Relative partial cross sections for single, double, and triple photoionization of C60 and C70

Partial cross sections for the photoion formation from C(60) and C(70) were determined from the yields of singly, doubly, and triply charged ions which were measured by mass spectrometry combined with tunable synchrotron radiation at hnu = 25-120 eV. The dependence of the detection efficiencies on the mass-to-charge ratio was evaluated by using the formula proposed by Twerenbold et al. Corrections of the detection efficiency were found to be critical for obtaining accurate partial cross sections for photoionization of fullerenes. Revisions were made of the partial cross-section curves for single and double photoionization of C(60) and C(70). The curve for triple photoionization of C(70) was newly proposed. The ratios between the cross sections for double and single photoionization increase with hnu and reach saturated values of 0.78 at 85 eV for C(60) and approximately 1.3 at 100 eV for C(70). In contrast, the ratios at 120 eV between the cross sections for triple and single photoionization of C(60) and C(70) amount to 0.14 and approximately 0.38, respectively. The formation mechanism of multiply charged fullerene ions was discussed in terms of valence-electron excitation to antibonding unoccupied orbitals and/or spherical standing waves inside the cavity of a fullerene. This excitation could be followed by Spectator Auger processes and transmission of the excess electronic energy among numerous vibrational degrees of freedom.     

Electron-impact ionization of silicon tetrachloride (SiCl4)

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on silicon tetrachloride (SiCl4) using two different experimental techniques, a time-of-flight mass spectrometer (TOF-MS) and a fast-neutral-beam apparatus. The energy range covered was from the threshold to 900 eV in the TOF-MS and to 200 eV in the fast-neutral-beam apparatus. The results obtained by the two different experimental techniques were found to agree very well (better than their combined margins of error). The SiCl3(+) fragment ion has the largest partial ionization cross section with a maximum value of slightly above 6x10(-20) m2 at about 100 eV. The cross sections for the formation of SiCl4(+), SiCl+, and Cl+ have maximum values around 4x10(-20) m2. Some of the cross-section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV followed by a broad second maximum at around 100 eV. This is similar to what has been observed by us earlier for another Cl-containing molecule, TiCl4 [R. Basner, M. Schmidt, V. Tamovsky, H. Deutsch, and K. Becker, Thin Solid Films 374 291 (2000)]. The maximum cross-section values for the formation of the doubly charged ions, with the exception of SiCl3(++), are 0.05x10(-20) m2 or less. The experimentally determined total single ionization cross section of SiCl4 is compared with the results of semiempirical calculations.     

Electron ionization of SiCl4

Relative partial ionization cross sections (PICS) for the formation of fragment ions following electron ionization of SiCl(4), in the electron energy range 30-200 eV, have been determined using time-of-flight mass spectrometry coupled with an ion coincidence technique. By this method, the contributions to the yield of each fragment ion from dissociative single, double, and triple ionization, are distinguished. These yields are quantified in the form of relative precursor-specific PICS, which are reported here for the first time for SiCl(4). For the formation of singly charged ionic fragments, the low-energy maxima appearing in the PICS curves are due to contributions from single ionization involving predominantly indirect ionization processes, while contributions to the yields of these ions at higher electron energies are often dominated by dissociative double ionization. Our data, in the reduced form of relative PICS, are shown to be in good agreement with a previous determination of the PICS of SiCl(4). Only for the formation of doubly charged fragment ions are the current relative PICS values lower than those measured in a previous study, although both datasets agree within combined error limits. The relative PICS data presented here include the first quantitative measurements of the formation of Cl(2) (+) fragment ions and of the formation of ion pairs via dissociative double ionization. The peaks appearing in the 2D ion coincidence data are analyzed to provide further information concerning the mechanism and energetics of the charge-separating dissociations of SiCl(4) (2+). The lowest energy dicationic precursor state, leading to SiCl(3) (+) + Cl(+) formation, lies 27.4 ± 0.3 eV above the ground state of SiCl(4) and is in close agreement with a calculated value of the adiabatic double ionization energy (27.3 eV).     

Differences between the internal energy depositions induced by collisional activation and by electron transfer of W(CO)6(2+) ions on collision with Ar and K targets

Doubly charged tungsten hexacarbonyl W(CO)(6) (2+) ions were made to collide with Ar and K targets to give singly and doubly charged positive ions by collision-induced dissociation (CID). The resulting ions were analyzed and detected by using a spherical electrostatic analyzer. Whereas the doubly charged fragment ions resulting from collisional activation (CA) were dominant with the Ar target, singly charged fragment ions resulting from electron transfer were dominant with the K target. The internal energy deposition in collisionally activated dissociation (CAD) evaluated with the Ar target was broad and decreased with increasing internal energy. The predominant peaks observed with the K target were associated with singly charged W(CO)(2) (+) and W(CO)(3) (+) ions: these ions were not the result of CA, but arose from dissociation induced by electron transfer (DIET). The internal energy deposition resulting from the electron transfer was very narrow and centered at a particular energy, 7.8 eV below the energy level of the W(CO)(6) (2+) ion. This narrow internal energy distribution was explained in terms of electron transfer by Landau-Zener potential crossing at a separation of 5.9 x 10(-8) cm between a W(CO)(6) (2+) ion and a K atom, and the coulombic repulsion between singly charged ions in the exit channel. A large cross section of 1.1 x 10(-14) cm(2) was estimated for electron capture of the doubly charged W(CO)(6) (2+) ion from the alkali metal target, whose ionization energy is very low. The term "collision-induced dissociation," taken literally, includes all dissociation processes induced by collision, and therefore encompasses both CAD and DIET processes in the present work. Although the terms CID and CAD have been defined similarly, we would like to propose that they should not be used interchangeably, on the basis that there are differences in the observed ions and in their intensities with Ar and K targets.     

Photoelectron spectroscopy of fullerene dianions C76(2-), C78(2-), and C84(2-)

We report laser photoelectron spectra of the doubly negatively charged fullerenes C(76) (2-), C(78) (2-), and C(84) (2-) at 2.33, 3.49, and 4.66 eV photon energy. From these spectra, second electron affinities and vertical detachment energies, as well as estimates for the repulsive Coulomb barriers are obtained. These results are discussed in the context of electrostatic models. They reveal that fullerenes are similar to conducting spheres, with electronic properties scaling with their size. The experimental spectra are compared with the accessible excited states of the respective singly charged product ions calculated in the framework of time dependent density functional theory.     

The anomalous shape of the cross section for the formation of SF3+ fragment ions produced by electron impact on SF6 revisited

The partial ionization cross section for the formation of SF(3) (+) fragment ions following electron impact on SF(6) is known to have a pronounced structure in the cross section curve slightly above 40 eV. We used the mass-analyzed ion kinetic energy (MIKE) scan technique to demonstrate the presence of a channel contributing to the SF(3) (+) partial ionization cross section that we attribute to the Coulomb explosion of doubly charged metastable SF(4) (2+) ions into two singly charged ions SF(3) (+) and F(+), with a threshold energy of about 45.5 eV. Thus the observed unusual shape of the SF(3) (+) partial ionization cross section is the result of two contributions, (i) the direct formation of SF(3) (+) fragment ions via dissociative ionization of SF(6) with a threshold energy of 22 eV and (ii) the Coulomb explosion of metastable SF(4) (2+) ions with a threshold energy of about 45.5 eV. A detailed analysis of the MIKE spectrum reveals an average kinetic energy release of about 5 eV in the Coulomb explosion of the SF(4) (2+) ions with evidence of a second channel corresponding to an average kinetic energy release of about 1.1 eV.     

Differential, partial and total electron impact ionization cross sections for SF(6)

Single and double differential ionization cross sections for the production of ions resulting from dissociative, single and double ionization of SF(6) by electron impact have been calculated using a semiempirical formulation based on the Jain-Khare approach. In addition, triple differential cross sections have been obtained for some of the doubly charged fragment ions at an incident electron energy of 100, 150, and 200 eV, respectively, and a fixed scattering angle of 30 degrees. As no previous data seem to exist for differential cross sections we have derived from these differential cross sections corresponding partial and total ionization cross sections from threshold up to 900 eV and compared those with the available theoretical and experimental data.     

Unimolecular and collision-induced reactions of doubly charged porphyrins

Results from a tandem mass spectrometry (MS/MS) study, obtained with a reverse-geometry mass spectrometer, of the unimolecular and collision-induced reactions of doubly charged free-base and metal containing alkyl-substituted porphyrins formed by electron ionization are reported. These doubly charged porphyrin ions dissociate to yield both singly and doubly charged product ions via a number of reactions. This article classifies the major reactions observed, illustrating each with the appropriate spectra. Supplementary data from the same porphyrins, acquired with a tandem quadrupole MS/MS instrument, are also presented. The potential utility of some of these reactions as new methods for porphyrin analysis is discussed.     

Doubly charged porphyrin ion tandem mass spectrometry: Implications for structure elucidation

Under electron ionization (EI) conditions, porphyrins yield unusually high intensities of doubly charged molecular and fragment ions. These doubly charged ions offer unique opportunities for the structure elucidation of porphyrins by tandem mass spectrometry (MS/MS). First, they fragment to a greater extent than the corresponding singly charged ions under both EI/MS and EI/MS/MS conditions. Second, doubly and singly charged porphyrin ions often fragment via different pathways, and can therefore yield different structural information. This paper describes several ways in which analyses of doubly charged porphyrin ions with a triple quadrupole tandem mass spectrometer can be useful in structure elucidation of porphyrins. The effect of the metal atom on the fragmentation of metalloporphyrins in an EI source is demonstrated by correlating the extent of doubly charged fragment ion formation to a stability index. Doubly charged porphyrin ions are shown to yield predominantly doubly charged daughter ions upon collisionally activated dissociation (CAD), and are also shown to fragment to a greater extent than corresponding singly charged porphyrin ions. Advantages and disadvantages of doubly charged porphyrin ion MS/MS for structure elucidation are discussed.     

Single and double photoionizations of methanal (formaldehyde)

Single and double photoionization spectra of formaldehyde have been measured at 40.81 and 48.37 eV photon energy and the spectrum of the doubly charged cation has been interpreted using high-level electronic structure calculations. The adiabatic double-ionization energy is determined as 31.7+/-0.25 eV and the vertical ionization energy is 33 eV. The five lowest excited electronic states are identified and located. The potential-energy surfaces of the accessible states explain the lack of stable H2CO2+ dications and the lack of vibrational structure. The experimental double-ionization spectrum can be decomposed into two distinct contributions, one from direct photoionization and the second from indirect double photoionization by an inner-valence shell Auger effect.     

The nature of collision-induced dissociation processes of doubly protonated peptides: comparative study for the future use of matrix-assisted laser desorption/ionization on a hybrid quadrupole time-of-flight mass spectrometer in proteomics.

Comparative MS/MS studies of singly and doubly charged electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) precursor peptide ions are described. The spectra from these experiments have been evaluated with particular emphasis on the data quality for subsequent data processing and protein/amino acid sequence identification. It is shown that, once peptide ions are formed by ESI or MALDI, their charge state, as well as the collision energy, is the main parameter determining the quality of collision-induced dissociation (CID) MS/MS fragmentation spectra of a given peptide. CID-MS/MS spectra of singly charged peptides obtained on a hybrid quadrupole orthogonal time-of-flight mass spectrometer resemble very closely spectra obtained by matrix-assisted laser desorption/ionization post-source decay time-of-flight mass spectrometry (MALDI-PSD-TOFMS). On the other hand, comparison of CID-MS/MS spectra of either singly or doubly charged ion species shows no dependence on whether ions have been formed by ESI or MALDI. This observation confirms that, at the time of precursor ion selection, further mass analysis is effectively decoupled from the desorption/ionization event. Since MALDI ions are predominantly formed as singly charged species and ESI ions as doubly charged, the associated difference in the spectral quality of MS/MS spectra as described here imposes direct consequences on data processing, database searching using ion fragmentation data, and de novo sequencing when ionization techniques are changed.     

The effect of charge state and the localization of charge on the collision-induced dissociation of peptide ions

The effect that charge state has on the collision-induced dissociation (CID) of peptide ions is examined in detail for several representative peptides under high-energy collision conditions. The CID spectra of singly and doubly charged precursor ions (generated by fast-atom bombardment and electrospray ionization, respectively) are compared for several peptides with similar primary structure. It is shown that for peptides that contain highly basic amino acids, the dissociation of doubly charged ions is strongly influenced by the position of these residues within the peptide and the general observations reported concerning the dissociation of singly charged ions can be extended to precursors with higher charge states. Based on the dissociation behavior of the doubly charged ions of these peptides, it is demonstrated that two charges can reside in close proximity in the precursor ions, overcoming possible repulsion effects, when favored by a high concentration of basic sites. In addition)’ this work illustrates that in the case of doubly charged ions..the charge state of some fragment ions can be determined directly from the mass-to-charge ratio assignments of the CID spectrum.     

High resolution and low-temperature photoelectron spectroscopy of an oxygen-linked fullerene dimer dianion: C(120)O(2-)

C(120)O comprises two C(60) cages linked by a furan ring and is formed by reactions of C(60)O and C(60). We have produced doubly charged anions of this fullerene dimer (C(120)O(2-)) and studied its electronic structure and stability using photoelectron spectroscopy and theoretical calculations. High resolution and vibrationally resolved photoelectron spectra were obtained at 70 K and at several photon energies. The second electron affinity of C(120)O was measured to be 1.02+/-0.03 eV and the intramolecular Coulomb repulsion was estimated to be about 0.8 eV in C(120)O(2-) on the basis of the observed repulsive Coulomb barrier. A low-lying excited state ((2)B(1)) was also observed for C(120)O(-) at 0.09 eV above the ground state ((2)A(1)). The C(120)O(2-) dianion can be viewed as a single electron on each C(60) ball very weakly coupled. Theoretical calculations showed that the singlet and triplet states of C(120)O(2-) are nearly degenerate and can both be present in the experiment. The computed electron binding energies and excitation energies, as well as Franck-Condon factors, are used to help interpret the photoelectron spectra. A C-C bond-cleaved isomer, C(60)-O-C(60) (2-), was also observed with a higher electron binding energy of 1.54 eV.     

Multiply charged cluster ions of Ar,Kr, Xe,N2, O2, CO2, SO2 and NH3: Production mechanism,appearance size and appearance energy

Clusters of Ar, Kr, Xe, N₂, O₂, CO₂, SO₂ and NH₃ formed by supersonic nozzle expansion have been studied by electron impact ionization mass spectrometry (up to 15000 amu). Besides mass spectra of singly charged ions showing the characteristic anomalous distributions, we have in particular investigated the properties of multiply charged cluster ions. Critical appearance sizes of doubly and triply charged cluster ions, n₂ and n₃ respectively, found in the present study confirm recent theoretical predictions about n₃/n₂ and their dependence on the properties of the cluster constituents. The appearance energies of multiply charged cluster ions determined are shifted way below the appearance energies of the respective monomer ions. These huge red shifts together with the observed linear threshold laws and large maximum ionization efficiencies indicate that multiply charged cluster ions are produced by sequential single ionization events of one incoming electron at different cluster sites. Furthermore, we have also obtained for the first time clear evidence that (for electron energies above the appearance energy of doubly charged ions) an appreciable amount of singly charged (also fragment) ions is produced via Coulomb explosion of unstable doubly charged ions in the ion source.     

A study of the decomposition of field ionized doubly charged molecular ions

The dissociation of field ionized doubly charged organic molecular ions into doubly charged fragment ions and neutral fragments is discussed. The kinetic energy released during the dissociation of the singly and doubly charged molecular ions rules out the possibility of a direct correlation between their mechanisms of formation. Further, the pressure as well as the temperature dependence revealed that the singly charged molecular ions are formed by direct ionization of the neutral molecules, while the doubly charged molecular ions are formed through a second ionization process of the adsorbed molecular ions on the field anode surface.     

A structural investigation of [C6H6]2+ and [C6H6]+˙ ions involved in charge exchange reactions

Mass-analysed ion kinetic energy spectra for collisional activation (CA) of [C₆H₆]⁺˙ formed via electron capture by [C₆H₆]²⁺ ions in collision with neutral benzene molecules have been compared for the C₆H₆ isomers benzene, 1,5-hexadiyne and 2,4-hexadiyne. Comparisons of fragment abundance and total CA fragment yields were also made for [C₆H₆]⁺˙ ions generated by electron ionization (EI). CA conditions of ion velocity and collision gas pressure were identical in these comparisons. In general the fragment abundance patterns for the ions formed by charge exchange were very similar to those for singly charged benzene ions generated by EI. However, significant variations in CA fragment yield (the ratio of the total CA fragment ion abundance to the abundance of the incident unfragmented ions) were observed. It is not clear from the results whether these variations reflect structurally different ions or ions of different internal energies. The CA spectra of [C₆H₆]⁺˙ ions derived from charge exchange reactions between the benzene dication and the target gases He, Ne, Ar, Kr and Xe have also been recorded and, once again, very similar fragment abundance patterns were observed along with large variations in total CA fragment yields. Charge exchange efficiency measurements are reported for reactions between the benzene dication and the targets He, Ne, Ar, Kr, Xe and C₆H₆ (benzene) and also for the doubly charged ions derived from the linear C₆H₆ isomers. In the latter case Xe and benzene targets were used. The energetics and efficiency measurements for the former reactions suggest that for targets such as He and Ne the processes probably involve excited states of the doubly charged ions. The efficiencies measured for the latter reactions were distinctly different for the three C₆H₆ isomers and may indicate a strong dependence of charge exchange cross-section on doubly charged ion structure.     

Mechanism of formation of doubly charged fragments from bis-benzyltetrahydroisoquinolines under electron impact

Some bis-benzyltetrahydroisoquinolines [α,α′-di-N,N-(1-benzyl-1,2,3,4-tetrahydroisoquinoline)-p-xylene and various substituted analogues] give rise to very abundant doubly charged fragment ions under electron impact, corresponding to the loss of the two benzyl groups. Substituent effects, ionization and appearance energy measurements and metastable transitions show that these doubly charged ions are formed (at least in part) from singly charged precursors by a heterolytic cleavage (charge separation).     

Ion-molecule reactions of ammonia clusters with C60 aggregates embedded in helium droplets

Helium nanodroplets are co-doped with C(60) and ammonia. Mass spectra obtained by electron ionization reveal cations containing ammonia clusters complexed with up to four C(60) units. The high mass resolution of Δm/m≈ 1/6000 makes it possible to separate the contributions of protonated, unprotonated and dehydrogenated ammonia. C(60) aggregates suppress the proton-transfer reaction which usually favors the appearance of protonated ammonia cluster ions. Unprotonated C(x)(NH(3))(n)(+) ions (x = 60, 120, 180) exceed the abundance of the corresponding protonated ions if n < 5; for larger values of n the abundances of C(60)(NH(3))(n)(+) and C(60)(NH)(n-1)NH(4)(+) become about equal. Dehydrogenated C(60)NH(2)(+) ions are relatively abundant; their formation is attributed to a transient doubly charged C(60)-ammonia complex which forms either by an Auger process or by Penning ionization following charge transfer between the primary He(+) ion and C(60). The abundance of C(x)NH(3)(+) and C(x)NH(4)(+) ions (x = 120 or 180) is one to two orders of magnitude weaker than the abundance of ions containing one or two additional ammonia molecules. However, a model involving evaporation of NH(3) or NH(4) from the presumably weakly bound C(x)NH(3)(+) and C(x)NH(4)(+) ions is at odds with the lack of enhancement in the abundance of C(120)(+) and C(180)(+). Mass spectra of C(60) dimers complexed with water complement a previous study of C(60)(H(2)O)(n)(+) recorded at much lower mass resolution.     

Capillary electrophoresis-electrospray mass spectra of the herbicides paraquat and diquat

The positive ion electrospray mass spectra of the quaternary ammonium salt herbicides paraquat and diquat are examined by on-line separation with capillary electrophoresis (CE) and by direct infusion of the analytes. The analytes are separated by CE in 7–10 min at pH 3.9 in 50% methanol-water by using several different separation buffer electrolytes. The capillary electrophoresis-electrospray ionization (CE-ES) mass spectra of paraquat and diquat consist primarily of doubly charged molecular ions, singly charged molecular ions, and singly charged deprotonated ions. The direct infusion spectra consist primarily of doubly charged molecular ions and singly charged deprotonated ions. The relative abundances of the doubly charged and deprotonated ions depend strongly on the presence or absence of ammonium ion in the CE separation buffer or the direct infusion solution. A deprotonation mechanism is proposed in which the free base ammonia is the deprotonating agent in the desolvating charged droplets or in the gas phase. The analytical potential of the CE-ES electrospray approach for environmental analyses is evaluated in terms of the precision of replicate injections, linear concentration range, and estimated detection limit.     

4d-->4f dipole resonance of the metal atom encapsulated in a fullerene cage: Ce@C82

The yield curves for photoions from Ce@C(82) are measured by using synchrotron radiation in the photon energy range from 90 to 160 eV. Parent Ce@C(82) (z+) and fragment ions C(60) (z+) and C(70) (z+) are observed in a mass spectrum (z=1 and 2). The yield curves for doubly charged ionic species exhibit broad resonance in the photon energy region of from 120 to 140 eV which is ascribed to the 4d-->4f giant dipole resonance of the encapsulated Ce atom. The total photoabsorption cross section of Ce@C(82) was determined from partial photoionization cross sections for formation of the parent and fragment ions to be 5.3(-1.1) (+1.8) and 19.6(-3.9) (+6.5) Mb at photon energies of 110 and 130 eV, respectively.