Multiple ionization and Coulomb explosion of mercury clusters in femtosecond laser fields

We report on studies of multiple ionization and fragmentation of free Hg_n (n ≤ 80) clusters in the femtosecond time domain at wavelengths ranging from 255 nm to 800 nm. After excitation by single laser pulses of an intensity of 5 * 10¹¹ W/cm² we observe prompt formation of multiply charged Hg_n clusters. The Hg_n cluster size distribution observed up to n ≈ 80 shows in additon to singly charged also doubly and triply charged clusters with a surprisingly high amount of doubly charged clusters. The measured cluster size distribution is nearly independent of laser wavelengths. For higher laser intensities (2 * 10¹² W/cm²) we observe multiply charged mercury atoms up to Hg⁵⁺. At 10¹³ W/cm² molecules and clusters eventually disappear due to Coulomb explosion and complete Fragmentation. Only atomic ions, singly and multiply charged, with high kinetic energies are then observed.     

Doubly charged sodium clusters: plasmon response and photoproduction

The optical response of doubly charged sodium clusters Na _n+2 ⁺⁺ was measured for n = 20, 40, and 58 valence electrons, for which the jellium model predicts spherical clusters. A new experimental scheme was developed which allows to separate doubly charged clusters of even mass from the singly charged with half the mass. The optical spectra are dominated by a plasmon-like resonance which is blue shifted and narrower than that of the singly charged clusters. The smallest doubly charged cluster observed was Na ₉ ⁺⁺ . The photo ionization cross section for singly charged clusters was found to be typically 2.6·10^-19cm² per Na atom for photon energies of around 6 eV, which is a factor of 400 smaller than the maximum in the plasmon absorption in the region of hω=2.6 eV.     

Cluster ions ejected from an Li-Mg alloy liquid metal ion source: Observation of MG 2 2+ and MG 3 2+

Ions ejected from a liquid metal ion source of an Li-Mg (10 atom %) alloy have been investigated by using a magnetic mass analyzer. In addition to singly charged homonuclear Li _n ⁺ (n ≤ 9) and Mg _n ⁺ (n ≤ 4) and heteronuclear Mg_mLi _n ⁺ (m, n ≤ 2) clusters, doubly charged diatomic and triatomic Mg clusters are observed. Discussion is focused on the observability and the formation mechanism of the doubly charged small Mg clusters. A postionization process is suggested for the formation of the doubly charged clusters.     

Production of doubly charged clusters (H2O)n · Ba2+ and (H2O)n · Ca2+ under low temperature fast atom bombardment conditions

Production of doubly charged ions of alkaline earth metals Ba²⁺ and Ca²⁺ and their doubly charged clusters with water molecules (H₂O)_n · Ba²⁺, (H₂O)_n · Ca²⁺ (n = 1, 2, 3) by means of low temperature fast atom bombardment technique is observed in the case of crystalline hydrates of BaCl₂ and CaCl₂ salts, formed during freezing of water-salt solutions. Reasons for a possibility of production of the doubly charged species in the case of the two indicated salts and their absence in the case of chlorides of some other divalent metals (Mg, Mn, Co, Cu, Zn) are discussed. As to singly charged secondary ions Me⁺, MeCl⁺, MeOH⁺, [(H₂O)_n · MeCl]⁺, [(H₂O)_n · MeOH]⁺ (where Me is metal), high efficiency of their production from crystalline hydrates was observed and possible explanation of the phenomenon is suggested.     

Comparison of collision‐ versus electron‐induced dissociation of sodium chloride cluster cations

The collision‐induced dissociation (CID) and electron‐induced dissociation (EID) spectra of the [(NaCl)_m(Na)_n]ⁿ⁺ clusters of sodium chloride have been examined in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. For singly charged cluster ions (n = 1), mass spectra for CID and EID of the precursor exhibit clear differences, which become more pronounced for the larger cluster ions. Whereas CID yields fewer product ions, EID produces all possible [(NaCl)_xNa]⁺ product ions. In the case of doubly charged cluster ions, EID again leads to a larger variety of product ions. In addition, doubly charged product ions have been observed due to loss of neutral NaCl unit(s). For example, EID of [(NaCl)₁₁(Na)₂]²⁺ leads to formation of [(NaCl)₁₀(Na)₂]²⁺, which appears to be the smallest doubly charged cluster of sodium chloride observed experimentally to date. The most abundant product ions in EID spectra are predominantly magic number cluster ions. Finally, [(NaCl)_m(Na)₂]^{+ .} radical cations, formed via capture of low‐energy electrons, fragment via the loss of [(NaCl)_n(Na)] ^. radical neutrals. Copyright © 2008 John Wiley & Sons, Ltd.     

Doubly charged ion mass spectra: VI—Amines

Doubly charged ion mass spectra of 22 amines (2–10 carbon atoms) were determined using an Hitachi RMU-7L double focusing mass spectrometer. Molecular ions were not observed in the spectra of aliphatic amines. The most intense product ion peaks in the spectra of lower molecular weight amines resulted from hydrogen elimination from the molecular ion; however, as amine molecular weight increased the largest peaks resulted from both hydrogen and heavy atom elimination from the molecular ion. Dominant ions in the doubly charged ion spectra of lower molecular weight aliphatic amines were from reactions of [C_nH₃N]²⁺ (n:=2, 3, 4) type ions. The spectra of higher molecular weight aliphatic amines spanned a wide mass range. Appearance energies for some of the more prominent ions were measured in the range from 25 to 49 eV. A geometry optimized quantum mechanical self-consistent field molecular orbital treatment was used to compute the energies and structural parameters of prominent ions in the doubly charged ion mass spectra.     

Structures,Unimolecular Fragmentations,and Reactivities of the Self‐Assembled Multimetallic/Peptide Complexes [Mnn(GlyGly‐H)2n−1]+ and [Mnn+1(GlyGly‐H)2n]2+

Complexes of Mn²⁺ with deprotonated GlyGly are investigated by sustained off‐resonance irradiation collision‐induced dissociation (SORI‐CID), infrared multiple‐photon dissociation spectroscopy, ion–molecule reactions, and computational methods. Singly [Mn_n(GlyGly‐H)_2n?1]⁺ and doubly [Mn_n+1(GlyGly‐H)_2n]²⁺ charged clusters are formed from aqueous solutions of MnCl₂ and GlyGly by electrospray ionization. The most intense ion produced was the singly charged [M₂(GlyGly‐H)₃]⁺ cluster. Singly charged clusters show extensive fragmentations of small neutral molecules such as water and carbon dioxide as well as dissociation pathways related to the loss of NH₂CHCO and GlyGly. For the doubly charged clusters, however, loss of GlyGly is observed as the main dissociation pathway. Structure elucidation of [Mn₃(GlyGly‐H)₄]²⁺ clusters has also been done by IRMPD spectroscopy as well as DFT calculations. It is shown that the lowest energy structure of the [Mn₃(GlyGly‐H)₄]²⁺ cluster is deprotonated at all carboxylic acid groups and metal ions are coordinated with carbonyl oxygen atoms, and that all amine nitrogen atoms are hydrogen bonded to the amide hydrogen. A comparison of the calculated high‐spin (sextet) and low‐spin (quartet) state structures of [Mn₃(GlyGly‐H)₄]²⁺ is provided. IRMPD spectroscopic results are in agreement with the lowest energy high‐spin structure computed. Also, the gas‐phase reactivity of these complexes towards neutral CO and water was investigated. The parent complexes did not add any water or CO, presumably due to saturation at the metal cation. However, once some of the ligand was removed via CO₂ laser IRMPD, water was seen to add to the complex. These results are consistent with high‐spin Mn²⁺ complexes.     

A REMPI study of solvation of small Hg n clusters by polar molecules

Free Hg _n (DME) _m clusters (where DME=dimethyl-ether,n=1, 2, 3,m=1÷5) formed in a supersonic expansion were studied by the REMPI (Resonance-Enhanced Multi-Photon Ionization) technique. A large decrease of ionization energies due to solvation of Hg _n clusters is observed. Preliminary results are discussed in terms of different equilibrium configurations of the electronic ground, excited and ionic states of clusters.     

Shell effects in singly and multiply charged silver and gold clusters

The mass spectra of silver- and gold-clusters, generated by a gas aggregation technique and ionized by electron impact, reveal anomalies in the relative abundance of both singly and multiply charged clusters. Concentration maxima for singly charged species Ag _n ⁺ and Au _n ⁺ (n=3, 9, 19, (21), 35) are in agreement with experimental data of Katakuse and the predictions from the electronic shell model. The observed anomalies in the abundance spectra of doubly charged silver and gold clusters as well as triply charged silver cluster ions are explained in terms of electronic shell closing.     

Fragmentation of the tryptophan cluster [Trp9-2H]2- induced by different activation methods

Electrospray ionization (ESI) of tryptophan gives rise to multiply charged, non‐covalent tryptophan cluster anions, [Trp_n–xH]^x?, in a linear ion trap mass spectrometer, as confirmed by high‐resolution experiments performed on a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The smallest multiply charged clusters that can be formed in the linear ion trap as a function of charge state are: x = 2, n = 7; x = 3, n = 16; x = 4, n = 31. The fragmentation of the dianionic cluster [Trp₉–2H]^2? was examined via low‐energy collision‐induced dissociation (CID), ultraviolet photodissociation (UVPD) at 266 nm and electron‐induced dissociation (EID) at electron energies ranging from >0 to 30 eV. CID proceeds mostly via charge separation and evaporation of neutral tryptophan. The smallest doubly charged cluster that can be formed via evaporation of neutral tryptophans is [Trp₇–2H]^2?, consistent with the observation of this cluster in the ESI mass spectrum. UVPD gives singly charged tryptophan clusters ranging from n = 2 to n = 9. The latter ion arises from ejection of an electron to give the radical anion cluster, [Trp₉–2H]^?.. The types of gas‐phase EID reactions observed are dependent on the energy of the electrons. Loss of neutral tryptophan is an important channel at lower energies, with the smallest doubly charged ion, [Trp₇–2H]^2?, being observed at 19.8 eV. Coulomb explosion starts to occur at 19.8 eV to form the singly charged cluster ions [Trp_x–H]^? (x = 1–8) via highly asymmetric fission. At 21.8 eV a small amount of [Trp₂–H–NH₃]^? is observed. Thus CID, UVPD and EID are complementary techniques for the study of the fragmentation reactions of cluster ions. Copyright © 2010 John Wiley & Sons, Ltd.     

Theory for the short-time response of small Hg n clusters after excitation

The short-time behavior of small Hg _n clusters immediately after single or double ionization is studied. We calculate self-consistently the ground state electronic energyE of ionized Hg _n clusters. Upon ionization changes of the potential energy surface (PES) occur, which govern the atomic motion in the cluster. These changes depend on cluster size and charge and are determined by the interplay between the localization of the holes within an ionic core and the polarization energy of the neutral rest of the cluster. In the case of single ionization of the cluster the PES results mainly from hole delocalization. In contrast, in the case of double ionization the PES is governed almost only by strong environment polarization. We use our theory to explain the physical origin of the oscillations in the ionization cross-section of singly and doubly excited Hg _n clusters observed in recent pump-probe experiments.     

Fragmentation by electron impact ionization and intracluster reactions of size selected (N2H4) n and (OCS) n clusters

The hydrogen-bonded (N₂H₄) _n clusters and the van der Waals (OCS) _n clusters are size selected in a scattering experiment with a He beam up to the cluster sizen=6. By measuring the angular distributions of the scattered clusters the complete fragmentation pattern of electron impact ionization is obtained. For Hydrazine the two main fragment masses are the protonated species (N₂H₄) _n?1H⁺ and with somewhat weaker intensities also the nominal ion mass (N₂H₄) _n ⁺ . The largest intensity is observed for the monomer ion N₂H ₄ ⁺ to which clusters up ton=5 fragment. For carbonylsulfide, completely different results are obtained. Aside from the fragments of the OCS monomer and the van der Waals cluster fragments (OCS) ₂ ⁺ and (OCS) ₃ ⁺ signals at mass S ₂ ⁺ , S ₃ ⁺ and S₂OCS⁺ are detected. This indicates a fast chemical reaction in the cluster according to: S + OCS → CO + S₂ which occurs for clusters of sizen ≥ 2. Peaks at S ₃ ⁺ and S₂OCS⁺ are seen for the first time forn ≥ 5 according to a further reaction of S₂ in the cluster.     

Self‐Assembled Multimetallic/Peptide Complexes: Structures and Unimolecular Reactions of [Mn(GlyGly−H)2n−1]+ and Mn+1(GlyGly−H2n]2+ Clusters in the Gas Phase

The unimolecular chemistry and structures of self‐assembled complexes containing multiple alkaline‐earth‐metal dications and deprotonated GlyGly ligands are investigated. Singly and doubly charged ions [M_n(GlyGly?H)_n‐1]⁺ (n=2–4), [M_n+1(GlyGly?H)_2n]²⁺ (n=2,4,6), and [M(GlyGly?H)GlyGly]⁺ were observed. The losses of 132 Da (GlyGly) and 57 Da (determined to be aminoketene) were the major dissociation pathways for singly charged ions. Doubly charged Mg²⁺ clusters mainly lost GlyGly, whereas those containing Ca²⁺ or Sr²⁺ also underwent charge separation. Except for charge separation, no loss of metal cations was observed. Infrared multiple photon dissociation spectra were the most consistent with the computed IR spectra for the lowest energy structures, in which deprotonation occurs at the carboxyl acid groups and all amide and carboxylate oxygen atoms are complexed to the metal cations. The N?H stretch band, observed at 3350 cm^?1, is indicative of hydrogen bonding between the amine nitrogen atoms and the amide hydrogen atom. This study represents the first into large self‐assembled multimetallic complexes bound by peptide ligands.     

Mass Spectrometry Study of Multiply Negatively Charged, Gas-Phase NaAOT Micelles: How Does Charge State Affect Micellar Structure and Encapsulation?

We report the formation and characterization of multiply negatively charged sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) aggregates in the gas phase, by electrospray ionization of methanol/water solution of NaAOT followed by detection using a guided-ion-beam tandem mass spectrometer. Singly and doubly charged aggregates dominate the mass spectra with the compositions of [Na_n-zAOT_n]^z– (n?=?1–18 and z?=?1–2). Solvation by water was detected only for small aggregates [Na_n-1AOT_nH₂O]^– of n?=?3–9. Incorporation of glycine and tryptophan into [Na_n-zAOT_n]^z– aggregates was achieved, aimed at identifying effects of guest molecule hydrophobicity on micellar solubilization. Only one glycine molecule could be incorporated into each [Na_n-zAOT_n]^z– of n?≥?7, and at most two glycine molecules could be hosted in that of n?≥?13. In contrast to glycine, up to four tryptophan molecules could be accommodated within single aggregates of n?≥?6. However, deprotonation of tryptophan significantly decrease its affinity towards aggregates. Collision-induced dissociation (CID) was carried out for mass-selected aggregate ions, including measurements of product ion mass spectra for both empty and amino acid-containing aggregates. CID results provide a probe for aggregate structures, surfactant-solute interactions, and incorporation sites of amino acids. The present data was compared with mass spectrometry results of positively charged [Na_n+zAOT_n]^z+ aggregates. Contrary to their positive analogues, which form reverse micelles, negatively charged aggregates may adopt a direct micelle-like structure with AOT polar heads exposed and amino acids being adsorbed near the micellar outer surface.      

Doubly charged ion mass spectra: III—N-alkanes

Doubly charged ion mass spectra have been obtained for 15 n-alkane hydrocarbons. Spectra were measured using a Nier-Johnson geometry Hitachi RMU-7L mass spectrometer operated at 1.6kV accelerating voltage. Fragment ions, which resulted from C? C bond rupture and extensive H loss, dominated the spectra. Molecular ions have not been observed. The most intense ions in the doubly charged ion mass spectra of n-alkanes were [C₂H₄]²⁺, [C₃H₂]²⁺, [C₄H₃]²⁺, [C₅H₂]²⁺, [C₆H₆]²⁺, [C₆H₈]²⁺, [C₇H₆]²⁺, [C₇H₈]²⁺, [C₈H₆]²⁺ and [C₈H₈]²⁺. Appearance energies for forming the prominent doubly charged fragment ions have been measured and range from 27.5 eV to energies greater than 60eV. A geometry optimized SCF approach has been used to compute the energies and structures of prominent ions in the doubly charged mass spectra.     

Quantum chemical studies on the role of water microsolvation in interactions between group 12 metal species (Hg2+, Cd2+, and Zn2+) and neutral and deprotonated cysteines

Interactions of group 12 metal(II) species (Hg²⁺, Cd²⁺, Zn²⁺, Hg(H₂O) _n ²⁺ , Cd(H₂O) _n ²⁺ , and Zn(H₂O) _n ²⁺ (n?=?1, 2) with neutral (RSH), deprotonated (RS^?), and doubly deprotonated cysteine species (abbreviated as ??H₂cys??, ??Hcys^???, and ??cys^2???, respectively) are examined with the Becke three-parameter Lee?CYang?CParr (B3LYP) hybrid functional after preliminary screening in a conformation analysis with the Parameterized Model number 3 (PM3) semiempirical method. Effects of water on aqueous solution are evaluated by microsolvation and polarized continuum model (PCM) approaches. In the most stable conformations of M(H₂cys)²⁺ and M(Hcys)⁺ complexes (M?=?Hg²⁺, Cd²⁺, and Zn²⁺), the SH group of the cysteine moiety is already deprotonated and undergoes strong binding with the metal ion. Among Hg(H₂cys)²⁺ complexes, cysteine complexes of Hg²⁺ without deprotonation of the SH group and mercury(II) carboxylato-type structures are at least 83 and 117?kJ/mol less stable in energy than the most stable complex (B3LYP/6-311++G(d,p)-SDD+d+f//B3LYP/6-31G(d)-SDD+d). Although Zn²⁺ binds more strongly than Hg²⁺ to a H₂cys molecule at the high-level CCSD(T)/6-311++G(d,p)-SDD+d+f//B3LYP/6-311++G(d,p)-SDD+d+f level, [Hg(H₂O)₂]²⁺ is stronger than [Zn(H₂O)₂]²⁺ because the deformation of [Zn(H₂O)₂]²⁺ required to bind to cys is much more than in [Hg(H₂O)₂]²⁺. Complexes with a deprotonated cysteine, M(Hcys)⁺ and M(cys), prefer a multidentate structure.     

Determination of trace mercury by solid substrate-room temperature phosphorimetry based on catalytic effect of Hg(2+) on formation of the ion association complex [Fe(bipy)(3)](2+)[(FinBr(4))(2)](2-)

A new solid substrate-room temperature phosphorimetry method for the determination of trace mercury has been established. It bases on the fact that in acetic acid medium, Hg²⁺ ion can catalyze the substitute reaction of CN⁻ ligand in [Fe(CN)₆]⁴⁻ by 2,2′-bipyridyl (bipy), and the resultant [Fe(bipy)₃]²⁺ cation can react with FinBr₄⁻ anion of tetrabromofluorescein (HFinBr₄) to form ion association complex [Fe(bipy)₃]²⁺[(FinBr₄)₂]²⁻ which can emit phosphorescent signal on filter paper substrate. Under the optimum condition, the linear dynamic range of this method is 1.6-16 fg per spot with a detection limit (LD) of 0.18 fg per spot (0.4 μl sample solution per spot), and the regression equation of working curve is ΔI_p=1.058+7.671 C_Hg²⁺ (fg per spot ), n=7, correlation coefficient is 0.9990. This method has been applied to the determination of trace mercury in hair and cigarette samples with satisfactory result. The reaction mechanism for formation of the ion association complex is also discussed.     

Electronic shell structure in multiply charged silver clusters

Silver clusters are generated by standard laser vaporization technique and ionized via multiphoton ionization. Time-of-flight mass spectrometry reveals singly, doubly and triply charged clusters, Ag _n ^z+ (z=1,2,3). The spectra show, for all charge states, intensity variations, indicating enhanced stabilities for cluster sizes with closed electronic configurations in accord with the spherical jellium model.     

Study of diketone/metal ion complexes by electrospray ionization mass spectrometry: Influence of Keto-Enol tautomerism and chelation

The formation and collisionally activated dissociation (CAD) behavior of a series of complexes containing cyclic or linear diketone ligands and alkali, alkaline earth, or transition metal ions are investigated. Electrospray ionization (ESI) is utilized for introduction of the metal ion complexes into a quadrupole ion trap mass spectrometer. The proximity of the carbonyl groups is crucial for formation and detection of ion complexes by ESI. For example, no metal ion complexes are observed for 1,4-cyclohexanedione, but they are readily detected for the isomers, 1,2-and 1,3-cyclohexanedione. Although the diketones form stable doubly charged complexes, the formation of singly charged alkaline earth complexes of the type (nL + M²⁺ ? H⁺)⁺ where L = 1,3-cyclohexanedione or 2,4-pentanedione is the first evidence of charge reduction. CAD investigations provide further evidence of charge reduction processes occurring in the gas-phase complexes. The CAD studies indicate that an intramolecular proton transfer between two diketone ligands attached to a doubly charged metal ion, followed by elimination of the resulting protonated ligand, produces the charge reduced complex. For transition metal complexation, the preference for formation of doubly charged versus singly charged complexes correlates with the keto-enol distribution of the diketones in solution.     

Macrocycle-mediated cation transport from binary Hg2+Mn+ mixtures in A 1 M HNO3CHCI31 M HNO3 liquid membrane system

The macrocycle-mediated flux of Hg²⁺ individually and of Hg²⁺ and Mⁿ⁺ (Mⁿ⁺ = K⁺, Tl⁺, Ag⁺, Sr²⁺, Cd²⁺, or Pb²⁺) in cation mixtures has been measured at 25°C in a I M HNO₃CHCl₃1 M HNO₃ liquid membrane system. Of the ten macrocycles used, 18-crown-6(18C6), dicyclohexano-18-crown-6(DC18C6), and 21-crown-7(21C7)were most effective in transporting Hg²⁺ individually. Relative cation fluxes in the metal ion mixtures correlated well with relative log K values for cation--macrocycle interaction and with relative partition coefficients for the distribution of the resulting complex between the aqueous and organic phases