Fragmentation Of Cluster Ions Research Articles

Intracluster Reaction, Fragmentation, and Structure of Monomethylamine, Dimethylamine, and Trimethylamine Cluster Ions

Ammonia, monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) clusters, formed in a supersonic expansion, were investigated using a multiphoton ionization time-of-flight mass spectrometer. The observed major product ions resulting from prompt fragmentation following ionization are (NH3)nH+, (CH3NH2)nH+, [(CH3)2NH]nH+, and [(CH3)3N]nH+. Detection of stable CH2NH2+ and (CH2NHCH3)+ immonium ions and other fragments provides evidence for molecular rearrangement and fragmentation within our observable time windows. Anomalously large relative intensities observed for (NH3)5H+, (CH3NH2)4H+, [(CH3)2NH]3H+, and [(CH3)3N]2H+ are attributed to enhanced stability of the cluster ions, due to the complete filling of hydrogen-bonding sites on the “central ions”. Fully optimized geometries and stabilization energies of these cluster ions have been predicted using ab initio molecular orbital methods. The experimental findings are in very good agreement with the calculated results.

Fragmentation and stopping of heavy cluster ions in a lithium target

Correlated stopping of N ion debris flowing at same velocity in a lithium target is worked out from the Basbas-Ritchie model of 2-cluster stopping in degenerate electron jellium. A very large energy loss enhancement is demonstrated. It is strongly modulated by the ions' topological arrangement. Relevance to inertial fusion driven by intense cluster ion beam is stressed.

Small carbon rings: dissociation, isomerization, and a simple model based on strain

Injected ion drift tube techniques have been used to examine the properties of carbon rings with 10–36 atoms. Previous studies have shown that the monocyclic ring is the dominant isomer for annealed clusters with 10–36 atoms, while for unannealed clusters a bicyclic ring first appears at n ≈ 22 and becomes the dominant isomer for n > 30. A detailed study of the annealing of the bicyclic ring has been performed for clusters with 24–36 atoms. Activation energies for the bicyclic to monocyclic transition are around 2.4eV and show a slight systematic decrease with increasing cluster size. The fragmentation of carbon cluster ions containing 6–30 atoms has been studied and dissociation energies estimated. A simple model for the strain energy of the carbon rings accounts for many of their physical properties, including isomerization to monocyclic rings, their fragmentation patterns, and dissociation energies. From simple estimates of their stabilities, planar graphite fragments should be lower in energy than the monocyclic rings for medium sized clusters ( n > 30), but little, if any, of this isomer is observed even after annealing. The low abundance of the graphite fragments is attributed to the large activation energy (induced by strain energy) for their formation from monocyclic rings.

Cluster ion stopping and fragmentation for ICF

Cluster ion beam with energy in the several tens of keV/a.m.u. range are considered as a novel direct driver for a simple fusion pellet made of deuterium+tritium fuel surrounded by a lithium pusher. The driver-pellet interaction is calculated through the hypothesis of maximum multifragmentation followed by highly correlated ion debris motion. One thus gets enhanced stopping and ablation pressure in the range of hundreds of Mbar.

Proton transfer dynamics and cluster ion fragmentation in phenol/ammonia clusters

Excited-state proton transfer dynamics are reported for the phenol(NH3)n cluster system. Excited-state proton transfer is shown to occur for this system by isotopic substitution of the hydroxyl proton and ammonia protons. The observed dynamics slow from 80 to 600 ps upon cluster deuteration. The effects of cluster vibrational energy and ammonia concentration in the expansion gas are also studied. The results of these experiments are compared with a previous study [Syage and Steadman, J. Chem. Phys. 95, 2497 (1991)] which used a single excitation energy and varied ionization energy. Our experiments indicate that a significant amount of cluster ion fragmentation occurs for this system and caution must be exercised in assigning the decays observed in a given ion mass channel to a particular parent cluster. Due to the difficulties of associating a given decay curve with a cluster of particular mass, a previously proposed model of proton transfer in isolated clusters [Hineman et al., J. Chem. Phys. 97, 3341 (1992)] which details the effect of cluster vibrational energy on the proton transfer rate cannot be applied to the phenol/ammonia system.

Infrared predissociation in cluster ions containing a guest chromophore. A gentle route to the dynamics and spectroscopy of heterogeneous systems

New observations of the infrared excitation and fragmentation of heterogeneous cluster ions of the form Y·X n + are presented. By ensuring that IP(Y) > IP(X n ) we show that it is possible to study the infrared spectroscopy of neutral chromophores (Y) in mass-selected clusters. By monitoring photofragmentation channels in conjunction with other aspects of cluster ion behaviour, such as intensity fluctuations (the presence of magic numbers), absorption spectra can be related to structure and decay dynamics. Preliminary results are presented for SF 67middot;Ar n +, SF 6·(CO 2) n +, SF 6·(NO) n +, and C 2H 4·(NO) n +.

Investigations on processes induced by single- and multiple-scattering of electrons in clusters involving excimer-induced fragmentation and a new metastable fragmentation channel observed for ArnO+* cluster ions

A new metastable decay reaction of ArnO+ cluster ions produced by the electron impact ionization of argonoxygen clusters has been discovered, leading to the evaporation of more than two monomers in the metastable time regime. It is estimated that an energy of approx. 1 eV has to be released from the intramolecular to intermolecular modes prior to this metastable decay. In contrast to previously observed excimer-induced fragmentation of pure argon cluster ions, it has been concluded that the localized metastable state of an ionic rather than a neutral species is involved in the fragmentation. A reaction mechanism involving an electronic transition from the a2Π ArO+* (O(3P)-Ar+(2P)) excited state to the X4Σ− ArO+ (O+(4S)-Ar(1S)) ground state of the ArO+ complex ion within the cluster and subsequent release of considerable amount of vibrational energy to cluster modes is suggested to account for the observed fragmentation. Based on these measurements, a lifetime of approx. 2 µsis proposed for the excited state a2Π ArO+*.

The fragmentation of proton-bound cluster ions and the gas-phase acidities of alcohols

The gas-phase acidities of 52 alkanols and cycloalkanols have been determined from studies of the unimolecular and collision-induced dissociation of proton-bound cluster ions [RO·H·OR′] −. It is found that CH 3OH is 0.6 kcal mol −1 more acidic (lower Δ H o acid value) than CD 3OH, while C 2H 5OH is 0.6 kcal mol −1 more acidic than C 2D 5OH. A detailed evaluation of the effect of methyl substitution α, β and γ to the oxygen on the acidities of the alcohols is presented, as well as an evaluation of the role of charge-induced dipole interactions on gas-phase acidities. The Δ o acid values for stereoisomeric methylcyclohexanols are reported. cis-Methyl groups lower the Δ H o acid values of the alcohols, the cis-2-methyl group showing the largest effect. The trans-2-methyl group also increases the acidity but trans-methyl groups in the 3 and 4 positions have no effect on the acidity of the cyclohexanol.

Benzyl alcohol–water and benzyl alcohol–ammonia clusters: Ion fragmentation and chemistry

Benzyl alcohol/ammonia, α,α-dimethylbenzyl alcohol/ammonia, and benzyl alcohol/water cluster ion fragmentation and chemistry are studied for isolated cold clusters by means of one and two-color mass resolved excitation spectroscopy, nozzle/laser timing delay, and deuteration experiments. Experiments lead to an identification of parent clusters for all fragment ion clusters observed. Three types of cluster ion fragmentation are observed for these systems: dissociation−solu+(solv)n→solu+(solv)k+m solv; acid-base chemistry−ArCH2OH+(B)n→ArCH2O(B)k+BmH+; and (benzyl) radical chemistry−ArCH2OH+(B)n→ArĊHOH(B)k+BmH+, ArCD2OH+(B)n→ArCDHOH+(B)k+BmB−d2 and ArCMe2OH+(B)n→ArĊMeOH+(B)k+BmṀe. Fragmentation reactions depend on cluster size, structure, and (weakly) on the vibrational energy deposited in the ion. Specifically, for benzyl alcohol+(NH3)1 only cluster radical chemistry and dissociation take place, while for higher order clusters, the acid-base reaction rate increases and this reaction becomes a major fragmentation pathway for benzyl alcohol+(NH3)4. For the benzyl alcohol(H2O)n system, cluster radical chemistry is not observed with n=1, only a weak α-hydrogen transfer reaction is observed with n=2, and acid base chemistry is not observed for clusters of any size. Cluster dissociative fragmentation is also a function of cluster size; large water and ammonia clusters dissociate much more easily than do n=1 clusters. The possible mechanisms for these fragmentation patterns are discussed.

Fragmentation and stopping of heavy cluster ions in a lithium target−Application to target implosion

Heavy-ion clusters are considered for driving directly a pellet for inertial confinement fusion. They are shown to be effective at much lower energy than requested for heavy atomic ions. Multifragmentation, arising from a Coulomb explosion, in the target, is discussed with a maximum entropy hypothesis. Direct driving pressure imparted to a deuterium–tritium (DT) pellet is estimated through the ion debris range in the target. The correlated stopping of N ions flowing close to the initial projectile trajectory is seen to be much larger than the uncorrelated one. Specific attention is paid to a few regular geometrics of cluster ion debris stopped in a lithium pusher. The calculations elaborate on the very high driving pressure obtained, for the case of a solid-state density of cluster ion projectiles around the pellet (moving tamper). A target containing 4 mg of deuterium and tritium fuel is demonstrated to be thus imploded in 5 nsec. A driving pressure of 500 Mbars yields an energy output of 630 MJ. This result can be obtained with an initial cluster kinetic energy smaller than 20 keV/amu.

Toluene–water clusters: Ion fragmentation and chemistry

Toluene/water cluster ion fragmentation is studied for isolated cold clusters by means of one- and two-color mass resolved excitation spectroscopy, time resolved pump (S1←S0) probe (I←S1) spectroscopy on the nanosecond time scale, and nozzle/laser delay timing experiments. These experiments lead to an identification of parent clusters for all fragment ion clusters observed. Fragmentation reactions depend on cluster size and on the energy deposited in the ion by the two photon I←S1←S0 excitation sequence. Fragments identified by these techniques include (H2O)xH+ (x=3,...,6) and toluene+(H2O)n−1 for toluene(H2O)n clusters and (H2O)xD+, (H2O)xH+, and toluene–d+3(H2O)n−1 for toluene–d3(H2O)n clusters. For n≤3 the preferred cluster fragmentation pathway is loss of a single H2O molecule, while for n≥4 the preferred cluster fragmentation pathway is generation of (H2O)nH+. Cluster ion fragmentation is prevalent in this system because of product stability (i.e., solvated protons and the benzyl radical) and because the I←S1 transition leaves the cluster ion in a very highly excited vibrational state (Δv≫0 for the I←S1 transition). The fragmentation of toluene+(H2O)3 to generate (H2O)3H+ and a benzyl radical takes place by two distinct pathways with generation times τ1<60 ns and τ2=480 ns. The toluene+(H2O)2 fragmentation from toluene+(H2O)3 has a generation time of τ<60 ns. The possible energetics, kinetics, and mechanisms for these fragmentations are discussed.

The infrared photofragmentation of Ar+2. Evidence of excited state population from dimer and cluster ionization

Argon dimer ions have been generated via three different techniques: (1) autoionization; (2) vertical ionization of neutral Ar2; (3) ionization and subsequent fragmentation of argon cluster ions. In experiments (2) and (3) the dimers and clusters are formed via the adiabatic expansion of argon in a supersonic beam. In each case Ar+2 ions have been mass selected and subjected to single-photon infrared excitation (912–1094 cm−1) using a line-tunable carbon dioxide laser in a crossed-beam arrangement. Only those Ar+2 ions with internal energies within 1000 cm−1 of a dissociation limit yield Ar+ photofragments, the kinetic energy spread of which has been measured using an electrostatic analyzer. The photofragment kinetic energy spectra of dimer ions formed by autoionization do not exhibit any dependence on the angle of laser polarization; it is proposed that such behavior is due to the presence of a high thermal rotational temperature (500 K). In contrast, the corresponding spectra of Ar+2 formed via vertical ionization, exhibit two quite distinct features, one of which shows a strong dependence on laser polarization angle. Calculations show that the latter behavior is most probably due to photodissociation out of an excited spin–orbit state of Ar+2. A very pronounced increase in Ar+2 infrared photodissociation signal is observed as a function of increasing nozzle stagnation pressure. To account for such behavior it is proposed that, following ionization, argon cluster ions fragment to give dimer ions in excited vibrational/rotational levels both in the electronic ground and an excited spin–orbit state.

Measurements of kinetic energy release following the unimolecular and collision-induced dissociation of argon cluster ions, Ar+n, for n in the range 2–60

A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision-induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (<10−7 s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale >10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n>4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar+15.

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 − 2NH 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.

Dynamics of cluster ion fragmentation

Experiments and calculations designed to probe the structure of inert-gas cluster ions are discussed. Examples are drawn from studies on the unimolecular fragmentation of large argon and krypton cluster ions. Preliminary results are presented from measurements of the kinetic-energy release associated with the unimolecular decay of argon cluster ions.

On ionisation induced unimolecular dissociation of sodium clusters

Fragmentation of sodium cluster ions (Na + ,x<42) was studied via photoionisation of neutral precursors. Expansions of metal vapor out of cylindrical and conical nozzles yielded supersonic beams with differing cluster compositions. Measurements of photoionisation efficiency curves in the 3–6 eV range for both types of expansion allow quantitative separation of direct ionisation and unimolecular dissociation contributions to specific ion signals. Data for Na 8 + and Na 7 + are analysed to yield lower limits on bond energies. Results obtained for larger clusters are also discussed.

Balloon-borne composition measurements of stratospheric negative ions and inferred sulfuric acid vapor abundances during the MAP/GLOBUS 1983 campaign

Balloon-borne composition measurements of stratospheric negative ions were carried out at altitudes between about 30 and 40 km using two improved mass spectrometer probes with high sensitivity and mass resolution mounted on a gondola carried by a 350,000 m 3 balloon. In order to minimize the risk of contamination, data were taken only at float altitude and during balloon descent. Besides the major ion species, HSO 4 - (H 2SO 4) l(HNO 3) m and NO 3 - (HNO 3) n, various minor ion species were detected including also mixed clusters with H 2O, SO 3 and possibly HOCl ligands. It appears that the SO 3 ligands were formed by electric field-induced collisional cluster ion fragmentation during sampling of the ions into the mass spectrometer. Sulfuric acid vapor abundances inferred from the present negative ion composition data reveal the presence of a sulfuric acid vapor layer with a pronounced maximum around 37 km with a H 2SO 4 vapor concentration of about 3 × 10 6 cm −3, which is in accord with a previous measurement by our group. The occurrence of a maximum suggests that sulfuric acid vapor is efficiently removed at heights above about 37 km. Potential removal processes include photolysis, OH-attack and eventually also reactions with meteor smoke particles.

The influence of stagnation pressure and electron energy on the unimolecular decomposition of CO 2 microclusters

Electron impact mass spectra of carbon dioxide clusters, generated by supersonic beam expansion, were investigated at various electron energies between 30 and 100 eV and stagnation pressures between 0.1 and 3.0 bar. The distribution of cluster ions (CO 2) N + ( N ≦ 8) was observed to depend on electron energy most strongly at high stagnation pressures. In contrast to monomeric CO 2 +, fragmentation of cluster ions to product ions (CO 2) N−2 O 2 + and (CO 2) N−1 X + (X = O, CO) is influenced by electron energy variations in the range studied. The dependence of relative ion intensities on the electron energy is strongest at high stagnation pressures. It is shown that phase transitions or evaporation reactions in the clusters can be employed for the interpretation of the experimental results. At low stagnation pressures the fragmentation behaviour of the clusters is mainly influenced by their intrinsic properties. At high stagnation pressures evaporation processes become more important.

Structure and bonding in small silicon clusters.

Accurate ab initio molecular-orbital calculations are performed to obtain the ground-state geometries and electronic configurations of the silicon clusters ${\mathrm{Si}}_{\mathrm{n}}$ for n=2--7,10 and ${\mathrm{Si}}_{\mathrm{n}}^{+}$ for n=2--6. The effects of polarization functions and electron correlation are included in these calculations. All the optimized structures are considerably reconstructed from those derived from microcrystal geometries. Ionization potentials and binding energies are calculated and used to interpret the distribution and fragmentation of small silicon cluster ions observed recently.

Photoelectron–photoion coincidence spectroscopy of gas-phase clusters

A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a single component of a gas-phase mixture has been developed. It utilizes a newly designed instrument which measures the ion mass in coincidence with the photoelectron kinetic energy. Initial experiments were carried out on Xe2 and Xe3 produced in mixture of clusters (plus monomer) in a free-jet supersonic expansion. These measurements determined the photoelectron appearance potential (i.e., the lowest binding energy for which photoelectrons are detected) to be 11.30(5) eV for Xe3. It was also found that fragmentation of cluster ions strongly affects the coincidence spectra. This was investigated by varying the stagnation pressure, and thus the beam composition, in order to assess fragmentation contributions to coincidence spectra of the cluster under study. One case studied in detail indicated the energy levels of Xe3 near the ionization potential of 11.6 eV, corresponding to 0.7 eV of internal energy in Xe+3, fragmented to form Xe+2 and Xe.