Formation and Fragmentation of Quadruply Charged Molecular Ions by Intense Femtosecond Laser Pulses

Charge State Influence on Stability and Isomerism in Dehydrogenated PAHs: Insights from Anthracene, Acridine, and Phenazine

In this study, we systematically explored the stability and isomerism of neutral and dehydrogenated polycyclic aromatic hydrocarbons (PAHs) in various charge states, focusing on anthracene, acridine, and phenazine. Our findings highlight key aspects that deepen the understanding of these molecules' reactivity and stability, relevant in both laboratory and astrophysical contexts. Structural symmetry and the presence of nitrogen atoms significantly impact PAH stability and reactivity. The optimal site for the first dehydrogenation varies with charge state, with notable differences in stability observed across different positions and charge states. For the loss of two hydrogens, there is a clear competition between low and high spin states, influenced by the positions of the hydrogens lost. Infrared spectral analysis reveals characteristic frequencies of conjugated Csp2-Csp2 bonds and variations across different charge states. The elimination of H2 typically occurs at adjacent carbons, forming bonds similar to triple bonds. Reaction networks for anthracene, acridine, and phenazine indicate preferred pathways for hydrogen loss, driven by the need to minimize charge repulsion and maintain aromaticity. Adjacent hydrogen loss is predominant in neutral and singly charged states, shifting to non-adjacent loss in higher charge states.

Ultrafast dynamics of fluorene initiated by highly intense laser fields

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

On the thermodynamic stability of polycations

We present a simple approximation to estimate the largest charge that a given molecule can hold until fragmentation into smaller charged species becomes more energetically favorable. This approximation solely relies on the ionization potentials, electron affinities of the parent and fragment species, and also on the neutral parent's dissociation energy. By parameterizing these quantities, it is possible to obtain analytical phase diagrams of polycationic stability. We demonstrate the applicability of this approach by discussing the maximal charge dependence on the size of the molecular system. A numerical demonstration for linear polyenes, monocyclic annulenes, and helium clusters is provided.

Charge-Dependent Metastable Dissociations of Multiply Charged Decafluorobiphenyl Formed by Femtosecond Laser Pulses

Femtosecond laser ionization is a unique means to produce multiply charged organic molecules in the gas phase. The charge-dependent chemical reactions of such electron-deficient molecules are interesting from both fundamental and applied scientific perspectives. We have reported the production of quadruply charged perfluoroaromatics; however, they were so stable that we cannot obtain information about their chemical reactions. In general, it might be difficult to realize the conflicting objectives of observing multiply charged molecular ion themselves and their metastable dissociations. In this study, we report the first example showing metastable dissociations of several charge states within the measurable time range of a time-of-flight mass spectrometer. Metastable dissociations were analyzed by selecting a precursor ion with a Bradbury-Nielsen ion gate followed by time-of-flight analysis using a reflectron. We obtained qualitative information that triply and quadruply charged decafluorobiphenyl survived at least in the acceleration region but completely decomposed before entering a reflectron. In contrast, three dissociation channels for singly and one for doubly charged molecular ions were discriminated by a reflectron and determined with the help of ion trajectory simulations.

Smallest Organic Tetracation in the Gas Phase: Stability of Multiply Charged Diiodoacetylene Produced in Intense Femtosecond Laser Fields

Coulomb explosion imaging, which is the reconstruction of a molecular structure by measuring the three-dimensional momenta of atomic ions formed by a Coulomb explosion of multiply charged molecular cations (MMCs), has been utilized widely. In contrast, intact MMCs, whose properties and reactions are interesting from both fundamental and applied scientific perspectives, themselves have been little explored to date. This study demonstrates that the four-atom molecule diiodoacetylene (DIA) can survive as a long-lived species in the gas phase after the removal of four electrons in intense femtosecond laser fields. The electron configurations of the equilibrium structures of the electronic ground states calculated by the complete active space self-consistent field (CASSCF) method reveal the stability of multiply charged DIA. The dissociation energies are estimated to be 3.01, 3.59, 2.57, 1.82, and 1.61 eV for neutral, cation radical, dication, trication radical, and tetracation, respectively. A fairly deep potential well suggests that a DIA tetracation is metastable toward dissociation, whereas the repulsive potential of a pentacation radical confirms its absence in the mass spectrum. With their sufficiently long lifetimes, minimum number of atoms, and simple dissociation paths, DIA MMCs are promising candidates for further experimental and theoretical investigations of multiply charged ion chemistry.

Charge Transfer and Metastable Ion Dissociation of Multiply Charged Molecular Cations Observed by Using Reflectron Time-of-Flight Mass Spectrometry

A multiply charged molecule expands the range of a mass window and is utilized as a precursor to provide rich sequence coverage; however, reflectron time-of-flight mass spectrometer has not been well applied to the product ion analysis of multiply charged precursor ions. Here, we demonstrate that the range of the mass-to-charge ratio of measurable product ions is limited in the cases of multiply charged precursor ions. We choose C₆ F₆ as a model molecule to investigate the reactions of multiply charged molecular cations formed in intense femtosecond laser fields. Measurements of the time-of-flight spectrum of C₆ F₆ by changing the potential applied to the reflectron, combined with simulation of the ion trajectory, can identify the species detected behind the reflectron as the neutral species and/or ions formed by the collisional charge transfer. Moreover, the metastable ion dissociations of doubly and triply charged C₆ F₆ are identified. The detection of product ions in this manner can diminish interference by the precursor ion. Moreover, it does not need precursor ion separation before product ion analysis. These advantages would expand the capability of mass spectrometry to obtain information about metastable ion dissociation of multiply charged species.

Selection of a Single Isotope of Multiply Charged Xenon (A Xez+ , A=128-136, z=1-6) by Using a Bradbury-Nielsen Ion Gate

The inclusion of an ion gate in a tandem mass spectrometer allows a specific precursor ion to be selected, and the fragment ions are then used for structure analysis and to investigate chemical reactions. However, the performance of an ion gate has been judged simply by whether or not the target ion was selected. In this study, we designed, manufactured, constructed, and characterized a Bradbury-Nielsen ion gate (BNG). The actual ion selection ability, i.e. the gate function, of the BNG was measured for isotopes of Xe^z+ (z=1-6). The gate function of the BNG was 36.5±0.5 ns in width and 3-13 ns in rise and fall times. The BNG provides a simple way to select multiply charged molecular cations of small organic molecules as well as large molecules such as proteins and peptides.