Metastable decay of rare gas cluster ions: mechanism and kinetics

Metastable decay of cluster ions has been discovered only recently. It was noted that one has to take this metastable decay into account when using mass spectrometry to probe neutral clusters, because ion abundance anomalies in mass spectra of rare gas and molecular clusters are caused by delayed metastable evaporation of monomers following ion production. Moreover, it was found that(i) the individual metastable reaction rates k depend strongly on cluster size and cluster ion production pathways and that(ii) there exists experimental evidence (k=k(t)) and a theoretical prediction that a given mass selected cluster ion generated by electron impact ionization of a nozzle expansion beam will comprise a range of metastable decay rates. In addition, it was discovered that metastable Ar cluster ions which lose two monomers in the μs time regime decay via sequential decay series Ar _n ⁺ *→Ar _n?1 ⁺ *→Ar _n?2 ⁺ * with cluster sizes 7≤n≤10 andn=3 (similar results were obtained recently in case of N₂ cluster ions). Conversely, the dominant metastable decay channel of Ar ₄ ⁺ * into Ar ₂ ⁺ was found to proceed predominantly via a single step fissioning process.     

Metastable decay of Ar n + involving single monomer evaporation and the loss of peculiar numbers of monomers

Production and stability of Ar _n ^+* ions (withn up to 420) formed by electron impact ionization of a supersonic Ar cluster beam were investigated with a double focussing sector field mass spectrometer. The present study confirms previous magic number determinations up to the 4th icosahedral shell. A systematic study of metastable dissociations (monomer evaporation, magic number evaporation) for singly charged cluster ions as a function of cluster size, internal excitation energy and time elapsed since ion formation gives new insight into the ionization process and subsequent reactions of the ions formed. At a well-defined threshold energy ofca. 28 eV, the magic number loss mechanism occurs simultaneously with the well known single monomer evaporation process which proceeds at all energies. The new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.     

Production and stability of oxygen cluster cations and anions,revisited

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O₂) _n ⁺ and (O₂) _n ^? ions, respectively. (O₂) _n ⁺ ions may also lose up top=(n?1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O₂) _n ^? ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O₂) _n ^? ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O₂ clusters.     

Benzene clusters in a supersonic beam

A supersonic beam is employed to produce benzene clusters (C₆H₆) _n up ton=40. Mass analysis is achieved after two-photon ionization in a reflectron mass spectrometer. Photon energy is chosen so that the internal energy of the cluster ions is less than 700 meV and a slow decay on the µs time scale is observed. By an energy analysis with the reflecting field it is found that the elimination of one neutral benzene monomer is the favoured dissociation process of the cluster ions. Information about the dissociation pathways of the cluster ions is essential if one is to obtain neutral cluster abundances from the ion mass spectrum. Furthermore an experimental method is presented to obtain pure intermediate state (S ₁←S₀) spectra of selected clusters without interferences from the other clusters present in the molecular beam. This method is based on the observation of the metastable decay of the corresponding cluster ion. When the metastable signal is recorded as a function of photon energy it reflects theS ₁←S ₀ intermediate state spectrum. Spectra are presented for the benzene dimer, trimer, tetramer and pentamer.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Abundance of Na cluster ions ejected from a liquid metal ion source

Sodium cluster ions Na⁺ _n withn ranging up to 25 have been observed from a liquid sodium ion source by using a magnetic mass analyzer. Ion intensity as a function of cluster size showed distinct steps and local maxima atn=3, 5, 11, 13 and 19 (magic numbers), and a pronounced odd-even alternation. The features in the ion abundance curve are attributed to the relative stability of cluster ions. The observed magic numbers are only partially explained by the electronic shell model, indicating need to include a consideration of atomic structure in a cluster.     

The behaviour of caesium iodide cluster ions produced by fast-atom bombardment

Metastable ion and collisionally-activated dissociation spectra of several cluster ions of the type [Cs_n+1I_n]⁺, formed by fast atom bombardment mass spectrometry (FABMS) of CsI, have been studied: It is found that particular cluster ions are more stable than others. Less stable ions undergo more unimolecular dissociation and have larger cross-sections for collisional dissociation than do the more stable ions, and this leads to unexpected anomalies in the FAB mass spectrum of CsI. The collision spectrum of [Cs₃₅I₃₄]⁺ (m/z 8966) has been acquired at an accelerating potential of 10 kV; the extent of both unimolecular and collisionally-activated dissociation of this ion is remarkably high and leads to collision/ transmission efficiencies of 85% for our tandem mass spectrometer. Clusters formed by FAB of an equimolar mixture of CsI and KI have also been studied by their collisionally-activated dissociation (CAD) and metastable ion (MI) spectra.     

Magic numbers in the mass distribution of the benzene+-argonn ions: evaporation dynamics and cluster structure

The intensity distribution of benzene⁺-Ar_n cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers atn=20 and 45) have been observed in the 15–60 size range. The evaporation dynamics of these species in the 2–50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflectron time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyondn=20, a sudden decrease of the evaporation energy is observed; in then=45–47 size range, the magic number is accounted for by the specific dynamics of then=46 and 47 clusters, in particular the possible loss of two argon atoms forn=47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters.     

Observation and investigation of metastable decay series of (N2) n + cluster ions

N₂ cluster ions are produced by electron impact ionization of a supersonic N₂ cluster beam and analyzed with a double focussing sector field mass spectrometer. It is found that metastable N₂ cluster ions lose more than one N₂ molecule in the μs time regime and decay predominantly via sequential series (N₂) _n ⁺ *→(N₂) _n?1 ⁺ *→...→N ₂ ⁺ , evaporating a single monomer in each of these successive decay steps. The metastable decay rates determined in detail for cluster sizes 2≤n≤6 andn=20 lie between 1 and 10⁶s^?1. These rates(i) depend strongly on the time elapsed after ion formation and on the respective parent cluster ion size, and(ii) exhibit a quasiperiodic pattern in magnitude.     

FTMS studies of mass-selected,large cluster ions produced by direct laser vaporization

A method is described for the production of large cluster ions by direct laser vaporization in a low-pressure FTMS. Production of high-mass carbon cluster ions (C_n⁺; 40 <n < 180) and bismuth-antimony (Bi_xSb_y⁺) cluster ions containing up to five metal atoms are reported. The observed distributions are compared with those obtained previously by both direct laser vaporization and molecular beam sources. Details of the mechanism for formation of these larger cluster ions by direct laser vaporization are discussed. The mass selectivity and long ion residence times obtainable in the FTMS may now be utilized in the study of these cluster ions. Results are presented from a limited study of the ion/molecule reactions and collision induced dissociation of the high-mass carbon cluster ions.     

Ion-pair evaporation from ionic liquid clusters

A differential mobility analyzer (DMA) is used in atmospheric pressure N₂ to select a narrow range of electrical mobilities from a complex mix of cluster ions of composition (CA)_n(C⁺)_z. The clusters are introduced into the N₂ gas by electrospraying concentrated (～20 mM) acetonitrile solutions of ionic liquids (molten salts) of composition CA (C⁺ = cation, A^? = anion). Mass analysis of these mobility-selected ions reveals the occurrence of individual neutral ion-pair evaporation events from the smallest singly charged clusters: (CA)_nC⁺→(CA)_{n? 1}C⁺+CA. Although bulk ionic liquids are effectively involatile at room temperature, up to six sequential evaporation events are observed. Because this requires far more internal energy than available in the original clusters, substantial heating (～10 eV) must take place in the ion guides leading to the mass analyzer. The observed increase in IL evaporation rate with decreasing size is drastic, in qualitative agreement with the exponential vapor pressure dependence predicted by Kelvin’s formula. A single evaporation event is barely detectable at n = 13, while two or more are prominent for n ≤ 9. Magic number clusters (CA)₄C⁺ with singularly low volatilities are found in three of the four ionic liquids studied. Like their recently reported liquid phase prenucleation cluster analogs, these magic number clusters could play a key role as gas-phase nucleation seeds. All the singularly involatile clusters seen are cations, which may help understand commonly observed sign effects in ion-induced nucleation. No other charge-sign asymmetry is seen on cluster evaporation.     

The fragmentation of metastable NH+3 ions and isotopic analogs: an example of tunneling through a rotational barrier

The fragmentation of metastable NH⁺₃ ions and isotopic analogs via the reaction NH⁺₃ → NH⁺₂ + H has been investigated using mass analysed ion kinetic energy spectrometry (MIKES). Kinetic energy release distributions and the metastable intensity were measured as a function of ion source temperature. Both the average kinetic energy release and the metastable intensity increase with ion source temperature. The data are consistent with the metastable reaction arising from tunneling through a rotational barrier. The experimental data are compared with the predictions of a tunneling model.     

A correlation between structure and reactivity in ion clusters

Observations on metastable peaks resulting from the unimolecular decomposition of ion clusters show that intensity variations as a function of cluster size can reveal the presence of stable cluster configurations. This technique has been used to confirm that (H₂O)₂₁H⁺ and (D₂O)₂₁D⁺ are stable ion clusters, and the method also provides evidence to suggest that Ar₁₉⁺ is a particularly stable species.     

Studies in cluster ion formation. Part I. Fast atom bombardment mass spectrometry of tetraalkylammonium halides: Factors influencing cluster ion size and stability

Positive and negative ion fast atom bombardment mass spectra of tetraalkylammonium halide salts (NR₄X, where X = Cl, Br, I and NR₄ = NMe₄, NEt₄) have been studied and intense cluster ion formation has been observed. The cluster ion intensity distributions were found to show enhancements at certain cluster numbers (n). The negative cluster ions of NMe₄X salts showed anomalous ion intensity regions, which differed from both the positive cluster ions of all NR₄X salts and also the corresponding negative clusters of NEt₄X salts. The influence of anion and cation size on cluster ion formation and abundances has been studied and it has been established that smaller anion and cation size favours the formation of larger cluster ions. The possible structures of the cluster ions exhibiting relative increased stabilities are discussed.     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Mass spectra of the bromofluorocyclotriphosphazenes

The mass spectra of geminal-, cis- and trans--bromofluorocyclotriphosphazenes (N₃P₃Br_nF_6−n, n = 2–5) have been obtained at an electron energy of 70 eV. When n = 2 or 3 there is a substantial difference between the geminal and non-geminal isomers for the process involving loss of a bromine atom from the parent ion. When n = 2–4 there is a distinction between the types of linear ions formed from geminal and non-geminal isomers. Cis- and trans-isomers could not be distinguished. The relative proportions of cyclic and linear ions are discussed. Groups of partially resolved ions were observed resulting from the decomposition of isotopically related metastable ions. Several new types of metastable transition were observed.     

Production and properties of CO2 cluster anions

Stoichiometric and non-stoichiometric negatively charged CO₂ cluster ions have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focussing sector field mass spectrometer. The observed abundance anomalies (“magic numbers”) in the mass spectra of (CO₂) _n ^? and (CO₂) _n O^? ions correlate with corresponding small and large metastable fractions of these ions (for loss of one CO₂ unit). Variation of the measured metastable fractions as a function ofn are related to corresponding changes in the monomer binding energies. In addition, we have observed for the first time (CO₂) _n O ₂ ^? ions (i.e. at electron energies above 8 eV with an energy resonance at about 14 eV) and we discuss possible production mechanisms for these ions. Relative electron attachment cross sections have been determined in the energy regime O<E≦20 eV for (CO₂) _n ^? , (CO₂) _n O^? and (CO₂) _n O ₂ ^? withn=1 to 20. The shape of the cross section function for (CO₂) _n O^? is strongly dependent on the cluster sizen.     

Organic cluster ions from continuous-flow fast atom bombardment

Cluster ions from fast atom bombardment of liquid alcohols and nitriles were examined using a continuous-flow technique. Protonated molecular M_nH⁺ species are the dominant cluster ions observed in molecules of formula M. The abundances of the M_nH⁺ cluster ions decrease monotonically with increasing n, and within a homologous series the M_nH⁺ abundance diminishes more rapidly for higher molecular mass compounds. Reaction products (ROH)_n(H₂O)H⁺ and (ROH)_n(ROR)H⁺ are observed also in the case of alcohols, and the ion abundances decrease with increasing n. Radiation damage yields fragment ions and ionic alkyl reaction products which are captured in solvent clusters. Semi-empirical molecular orbital methods were used to examine the energetics of cluster ion formation and decomposition pathways. Metastable decomposition processes exhibit only evaporative loss of monomers, with the probability of loss increasing sharply with n. The evaporative ensemble model of Klots was used to predict the cluster size-dependent trends of metastable dissociation processes observed for alcohol and nitrile cluster ions.     

Electron bombardment fragmentation of size selected molecular clusters

(CO₂) _n , (NO) _n and (NH₃) _n clusters are generated in a supersonic molecular beam and size selected by scattering from an He beam. By measurements of angular dependent mass spectra, TOF distributions and the angular dependence of the scattered signal quantitative information on the fragmentation probability by electron impact is derived. The van der Waals systems (CO₂) _n and (NO) _n appear only at masses which are simply multiples of the monomer mass. The preferred cluster ion is the monomer ion for all investigated cluster sizes withn=2 to 4. The fragment pattern for the quasi-hydrogen bonded (NH₃) _n -cluster shows, beside a large number of fragment masses, a preference for protonated ions. The results are explained in terms of simple models based on the structural change from the neutral to the ionized configuration and the fragmentation pattern of the monomer followed by ionmolecule reactions.     

Tandem mobility mass spectrometry study of electrosprayed tetraheptyl ammonium bromide clusters

Multiply charged electrospray ions from concentrated solutions of Heptyl4N+Br- (designated A+B- hereafter) in formamide are analyzed mass spectrometrically (MS) following mobility selection in ambient air in a differential mobility analyzer (DMA). Most of the sharp mobility peaks seen are identified as (AB)(n)A+ clusters, with 0 < or = n < ot = 5. One anomalously abundant and mobile ion is identified as NH4+(AB)4. Six ions in the (AB)n(A+)2 series are also identified, completing and correcting earlier mobility data for singly and doubly charged ions up to masses of almost 9000 Da. The more mobile of two broad humps seen in the mobility spectrum includes m/z values approximately from 2500 up to 12,000 Da. It is formed primarily by multiply charged (AB)n(A+)z clusters with multiple ammonium bromide adducts. Because of overlapping of many peaks of different m/z and charge state z, only a few individual species can be identified by MS alone in this highly congested region. However, the spectral simplification brought about by mobility selection upstream of the MS reveals a series of broad modulations in m/z space, with all ions resolved in the second, third, ...sixth modulation being in charge states z = 2, 3, ...6, respectively. Extrapolation of this trend beyond the sixth wave fixes the ion charge state (in some cases up to z = 15) and mass (beyond m = 175,000 u). This wavy structure had been previously observed and explained in terms of ion evaporation kinetics from volatile drops, though without mass identification. All observations indicate that the clusters are formed as charged residues, but their charge state is fixed by the Iribarne-Thomson ion evaporation mechanism. Consequently, the measured curve of cluster diameter versus z yields the two parameters governing ion evaporation kinetics. Clusters with z > 1 and electrical mobility Z > 0.495 cm2/V/s are metastable and evaporate a singly charged cluster, probably (AB)2A+, between the DMA and the MS. Plotting the electrical mobilities Z of the clusters in the form (z/Z)1/2 versus m(1/3) (both proportional to cluster diameter) collapse the data for all cluster sizes and charge states into one single straight line for Z below 0.495 cm2/V/s. This linear relation reveals a uniform apparent cluster density of 0.935 g/cm3 and an effective hard-sphere diameter of the air molecules of 0.44 nm. An anomalous mobility increase is observed at diameters below 3 nm.