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Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer ( m/ Δm ≅ 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He. The yield of He shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 ± 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters HeKr indicate closure of a solvation shell at m = 12. [ABSTRACT FROM AUTHOR]

Helium nanodroplets, formed in a supersonic expansion, are doped with C60 in a pickup cell. In some experiments, they are co-doped with water. Electrons are attached to the doped droplets; the yield of anions is recorded as a function of electron energy. The C60 - yield extends to much higher energies than in experiments involving isolated, hot fullerenes; we attribute the difference to the low temperature of the neutral precursors and the efficient cooling of the nascent anions by the helium droplet, which quench thermally activated autodetachment. The yields of (C60)2 -O60D2O- anions reveal another important factor, namely depletion of the anion signal by dissociation which is energetically more facile than autodetachment. [ABSTRACT FROM AUTHOR]

Neon dimer ions undergo spontaneous dissociation (metastable decay) several microseconds after formation by electron impact ionization of neon clusters. In this contribution we compare the kinetic energy release distribution (KERD) of the previously reported isotopomer 20Ne2 + with that of 22Ne2 +. The heavy isotopomer shows the same two components in the KERD as the lighter ones. However, the high-energy component that is due to electronic pre-dissociation is reduced in intensity. The decrease is attributed to a reduced predissociation rate from the II(1/2u) state into I(3/2u). [ABSTRACT FROM AUTHOR]

The recent addition of (i) a third sector field to our two sector field mass spectrometer (resulting in a BE1E2 field configuration) and of (ii) a high performance electron gun enables us now to study in detail the time dependence of the kinetic energy release distribution (KERD) over a relatively wide range of cluster ion lifetimes. Using this newly constructed device we have studied here for the first time KERDs and deduced binding energies BEs (using finite heat bath theory) of large rare gas cluster ions (an upper size limit in earlier studies arose from the fact that different naturally occurring isotopes will contribute to a chosen metastable peak when the size exceeds a certain value) and in addition of fullerene ions smaller and larger than C60+ (here again contaminating coincidences did not allow such studies earlier). Moreover, high precision KERD measurements for the decay of rare gas dimer ions in conjunction with model calculations (using recently calculated potential energy curves for the rare gas dimer ions) also enable us to obtain information on the dynamics and the mechanisms of the underlying spontaneous decay reactions Rg2+∗ → Rg+ + Rg. In addition, we are also reporting here a novel method (unified breakdown graph method) to determine cluster ion binding energies using a recently constructed tandem mass spectrometer BESTOF allowing us to measure fragmentation patterns arising from the unimolecular decay of molecular cluster ions induced by surface collisions. The fragmentation and reaction patterns of protonated ethanol cluster ions investigated here clearly demonstrate in contrast to some of the earlier cluster ion studies that unimolecular dissociation kinetics determines the formation of product ions in the surface-induced decomposition. [ABSTRACT FROM AUTHOR]

We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar5 + to Ar15 +, it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions. [ABSTRACT FROM AUTHOR]

Na clusters in the size range of 50–400 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. [ABSTRACT FROM AUTHOR]

Three different types of electron impact ionization experiments have been performed, involving neutral and charged C and C. 1) We have determined absolute partial ionization cross sections for formation of parent ions C and C in charge states up to z = 4, and of singly and multiply charged fragments of size n ≥ 44 and n ≥ 50 from C and C neutral precursors, respectively. 2) Previous appearance energy measurements of C 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 C has been crossed with an intense electron beam; the induced reactions (fragmentation, post- ionization, and dissociative post-ionization) have been analyzed. [ABSTRACT FROM AUTHOR]

Resonance-enhanced multiphoton ionization (REMPI) of the $$\tilde B$$ and $$\tilde C'$$ states of NH in a pulsed supersonic molecular beam was observed at 304-340 nm, using the multiphoton-ionization mass-spectrometer system described herein. The observed features are the result of a 2+1 MPI process and correspond quite well with the reported 3+1 MPI of NH in the literature from the ground state of opposite parity. [ABSTRACT FROM AUTHOR]

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, 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 Δ F( N)= F( N−1)−2 F( 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. [ABSTRACT FROM AUTHOR]

The structure and dynamics of lattice defects in copper are investigated. The defects have been produced by irradiation with heavy ions of 350 keV between 85 K and 300 K. Three different defect configurations are distinguished by the time differential perturbed angular correlation technique (DPAC) withIn as radioactive probe. Their electric quadrupole coupling constants ν= e Qq/ h are ν=116(2)MHz, ν=181(3)MHz and ν=52(1)MHz. The defects characterized by ν and ν are observed in the temperature range of recovery stage III. The appearance of ν is closely correlated with recovery stage V. The symmetry axis of the electric field gradient associated with this defect-In configuration points along a crystallographic 〈111〉 direction. We attribute this defect to a planar faulted loop, presumably of vacancy type. The influence of irradiating particles and dose on the defect configurations is analyzed. [ABSTRACT FROM AUTHOR]

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=98. [ABSTRACT FROM AUTHOR]

Multiply charged ammonia cluster ions are produced by adiabatic nozzle expansion and subsequent ionization by electron impact. They are analyzed in a double focussing sector field mass spectrometer (reversed geometry). Doubly charged clusters are only detected above a critical size of 51 and triply charged clusters above 121. Some of these multiply charged ions decay via metastable dissociation processes in the experimental time window accessible. Doubly charged ammonia clusters with sizes of n≧51 lose one neutral monomer or, roughly ten times less probable, two neutral monomers. Conversely, triply charged ammonia clusters with sizes 110≦ n≦120 show an extremely asymmetric Coulomb dissociation resulting in doubly charged cluster ions of about 90% of the initial mass [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

The superior stability of closed-shell icosahedral structures is evident from size distributions of argon, krypton and xenon cluster ions in the size range 100≲ n≲1000. [ABSTRACT FROM AUTHOR]

Metastable decay of (N), formed in a supersonic jet and ionized by electron impact, has been analyzed for n≤50. The probability for decay of (N) into (N), plotted versus x, exhibits pronounced oscillations. The 'period' of these oscillations increases with increasing precursor size n, but converges to an average value of approximately 4.7 beyond n=25. [ABSTRACT FROM AUTHOR]

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 ( 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. [ABSTRACT FROM AUTHOR]

Neon dimer ions undergo spontaneous dissociation (metastable decay) several microseconds after formation by electron impact ionization of neon clusters. In this contribution we compare the kinetic energy release distribution (KERD) of the previously reported isotopomer 20Ne2 + with that of 22Ne2 +. The heavy isotopomer shows the same two components in the KERD as the lighter ones. However, the high-energy component that is due to electronic pre-dissociation is reduced in intensity. The decrease is attributed to a reduced predissociation rate from the II(1/2u) state into I(3/2u). [ABSTRACT FROM AUTHOR]