Photoelectron spectroscopy of hydrated adenine anions

We report the observation of hydrated adenine anions, A(-)(H(2)O)(n), n=1-7, and their study by anion photoelectron spectroscopy. Values for photoelectron threshold energies, E(T), and vertical detachment energies are tabulated for A(-)(H(2)O)(n) along with those for hydrated uracil anions, U(-)(H(2)O)(n), which are presented for comparison. Analysis of these and previously measured photoelectron spectra of hydrated nucleobase anions leads to the conclusion that threshold energies significantly overstate electron affinity values in these cases, and that extrapolation of hydrated nucleobase anion threshold values to n=0 leads to incorrect electron affinity values for the nucleobases themselves. Sequential shifts between spectra, however, lead to the conclusion that A(-)(H(2)O)(3) is likely to be the smallest adiabatically stable, hydrated adenine anion.     

Photoelectron spectroscopy of hydrated hexafluorobenzene anions

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6(-)(H2O)n (n=0-2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6(-)(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O-H...F ionic hydrogen bonds. Whereas in the case of C6F6(-)(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6(-)(H2O)1.     

Photoelectron spectroscopy of nickel-benzene cluster anions

(Nickel)(n)(benzene)(m) (-) cluster anions were studied by both mass spectrometry and anion photoelectron spectroscopy. Only Ni(n)(Bz)(m) (-) species for which n > or =m were observed in the mass spectra. No single-nickel Ni(1)(Bz)(m) (-) species were seen. Adiabatic electron affinities, vertical detachment energies, and second transition energies were determined for (n,m)=(2,1), (2,2), (3,1), and (3,2). For the most part, calculations on Ni(n)(Bz)(m) (-) species by B. K. Rao and P. Jena [J. Chem. Phys. 117, 5234 (2002)] were found to be consistent with our results. The synergy between their calculations and our experiment provided enhanced confidence in the theoretically implied magnetic moments of several nickel-benzene complexes. The magnetic moments of small nickel clusters were seen to be extremely sensitive to immediate molecular environmental effects.     

Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS _m ^? cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n _max) is generally expressed as n _max =2m ? 1 in each series of the number of S atoms (m). For Fe?S and Mn?S, it was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag?S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (_n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe?S and Mn?S, the PES spectra of Fe_nS _m ^? /Mn_nS _m ^? show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS _n ^? /Mn_nS _n ^? has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.     

Photoelectron spectroscopy of jet-cooled aluminium cluster anions

Aluminium cluster anions (Al _n ^? ) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for “shell” effects.     

Photoelectron spectroscopic study of iron-pyrene cluster anions

Iron-pyrene cluster anions, [Fe(m)(pyrene)(n)](-) (m = 1-2, n = 1-2) were studied in the gas phase by photoelectron spectroscopy, resulting in the determination of their electron affinity and vertical detachment energy values. Density functional theory calculations were also conducted, providing the structures and spin multiplicities of the neutral clusters and their anions as well as their respective electron affinity and vertical detachment energy values. The calculated magnetic moments of neutral Fe(1)(pyrene)(1) and Fe(2)(pyrene)(1) clusters suggest that a single pyrene molecule could be a suitable template on which to deposit small iron clusters, and that these in turn might form the basis of an iron cluster-based magnetic material. A comparison of the structures and corresponding photoelectron spectra for the iron-benzene, iron-pyrene, and iron-coronene cluster systems revealed that pyrene behaves more similarly to coronene than to benzene.     

Photoelectron spectroscopy of chromium-doped silicon cluster anions

The photoelectron spectra of chromium-doped silicon cluster anions, CrSi-(n), were measured over the size range, n=8-12. Their vertical detachment energies were measured to be 2.71, 2.88, 2.87, 2.95, and 3.18 eV, respectively. Our results support theoretical calculations by Khanna, Rao, and Jena [Phys. Rev. Lett. 89, 016803 (2002)] which found CrSi12 to be an enhanced stability (magic) cluster with its chromium atom encapsulated inside a silicon cage and with its magnetic moment completely quenched by the effects of the surrounding cage.     

Photoelectron imaging spectroscopy of nitroethane anions

We present low-energy velocity map photoelectron imaging results for bare and Ar solvated nitroethane anions. We report an improved value for the adiabatic electron affinity of nitroethane of (191 ± 6) meV which is used to obtain a C-NO(2) bond dissociation energy of (0.589 ± 0.019) eV in nitroethane anion. We assign a weak feature at (27 ± 5) meV electron binding energy to the dipole-bound anion state of nitroethane. Photoelectron angular distributions exhibit increasing anisotropy with increasing kinetic energies. The main contributions to the photoelectron spectrum of nitroethane anion can be assigned to the vibrational modes of the nitro group. Transitions involving torsional motion around the CN bond axis lead to strong spectral congestion. Interpretation of the photoelectron spectrum is assisted by ab initio calculations and Franck-Condon simulations.     

Photoabsorption of sulfur dioxide cluster anions

Photodissociation and photodetachment of negatively charged sulfur dioxide clusters (SO₂) _n ^? ,n=2–11, were investigated in the wavelength range from 458 to 660 nm. Electrons obtained from the interaction of photons with clusters were found to be produced in two photon processes forn≥3. Hence their detachment threshold energy is increased by at least 0.7 eV with respect to the dimer. Wavelength dependent depletion spectra indicate that the clusters are composed of a dimer anion chromophore solvated by neutral molecules. The spectral position of the absorption band is maintained and the shape evolves continuously with cluster size. However, a narrowing of the band with increasing cluster size is observed.     

Photodissociation of sulfur dioxide cluster anions

Photodissociation of negatively charged sulfur dioxide clusters (SO₂) _n ^? , 2≦n≦11, was investigated in the wavelength range between 458 nm and 600 nm using a tandem mass spectrometer. The spectral position of the absorption band remains unchanged, however it exhibits narrowing with increasing cluster size. The smooth evolution of the spectra shows that the clusters are composed of a dimer anion core surrounded by neutral molecules. The analysis of the fragmentation products reveals that the absorption of a photon is followed by statistical evaporation of neutrals with a mean energy loss of 0.28±0.05 eV per evaporated monomer in the large cluster limit.     

Al-H bond formation in hydrated aluminum oxide cluster anions

Quantum chemical calculations have been performed to investigate the interaction of a water molecule with gas phase aluminum oxide cluster anions. While oxygen-rich clusters (AlxOy-,xy) generate metal hydrides. These hydride species are, in many cases, 30-35 kcal/mol more stable than their hydroxide counterparts. Our observations on such competing reaction pathways may be useful to understand the catalytic role of alumina nanoparticles in many chemical reactions.     

Photoelectron spectroscopy of silicon doped gold and silver cluster anions

Photoelectron spectra of low temperature silicon doped gold cluster anions Au(n)Si(-) with n = 2-56 and silver cluster anions Ag(n)Si(-) with n = 5-82 have been measured. Comparing the spectra as well as the general size dependence of the electron detachment energies to the results on undoped clusters shows that the silicon atom changes the apparent free electron count in the clusters. In the case of larger gold clusters (with more than about 30 gold atoms) the silicon atom seems to consistently delocalize all of its four valence electrons, while in the case of the silver clusters a less uniform behavior is observed. Here the silicon atoms act partly as electron donors, partly as electron acceptors, without following an obvious simple principle. Additionally some structural information can be obtained from the measured spectra: while Ag(54)Si(-) seems to adopt an icosahedral structural motif, Au(54)Si(-) seems to take on a low symmetry structure, much like the corresponding pure 55 atom clusters. This indicates that for such larger clusters the incorporation of a single silicon atom does not change the ground state geometry significantly.     

Photoelectron spectroscopic study of iron–benzene cluster anions

Iron–benzene cluster anions, (n = 1–7, m = 1–4), were generated via laser vaporization and studied using mass spectrometry, anion photoelectron spectroscopy and in one case by density functional theory. Based on these studies, we propose that and Fe₁Bz₁ as well as and Fe₂Bz₁ exhibit half-sandwich structures, that and Fe₁Bz₂, and Fe₂Bz₂, as well as Fe₃Bz₂ and Fe₄Bz₂ are sandwich structures, and that and Fe₂Bz₃ and larger species form ‘rice-ball’ structures which in each case consist of benzene molecules surrounding an iron cluster core.     

Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene)(n)(-) (n=3-150), and its related aromatic cluster anions, (acenaphthene)(n)(-) (n=4-100) and (azulene)(n)(-) (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene)(n) (-) clusters, two isomers coexist over a wide size range: isomers I and II-1 (28 < or = n < or =60) or isomers I and II-2 (n > or = ~60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene)(n) (-) and (azulene)(n) (-) clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7 < or = n < or = 100) in all the anionic clusters depend linearly on n(-13) and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene)(n)(-) clusters with n > or = approximately 30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene)(n)(-) and (azulene)(n)(-) and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric pi-conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.     

Oxygen adsorption on hydrated gold cluster anions: experiment and theory

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.     

Photoelectron imaging of copper and silver mono- and diamine anions

The results of photoelectron imaging experiments of Cu and Ag mono- and diamine anions are reported. The photoelectron images were recorded at two photon energies, 800 and 527 nm. The vertical detachment energies of CuNH(2) (-) and AgNH(2) (-) are lower than those of the respective atomic metal ion and are measured to be 1.11+/-0.05 and 1.23+/-0.05 eV, respectively. By contrast, the electron detachment energies for Cu(NH(2))(2) (-) and Ag(NH(2))(2) (-) are higher than those of the corresponding metal ion and are determined to be 1.48+/-0.05 and 1.85+/-0.05 eV, respectively. Energy-dependent photoelectron anisotropy parameters are also reported. The photodetachment of the Cu and Ag mono- and diamine anions exhibit a cos(2) theta angular dependence relative to the direction of the laser polarization. The nature of the chemical bonding and the symmetry of the highest occupied molecular orbitals are discussed in relevance to the measured anisotropy parameters.     

Photoelectron imaging of negative ions: atomic anions to molecular clusters

The negative ion photoelectron imaging technique is illustrated using two relatively simple atomic and molecular anion systems, and then applied to the study of a cluster system. Photoelectron images of I- and CS2- at 267 nm and 800 nm respectively are presented. Photoelectron spectra and angular distributions are obtained from the images and the concepts underlying these and their interpretation are outlined. The imaging technique is then applied to (CS2)n - (n = 2-4) cluster anions, for which 400 nm images are presented. Features of these images are highlighted and discussed with reference to solvation effects and structural properties of the cluster anionic moiety. Photoelectron signatures of different forms of the cluster core are discussed. These core structures are anionic monomer units solvated by the remaining n - 1 CS2 molecules or covalent dimer units solvated by the remaining n - 2 molecules. Images of the n = 2 anion at 400, 530 and 800 nm reveal information about the electron detachment processes within the different cluster types and both direct detachment and autodetachment are seen. The direct transitions are seen from clusters with either core type, while autodetachment is only seen from clusters with the covalent dimer core. The imaging work also reveals evidence of a previously unreported electronic transition within the direct detachment band due to the covalently bound core type.     

Reactions of carbon dioxide with metal carbonyl anions

The reactivity of a series of metal carbonyl anions with CO₂ has been found to parallel their relative nucleophilicities. The highly nucleophilic species, C₅H₅Fe(CO)^?₂, reacts readily to give the dimer, (C₅H₅Fe(CO)₂)₂, and carbonate while Co(CO)^?₄ is unreactive. The reaction of ¹³CO₂ with C₅H₅Fe(CO)^?₂ results in the formation of the ¹³CO enriched dimer.     

Photoelectron velocity-map imaging signature of structural evolution of silver-doped lead Zintl anions

A set of silver-doped lead Zintl anions, Ag@Pb(n) (-) (n = 5-12), have been studied using photoelectron velocity-map imaging spectroscopy and quantum chemical calculation. The structures of Ag@Pb(n) (-) (n = 7-9, 11) built upon a square pyramid base, hitherto not considered, were assigned. Overall agreement between the experimental and calculated photoelectron spectra as well as vertical detachment energies allows for structural evolution to be established. The silver atom prefers to stay outside in the n ≤ 6 clusters and intends to be encapsulated by the lead atoms in n > 6. A stable endohedral cage with bicapped square antiprism structure is formed at n = 10, the endohedral structure of which persists for the larger clusters. Especially, these Ag@Pb(n) (-) anions have been found to undergo a transition between square pyramid and pentagonal pyramid molecular structures at n = 11.     

Photoelectron spectroscopy of radical anions

The photoelectron spectroscopy of a number of radical anions has been investigated. We find the following electron affinities: EA(C₃) =1.981 ±0.020 eV, EA(C₃H) = 1.858 ±0.023 eV, EA(C₃H₂) = 1.794 ± 0.025 eV, EA(C₃O) = 1.34±0.15 eV, EA(C₃O₂) = 0.85±0.15 eV, EA(C₄O)= 2.05±0.15 eV, and EA(CS₂) = 0.895± 0.020 eV. The structure and bonding for each of these ions is discussed.