Photoionisation studies of homogeneous argon and krypton clusters using TPEPICO

The photoionisation threshold region of homogeneous Argon and Krypton clusters Ar _n and Kr _n forn up to 24 formed in a free jet expansion has been studied in detail, using the threshold photoelectron photoion coincidence (TPEPICO) time of flight technique. Measurements performed at a variety of different expansion conditions (nozzle temperature and stagnation pressure) demonstrate that fragmentation of larger clusters contributes substantially to the shape of the TPEPICO spectra even for the smallest clusters and at all photon energies higher than about 200 meV to 400 meV above the ionisation threshold. The determination of ionisation potentials for these cluster ions is discussed and careful estimates are given and compared with recent theoretical values.     

Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH₃) _n and (ND₃) _n clusters withn up to 8. The fourth harmonic (～ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH₃ while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH₃) ₂ ⁺ cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH₃NH₂NH₄.     

TPEPICO studies near ionization threshold of argon and krypton clusters

Single photon ionization of argon and krypton clusters has been studied in the region between threshold and the ionization potential of the corresponding atom. Synchrotron radiation from the electron storage ring BESSY is used to ionize the clusters; threshold-photo-electron-photoion-coincidence (TPEPICO)-time-of-flight technique is used to detect ions correlated with the emission of zero-kinetic-energy-electrons. The spectra of the clusters in the range ofn=2 to 15 are discussed in view of the extensive fragmentation taking place in these systems. In order to characterize the properties of the clusters a method using scaling laws is applied. The principles and the deduction of Hagena's scaling parameter Γ* are briefly reviewed. Using Γ* an experimentally derived mean cluster size for molecular beams can be assigned. This allows one to clearly demonstrate the systematic variations of the measured spectra due to cluster fragmentation. As a general feature it is observed that, in the range studied, the peak in the measured ionization rate for a cluster ion (fragment) of a given size shifts to higher photon energies as the mean cluster size is increased.     

The photoionization dynamics of Ne clusters

Photoionization experiments have been performed to investigate Ne clusters after absorbing a single photon of known energy. The photon energies used were below the threshold energy of the Ne atom. Masses and initial velocities of the clusters were determined by time-of-flight techniques with a resolution ofm/Δm ≈ 500 (t/Δt ≈ 1000). The mass spectra of Ne clusters differ significantly from those reported for other rare gas clusters. An analysis of the isotopic composition of the clusters showed an enrichment of²²Ne, which is attributed to a photo-induced fragmentation process.     

On the determination of monomer dissociation energies of small water clusters from photoionization experiments

Recently, monomer dissociation energies of neutral water clusters were estimated via a thermodynamic cycle that utilized the measured appearance energies of vacuum ultraviolet (VUV) photoionized water clusters and the previously reported dissociation energies of protonated water clusters. The thermodynamic cycle incorrectly assumed that the energy difference between the (H2O)n+ --> (H2O)n-1H+ + OH* asymptotes (the relaxation energy) was zero. We show that these relaxation energies are large and cannot be neglected in the analysis. Thus, the neutral water cluster monomer dissociation energies cannot be directly determined from the measured ionization potentials because they are themselves involved in the appropriate thermodynamic cycle.     

On the structure of water—alcohol and ammonia—alcohol protonated clusters

Collision-induced dissociation (CID) of protonated ammonia-alcohol and water-alcohol heteroclusters was studied using a triple quadrupole mass spectrometer with a corona discharge atmospheric pressure ionization source. CID results suggested that the ammonia-alcohol clusters had NH: at the core of the cluster and that hydrogen-bonded alcohol molecules solvated this central ion. In contrast, CID results in water-alcohol clusters showed that water loss was strongly favored over alcohol loss and that there was a preference for the charge to reside on an alcohol molecule. The results also indicated that a loose chain of hydrogen-bonded molecules was formed in the water-alcohol clusters and that there appeared to be no rigid protonation site or a fixed central ion. (J Am Soc Mass     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Two-color threshold photoionization spectroscopy of jet-cooled indole clusters

We report the results of a comprehensive investigation of the two-color threshold photoionization of jet-cooled indole clusters. Using two-color photoionization spectroscopy, we have probed both the neutral excited levels and the ground ionic states of indole clusters containing non-polar (Ar, CH₄, CF₄, C₆H₆) and polar (H₂O, MeOH, EtOH, NH₃, N(CH₃)₃) solvent species. These studies have allowed the determination of accurate cluster ionization energies (IEs) as well as the assignment of electronic absorption features to clusters of known composition. The determination of the cluster IE, which is typically lower than that of bare indole, has allowed us to investigate the importance of charge-induced dipole and charge-dipole attractive forces in the binding of the ion-neutral clusters. In addition, we have found that the shape of the photoionization efficiency (PIE) spectra gives valuable information regarding the relative shape and/or position of the potential energy surfaces of the neutral excited and ground ionic states of the clusters. We have also identified two distinct conformational isomers of the indole-(H₂O)₁ hydrogen bonded cluster using the techniques of electronic spectroscopy, two-color threshold ionization spectroscopy and mass analysis.     

IR dissociation of ammonia clusters

Dissociation spectra of NH₃ clusters have been recorded using a cw CO₂ laser. For the dimer two absorption bands have been found at 979 and 1004 cm⁻¹, which originate from the excitation of two non-equivalent NH₃ molecules. A tunneling motion is held responsible for the observed structure on one of these bands. The symmetry group of the NH₃ dimer is presented considering the tunneling motion solely. Heavier NH₃ clusters dissociate at frequencies between 1020 and 1100 cm⁻¹. The dissociation spectrum of the SiH₄-NH₃ complex shows one peak centered at 972.3 cm⁻¹.     

Study of the fragmentation of small sulphur clusters

The fragmentation of sulphur clusters caused by electron impact ionization was studied. For this purpose, a beam ofS _n -clusters withn≦8 was generated in a gas aggregation source and ionized by electrons of variable energy. Special care was taken to maintain constant nucleation conditions so that the neutral cluster composition remained unchanged. It was found that the cluster ion mass spectra drastically depend on the electron energy. Even near threshold fragmentation processes contribute significantly to the dependence of the ion intensities on the electron energy.     

Theory of fragmentation of metal clusters

The theory of shell correction developed for discussing fissing of heavy nuclei is applied to symmetric fragmentation of charged metal clusters. Assuming the effective potential of an anisotropic harmonic oscillator, we calculate the shell correction. We also calculate the energy of deformed charged droplets in the two-dimensional parameter space describing the deformation of droplets such as elongation and neck formation. The symmetric fragmentation of microclusters of alkali and noble metals is discussed.     

Carbon-carbon bond cleavage in the photoionization of ethanol and 1-propanol clusters

Tunable VUV laser was used to initiate the ion-molecule reactions in the clusters of ethanol and 1-propanol by photoionization in the region between 10.49 to 10.08 eV. Ionic products were detected by the time-of-flight mass spectrometer. In addition to the protonated clusters from proton transfer reactions, the products corresponding to beta carbon-carbon bond cleavage were found to be one of the major products for small sizes of clusters. A comparison with photoionization of methanol clusters and the results of ab initio calculation has been made.     

Calculation of photoionization cross sections of small metal clusters

In this paper we present photoionization cross sections of the Cu and Al dimers and tetramers. The local spin density method is applied to calculate the electronic structure and the ground state potential. The cross sections are calculated using the continuum multiple scattering method, and a basic analysis of the photoionization process is given within the independent particle picture.     

Structures and dynamics of protonated ammonia clusters

The structures and infrared spectra of protonated ammonia clusters NH(4+)(NH3)n, for n < or = 8, are investigated using density functional-theory (DFT) calculations and semiempirical DFT/molecular dynamics simulations. For n < 5 the clusters are found to be mostly stable up to 100 K, while the larger clusters (n > or = 5) isomerize. Temperature effects are taken into account by performing ab initio molecular dynamics simulations with the computationally tractable self-consistent charges density functional tight-binding method. The infrared spectra at 10 K for the most stable isomers for n = 3-8 compare qualitatively with predissociation experiments, and using a common scaling factor almost quantitative agreement is found. For n > or = 6 the notion of multiple isomers present under the experimental conditions is supported. Of the 13 stable structures for n = 8 only three are found to survive at 100 K. All other clusters isomerize. Cluster structures are inferred from the analysis of the cumulative radial distribution function of the ammonia molecules surrounding the NH(4+) core. The infrared spectra are found to be typical for the structure of the clusters, which should help to relate the experimentally measured infrared spectra to the number and identity of the contributing isomers. For clusters that reorganize to a more stable isomer during the dynamics, the infrared spectrum is generally similar to that of the stable isomer itself. The clusters are found to preferably form globular structures, although chain-like arrangements are also among the low-energy configurations.     

Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

In this work, we report on the vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH(+) (n = 1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H(+) (n = 2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH(+), (CH3OH)2(+), (CH3OH)nH(+) (n = 1-9), and (CH3OH)n(H2O)H(+) (n = 2-9) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for the methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.     

On the photoionization of large molecules

There is no apparent limit to the size of a molecule for which photoionization can occur. It is argued that it is difficult to obtain useful photoionization mass spectra of peptides (above ～ 2000 u), proteins, and oligonucleotides, because of the high internal energy of these polar molecules as a result of the desorption event and because vibrationally excited radical cations readily fragment. Evidence to support this hypothesis is presented from the 118-nm single-photon ionization (SPI) mass spectra of the cyclic decapeptide gramicidin S and of fullerenes, from null SPI results with the linear peptides substance P and gramicidin D and oligonucleotides, and from a variety of data found in the literature. The literature data include mass spectra from jet-cooled peptides, perfluorinated polyethers, collisional ionization of small neutral peptides, and the ultraviolet photoelectron spectroscopy of polymeric solids.     

Statistical fragmentation of hot atomic metal clusters

Fragmentation processes of highly excited neutral and charged atomic metal clusters are studied in the framework of an equilibrium statistical model. In the particular case of hot (near and above melting) neutral and charged sodium clusters of 100 and 200 atoms, a microcanonical Metropolis sampling is used to compute mass (or charge) correlation functions as a function of the excitation energy. This method allows to take the strong anharmonicities in the internal phonon spectrum realistically into account which are linked to the internal structural changes like melting. It is found that, at high enough excitation energy, the system exhibits a phase transition. This phase transition is specific for fragmenting finite systems. From the shape of the caloric curve one sees that the two phases involved are connected by a van der Waals loop characterizing a first order phase transition. Here we observe an enhanced fission and multifragmentation into two or more charged clusters with more than 10 atoms each. Various fragment correlations are studied.     

Electron impact fragmentation of size-selected krypton clusters

Clusters of krypton are generated in a supersonic expansion and size selected by deflection from a helium target beam. By measuring angular distributions for different fragment masses and time-of-flight distributions for fixed deflection angles and fragment masses, the complete fragmentation patterns for electron impact ionization at 70 eV are obtained from the dimer to the heptamer. For each of the neutral Kr(n) clusters studied, the main fragment is the monomer Kr(+) ion with a probability f(n)(1) > 90%. The probability of observing dimer Kr(2)(+) ions is much smaller than expected for each initial cluster size. The trimer ion Kr(3)(+) appears first from the neutral Kr(5), and its fraction increases with increasing neutral cluster size n, but is always much smaller than that of the monomer or dimer. For neutral Kr(7), all possible ion fragments are observed, but the monomer still represents 90% of the overall probability and fragments with n > 3 contribute less than 1% of the total. Aspects of the Kr(n) cluster ionization process and the experimental measurements are discussed to provide possible reasons for the surprisingly high probability of observing fragmentation to the Kr(+) monomer ion.     

Double photoionization and dication fragmentation of CF3I: Experiment and theory

The double photoionization of CF(3)I and the electronic structure and the dissociation dynamics of the CF(3)I(++) dication have been investigated using large ab initio calculations and coincidence techniques. The double photoionization spectrum of CF(3)I consists of a continuous background with a number of narrow bands superimposed. The spectrum is attributed here to the population of groups of close lying electronic states interacting mutually by spin-orbit, spin-spin, and rovibronic couplings. At energies near the vertical double ionization threshold, CF(3) (+)+I(+) ionic fragments are produced. At higher energies, a very specific dissociation with double charge retained on one fragment, CF(3)I(++)-->CF(2)I(++)+F becomes dominant and is attributed to a specific group of dication electronic states.