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

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.

Chemical properties of inner and surficial regions of hydrogen-bonded clusters of biological molecules: ultraviolet photodissociation and water adsorption analyses in the gas phase

The chemical and physical properties of cold, gas-phase hydrogen-bonded clusters of L-alanine (L-Ala), L-trialanine (L-Ala₃), L-tetraalanine (L-Ala₄), and tryptophan (Trp) enantiomers were investigated using tandem mass spectrometry with an electrospray ionization source and cold ion trap. From the ultraviolet (UV) photodissociation spectra at 265-290 nm, the electronic structures of homochiral H⁺(L-Trp)(L-Ala) at 8 K were found to be different from those of heterochiral H⁺(D-Trp)(L-Ala) and protonated Trp. The number of water molecules adsorbed on the surface of gas-phase H⁺(D-Trp)(L-Ala) was larger than that of H⁺(L-Trp)(L-Ala), indicating stronger intermolecular interactions of L-Ala with H⁺(L-Trp) than those with H⁺(D-Trp). The product ion spectrum obtained by 265 nm photoexcitation of H⁺(L-Trp)(L-Ala₃)(H₂O)_n formed via gas-phase water adsorption on H⁺(L-Trp)(L-Ala₃) showed that the evaporation of water molecules was the main photodissociation process. In the case of H⁺(L-Trp)(L-Ala₄)(H₂O)_n, signals of H⁺(L-Ala₄) (H₂O)_n formed via L-Trp evaporation were observed in the product ion spectra, and the cross-section for UV photoinduced L-Trp evaporation became larger as the number of adsorbed water molecules increased. This observation indicates that water molecules were selectively adsorbed on the H⁺(L-Ala₄) side of H⁺(L-Trp)(L-Ala₄) and weakened the intermolecular interactions between L-Trp and H⁺(L-Ala₄) in the hydrogen-bonded cluster.

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.