Abstracts

Kinetic Energy Release Distributions and Evaporation Energies for Metastable Fullerene Ions,
S. Matt, O. Echt, M. Sonderegger, R. David, P. Scheier, J. Laskin, C. Lifshitz, and T. D. Märk
Kinetic energy release distributions for spontaneous (metastable) decay of singly, doubly and triply charged fullerene ions have been measured for C₆₀^z+ and C₅₈^z+. The data are analyzed in terms of generalized Maxwell distributions. Based on recently measured values of the Gspann parameter g , energies for evaporation of C₂ are derived. An average evaporation energy E_a = 11.6 eV is obtained for g = 37.6; E_a = 10.1 eV is obtained for g = 33. No significant dependence of E_a on the size or charge state of the ions is observed within the experimental error of about -11 to +5 %.

Kinetic Energy Release for Metastable Fullerene Ions,
S. Matt, M. Sonderegger, R. David, O. Echt, P. Scheier, J. Laskin, C. Lifshitz, and T. D. Märk
Kinetic energy release distributions for spontaneous (metastable) decay of fullerene ions, C_n^z+, have been determined for parent sizes n = 58, 60, and 70, and charge states z = 1, 2, 3 and 4. Extensive numerical modeling has been applied to verify that the shapes of fragment ion peaks in our MIKE spectra are not affected by discrimination, making it possible to derive the center-of-mass total kinetic energy release (KER) distributions directly from the spectra. Measurements involving C₃H₈⁺ and C₃H₇⁺ ions confirm the accuracy of the approach.

Evaporation Rates for Na-Clusters
J. Borggreen, F. Chandezon, O. Echt, H. Grimley, K. Hansen, P.M. Hansen, C. Ristori
Na clusters in the size range of 50 to 400 atoms are led to evaporate in a heat bath with controlled temperature. For specific cluster sizes just above closed shells, decay rates are measured as a function of temperature. They agree with an Arrhenius law and allow absolute separation energies to be extracted.

Reply to the Comment by Stampfli and Märk,
Klavs Hansen and Olof Echt
The authors of [1] argue that the time dependence of the electron yield can have a power-law behavior as observed, i.e. with a power less than one, if only the energy distribution is sufficiently different from a flat one. In Fig.1 of [1] the decay on the experimentally sampled time scale involves molecules that have energies at the decreasing edge of the energy distribution (E > 40 eV); this produces a quasi-power-law decay with exponent less than one. The discreteness of the photon energy can be ignored as also noted in the Comment. The assumptions underlying Fig. 1 [1] are, however, contradicted by the experimental data.

High Resolution Electron Ionization Study of CO, (CO)₂, and (CO)₃: Appearance Energies and Bond Dissociation Energies
D. Muigg, G. Denifl, A. Stamatovic, O. Echt and T.D. Märk
Electron ionization cross sections for CO, Co dimers and trimers have been investigated near the threshold with a newly constructed crossed beam apparatus using a hemispherical electron monochromator (HEM) to monochromatize the primary electron beam. The dissociative attachment cross section curve for CO used to test the new set-up shows, in accordance with the high resolution study of Stamatovic and Schulz, a vertical onset at the thermochemical threshold (9.63 eV) of the lowest possible DA channel O^- (²P) + C(³P) in excellent agreement with the value of 9.63 eV derived from known thermochemical data, thereby indicating an energy scale accuracy of better than 10 meV. The appearance energies (AE) of some rare gases (Ar, Kr, Xe) and molecules (N₂, O₂, N₂O) measured for calibration purposes agreed with the known ionization energies (IE) of these compounds within 10 meV.
Using a novel data handling procedure, involving a simultaneous non-linear weighted least-squares fit of two functions, the following appearance energies were obtained from the measured ionization cross section curves using the AE of CO⁺ as reference: AE((CO)₂⁺) = 13.194 ± 0.053 eV and AE((CO)₃⁺) = 12.98 ± 0.34 eV. These values are in excellent agreement with ionization energies obtained in a high resolution photoionization experiment by Ng and co-workers yielding 13.05 ± 0.04 eV and 12.91 ± 0.04 eV, respectively.

Clusters and Cluster Ions
O. Echt and T. D. Märk
Mass spectrometry has provided a wealth of information about atomic clusters. Experiments which helped to unravel the geometric and electronic structure of clusters as well as dissociative and non-dissociative reactions of singly and multiply charged cluster ions are summarized.

On the Efficiency of Thermionic Emission from C₆₀,
Rongping Deng and Olof Echt
We have determined the number of delayed electrons emitted from an ensemble of gas-phase C₆₀, excited at 355 nm with light from an unfocused pulsed Nd:YAG laser. Delayed electrons are detected by single particle counting and integrated over a time interval of 0.1 µs £ t £ 80 µs. At low laser fluence I, the electron number scales as I^p with p = 6.0 ± 0.4. It approaches saturation at a maximum laser fluence of 100 mJ/cm², equivalent to a power density of 14 MW/cm², where delayed electron emission reaches a probability of 2.6 ± 1.1 % per multiphoton-excited C₆₀. This experimental value is interpreted as a lower limit to the quantum efficiency for electron emission from C₆₀; it provides an upper limit to the efficiency of competing reactions such as dissociation of C₆₀ into C₅₈ + C₂. The dependence of the electron number on the temperature of the fullerene source is in good agreement with the number density of C₆₀ in the molecular beam, as computed from published equilibrium vapor pressures.

Relative Dissociation Energies of Singly and Doubly Charged Fullerene Ions, C_n^z+, for n = 52 to 70,
S. Matt, R. Wörgötter, P. Scheier, C. E. Klots, O. Echt and T. D. Märk
Relative activation energies for dissociation of fullerene ions C_n^z+ into C_n-2^z+ + C₂ have been determined for z = 1 and 2, covering the size range 52 £ n £ 70. These data are derived from measured appearance energies of fragment ions, obtained by electron impact ionization of C₇₀, with help of the finite heat bath theory. The present results are compared with experimental and theoretical values for C₂ loss reported in the literature.

Sequential Reaction Channels of Metastable C₆₀⁴⁺,
B. Dünser, O. Echt, P. Scheier, and T. D. Märk
We employ a two-sector-field mass spectrometer to identify sequential, unimolecular dissociation channels of C₆₀⁴⁺. In addition to sequential "monomer evaporations" (loss of two C₂ units), we observe two novel sequential channels which involve fission-like events: Loss of C₂⁺ followed by evaporation, and sequential loss of two C₂⁺ units.

Dissociation of Singly and Multiply Charged Fullerenes: Emission of C₄, or Sequential Emission of C₂?
M. Foltin, O. Echt, P. Scheier, B. Dünser, R. Wörgötter, D. Muigg, S. Matt, and T. D. Märk
We have obtained direct mass spectrometric evidence that fullerene ions C₆₀^z+ (z = 1, 2, or 3) and C₅₈^z+ (z = 1, 2) undergo unimolecular dissociation by sequential emission of two C₂ units, on a time scale of 10^-5 s. Moreover, a comparison of experimental and theoretical breakdown graphs reveals that unimolecular formation of C₅₆⁺ from the C₆₀⁺ parent ion within a given observational time window is dominated by successive loss of C₂; direct C₄ loss does not contribute significantly. This conclusion is not affected by uncertainties in our knowledge of the energetics of C₂ versus C₄ loss.

Photoexcited C₆₀: Fragmentation and Delayed Ionizotion,
Rongping Deng, Geoffrey Littlefield and Olof Echt
We investigate two reaction channels of C₆₀, excited by a pulsed laser at 355 nm: i) The relative and absolute yields of even-sized fragment ions are measured over a wide range of laser fluences. At low fluence we find a dramatic dependence of the abundance ratio of C_n-2⁺ versus C_n⁺. This result argues against the notion that unimolecular emission of C₄ or larger fragments contributes significantly to the production of even-sized fragment ions. ii) We count the number of delayed electrons emitted from C₆₀, or one of its products, after photo-excitation. For high laser fluences, this number reaches a value of (2.6 ± 1.1) % per photoexcited C₆₀. This sets a lower bound to the ratio of effective rate constants, k_e(E^*)/å k_j(E^*), where k_e refers to electron emission, and the sum in the denominator extends over all reaction channels.

Electron impact ionization and dissociation of neutral and charged fullerenes,
S. Matt, O. Echt, T. Rauth, B. Dünser, M. Lezius, A. Stamatovic, P. Scheier and T. D. Märk
Three different types of electron impact ionization experiments have been performed, involving neutral and charged C₆₀ and C₇₀. 1) We have determined absolute partial ionization cross sections for formation of parent ions C₆₀^z+ and C₇₀^z+ in charge states up to z = 4, and of singly and multiply charged fragments of size n ³ 44 and n ³ 50 from C₆₀ and C₇₀ neutral precursors, respectively. 2)  Previous appearance energy measurements of C₇₀ have been improved and extended to z = 5; ionization energies are found to depend linearly on the charge state of the precursor, in agreement with theoretical predictions. 3) A beam of mass selected C₆₀²⁺ has been crossed with an intense electron beam; the induced reactions (fragmentation, post-ionization, and dissociative post-ionization) have been analyzed.

Thermionic Emission and Fragmentation of C₆₀
K. Hansen and O. Echt
We analyze the rate of delayed (thermionic) ionization of photo-excited C₆₀ molecules. The rate has a power law dependence, indicating the presence of a continuum of rate constants. The value of the exponent provides information about the competition between delayed ionization and unimolecular fragmentation; it is equal to the ratio of the ionization energy and the activation energy for fragmentation. This result provides a novel method to measure the controversial bond dissociation energy of neutral C₆₀. We obtain a value of 11.9 ± 1.9 eV.

Appearance and ionization energies of multiply-charged C₇₀ parent ions produced by electron impact ionization
S. Matt, O. Echt, R. Wörgötter, V. Grill, P. Scheier, C. Lifshitz and T.D. Märk
Using an improved crossed beams/mass spectrometer apparatus we have extended previous electron impact ionization studies concerning ionization cross sections and appearance energies of up to quadruply-charged ions of C₇₀ to ions with charge states up to 6. A novel refined data procedure, involving a simultaneous non-linear weighted least squares fit of two functions ((NWLT), allows to objectively extract the cross section threshold. The obtained ionization energies depend linearly on the precursor charge state and compare well with theoretical predictions and the molecular capacitor scaling law. The corresponding maximum cross sections of C₇₀⁵⁺ and C₇₀⁶⁺ are about 1.5 10^-18 cm² and 1 10^-20 cm², respectively.

Direct Evidence for the Sequential Decay C₆₀^z+ --> C₅₈^z+ --> C₅₆^z+ --> …
P. Scheier, B. Dünser, R. Wörgötter, D. Muigg, S. Matt, O. Echt, M. Foltin, and T. D. Märk
Using a two sector field mass spectrometer in combination with a crossed beams ion source we have obtained direct experimental evidence that C₆₀ fragment ions such as C₅₈, C₅₆, C₅₄,… produced by electron impact ionization of C₆₀ may be formed by unimolecular decay of the C₆₀ parent ion involving sequential loss of C₂. Moreover, by comparing experimental and theoretical breakdown graphs the overriding conclusion is that in the case of C₅₆^z+ production (with z = 1, 2, 3) sequential loss of two C₂ units dominates over the loss of a single C₄ unit.

Convergence of Cluster Properties towards Bulk Behavior: How Large is Large?
Olof Echt
Recent progress in experimental techniques has made it possible to explore properties of clusters containing hundreds or thousands of monomers from two different venues: Preparing ever smaller particles in solution, or growing ever larger particles in the gas phase. Results obtained through these approaches will be used to illustrate the evolution of bulk properties in clusters with increasing size. In many cases, properties approximately scale as the inverse cluster radius and, hence, will have converged to the bulk limit within about 10% when the cluster size exceeds 10³ monomers, equivalent to a radius of 1 to 2 nm. However, there are several properties that do not exhibit a simple 1/r dependence.

Interaction of Electrons with C₆₀⁺ and C₆₀^-: Post-Ionization, Dissociation, and Kinetic Energy Release
T. Rauth, O. Echt, P. Scheier and T. D. Märk
We report on the interaction of energetic electrons with size- and charge-selected fullerene ions in the second field-free region of a double-focusing sector-field mass spectrometer. C₆₀²⁺ and C₆₀⁺ are formed from C₆₀⁺ and C₆₀^-, respectively. The kinetic energy released in the formation of fragment ions, C₅₈²⁺ and C₅₈⁺, is measured.

Resonant Formation of SF₆ Containing Cluster Anions in a Pick-up Experiment Involving Argon and Nitrogen Clusters
V. Grill, O. Echt and T. D. Märk
Electron attachment to species formed in an experiment involving either Ar or N₂ clusters and SF₆ background gas leads to formation of cluster anions M_m(SF₆)_n^- containing as many as twenty monomers (M = Ar or N₂) and up to five SF₆ molecules, including "pure" (SF₆)_n^-. Ions form resonantly near 0 eV. In case of N₂ clusters, another resonance near 1.8 eV is observed which is related to (intra-cluster) autoscavenging, involving the ^2P _g transient negative ion state of solvated N₂. These observations, and the absence of SF₅ containing clusters, make it possible to unambiguously identify the mechanism through which the observed ions form: In a first step, mixed neutral clusters M_x(SF₆)_y arise from pick-up reactions, subsequent electron attachment to these species results in mixed and pure cluster ions.

Evolution of Cluster Properties towards Bulk Behavior
Olof Echt
This review discusses the evolution of bulk properties in clusters of increasing size n. In many cases, clusters beyond n » 10³ (cluster radius larger than 1 to 2 nm) are "nearly" bulk like. Spectral shifts in electronic transitions in solvated molecules, the ionization energy and band structure of metal clusters, and the melting transition, are considered in detail.

Multiply Charged Clusters
O. Echt and T. D. Märk
We present a comprehensive review of the literature published before 1992.

Thermionic Emission from Free, Phot-Excited Tungsten Clusters
Thomas Leisner, Kalliopi Athanassenas, Dietmar Kreisle, Ekkehard Recknagel, and Olof Echt
We report on delayed electron emission from free tungsten clusters, excited by light from a Q-switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of ~ 50 ns to 5 µs after the laser pulse, without interference from prompt ions. All clusters of size 5 £ n £ 40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate, for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds within « 50 ns, i.e. that the observed phenomenon is the (molecular) analogon of thermionic emission. Semi-quantitative agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed de-excitation channels, namely emission of atoms or photons, are not significant under our experimental conditions.

Thermionic Emission from Fullerenes
Gerhard Walder, Keith W. Kennedy, and Olof Echt
We investigate the time dependence of carbon cluster ions, formed via thermionic emission from photo-excited fullerenes (C₆₀ and C₇₀). By pulsing the extraction field, we are able to observe delayed ions formed as late as 100 µs after excitation at 532 nm, 355 nm, or 266 nm. All even-sized clusters in the range 36 £ n £ 70 undergo thermionic emission..

Delayed Electron Emission from Higher Fullerenes C_n, n < 96, Following Excitation at 1064 nm
Keith W. Kennedy and Olof Echt
Excitation of gas-phase fullerenes with laser pulses at 1064 nm yields large amounts of higher fullerene ions C_n⁺, 72 £ n £ 96, even though their neutral precursors are only minor contaminants in the vapor over the toluene extract of soot. The ions are particularly prominent in the size distribution of delayed ions recorded at low fluence. Comparison with data obtained at 532 nm suggests that the size dependent variations in the onset of photoabsorption are responsible for the observed phenomena. We propose that delayed ionization may be utilized to distinguish between higher fullerenes and other, more loosely bound, carbon aggregates.

Production of Non-Stoichiometric SF₆ Cluster Anions by Electron Attachment to SF₆ Clusters
T. Rauth, O. Echt and T.D. Märk
We report on the detection of (SF₆)_nX^- (X = SF₅, SF₄, SF₃) ions which form by electron attachment to SF₆ clusters in a molecular beam. The formation of dome species, e.g. SF₆SF₅^- with resonances at 6 and 10 eV, is unexpected because attachment to the bare SF₆ molecule does not yield the analogous monomer fragment ion in this energy range.

Thermionic Emission from Fullerenes
Gerhard Walder and Olof Echt
Fullerenes in the gas phase, excited by a pulsed laser, exhibit thermionic emission. We analyze the time dependence of the ion yield of C_n⁺ in a mass spectrometer. By pulsing the extraction field of the ion lens, we are able to observe delayed ions formed as late as 50 µs after excitation by the 2nd, 3rd or 4th harmonic of a Q-switched YAG laser. The enhanced sensitivity of this new technique allows us to detect delayed ions not only from excited C₆₀ and C₇₀, but from all other even-sized clusters in the size range 36 £ n £ 70. Our results do not confirm the assertion that thermionic emission from C₆₀ and C₇₀ can be characterized by just 2 or 3 distinct rate constants.

Multiply Charged Atomic Clusters
Olof Echt
In this contribution I shall highlight some general trends, some discrepancies, and some findings pertaining to fission of atomic clusters. For a much more detailed discussion, the reader is referred to a recent review by Echt and Märk. A brief comparison with nuclear clusters will conclude this chapter.

Thermionic Emission of Free Tantalum Clusters
T. Leisner, K. Athanassenas, O. Echt, D. Kreisle, and E. Recknagel
In this paper we report on the thermionic emission of electrons from hot metal clusters. Tantalum clusters were exposed to a 10ns laser pulse, and the delayed emission of electrons from the clusters was recorded as a function of cluster size as well as the time after the excitation. A large yield of Ta_n⁺ cluster ions, which were born as late as microseconds after the laser pulse, has been detected for n > 3. The data are discussed in terms of a simple model based on the Richardson Dushman - equation and compared to the results already obtained for tungsten clusters.

Hot Tungsten Clusters: Competition Between Atom Ejection and Thermionic Emission
T. Leisner, K. Athanassenas, O. Echt, O. Kandler, D. Kreisle, and E. Recknagel
This is a first report concerning the thermionic emission of electrons from hot metal clusters. Tungsten clusters were exposed to a 10ns 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 for n > 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.

Electronic Shell Structure in Multiply Charged Silver Clusters
O. Kandler, K. Athanassenas, O. Echt, D. Kreisle, T. Leisner, and E. Recknagel
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.

The Influence of Shells, Electron Thermodynamics, and Evaporation on the Abundance Spectra of Large Sodium Metal Clusters
S. Bjųrnholm, J. Borggreen, O. Echt, K. Hansen, J. Pedersen, and H. D. Rasmussen
Measurements of the mass abundance spectra of sodium clusters containing up to 600 atoms are presented. The clusters are produced in a seeded supersonic expansion of Ar or Kr gas, and the spectra are obtained by a time-of-flight technique. The sawtooth features in the spectra are interpreted as evidence of a regular spherical shell structure with magic numbers, N_o, scaling approximately with the cube root of the number of sodium atoms. Altogether twelve shell closings are observed, N,= 2, 8, 20, 40, 58, 92, 138, 196, 260, 344, 440 and 558. There is also a pronounced odd-even staggering all the way up to N = 70. The experimentally observed intensity changes for the clusters around the magic numbers are discussed in terms of the electronic free energy, F(N), calculated at finite temperature, and the second differences of the free energy D ₂F(N)= F(N - 1) - 2F(N)+ F(N + 1). The processes behind the non-uniform abundance distributions, and the thermodynamics of finite electron systems with non-uniform level spacings are discussed on this basis.

Mean-Field Quantization of Several Hundred Electrons in Sodium Metal Clusters
S. Bjųrnholm, J. Borggreen, O. Echt, K. Hansen, J. Pedersen, and H. D. Rasmussen
Measurements of the mass-abundance spectra of sodium clusters from supersonic expansions are presented. The spectra show evidence of a regular spherical shell structure with magic numbers N_o scaling approximately with the cube root of the number of sodium atoms N, and hence the number of delocalized (valence) electrons in the cluster. Altogether twelve shell closings are observed, adding to the previously known shell closings, N_o = 2, 8, 20, 40, 58, 92, and 138, the 8-12th magic numbers N_o = 196, 260, 344, 440, and 558. The implications of shell structure in such large systems are discussed.

Magic Numbers in Mass Spectra of Large van der Waals Clusters
Olof Echt, Otto Kandler, Thomas Leisner, Werner Miehle and Ekkehard Recknagel
Mass spectra of rare-gas clusters (Ar, Kr and Xe), produced in the neutral state and ionized by electron impact, exhibit steep intensity drops at n = 147 (148 for Ar), 309, 561 and 923, indicating that very large cluster ions relax successfully into complete shell icosahedra on the time scale of the experiment. This also holds true for large CO and CH₄ clusters, which feature magic numbers at n = 147 and 309. Furthermore, a series of intensity anomalies, common to Ar, Kr, Xe, CO and CH₄, is observed in the range 147 < n < 309. They closely correlate with the number of atoms for which closure of an outer icosahedral subshell is possible.

Mass Spectrometric Evidence for Icosahedral Structure in Large Rare Gas Clusters: Ar, Kr, Xe
W. Miehle, O. Kandler, T. Leisner and O. Echt
Clusters of argon, krypton, and xenon are grown in a free jet and ionized by electron impact. The size of these clusters, (Rg)_n⁺, extends up to n » 1000. Individual cluster sizes are mass resolved up to n » 570 in the case of Ar. The well known, but puzzling differences in the size distributions of Kr and Xe clusters disappear beyond n » 130, while those between Ar and Xe disappear beyond n » 220. The most pronounced "magic numbers" in the distributions of large cluster ions occur at n = 147 (148 for Ar), 309, and 561, in striking agreement with the number of atoms required to build icosahedral clusters with 3, 4, and 5 complete coordination shells, respectively. Closure of the 6th icosahedral coordination shell is indicated by another strong intensity drop at n » 923 in the unresolved part of the spectra. Several additional intensity extrema are observed between major shell closures. A simple structural model, assuming an icosahedral core decorated by the additional atoms, accounts for these anomalies reasonably well up to n = 561.

Carbon Monoxide Clusters: Critical Size and Magic Numbers
O. Kandler, T. Leisner, O. Echt, and E. Recknagel
Fully resolved mass spectra of carbon monoxide clusters have been recorded in the size range n £ 320. Intensity anomalies in these spectra beyond n = 135 are strikingly similar to those being observed in krypton and xenon spectra. Particularly pronounced intensity drops occur at n=147 and 309. For the first time, these data provide evidence for icosahedral structure in large molecular cluster ions. Concerning doubly charged CO clusters, their lower size limit has been measured to be n_c = 98.

Dissociation Channels of Multiply Charged van der Waals Clusters
O. Echt, D. Kreisle, E. Recknagel, J.J. Saenz, R. Casero and J.M. Soler
We have analyzed the reaction channels for delayed, unimolecular dissociation of multiply charged clusters made from CO₂ and C₂H₄. The smallest observable triply and quadruply charged clusters feature a large probability for fission into at least two charged fragments. The size distribution of heavy, doubly charged fragments from (CO₂)_n³⁺, 109 £ n £ 123, has been analyzed in detail, it strongly peaks at 92 % of the precursor size. Delayed fission of doubly charged clusters, however, does not occur on the time scale of the experiment, which is 10 us £ t £ 100 us with respect to ionization. A simple model, based on a liquid drop approximation, is proposed. It allows to calculate the height of the fission barrier for any given precursor ion and fission channel from readily available bulk data. Calculated critical sizes are presented for 2-, 3- and 4-fold charged clusters of 28 different, van der Waals bound or hydrogen bonded compounds; they agree reasonably well with experimental values. The model also accounts for the strongly asymmetric fission reaction of triply charged CO₂ clusters, and for the strong size dependence of their fission rate.

Quantum Effects in the Decomposition of Nitrogen Clusters
T. Leisner, O. Echt, O. Kandler, Yan Xue-Jian and E. Recknagel
We have analyzed metastable decompositions of nitrogen cluster ions, (N₂)_n⁺, n £ 50, produced in a supersonic beam with subsequent electron impact ionization. The dissociation probability, plotted versus the number of desorbed N₂ molecules, exhibits an oscillatory pattern. This is attributed to radiationless relaxation of vibrationally excited N₂ molecules in the cluster. The number of molecules being lost per quantum rapidly increases in the size range 20 £ n £ 25, but attains a constant value, D n » 4.7, above.

Electron Attachment to Water Clusters under Collision-Free Conditions
Martin Knapp, Olof Echt Dietmar Kreisle, and Ekkehard Recknagel
Long-lived water cluster anions (H₂O)_n^-, n ³ 11, can be formed by electron attachment to preformed, cold water clusters. Attachment occurs resonantly at energies very close to 0 eV. Obviously, the adiabatic electron affinity of these clusters is nonnegative, and electron trapping is not impeded by a potential barrier. Under comparable expansion conditions, attachment to cold, pre-existing clusters yields the same minimum cluster anion size as injection of low-energy electrons into the condensation zone of neat water vapor, as reported previously by Haberland and co-workers. The smallest cluster anions being observed show a high probability for electron detachment in the field free drift tube of the time-of-flight mass spectrometer.

Formation of Long-Lived CO₂^-, N₂O^-, and their Dimer Anions, by Electron Attachment to van der Waals Clusters
M. Knapp, O. Echt, D. Kreisle, T. D. Märk and E. Recknagel
CO₂^- and N₂O^- anions, which cannot be formed by electron attachment to the corresponding molecules in the gas phase, are formed by dissociative attachment to clusters of CO₂ and N₂O, respectively. The relative yields of the monomer and dimer anions are presented and discussed.

Time-Dependent Size Distribution of XenonCluster Ions
D. Kreisle, O. Echt, M. Knapp, and E. Recknagel
The evaporation of monomers following electron-impact ionization causes a significant enhancement of the intensity anomalies ("magic numbers") in mass spectra of xenon clusters. The dissociation rates are measured on a microsecond time scale for clusters containing between 10 and 79 atoms. The highest dissociation probability, 99 ± 1 %, is found for Xe₅₇⁺, which is two atoms larger than a closed-shell icosahedral cluster. Each anomaly in the dissociation rates corresponds to an intensity anomaly (magic number) in the mass spectrum. Recent Monte Carlo simulations for Xe_n⁺, n » 30, are in full agreement with our experimental results.

Magic Numbers for Sphere Packings: Experimental Verification in Free Xenon Clusters
O. Echt, K. Sattler, and E. Recknagel
The existence of magic numbers for atomic microclusters has been found experimentally for the first time. The magic numbers n^* manifest themselves in the mass spectra of free xenon clusters, nucleated in the gas phase. The observed numbers n^* = 13, 55, and 147 coincide with the numbers of spheres required for complete-shell icosahedra. The appearance of further magic numbers (19, 25, 71, and 87) is only partially explained by previous calculations.

Evidence for Coulomb Explosion of Doubly Charged Microclusters
K. Sattler, J. Mühlbach, O. Echt, P. Pfau, and E. Recknagel
Mass spectra of Pb, Nal, and Xe microclusters show that below a critical number of atoms per cluster, n^**, doubly charged particles are not stable. These numbers are 30 for Pb_n, 20 for (NaI)_n, and 52 for Xe_n. The two positive charges generated at one atom by electron bombardment are likely to move to opposite sides of the cluster which then explodes into singly charged fragments as long as the Coulomb repulsion energy is greater than the binding energy.