Generation Of Multiply Charged Ions Research Articles

Study of atoms and multiply charged ions features in the nanosecond laser produced Mo plasma in vacuum using optical emission spectroscopy and time-of-flight electrostatic energy analyzer

The species including atoms and multiply charged ions in the laser produced molybdenum (Mo) plasma are investigated in this work using optical emission spectroscopy and time-of-flight electrostatic energy analyzer (TOF-EEA). Nanosecond laser (5 ns, 1064 nm,) pulses were focused on the Mo target surface with a spot size of 0.4 mm2, energy of ∼150 mJ/pulse (corresponding to a power density of ∼7.5 GW cm−2) to generate the Mo plasma in vacuum environment. Time-resolved spectral analysis was carried out to investigate the temporal evolution of continuous background, atomic, and monovalent ionic spectral signals. The Saha–Boltzmann method is applied for spectral fitting, providing insight into the temporal evolution of electron temperature (T e) and electron density (n e). Over the time from 40 ns to 500 ns, the T e decreases from 3.6 eV to 0.52 eV, and the n e decreases from 2.5 × 1020 cm−3–1.0 × 1015 cm−3. Linear fitting extrapolation predicts the T e and n e could be even up to 6.3 eV and 2.5 × 1022 cm−3, respectively, at the early stage of 10 ns. This indicates the generation of multiply charged ions during the laser ablation process. The multiply charged ions up to 6 charge states were observed by the TOF-EEA and the energy distributions for the different charged ions were also obtained. It was found the ion kinetic energy is positively related to the number of charge state indicates the existence of acceleration electric field. The equivalent accelerating potential is determined as approximately 570 V at the current laser power density. This research provides a significant reference for the establishment of models for laser ablation plasmas and a profound understanding of the underlying physical processes.

Generation of multiply charged ions in accordance with geometry and magnetic field in Hall thruster plasmas

The production of multiply charged ions in Hall effect plasmas has been investigated. The factors that significantly affect plasma characteristics, such as geometry, magnetic field configuration, and magnetic mirror ratio, were analyzed. Among the co-current and counter-current configurations of a cylindrical Hall thruster (CHT) and an annular Hall thruster (AHT), the co-current configuration of the CHT shows the largest fractions of multiply charged ions. In particular, for the CHT, the region with high magnetic flux density had a significant influence on the fraction of multiply charged ions, but the fraction of multiply charged ions was not significantly different depending on magnetic mirror ratios. In addition, the configuration having a similar magnetic field in the CHT demonstrated comparable fractions of multiply charged ions and thrust even with different channel lengths. Hence, the broad region with the high magnetic flux density could enhance the probability of further ionization inside the discharge channel.

Generation of multiply charged ions from homogeneous MALDI microcrystals

The homogeneity of matrix assisted laser desorption/ionization (MALDI) microcrystals, laser fluence, and the proton affinity (PA) of matrices determine the generation of multiply charged ions. Here we adopt three structurally similar matrices, i.e. α-cyano-4-hydroxycinnamic acid (CHCA), 4-chloro-α-cyanocinnamic acid (Cl-CCA), and (E)-propyl α-cyano-4-hydroxyl cinnamylate (CHCA-C3) to examine the homogeneity of microcrystals by measuring the relative standard deviation (RSD) values of the matrices with the homogeneous sample preparation method, e.g. forced dried droplet (FDD). The average RSD values of CHCA, Cl-CCA, and CHCA-C3 matrices are ∼15%, ∼10%, and ∼20%, respectively. With the same CHCA matrix, we found the charge state of myoglobin ions can obtain more charges (+6) with the FDD method as compared to +5 charges obtained by the inhomogeneous sample preparation method, i.e. dried droplet (DD). The laser fluence also affects the generation of multiply charged ions. For instance, the maximum charge states of myoglobin and cytochrome c ions are +6 with CHCA and +4 with CHCA-C3 at threshold laser fluence and decreases at elevated laser fluence. The charge state of myoglobin ions with Cl-CCA matrix is +3 at threshold laser fluence and increases to the maximum charge state of +6 and eventually reduces to +5 with elevated laser fluence. Moreover, we found low PA value of matrix results in high charge state of analyte ions and vice versa. The matrix PA modulates the proton transfer reaction, leading to the formation of multiply charged ions. The homogeneous samples help control the charge states of ions and generate the multiply charged protein ions which can be used as potential mass calibration standards to search suitable protein samples at high m/z ratios with a MALDI time-of-flight mass spectrometer.

Atmospheric Pressure Ultraviolet Laser Desorption and Ionization from Liquid Samples for Native Mass Spectrometry.

Understanding protein structure is vital for evaluating protein interactions with drugs, proteins, and other ligands. Native mass spectrometry (MS) is proving to be invaluable for this purpose, enabling analysis of “native-like” samples that mimic physiological conditions. Native MS is usually performed by electrospray ionization (ESI) with its soft ionization processes and the generation of multiply charged ions proving favorable for conformation retention and high mass analysis, respectively. There is scope to expand the currently available toolset, specifically to other soft ionization techniques such as soft laser desorption, for applications in areas like high-throughput screening and MS imaging. In this Letter, observations made from native MS experiments using an ultraviolet (UV) laser-based ion source operating at atmospheric pressure are described. The ion source is capable of producing predominately multiply charged ions similar to ESI. Proteins and protein complexes were analyzed from a native-like sample droplet to investigate the technique. Ion mobility-mass spectrometry (IM-MS) measurements showed that folded protein conformations were detected for ions with low charge states. This observation indicates the source is suitable for native MS analysis and should be further developed for higher mass analysis in the future.

Gas phase ionisation of carbon disulfide clusters at terawatt laser intensity: Generation of singly and multiply charged atomic and molecular ions

Gas phase ionisation of carbon disulphide clusters (CS2)n with different laser wavelengths of terawatt intensity has been studied using time-of-flight mass spectrometry. In the mass spectra, series of singly charged cluster fragments such as C+, S+, CS+, [C(CS)]+, [S(CS)]+, [S2(CS)]+, [C(CS2)]+, [C2(CS2)]+, [C3(CS2)]+, [S(CS2)]+, S2+, S3+, [CS(CS2)]+ etc. were observed at 266 nm photoionisation. Higher cluster fragments formation has been rationalised based on various ion-molecule reactions that occur within the cluster following ionisation. One of the prominent cluster fragment was S2+ whose formation mechanism has been explained based on two competitive ion-molecule reactions. Photoionisation at 355 nm leads to formation of doubly charged carbon and iodine ion along with singly charged cluster fragments. An increase in charge state of carbon and sulphur ions (up to +4 state) was observed on changing the laser wavelength to 532 nm, indicating efficient interaction of CS2 clusters at longer wavelength. In order to elucidate the mechanism of formation of multiply charged atomic ions at 355 and 532 nm, energy of electrons released on ionisation of CS2 cluster at different wavelengths was measured. Based on these experimental results, energisation of electrons within the cluster was observed due to influence of electric field of the laser pulse. The extent of energisation was found to be more at longer wavelength indicating role of electron impact ionisation for generation of multiply charged ions at this laser intensity conditions. Apart from multiply charged atomic ions, doubly charged molecular ion (CS22+) was also observed at 532 nm. Generation of CS22+ is explained based on similar ionisation mechanism as described for atomic multiply charged ions. However, lifetime of the doubly charged ion plays an important role for generation of the ions under our experimental conditions.

Matrix-assisted ionization vacuum for protein detection, fragmentation and PTM analysis on a high resolution linear ion trap-orbitrap platform

Matrix-assisted ionization vacuum (MAIV) is a novel ionization technique that generates multiply charged ions in vacuum without the use of laser ablation or high voltage. MAIV can be achieved in intermediate-vacuum and high-vacuum matrix-assisted laser desorption/ionization (MALDI) sources and electrospray ionization (ESI) sources without instrument modification. Herein, we adapt MAIV onto the MALDI-LTQ-Orbitrap XL platform for biomolecule analysis. As an attractive alternative to MALDI for in solution and in situ analysis of biomolecules, MAIV coupling to high resolution and accurate mass (HRAM) MS instrument has successfully expanded the mass detection range and improved the fragmentation efficiency due to the generation of multiply charged ions. Additionally, the softness of MAIV enables potential application in labile post-translational modification (PTM) analysis. In this study, proteins as large as 18.7 kDa were detected with up to 18 charges; intact peptides with labile PTM were well preserved during the ionization process and characterized MS/MS; peptides and proteins in complex tissue samples were detected and identified both in liquid extracts and in situ. Moreover, we demonstrated that this method facilitates MS/MS analysis with improved fragmentation efficiency compared to MALDI-MS/MS.

Investigation and optimization of parameters affecting the multiply charged ion yield in AP-MALDI MS

Liquid matrix-assisted laser desorption/ionization (MALDI) allows the generation of predominantly multiply charged ions in atmospheric pressure (AP) MALDI ion sources for mass spectrometry (MS) analysis. The charge state distribution of the generated ions and the efficiency of the ion source in generating such ions crucially depend on the desolvation regime of the MALDI plume after desorption in the AP-to-vacuum inlet. Both high temperature and a flow regime with increased residence time of the desorbed plume in the desolvation region promote the generation of multiply charged ions. Without such measures the application of an electric ion extraction field significantly increases the ion signal intensity of singly charged species while the detection of multiply charged species is less dependent on the extraction field. In general, optimization of high temperature application facilitates the predominant formation and detection of multiply charged compared to singly charged ion species. In this study an experimental set-up and optimization strategy is described for liquid AP-MALDI MS which improves the ionization efficiency of selected ion species up to 14 times. In combination with ion mobility separation, the method allows the detection of multiply charged peptide and protein ions for analyte solution concentrations as low as 2fmol/μL (0.5μL, i.e. 1fmol, deposited on the target) with very low sample consumption in the low nL-range.

Numerical simulation and experimental investigation of the production of multiply charged ions by the ionization of benzene cluster with a moderate intensity laser

The production of multiply charged ions by the interaction of intense femtosecond laser with clusters has been widely reported. Recently, many groups discovered the multiply charged ions when the cluster was irradiated by a 532 nm nanosecond laser with the intensity as low as 1010 W/cm2. Although this interesting phenomenon could be explained by the mechanism of multiphoton ionization triggered-inverse bremsstrahlung heating-electron impact ionization, there is a lack of numerical simulation to explain the generation of multiply charged ions. In this paper, numerical simulation is performed to study the generation process of multiply charged ions in the moderate intensity laser. Firstly, the electron energy is calculated according to ponderomotive potential. Secondly, the cross section of electron impact ionization is calculated on the basis of Lotz formula. Finally, the evolution of multiply charged ions in the cluster is calculated with the kinetic reaction rate equation. The effects of cluster size and electron density on multiply charged ions are investigated in detail. Simulation results show that the ionization process is completed and the balance among C2+, C3+ and C4+ is achieved in 0.7 ns. The relative intensity sequence of multiply charged ions is C2+ C3+ C4+, which is consistent with the experimental results. In addition, numerical simulation results show that the charge state of ions is increased with the increase of cluster size, which is consistent with the experimental results.

Optimization in Detecting Multiply-charged Protein Ions using MALDI TOF-MS

The effects of trifluoroacetic acid (TFA) were evaluated on the generation of multiply charged ions of cytochrome c in a 2-nitrophloroglucinol (2-NPG) matrix in high-vacuum, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The presence of 1% TFA in the 2-NPG matrix solution was more effective in generating multiply charged protein ions than matrix solutions containing 0.1% or 0% TFA. Regarding the matrix itself, with 1% TFA, 2-NPG was significantly more effective in generating multiply charged ions than 2,5-dihydroxybenzoic acid (2,5-DHB). The maximum charge state of cytochrome c was +8 when using a 2-NPG matrix containing 1% TFA.

Multiple ionization of diethyl ether cluster by nanosecond laser: generation of multiply charged ions and the electron energy distribution

The interaction of diethyl ether clusters and laser was studied at laser intensity of 1011 W/cm2 using 5 ns, 1064 nm laser pulses. Multiply charged ions O6+ and C4+ were observed and the ratio of O6+/C4+ nearly kept constant in the range of laser intensity studied. The energy distribution of electrons ejected was measured via retarding potential method. At the laser intensity of 4.0×1011 W/cm2, the mean electron energy of 56eV and the maximum electron energy of 102eV were observed. It indicates that the multiply charged ions are mainly produced from energetic electron impact ionization. In addition, our calculated results show that the screening effect inside cluster can significantly decrease the ionization potentials of multiply charged ions, thus facilitating the ionization especially at low laser intensity. The above results further support our three stage model of “multiphoton ionization ignition-inverse bremsstrahlung heating-electron impact ionization”.

Electrospray Ionization Mass Spectrometry of Oligonucleotides

Because of the high molecular weights and thermal lability of biomolecules such as nucleic acids and protein, they can be difficult to analyze by mass spectrometry. Such analyses require a "soft" ionization method that is capable of generating intact molecular ions. In addition, most mass analyzers have a limited upper mass range that is not sufficient for studying these large molecules. ESI-MS can be used to analyze molecules with a molecular weight that is larger than the mass-to-charge ratio limit of the analyzer. This unit describes how ESI allows for analysis of high-molecular-weight compounds through the generation of multiply charged ions in the gas phase. It discusses analyzer configurations and solvent selection, and gives protocols for sample preparation. For applications of ESI-MS, the unit discusses molecular weight determination, sequencing, and analysis of oligonucleotide mixtures by LC-MS.

Development and Characterization of an Ionization Technique for Analysis of Biological Macromolecules: Liquid Matrix-Assisted Laser Desorption Electrospray Ionization

We have developed an atmospheric pressure ionization technique called liquid matrix-assisted laser desorption electrospray ionization (liq-MALDESI) for the generation of multiply charged ions by laser desorption from liquid samples deposited onto a stainless steel sample target biased at a high potential. This variant of our previously reported MALDESI source does not utilize an ESI emitter to postionize neutrals. Conversely, we report desorption and ionization from a macroscopic charged droplet. We demonstrate high mass resolving power single-acquisition FT-ICR-MS analysis of peptides and proteins ranging from 1 to 8.6 kDa at atmospheric pressure. The liquid sample acts as a macroscopic charged droplet similar to those generated by electrospray ionization, whereby laser irradiation desorbs analyte from organic matrix containing charged droplets generating multiply charged ions. We have observed a singly charged radical cation of an electrochemically active species indicating oxidation occurs for analytes and therefore water; the latter would play a key role in the mechanism of ionization. Moreover, we demonstrate an increase in ion abundance and a concurrent decrease in surface tension with an increase in the applied potential.

Cluster-assisted generation of multiply charged ions in nanosecond laser ionization of seeded furan beam at 532 and 1064 nm

The multiply charged ions of C q + (q ≤ 4), O q + (q ≤ 6) were produced when a furan cluster beam interacted with nanosecond 1064 and 532 nm lasers at intensities of 1010–1012 W/cm2. It is shown that O6+ and C4+ ions were the dominant multiply charged species at 1064 nm, while C2+, C3+, O2+, O3+ ions were the main multiply charged species at 532 nm. By varying the electric field in the extraction region of the time of flight mass spectrometer (TOFMS), two types of ions were extracted, one of which had large kinetic energy and narrow space distribution, and the other had small kinetic energy and broad space distribution. The formation channels for He-like ions of C4+ and O6+ are discussed.

Generation of multiply charged atomic ions of halogens using second harmonic of nanosecond Nd:YAG laser

Abstract Using the second harmonic output (532 nm) of a commercially available pulsed Nd:YAG laser interacting with pure and mixed clusters of iodo compounds, multiply charged ions of halogen group (F, Cl, Br and I) atoms have been generated based on the phenomenon of low intensity Coulomb explosion. This method provides a useful rugged tool for generation of multiply charged ions which can then be used for various applications. The effect of boiling point (vapour pressure) of the compound forming clusters has been correlated with the highest charged state of ion observed in time of flight mass spectrometer.

High power impulse magnetron sputtering: Current-voltage-time characteristics indicate the onset of sustained self-sputtering

The commonly used current-voltage characteristics are found inadequate for describing the pulsed nature of the high power impulse magnetron sputtering (HIPIMS) discharge; rather, the description needs to be expanded to current-voltage-time characteristics for each initial gas pressure. Using different target materials (Cu, Ti, Nb, C, W, Al, and Cr) and a pulsed constant-voltage supply, it is shown that the HIPIMS discharges typically exhibit an initial pressure dependent current peak followed by a second phase that is power and material dependent. This suggests that the initial phase of a HIPIMS discharge pulse is dominated by gas ions, whereas the later phase has a strong contribution from self-sputtering. For some materials, the discharge switches into a mode of sustained self-sputtering. The very large differences between materials cannot be ascribed to the different sputter yields but they indicate that generation and trapping of secondary electrons play a major role for current-voltage-time characteristics. In particular, it is argued that the sustained self-sputtering phase is associated with the generation of multiply charged ions because only they can cause potential emission of secondary electrons, whereas the yield caused by singly charged metal ions is negligibly small.

Formation of a Micropinch and Generation of Multiply Charged Ions at the Front of a Current-Carrying Plasma Jet

The formation of a neck in the cathode plasma jet in the initial stage of a low-voltage vacuum spark is investigated experimentally and theoretically. X-ray bursts corresponding to an electron temperature of 150–300 eV are detected. With the use of a pinhole camera, it is found that an emitting region less than 1 mm in size is located near the cathode. The free expansion of a current-carrying cathode plasma jet with a current growing in accordance with the experimentally observed time dependence is simulated using a hydrodynamic model. It is shown that the neck forms at the front of the plasma jet due to the plasma compression by the magnetic self-field. In the constriction region, the plasma is rapidly heated and multiply charged ions are generated. The calculated spatial and temporal variations in the electron temperature and average ion charge are close to the measured dependences over a wide range of the discharge parameters.

Multiphoton ionization of xenon in the vuv regime

In a recent experiment at the VUV free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several VUV photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual VUV laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the VUV multiphoton ionization of xenon ions.

Generation of multiply charged ions at low and high laser-power densities

The PERUN and PALS iodine laser systems and a Nd : YAG laser have been used for studies of ions emitted from various single element laser plasmas, at both low (IL∼109 W cm−2) and high (IL∼1016 W cm−2) laser intensities. Sufficiently high current densities of low-charge ions, with energies only of hundreds of electronvolt, were produced for starting tests of a considered new hybrid ion source, based on coupling of laser ion source to electron cyclotron resonance plasma. Highly charged ions with charge states about 50+ and with energies even higher than 20 MeV were generated at the highest laser intensities (IL>2×1014 W cm−2). Seven ion sub-groups at least (faster and slower) were recorded and distinguished at these conditions. The existence of an `optimum focus position' in front of the target surface is consistent with the idea that laser interaction with a prepulse-produced plasma improves the conditions for self-focusing. The experimental arrangements and ion diagnostics used for the various laser ion generation experiments are briefly reviewed, as well as our both older and more recent results on the characteristics of the ions, produced at the various laser irradiances, are presented. The results of laser ion implantation experiments using these ions directly (without additional electrostatic acceleration) are also mentioned.

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.

Generation of multiply charged ions from a cathode jet of a low-energy vacuum spark

Ion parameters in a cathode plasma jet of a vacuum spark with a discharge energy of a few joules were measured. The studies were performed by a time-of-flight method with the help of an energy analyser. It was found that the discharge plasma generated a short-run beam of multiply charged ions of the cathode material at the initial stage of the discharge firing. The ions had charge states from Z = + 1 up to Z = + 18. When the initial capacitor voltage was varied within the ranges of 0.3-2.5 kV, the mean ion charge increased from +4.3 to +9.0.