Photofragmentation of mass resolved carbon cluster ions

The results of a detailed study of the photodissociation of carbon cluster ions, C ₃ ⁺ to C ₂₀ ⁺ , are presented and discussed. The experiments were performed using internally cold cluster ions derived from pulsed laser evaporation of a graphite target rod in a helium buffer gas followed by supersonic expansion. The mass selected clusters were photodissociated using 248 nm and 351 nm light from an excimer laser. Photofragment branching ratios, photodissociation cross sections and data on the laser fluence dependence of photodissociation are reported. For almost all initial clusters, C _n ⁺ , the dominant photodissociation pathway was observed to be loss of a C₃ unit to give a C _n?3 ⁺ ion. This observation is interpreted as indicating that dissociation occurs by a statistical unimolecular process rather than by direct photodissociation. The photodissociation was found to be linear with laser fluence forn>5 with 248 nm and 351 nm light; quadratic forn=5 for 248 nm and 351 nm; and linear forn=4 at 248 nm. Dissociation energies for the carbon cluster ions implied by these results are discussed. The photodissociation cross sections were found to change dramatically with cluster size and with the wavelength of the photodissociating light.     

Studies of cluster anions C n X− (X=N,P, As,Sb, Bi) produced by laser ablation

A series of cluster anions C _n X^? are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For C _n N^?, only anions with oddn can be observed; For C _n P^? and C _n As^?, cluster anions with evenn are produced but with lower signal intensities; For C _n Sb^?, the signal intensity of clusters does not show even/odd alternation; Finally, for C _n Bi^?, the intensities of cluster anions with evenn are higher than those with oddn. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of C _n X^? from C _n N^? to C _n Bi^? can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hückel model was applied to account the structural feature of these clusters.     

Studies of Sb and Bi cluster produced by laser desorption

Direct production of cations and anions of metal clusters of Sb and Bi by laser evaporation in a vacuum has been studied. Bulk sample substrates are irradiated by 1064, 532 and 355 nm beams at variable intensity, and the ions produced are accelerated and identified by time-of-flight mass spectrometry. At 1064 nm, the cation distributions show that Sb ₃ ⁺ and Bi ₃ ⁺ are the most abundant species, while the monomer and dimer cations are almost non-existent. The anion spectra indicate very low yields of Sb^? and Bi^? with dominant dimer anion species. These patterns persist with laser power variation within the stable operation domain. With lower incident laser wavelength, the mass distributions are modified, favouring the production of the light cluster ions. In no circumstances were Sb and Bi ions withn>5 observed. Many of the observed phenomena can be explained if one assumes that for these elements, clusters withn<6 are formed on the substrate surface. Cluster ions are produced via a prompt desorption process, and are subjected to photon induced reactions due to the same incident laser beam. However, more detailed investigation of the desorption properties will be necessary to confirm such a desorption mechanism.     

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.     

激光溅射金属等离子体与醇类分子束反应的研究：产物离子对分子束状态的依赖性

The gas phase reactions of metal plasma with alcohol clusters were studied by time of flight mass spectrometry （TOFMS） using laser ablation-molecular beam （LAMB） method. The significant dependence of the product cluster ions on the molecular beam conditions was observed. When the plasma acted on the low density parts of the pulsed molecular beam, the metal-alcohol complexes M^＋An （M=Cu, Al, Mg, Ni and A=C2H5OH, CH3OH） were the dominant products, and the sizes of product ion clusters were smaller. While the plasma acted on the high density part of the beam, however, the main products turned to be protonated alcohol clusters H^＋An and, as the reactions of plasma with methanol were concerned, the protonated water-methanol complexes H3O^＋（CH3OH）n with a larger size （n≤12 for ethanol and n≤24 for methanol）. Similarly, as the pressure of the carrier helium gas was varied from 1 × 10^5 to 5 × 10^5 Pa, the main products were changed from M^＋An to H^＋An and the sizes of the clusters also increased. The changes in the product clusters were attributed to the different formation mechanism of the output ions, that is, the M^＋An ions came from the reaction of metal ion with alcohol clusters, while H^＋An mainly from collisional reaction of electron with alcohol clusters.     

Bare phosphorus and binary phosphide cluster ions generated by laser ablation

Both positive and negative phosphorus cluster ions were generated from the laser ablation of a red phosphorus sample. The mass distribution of phosphorus cluster ions was found to be very sensitive to the power density of the ablation laser. The P ₇ ⁺ species exhibits the highest signal intensity in the recorded mass spectra of bare phosphorus cluster cations, as does P ₅ ^- among the anions. Their special structural stability can be attributed to their planar configuration and their aromatic character. As the phosphorus cluster size increases, the odd/even alternation of the signal intensity becomes more pronounced. For the P _n ⁺ species with n > 24, the relative abundance varies in the order of 8 and P _n ⁺ with n = 8k + 1 (k = 3–11) are more intense than their neighbors. For comparison, some binary phosphide cluster ions, including C_nP _m ^- , Si_nP _m ^- , B_nP _m ⁺ and Al_nP _m ⁺ , were produced as well. The mass distribution of binary phosphide cluster ions changes with different components. From analysis of the recorded mass spectra of the phosphide cluster ions, the larger clusters may be in a polyhedral configuration and tend to have all valence electrons paired.     

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.     

Substitution reactions in the secondary ion mass spectrometry of N-methylpyridinium halides

Secondary ion mass spectra of N-methylpyridinium halides (C⁺X^?, where C⁺ is a pyridinium cation and X^? is a halogen anion) exhibit the C⁺ ions, a series of cluster ions ((C⁺)_n(X^?)_n–1) and, furthermore, remarkable [CX – R]⁺ ions (R = H or Me). The mechanism of the formation of [CX – R]⁺ ions was investigated by the use of deuterated compounds and B/E and B²/E constant linked-scan measurements. A possible explanation is proposed in which the ions are produced through substitution reactions between species constituting the C₂X⁺ cluster ions in the gas phase.     

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.     

Optical response of cesium coated C60

Photoabsorption spectra are reported for Cs _n ⁺ and C₆₀Cs_n ⁺ + clusters for n=40, 60, 120 and 310. The spectra were obtained by heating the mass selected clusters in a beam by means of photoabsorption until they evaporated metal atoms. The resulting mass loss was observed in a time-of-flight mass spectometer. The plasmon-like resonance in pure Cs clusters shifts to lower energies with decreasing cluster size. The collective electronic excitations in clusters containing C₆₀ are split in energy as would be expected for fullerene molecules coated with layers of metal.     

Reactions of small carbon cluster ions with HCN: a structural probe

The results of a detailed study of the primary and secondary reactions of carbon cluster ions, C _n ⁺ (3≤n≤20), with HCN are presented and discussed. The experiments were performed in a Fourier transform (ICR) mass spectrometer, using direct laser vaporization of graphite to form the carbon cluster ions. Evidence for two structural forms of then=7, 8 and 9 cluster ions is obtained from their differing reactivity with HCN. The C ₇ ⁺ ion is anomalous in its behavior in many respects, which is interpreted by an isomerization mechanism. The HCN reactions offer a contrast to the reactions with nonpolar neutrals studied previously. All HCN reactions produced ions of the type C_nX⁺ (primary product) or C_nXY⁺ (secondary product) where X, Y=H, CN or HCN. Fragmentation of the original carbon cluster was not observed, while radiative association is an important reaction channel. Low-energy collision-induced dissociation studies of the product ions support the mechanism of insertion into the H-CN bond and formation of covalent bonds at the carbene site for the primary reactions. In most secondary reactions however, the HCN associates weakly with the ion, rather than binding covalently.     

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO₂) _k ^? (4≦k≦25) and (O₂) _k ^? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO₂) _m and (O₂) _m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s ³ P _{2, 0}) Penning ionization. For both CO ₂ ^? and O₂-clusters, the negative and the positive ion mass spectra are very different. For (CO₂) _k ^Emphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF ₆ ^? formation in collisions with SF₆. For (O₂) _k ^? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O ₄ ^? as dominant species).     

Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N-Acetylcysteine and nine N-acetylcysteine conjugates of synthetic origin were characterized by positive- and negative-ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S-substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive-ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence-specific fragment ions (A⁺, B⁺, [Y + 2H]⁺, Z⁺), immonium ions (I⁺) and a diagnostic fragment ion for mercap-turic acids (R_M⁺) are detected. The negative-ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side-chain specific cleavage ([R_XS]^?) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]⁺ or [nM - H]^? ion type (n = 1-3) are observed. The analysis of an equimolar mixture of eight N-acetylcysteine conjugates shows different quasi-molecular ion yields for the positive- and negative-ion spectra.     

Encasing of Na+ ion in dimer‐formed acetic acid clusters

Peaks with anomalous abundance found in the mass spectra are associated with ions with enhanced stability. Among the scientific community focused on mass spectrometry, these peaks are called ‘magic peaks’ and their stability is often because of suggestive symmetric structures. Here, we report findings on ionised Na‐acetic acid clusters [Na⁺‐(AcA)_n] produced by Na‐doping of (AcA)_n and UV laser ionisation. Peaks labelled n = 2, 4, 8 are clearly distinguishable in the mass spectra from their anomalous intensity. Ab initio calculations helped elucidate cluster structures and energetic. A plausible interpretation of the magic peaks is given in terms of (AcA)_n formed by dimer aggregation. The encasing of Na⁺ by twisted dimers is proposed to be the origin of the enhanced cluster stability. A conceivable dimer‐formed tube‐like closed structure is found for the Na⁺‐(AcA)₈. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

The gas phase “matrix isolation” spectroscopy of CH3F

We report infrared photodissociation spectra for Ne, Ar, Kr, N₂ and CH₄ clusters which contain CH₃F chromophores. The CH₃Fv ₃ mode is excited with a line tunable CO₂ laser. Mass spectrometer detection of changes in the cluster beam intensity serve to partially distinguish the spectra of different size neutral clusters. Many spectra consist of rather broad, inhomogeneous profiles. For intermediate size Ar_nCH₃F clusters a sharp, narrow peak is observed in the spectrum. We assign this peak as due to a cluster in which a central CH₃F molecule is surrounded by at least a full shell of Ar atoms packed in a contracted icosohedral geometry. Because the Ar atoms in a gas phase cluster are unconstrained by an extended crystalline structure, the CH₃F dipole is more fully stabilized (and thus red-shifted) than in a solid matrix. The dependence of the observed spectrum on cluster size is discussed. For comparison, no comparable narrow spectral features are observed in Ar_nC₂H₄ cluster spectra. Clear evidence is also presented that the fragmentation of the neutral clusters upon electron impact ionization is fairly specific. Finally, we note that ionization of Ar_nCH₃F clusters sometimes produces Ar_nF⁺ ions. This is a fragmentation process which does not occur in free CH₃F.     

Hot tungsten clusters: competition between atom ejection and thermionic emission

This is a first report concerning the thermionic emission of electrons from hot metal clusters. Tungsten clusters were exposed to a 10 ns laser pulse, and the delayed emission of electrons from the clusters was recorded as a function of time after the excitation. A large yield of W _n ⁺ cluster ions, which were born as late as microseconds after the laser pulse, has been detected forn>4. Tungsten cluster ions created via thermionic emission show no measurable metastable decay during the flight time in the mass spectrometer. This indicates that they are colder than expected, if evaporative cooling after prompt ionization would prevail.     

Positive ion — negative ion coincidence spectroscopy of O2 and H2 using synchrotron radiation

Kinetic studies of the reactions of neutral lead clusters with NO₂, NO and O₂ were performed at 300 K. Reaction with NO₂ is rapid, with the observed second-order rate constants for most clusters being between 0.2 and 5 × 10^?11 cm³/s. There is a general trend of increasing rate with cluster size, although a few clusters display unusually high or low rates compared to ones of neighboring size. The reactions with NO are considerably slower by factors ranging from about 5 to 10. Reaction products are observed by laser ionization at 193 and 222 nm in conjunction with time-of-flight mass spectrometry. At lower fluence, the association products Pb _x (NO₂)⁺ and Pb _x (NO₂) ₂ ⁺ are observed in the case of reactions with NO₂. At higher laser fluence, Pb _x ⁺ and Pb _x O _x ^?1+ dominate the mass spectra of Pb _x reactions with NO₂, showing that the products fragment to more stable oxides. No reaction with oxygen was observed for any cluster, setting upper limits on the rates of 5 × 10^?14 cm³/s.     

Positive ion–molecule reactions in OCS/hydrocarbon mixtures

The positive ion–molecule reactions of OCS have been investigated in an ion cyclotron resonance spectrometer. A variety of reactions in OCS/hydrocarbon mixtures have been investigated for various C₁? C₄ hydrocarbons—alkanes, alkenes and alkynes. The formation of organosulfur ions is found in reactions in OCS/hydrocarbon (C_n) mixtures with n <4. Formation of organosulfur ions is observed from hydrocarbon ions reacting with OCS and [OCS]⁺˙ and S⁺˙ reacting with the hydrocarbons. The proton affinity of OCS has been determined to be 688.7±8 kJ mol^?1 while that of CS₂ is measured to be 712.1±8 kJ mol^?1. Comparison with the proton affinity of CO₂ shows that the proton affinity increases as sulfur is substituted for oxygen.     

Proton transfer and isotope‐induced reaction in aniline cluster ions

The proton transfer (PT) and other intraclusters reactions occurring after electron ionization of aniline clusters (PhNH₂)_N are investigated by the time‐of‐flight mass spectrometry. The mass spectra are recorded for different expansion conditions leading to the generation of different cluster sizes. Several fragment ions are shown to originate from intracluster reactions, namely, [Ph]⁺, [PhNH₃]⁺ and [Ph–N–Ph]⁺. Reaction schemes are proposed for these ions starting with the PT process. The mass region beyond the monomer mass is dominated by cluster ions (PhNH₂)_n⁺ accompanied by satellites with ±H and +2H. In experiments with deuterated species, new fragment ions are identified. The aniline isotopomer d₅‐PhNH₂ yields the fragment ions (PhNH₂)_n?(N–Ph–NH₂)⁺. Analogical series is observed in experiments with d₇‐PhND₂, and additional fragments occur corresponding to (PhND₂)_n?(D₂N–ND–Ph–ND–ND₂)⁺ ions. The possible reaction pathways to these ions and the unusual isotope effects are discussed. Copyright © 2015 John Wiley & Sons, Ltd.