Multiply Charged Molecular Ions Research Articles

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.

MALDI‐MS/MS of N‐Terminal TMPP‐Acyl Peptides: A Worthwhile Tool to Decipher Protein N‐Termini

Acylation with tris(trimethoxyphenyl)phosphonium (TMPP), a chemical reagent specifically targeting peptide N-termini, was found to provide very valuable sequencing information provided that the derivatized charged peptide was engaged in MALDI MS/MS analyses combining LID and CID activations. In addition to readable N-terminal sequence ion series, fragmentation of aliphatic residues side-chain distinguished isomeric leucine and isoleucine. Unexpected cysteine acylation was also evidenced.

Negative Ion Mode nanoLC-ESI-MS/MS Analyses of Permethylated Sulfated Glycans.

We have developed enabling techniques for sulfoglycomics based on MALDI-MS mapping and MS/MS sequencing of permethylated sulfated glycans. We then extended further the analytical workflow to C18 reverse phase (RP)-nanoLC-nanoESI-MS/MS analyses of permethylated sulfated glycans in the negative ion mode. The advantages are that extra sulfates on permethylated di- and multiply sulfated glycans will survive in nanoESI conditions to allow detection of multiply charged intact molecular ions, and more comprehensive MS/MS can be performed in an automated fashion at higher sensitivity, compared with MALDI-MS/MS. Parallel higher energy collision dissociation (HCD) and ion trap collision induced dissociation (CID)-based MS2, coupled with product-dependent MS3 in data dependent acquisition mode proved to be highly productive when applied to resolve and identify the isomeric sulfated glycan structures. In-house glycomic data mining software, GlyPick, was developed and used to automate the downstream process of identification and relative quantification of target sulfated glycotopes based on summed intensity of their diagnostic MS2 ions extracted from thousands of HCD-MS2 and/or CID-MS2 data.

How does the packing density of a metal screen affect the mechanism for catching highly charged nanoparticles?

Highly charged aerosol nanoparticles of a molecular size (< 5 nm) exhibit complex behaviors caused by random (Brownian) motion and electrostatic interactions (image force) with solid surfaces. The classical single fiber theory, however, can only partially describe the complex filtration mechanism of highly charged nanoparticles. We began this study by employing the Langevin Dynamics (LD) Method to simulate the random motion and deposition of highly charged nanoparticles on a single fiber. The single fiber efficiencies predicted by the LD simulation did not agree with data from experiments using electrosprayed highly charged molecular ions. To explain this mismatch and search for other factors that could potentially affect the capturing mechanism, we investigated the influence of the mesh structure. While the image force exerted a significant influence as the packing density decreased, the influence tended to oppose the theoretical predictions. These results suggested that the fiber-fiber interaction in a wire screen with high packing density significantly influences the collection of multiply charged molecular ions. A new theoretical model describing the influence of inter-fiber interaction will be required to clarify how highly charged molecular ions are captured by image force.

Ambient Ionization and FAIMS Mass Spectrometry for Enhanced Imaging of Multiply Charged Molecular Ions in Biological Tissues.

Ambient ionization mass spectrometry imaging (MSI) has been increasingly used to investigate the molecular distribution of biological tissue samples. Here, we report the integration and optimization of desorption electrospray ionization (DESI) and liquid-microjunction surface sampling probe (LMJ-SSP) with a chip-based high-field asymmetric waveform ion mobility spectrometry (FAIMS) device to image metabolites, lipids, and proteins in biological tissue samples. Optimized FAIMS parameters for specific molecular classes enabled semitargeted detection of multiply charged molecular species at enhanced signal-to-noise ratios (S/N), improved visualization of spatial distributions, and, most importantly, allowed detection of species which were unseen by ambient ionization MSI alone. Under static DESI-FAIMS conditions selected for transmission of doubly charged cardiolipins (CL), for example, detection of 71 different CL species was achieved in rat brain, 23 of which were not observed by DESI alone. Diagnostic CL were imaged in a human thyroid tumor sample with reduced interference of isobaric species. LMJ-SSP-FAIMS enabled detection of 84 multiply charged protein ions in rat brain tissue, 66 of which were exclusive to this approach. Spatial visualization of proteins in substructures of rat brain, and in human ovarian cancerous, necrotic, and normal tissues was achieved. Our results indicate that integration of FAIMS with ambient ionization MS allows improved detection and imaging of selected molecular species. We show that this methodology is valuable in biomedical applications of MSI for detection of multiply charged lipids and proteins from biological tissues.

Molecular Dications in Planetary Atmospheric Escape

Fundamental properties of multiply charged molecular ions, such as energetics, structure, stability, lifetime and fragmentation dynamics, are relevant to understand and model the behavior of gaseous plasmas as well as ionosphere and astrophysical environments. Experimental determinations of the Kinetic Energy Released (KER) for ions originating from dissociations reactions, induced by Coulomb explosion of doubly charged molecular ions (molecular dications) produced by double photoionization of CO2, N2O and C2H2 molecules of interest in planetary atmospheres, are reported. The KER measurement as a function of the ultraviolet (UV) photon energy in the range of 28–65 eV was extracted from the electron-ion-ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques at the ELETTRA Synchrotron Light Laboratory Trieste, Italy. These experiments, coupled with a computational analysis based on a Monte Carlo trajectory simulation, allow assessing the probability of escape for simple ionic species in the upper atmosphere of Mars, Venus and Titan. The measured KER in the case of H+, C+, CH+, CH2+, N+, O+, CO+, N2+ and NO+ fragment ions range between 1.0 and 5.5 eV, being large enough to allow these ionic species to participate in the atmospheric escape from such planets into space. In the case of Mars, we suggest a possible explanation for the observed behavior of the O+ and CO22+ ion density profiles.

Nonsequential and Sequential Fragmentation ofCO23+in Intense Laser Fields

We experimentally studied the three-body fragmentation dynamics of CO(2) initiated by intense femtosecond laser pulses. Sequential and nonsequential fragmentations were precisely separated and identified for CO(2)(3+) to break up into O(+) + C(+) + O(+) ions. With accurate measurements of three-dimensional momentum vectors of the correlated atomic ions and calculations of the high-level ab initio potential of CO(2)(3+), we reconstructed the geometric structure of CO(2)(3+) before fragmentation, which turns out to be very close to that of the neutral CO(2) molecule before laser irradiation. Our study indicated that Coulomb explosion is a promising approach for imaging geometric structures of polyatomic molecules if the fragmentation dynamics can be clearly clarified and the appropriate dissociation potential is provided for multiply charged molecular ions.

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

We investigated the formation and fragmentation of multiply charged molecular ions of several aromatic molecules by intense nonresonant femtosecond laser pulses of 1.4 mum with a 130 fs pulse duration (up to 2 x 10(14) W cm(-2)). Quadruply charged states were produced for 2,3-benzofluorene and triphenylene molecular ion in large abundance, whereas naphthalene and 1,1'-binaphthyl resulted only in up to triply charged molecular ions. The laser wavelength was nonresonant with regard to the electronic transitions of the neutral molecules, and the degree of fragmentation was strongly correlated with the absorption of the singly charged cation radical. Little fragmentation was observed for naphthalene (off-resonant with cation), whereas heavy fragmentation was observed in the case of 1,1'-binaphthyl (resonant with cation). The degree of H(2) (2H) and 2H(2) (4H) elimination from molecular ions increased as the charge states increased in all the molecules examined. A striking difference was found between triply and quadruply charged 2,3-benzofluorene: significant suppression of molecular ions with loss of odd number of hydrogen was observed in the quadruply charged ions. The Coulomb explosion of protons in the quadruply charged state and succeeding fragmentation resulted in the formation of triply charged molecular ions with an odd number of hydrogens. The hydrogen elimination mechanism in the highly charged state is discussed.

The ejection of triatomic molecular hydrogen ions H3+ produced by the interaction of benzene molecules with ultrafast laser pulses

The ejection process of triatomic molecular hydrogen ions produced by the interaction of benzene with ultrafast laser pulses of moderate strong intensity ( approximately 10(14) W/cm(2)) is studied by means of TOF mass spectrometry. The H(3) (+) formation can only take place through the rupture of two C-H bonds and the migration of hydrogen atoms within the molecular structure. The H(3) (+) fragments are released with high kinetic energy (typically 2-8 eV) and at laser intensities >or=10(14) W/cm(2), well above that required for the double ionization of benzene, suggesting that its formation is taking place within multiply charged parent ions. The relative ejection efficiency of H(3) (+) molecular hydrogen ions with respect to the atomic ones is found to be strongly decreasing as a function of the laser intensity and pulse duration (67-25 fs). It is concluded that the H(3) (+) formation is only feasible within parent molecular precursors of relatively low charged states and before significant elongation of their structure takes place, while the higher multiply charged molecular ions preferentially dissociate into H(+) ions. The ejection of H(2) (+) ions is also discussed in comparison to the production of H(3) (+) and H(+) ions. Finally, by recording the mass spectra of two deuterium label isotopes of benzene (1,2-C(6)H(4)D(2), 1,4-C(6)H(4)D(2)) it is verified that the ejection efficiency of some molecular fragments, such as D(2)H(+), DH(+), is dependent on the specific position of hydrogen atoms in the molecular skeleton prior dissociation.

Ionization of Anthracene Followed by Fusion in the Solid Phase under Intense Nonresonant Femtosecond Laser Fields

Fused anthracene cation and a series of anthracene-C n H m cluster-like products were observed on irradiatior with nonresonant femtosecond pulses at 1.4 μm. Ionization of polycrystalline anthracene revealed strikingly different features such as the absence of multiply charged molecular ions, the appearance of H + and C + a low intensity without the formation of C n H m fragment ions, C n H m -attached and detached molecular ions, and multimer cation formation, compared with the gas-phase experiments. The fusion reactions of anthraceno and C n H m fragments indicated the presence of novel reactions among high-density cations in the crystal produced by femtosecond laser irradiation.

Explosive Desorption and Fragmentation of Molecular Ion from Solid Fullerene by Intense Nonresonant Femtosecond Laser Pulses

Desorption of C 60 (+) and its dimer cation was investigated on irradiation with nonresonant femtosecond laser pulses at 1.4 mum. Ionization of solid C 60 revealed strikingly different features, such as the absence of multiply charged molecular ions, the emission of C (+) at low laser intensity, C 2 attachments, delayed ionization, and dimer cation formation, as compared with the gas phase experiments. The large kinetic energy distribution of ions found in this study indicated that the desorption process was mainly driven by an electrostatic mechanism rather than by thermal, photochemical, or volume expansion mechanisms. Singly charged C 60 emission by a Coulomb explosion due to the high density of C 60 (+) is suggested.

High LET highly charged ion-induced ionization and fragmentation of water molecules and clusters

Kinematically complete experiments lead to very substantial progress in the understanding of decay dynamics of multiply charged molecular ions. This paper is devoted to the study of dissociative ionization of water molecules and clusters induced by highly charged ions. We will present first how experimental results determined in the gas phase can provide information about the physical stage of liquid water radiolysis. Second, we will discuss strong bond cleavage selectivity in ion-induced dissociation of deuterium tagged water molecule. Finally, we will present first promising results about water cluster dissociative ionization. Stability, energetics and charge mobility in the charged cluster will be briefly discussed.

Charge separation mass spectrometry study of the chloroform molecule following valence and Cl 2p photoionization

Chloroform (HCCl 3) molecule has been studied using time-of-flight mass spectrometry in the multi-coincidence mode and synchrotron radiation as the source of photons in the range from 34 to 230 eV, which encompasses the valence level and the Cl 2p edge. Photoionization branching ratios have been measured under efficient ion extraction. Photoelectron photoion coincidence techniques (PEPIPICO, and PEPIPIPICO) have been used in the elucidation of the fragmentation mechanisms of multiply charged molecular ions, formed following the absorption of energetic photons. Observations concerning the molecular fragmentation of the HCCl 3 2+ were attributed to the role of the fast dissociation leading to the release of H atom. It is shown that this unstable doubly charged parent molecule, fragments preferentially via the symmetric mechanism m 2+ → m 1 + + m 2 + + m 3 which is mostly driven by Coulomb forces and has the Cl + + Cl + + m 3 as its major outcome. The dissociation mechanisms for the trication HCCl 3 3+ have also been investigated and are seen to dissociate preferentially into singly charged fragments.

Infinite zig-zag and cyclic-tetranuclear isomeric imidazolate-bridged polynuclear copper(II) complexes: Magnetic properties, catalytic activity and electrospray mass and tandem mass spectrometry characterization

Two mononuclear copper(II) complexes 1 and 2 with the unsymmetrical tridentate ligands 2- and 4-[((imidazol-2-ylmethylidene)amino)ethyl]pyridine have been prepared. In alkaline solution, deprotonation of the imidazole moiety in 1 and 2 promotes self-assembly, which yielded two structurally different species. Depending on the binding site in the imidazole ring, a polymeric complex with an infinite zig-zag-chain 3, or a cyclic-tetranuclear complex 4 is formed, as shown by spectroscopic and spectrometric analysis. Herein, structural characterization of these isomeric polynuclear complexes was performed by electrospray mass (ESI-MS) and tandem mass spectrometric experiments (ESI-MS/MS). Each isomer was shown to be stable in methanolic solutions and to display unique mass spectra with characteristic multiply charged molecular and fragment ions, corroborating previous data by EPR measurements. Magnetic data in the solid state fit a typical curve for an one-dimensional infinite regular chain system, with J = −(32.4 ± 1.2) cm −1 and g = 2.03 for 3, and that of a cyclic-tetranuclear structure with J = −(55.5 ± 0.4) cm −1 and g = 2.29 for 4. In the oxidation of 3,5-di- tert-butylcatechol (3,5-DTBC) by molecular oxygen, both complexes were shown to act as efficient catalysts, exhibiting very similar ratios: k cat/ K M = 9.12 × 10 6 mol −1 dm 3 min −1 for 3 and 8.73 × 10 6 mol −1 dm 3 min −1 for 4. These similar ratios indicate that interactions between the metal centres in 3 or 4 and the substrate in solution occur predominantly at the outside of the catalyst framework.

Deformation, nuclear motion and fragmentation of core-excited CO 2 probed by multiple-ion coincidence momentum imaging

The nuclear motion and geometry in core-excited CO 2 are probed using a multiple-ion-coincidence imaging technique. We demonstrate that CO 2 has a linear stable geometry in the C/O 1s −1 core-ionized state and a bent geometry in the C/O 1s −1π* core-excited state. The molecules in the C/O 1s −1π* A 1 and B 1 Renner–Teller states are probed to be bent in the A 1 state and linear in the B 1 state. The O O correlation angle distributions are well reproduced using a Coulomb explosion model which takes account of the zero point bending motion in the ground state, the classical bending motion along the potential energy curve of the core-excited state within the core-hole lifetime and the initial inhomogeneous charge distribution in the multiply charged molecular ion just before the dissociation. When the photon energy is tuned to be higher (lower) energy than the 1s −1π* resonance center, the events for the 1s −1π* A 1 states that result in the low O O correlation angle distribution are suppressed (enhanced).

Optimal Control of Multiphoton Ionization Processes in I2 Molecules with Time-Dependent Polarization Pulses

We have developed a closed-loop pulse shaping system with a spatial light modulator, where even a time-dependent polarization pulse can be generated and controlled. An outline of the developed pulse shaping system is described. We apply the developed pulse shaping system to the active control of multiphoton ionization processes in aligned I2 molecules. We perform two kinds of control experiments. First, we show the ability to selectively produce specific multiply-charged molecular ions. Second, we investigate a correlation between a femtosecond time-dependent polarization pulse and the production efficiency of evenly- or oddly-charged molecular ions. We achieve much better controllability of the correlation with a time-dependent polarization pulse than with a pulse having a fixed ellipticity. The results suggest the existence of an unknown tunnel ionization mechanism which is characteristic of an elliptically polarized pulse. Our experiments point to new directions in optimal control studies with molecular systems.

Cluster ion control by simultaneous irradiations of femtosecond laser and nanosecond laser pulses

Generation of multiply charged ions and molecular ions have been investigated using simultaneous irradiation of high intensity and ultrashort pulse of Ti:sapphire laser and fourth harmonics of Q-switched nanosecond pulse of Nd:YAG laser on carbon targets [Morimoto et al., in: Proceedings of the 13th International Conference on High-Power Particles Beams (BEAMS2000),Vol. PB-89, Nagaoka, 2000, p. 359; Toyoda et al., in: Proceedings of the 8th International Symposium on Gas Flow and Chemical Lasers and High-Power Laser Conference (GCL-HPL2000), Vol. P1.60, 2000, p. 101]. The ion current waveforms have been analyzed by means of time-of-flight (tof) mass measurement. Simultaneous irradiation of high intensity and ultrashort pulse of Ti:sapphire laser and fourth harmonics of Q-switched nanosecond pulse of Nd:YAG laser on carbon targets was found to generate molecular ions of carbon.

Ionic photofragmentation and photoionization of dimethyl ether in the VUV and soft X-ray regions (8.5–80 eV) – absolute oscillator strengths for molecular and dissociative photoionization

The branching ratios for molecular and dissociative photoionization of dimethyl ether (CH 3OCH 3, DME) have been measured in the VUV and soft X-ray regions using dipole (e,e+ion) coincidence spectroscopy (∼1 eV FWHM) at equivalent photon energies from the first ionization threshold up to 80 eV. The absolute partial oscillator strengths (cross-sections) for molecular and dissociative photoionization have been determined from recently published absolute photoabsorption oscillator strength data [R. Feng, G. Cooper, C.E. Brion, Chem. Phys. 260 (2000) 391] together with the photoionization branching ratios and the (multi-dissociative-corrected) photoionization efficiency obtained from time-of-flight mass spectra reported in the present work. No stable multiply charged molecular ion(s) from DME have been found in the present work. However, the fact that the photoionization efficiency has been measured as greater than unity above ∼30 eV indicates the existence of multi-dissociative products from Coulomb explosion of multiply charged ions. Appearance potentials of all ion products from DME are also reported. The presently reported results are compared with the previously published data where possible.

On the Multielectron Dissociative Ionization of Some Cyclic Aromatic Molecules Induced by Strong Laser Fields

The interaction of strong laser fields (4 × 1016 W/cm2) with cyclic aromatic molecules (furan, pyrrole, pyridine, and pyrazine) has been studied at λ = 790 nm. The fragmentation processes of multiply charged transient molecular ions were analyzed by means of a time-of-flight (TOF) spectrometer. Most ion peaks consisted of one broad and one sharp component. From the analysis of the peak profiles the kinetic energies of the molecular and atomic fragment ions have bean measured. On the basis of these energy values and the estimated charge separation distances, it is concluded that, for furan, it is probable that Coulomb explosion takes place in multiple charged parent ions with deformed molecular structure. The doubly charged parent ions of furan and pyrrole fragments through charge separation processes. At higher charged transient parent ions the fragmentation proceeds through direct (instantaneous) process. Moreover, there is evidence that, at least for furan, the ionization process takes place via a direct field ionization mechanism. Pyridine and pyrazine multiple charged ions seem to follow different dissociation routes. The abundance ratios C+/A+ (A = N, O) for the singly charged atoms were found to be equal to the stoichiometric ratio for all the molecules studied, while a divergence has been observed for the C2,3+/A2,3+ (A = N, O) ratios.

Electrospray mass spectrometry of isomeric tetrakis(N-alkylpyridyl)porphyrins and their manganese(III) and iron(III) complexes

Manganese(III) complexes of isomeric tetrakis(N-alkylpyridyl)porphyrins (N- alkyl = N- methyl , M or N- ethyl , E ), MnTM ( E )-2(3,4)- PyP5+, are being developed as superoxide dismutase (SOD) mimics. Simultaneously, techniques for their purification, identification and characterization are being pursued. Electrospray mass spectrometry ( ESMS ) proved to be an excellent method for identification and characterization of this group of water-soluble cationic porphyrins. The multiply charged parent ion is observed for both the metal-free ligands and their corresponding manganese complexes. The other major peaks in the mass spectra result from loss of N-alkyl groups, reduction of the metal center, axial coordination of chloride or hydroxo ion in the case of the Fe porphyrin, loss of metal and deprotonation of pyrrolic nitrogens. As a result of inductive and resonance effects, which stabilize the ortho isomer, almost no loss of N-alkyl groups from the manganese complex or from its parent ligand was observed. The relative intensity of the multiply charged molecular ion MnIIITM -3(4)- PyP5+/5 was 100% in the case of the meta and para isomers. Although manganese porphyrins display a low preference toward axial ligation, favorable electrostatics at the metal center of the ortho isomer gives rise to 100% relative intensity of the species that has chloride axially ligated at the manganese site, MnIIITM ( E )-2- PyPCl4+/4. When the stronger preference of iron porphyrins toward axial ligation combines with the ortho effect, the monohydroxo iron porphyrin FeIIITM -2- PyP ( OH )4+/4 dominates the ESMS of an aqueous acetonitrile solution at pH 7.8.