single pulse intensity was achieved. These conditions resulted in mean depth resolutions of ~37 and ~30 nm for the Cr and Ni layers, respectively. It is demonstrated on the thin-film standard that the double-pulse excitation scheme substantially enhances the chemical depth profiling resolution of LIMS with respect to the single-pulse scheme. The post-ionization allows for extraordinarily low ablation rates and for quantitative and stoichiometric analysis of nm-thick films/coatings.

Enhancing frustrated double ionization with no electronic correlation in triatomic molecules using counter-rotating two-color circular laser fields

We demonstrate significant enhancement of frustrated double ionization (FDI) in the two-electron triatomic molecule D$_{3}^{+}$ when driven by counter-rotating two-color circular (CRTC) laser fields. We employ a three-dimensional semiclassical model that fully accounts for electron and nuclear motion in strong fields. For different pairs of wavelengths, we compute the probabilities of the FDI pathways as a function of the ratio of the two field-strengths. We identify a pathway of frustrated double ionization that is not present in strongly-driven molecules with linear fields. In this pathway the first ionization step is "frustrated" and electronic correlation is essentially absent. This pathway is responsible for enhancing frustrated double ionization with CRTC fields. We also employ a simple model that predicts many of the main features of the probabilities of the FDI pathways as a function of the ratio of the two field-strengths.

Angular dependence of strong field sequential double ionization for neon and acetylene simulated with time-dependent configuration interaction using CIS and CISD-IP.

Strong field ionization is fundamentally important for attosecond spectroscopy and coherence control. However, the modeling beyond the single active electron approximation is still difficult. Time-dependent configuration interaction with singly excited configurations and a complex absorbing potential (TDCIS-CAP), can be used to simulate single and double ionization by intense laser fields. When the monocation does not have degenerate states, TDCIS-CAP starting from a Hartree-Fock calculation of the cation is suitable for simulating the second ionization step. When the monocation has two or more degenerate states, the simulations should treat these degenerate states equivalently. CISD-IP (single and double excitation configuration interaction with ionization) can be used to treat degenerate states of the cation on an equal footing by representing the cation wavefunctions with ionizing single (1 hole) and double (2 holes/1 particle) excitations from the neutral molecule. Since CISD-IP includes single excitations for each of the monocation states, time dependent CISD-IP with a complex absorbing potential (TDCISDIP-CAP) can also be used to simulate ionization to the dications states. In this work, TDCIS-CAP and TDCISDIP-CAP have been used to simulate the angular dependence of ionization of the neon cation and acetylene cation. In both cases, the second electron is ionized predominantly from an orbital perpendicular to the orbital involved in the first ionization. The TDCISDIP-CAP simulations show some features involving interactions between the monocation states that are not seen in the TDCIS-CAP simulations.

Narrow-band pulsed electron source based on near-threshold photoionization of Cs in a magneto-optical trap

The newly developed method of time-of-flight (ToF) momentum microscopy was used to analyse the cold electron emission from a Cs 3D magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (6P3/2 pumped with 852 nm laser and 7S1/2 with 1470 nm) and a tuneable femtosecond Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs-laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field induced ionization of highly-excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. ToF analysis allowed us to disentangle the ionization paths and the dependence of the spatio-temporal distribution of the cold electrons on the polarization of the ionizing laser.

Theoretical calculations on photoelectron angular distribution of sequential two-photon double ionization for Ar atom

With the development of the intense light source, such as free electron lasers, the experiments on the nonlinear process in atomic photo absorption in the XUV and X-ray region became more and more feasible. As one of the simplest possible nonlinear processes, the sequential two-photon double ionization, in which the first photon produces an ion which is subsequently ionized by the second photon, attracts increasing attention of theorists and experimentalists. Study on the angular distributions and angular correlations of the photoelectrons in the sequential two-photon double ionization process are especially attractive, which provides valuable information about the electronic structure of atom or molecule systems and allows the obtaining of additional information about mechanism and pathway of the two-photon double ionization. In this paper, the expression for the photoelectron angular distribution in a sequential two-photon process is given based on the multi-configuration Dirac-Fock method and the density matrix theory. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this code, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically. The cross section, magnetic cross section, alignment of residual ions and the asymmetry parameter of the photoelectron angular distribution, each as a function of photon energy, for the first and the second step of sequential two-photon double ionization of argon are presented. The calculations predict that the alignment has a maximum value and the asymmetry parameter has a minimum value in the region of the cooper minimum. The angular distribution of the first step ionization for Ar atom and the second step ionization for Ar<sup>+</sup> ion are given at 33.94 eV and 55.34 eV photon energy, respectively. In addition, the difference in property between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization is discussed. The present calculated results are compared with other available results, showing that they are in good agreement with each other. The results of this paper will be helpful in studying nonlinear processes in the XUV range.

Revealing the multiorbital and excitation effects in dissociative double ionization of CO molecules with angular streaking measurement

SynopsisWe investigate the sequential double ionization dynamics for the dissociative channel C+ + O+ of CO molecules in strong circularly polarized laser fields. By using four-particle coincident measurement, the obtained ionic angular emissions and molecular frame photoelectron angular distributions of each ionization steps for various pathways reveals the significant effects of multiorbital ionization and laser-driven excitation.

Semiclassical two-step model with quantum input: Quantum-classical approach to strong-field ionization

We present a mixed quantum-classical approach to strong-field ionization - a semiclassical two-step model with quantum input. In this model the initial conditions for classical trajectories that simulate electron wave packet after ionization are determined by the exact quantum dynamics. As a result, the model allows to overcome deficiencies of standard semiclassical approaches in describing the ionization step. The comparison with the exact numerical solution of the time-dependent Schr\"{o}dinger equation shows that for ionization of a one-dimensional atom the model yields quantitative agreement with the quantum result. This applies both to the width of the photoelectron momentum distribution and the interference structure.

Characterization of liquid–liquid extraction fractions from lignocellulosic biomass by high performance liquid chromatography hyphenated to tandem high-resolution mass spectrometry

The conversion of lignocellulosic biomass is a major challenge in the field of renewable energies and bio-based chemicals. The diversity of biomasses and processes leads to complex products having a wide range of polarities and molecular weights. Nowadays, the molecular description of these oxygenated matrices is still largely incomplete and new analytical strategies are required to have a better understanding of biomass products properties. The present study proposes a reliable protocol based on successive liquid–liquid extractions prior to high performance liquid chromatography hyphenated to high-resolution tandem mass spectrometry (HPLC/MSn) using a linear ion trap-Fourier transform ion cyclotron resonance mass spectrometer (LTQ/FT-ICR). The protocol allowed to fractionate an industrial sample coming from the sulfuric acid-based pretreatment of a wheat straw into four key chemical families: carbohydrates, organic acids, phenols and neutral compounds. Each fraction was separately analyzed, which limited matrix effects during mass spectrometry ionization step. Electrospray and atmospheric pressure chemical ionization sources were used in both positive and negative modes in order to ionize and detect a maximum of compounds. Thanks to HPLC/MSn, structures of heavy lignin-carbohydrate complexes (LCC) were elucidated (up to 600 g/mol) as well as carbohydrate oligomers having acid functionalities. Mono, di, tri and tetra-aromatic compounds coming from lignin were also detected. The results reported in this paper demonstrate the complexity of pretreated biomass samples and propose an analytical approach from sample simplification to data treatment in order to describe the biomass composition.

Strong-field photoelectron holography beyond the electric dipole approximation: A semiclassical analysis

Strong-field photoelectron holography denotes the interference of various electron paths in laser-induced ionization of atoms, leading to interference patterns in the final momentum distribution. For a quantitative description of holography beyond the electric dipole approximation and in the presence of the Coulomb potential, we develop a semiclassical model in which the initial conditions of outgoing electrons are set according to the beyond-dipole strong-field approximation for the ionization step. The phases associated with trajectories are evaluated following the prescription for semiclassical propagators. The validity of the method is confirmed by comparison to the numerical solution of the time-dependent Schr\"odinger equation in two spatial dimensions. The semiclassical model reproduces correctly the backward and forward shifts of the photoelectron momenta along the laser propagation axis that arise from beyond-dipole dynamics. The position of the central holographic interference fringe can be estimated already from a simplified Coulomb-free interference model that provides closed-form expressions for the beyond-dipole shifts. In three dimensions, Coulomb focusing causes a breakdown of the semiclassical model for final momenta with directions close to the polarization axis. We implement a beyond-dipole regularization procedure based on the concept of glory scattering, which was recently used to describe Coulomb focusing in the dipole approximation. While the position of the central maximum and higher-order fringes can already be obtained approximately by simpler semiclassical modeling, this glory model is able to predict the shape of the distribution in the close vicinity of the central maximum. The violation of the dipole approximation in holography should be observable with mid-infrared fields, for which the forward/backward shifts can be comparable with the fringe spacing.

Two-photon sequential double ionization of argon in the region of Rydberg autoionizing states of Ar+

A theoretical study of two-photon sequential double ionization of Ar is presented for the photon energies in the region of Rydberg autoionizing states 3p4 (1D) nl of the ion Ar+ overlapping with the particle-hole autoionizing states 3s3p6np of neutral Ar. The atomic and ionic autoionizing states lead to sharp variations of the angular correlation function between the two outgoing electrons, as well as in the angular distributions of the first and the second emitted electrons. A strong influence of the second step ionization on the first ionization step is demonstrated.

Nebulization Prior to Isolation, Ionization, and Dissociation of the Neutral Serine Octamer Allows Its Characterization

AbstractNebulization of enantiomerically mixed serine solutions, followed by ion deflection and gentle on‐line protonation, is used to isolate and characterize the strongly preferred homochiral neutral serine octamer (Ser8). The separation of the nebulization and ionization steps and the use of the enantiomeric l‐serine ion (l‐d3Ser)8H+ as a soft protonating agent ensured that neutral (d‐Ser)8 was characterized without interference from serine octamer ((d‐Ser)8H+) ions. MS/MS experiments showed that the ionized neutral octamer and the protonated octamer formed directly by ESI have the same structures. Experiments in which the temperature of the heated transfer tube was varied indicate dissociation of the neutral octamer to the dimer. Field‐free nebulization of racemic serine forms the separate homochiral species with very high chiral selectivity. We suggest that the homochiral preference is a property of (d‐Ser)8 and that the preference observed earlier for (d‐Ser)8H+ is simply an expression of this feature of the neutral cluster.

Nebulization Prior to Isolation, Ionization, and Dissociation of the Neutral Serine Octamer Allows Its Characterization.

Nebulization of enantiomerically mixed serine solutions, followed by ion deflection and gentle on-line protonation, is used to isolate and characterize the strongly preferred homochiral neutral serine octamer (Ser8 ). The separation of the nebulization and ionization steps and the use of the enantiomeric l-serine ion (l-d3 Ser)8 H+ as a soft protonating agent ensured that neutral (d-Ser)8 was characterized without interference from serine octamer ((d-Ser)8 H+ ) ions. MS/MS experiments showed that the ionized neutral octamer and the protonated octamer formed directly by ESI have the same structures. Experiments in which the temperature of the heated transfer tube was varied indicate dissociation of the neutral octamer to the dimer. Field-free nebulization of racemic serine forms the separate homochiral species with very high chiral selectivity. We suggest that the homochiral preference is a property of (d-Ser)8 and that the preference observed earlier for (d-Ser)8 H+ is simply an expression of this feature of the neutral cluster.