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The static electric dipole polarizabilities of rhodium clusters Rhn, n=5–28, have been measured via a molecular beam deflection method. Uniform high-field beam deflections, indicative of induced polarization, were observed for all Rhn except Rh7 and Rh10 which by contrast exhibited beam broadening and anomalously high effective polarizabilities. Analysis of the beam deflection profile of Rh7 indicates that it possesses a permanent dipole moment of 0.24±0.02 D. Unlike the other clusters in the n=5–28 size range, the polarizability of Rh10 is observed to decrease with increasing source temperature. We attribute this temperature dependence to paraelectric behavior, suggesting that Rh10 is a fluxional molecule possessing a dipole moment that spatially fluctuates, uncorrelated with overall rotation. [ABSTRACT FROM AUTHOR]

Magnetic moments of bare cobalt clusters Con (n=7–32) and benzene-capped cobalt clusters Con(bz)m have been measured at temperatures ranging from 54 to 150 K using a molecular beam deflection method. It was observed that Co12–32 produced at temperatures greater than ∼100 K display high-field-seeking behavior at all temperatures in the range investigated, indicating that they are superparamagnetic species. At temperatures below ∼100 K, the field-on beam profiles of Co7–11 and some larger clusters displayed substantial symmetric broadening, indicating that some fraction of the clusters in the beam were no longer superparamagnetic, but rather were in a blocked (locked-moment) state. In the superparamagnetic regime (T=150 K) Con clusters in the n=7–32 size range were found to possess per-atom moments ranging from 1.96±0.04μb(Co24) to 2.53±0.04μb(Co16), significantly above the bulk value of 1.72μb. Locked-moment isomers were found to display moments of ∼1μb per atom. Cobalt clusters containing a layer of adsorbed benzene molecules were found to possess significantly lower moments per cobalt atom than the corresponding bare cobalt clusters. [ABSTRACT FROM AUTHOR]

The static electric dipole polarizabilities of Cu9–Cu61 have been measured via a molecular beam deflection method. The clusters display per-atom polarizabilities that decrease monotonically with size, from ∼16 Å3 per atom Cu9­10 to ∼5 Å3 (Cu45­61). Absent are any discernible discontinuities or odd—even alternations due to electronic shell filling or electron pairing effects. For the smallest clusters, the experimental polarizabilities are ∼3 times larger than those predicted classically for conducting ellipsoids, and approach the classical values only for clusters containing more than ∼45 atoms. [ABSTRACT FROM AUTHOR]

The static electric dipole polarizabilities of Nb[SUB2]-Nb[SUB27] have been measured via a molecular beam deflection method. The clusters display per-atom polarizabilities that vary between 5.4 ± 0.3 Å[SUP3] per atom (Nb[SUB10]) to 37.7 ± 0.3 Å[SUP3] (Nb[SUB13]). The factor of ∼7 variation observed in the per-atom polarizabilities is the largest observed for any metal cluster system investigated thus far and implies large size-dependent variations in their electronic and/or geometric structures. [ABSTRACT FROM AUTHOR]

Magnetic moments have been measured for bare, isolated nickel clusters Ni[sub n] and their association complexes with carbon monoxide, oxygen, and hydrogen using a molecular beam deflection method. The moments measured for bare Ni[sub n] are in general agreement with those previously reported by Apsel et al. [Phys. Rev. Lett. 76, 1441 (1996)], lying slightly lower overall. It is found that adsorbed carbon monoxide and hydrogen decrease magnetic moments of Ni[sub n]. The reduction in moments can be substantial for smaller clusters: the moment of Ni[sub 8] is decreased by approximately 6 bohr magnetons by a single absorbed CO molecule. In general, the adsorbate-induced changes in magnetic moments diminish with increasing cluster size. The present results are in qualitative accord with previous measurements of the magnetization quenching effects of CO and H on nickel nanoparticles and thin nickel films. Atomic oxygen decreases cluster moments of some nickel clusters and increases those of others, an effect attributed to adsorbate-induced reconstruction. The experimental results are compared to detailed electronic structure calculations of nickel clusteradsorbate complexes and to the predictions of the semiempirical bond order-rigid band model of Fourier and Salahub [Surf. Sci. 238, 330 (1990)]. [ABSTRACT FROM AUTHOR]

Stern–Gerlach magnetic deflection measurements were performed on Ni[sub n] and Ni[sub n]CO (n=8–18) in a molecular beam experiment. Adsorbed carbon monoxide was found to reduce the magnetic moments of most nickel clusters in this size range. For Ni[sub 8], the reduction was ∼5 bohr magnetons, while smaller reductions were observed for the other clusters. The present results are in qualitative accord with previous experiments and calculations of the magnetization quenching effects of CO on both nickel nanoparticles and thin nickel films. The magnitude of the changes in cluster magnetic moments of Ni[sub 8], Ni[sub 15], and Ni[sub 18] were larger than those predicted by the local quenching (bond number) model, indicating that the CO-induced perturbation to cluster electronic structure in these clusters is distributed among many nickel atoms. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The infrared photodissociation spectra of Fen(CH3OH)m, Fen(CH3OD)m, Fen(CD3OH)m, and Fen(CD3OD)m (n=5–15; m=1,2) have been recorded from 920–1090 cm-1. Analysis of the spectral features indicates that in all cases, molecularly adsorbed methanol is the absorbing chromophore, with no evidence of methoxy formation. The observed frequencies for the C–O stretching vibration and other vibrational modes are shifted ∼40 cm-1 to the red of the corresponding fundamentals of gas-phase methanol. The spectral shifts and thus the magnitude of the cluster–methanol interaction are observed to be independent of iron cluster size over the size range studied, implying an insensitivity of the interaction to the underlying cluster structure. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation spectra of the copper trimer and its argon and krypton adducts have been recorded in the visible. For Cu3, we observe a number of resolved vibronic transitions to the blue of the A←X origin at 539.8 nm in accord with the observations of Morse et al. [J. Chem. Phys. 79, 5316 (1983)]. By contrast, the spectra of Cu3Ar and Cu3Kr consist of continuous absorption bands with maxima at 516 and 517 nm, respectively. While the oscillator strengths of the Cu3Ar and Cu3Kr absorption bands are nearly an order of magnitude larger than that derived for Cu3 from its photodissociation spectrum, they are in much closer agreement with the oscillator strength determined from the Cu3 A→X radiative lifetime. [ABSTRACT FROM AUTHOR]

The photodissociation action spectra of NinArm (n=3,4,5, and 7) have been recorded from 306 to 674 nm. The spectra are continuous, with cross sections which increase monotonically into the ultraviolet. It is observed that the photodissociation spectra recorded for Ni3Arm, Ni4Arm, and Ni5Arm are independent on m, indicating a negligible influence of the weakly bound argon atoms on the absorption spectra of the underlying clusters. The oscillator strengths increase monotonically with increasing cluster nuclearity (n) in accord with the Thomas–Reiche–Kuhn sum rule. The results are discussed from both molecular and classical electrodynamic points of view. [ABSTRACT FROM AUTHOR]

The optical absorption spectra of niobium clusters containing 7 to 20 atoms have been measured from 336 to 634 nm by way of photodissociation action spectroscopy of the corresponding van der Waals complexes with argon atoms: Nbn Arm → hν Nbn + m Ar. The clusters in this size range do not display discrete absorption bands characteristic of molecular behavior, but rather absorption cross sections which increase monotonically with decreasing wavelength. This behavior is in qualitative accord with the absorption behavior predicted by the spherical Mie model for small niobium spheres, however, the measured cross sections are 2–5 times larger than predicted over this wavelength range, with the smallest clusters displaying the largest deviations. Interpreted within the classical electrodynamic framework, these observations suggest that the absorption spectra derive oscillator strength from an incipient surface plasmon, redshifted from its predicted resonance frequency in the vacuum ultraviolet. [ABSTRACT FROM AUTHOR]

The photoionization efficiency spectra of nickel clusters containing adsorbed ammonia have been recorded near threshold. Ionization potentials (IPs) have been assigned for 97 selected Nin(NH3)m species, with n up to 56 atoms. We observe that nickel cluster IPs decrease upon ammonia adsorption, with the magnitude of the IP shifts varying linearly with the number of adsorbed molecules, in good agreement with a simple electrostatic model. Deviations from this behavior are observed for Ni5(NH3)m and Ni6(NH3)m clusters, in which adsorption of the first ammonia molecule produces a significantly larger IP drop than do subsequently adsorbed molecules. Such behavior may indicate the presence of unique adsorption sites on Ni5 and Ni6 preferentially occupied by the first ammonia molecule adsorbed onto these clusters, or alternatively, may indicate cluster rearrangement occurring as the result of chemisorption. Our results are discussed in light of studies of ammonia adsorption onto single crystal nickel surfaces. [ABSTRACT FROM AUTHOR]

The photoionization efficiency (PIE) spectra for niobium clusters up to Nb76 are measured using laser ionization time-of-flight mass spectrometry. Ionization potentials (IPs) assigned from the PIE spectra evolve with the same general pattern observed for other transition metal clusters (Mn) studied to date: a rapid yet nonmonotonic decrease in IP to n[bar_over_tilde:_approx._equal_to]15 followed thereafter by a relatively slow and smooth evolution. The measured IPs evolve with cluster radius R according to the predictions of the conducting spherical droplet model if the limiting (R→∞) IP is chosen to be 0.5 eV lower than the bulk polycrystalline work function. Kinetics experiments using D2 as the coreactant indicate the existence of two structural forms, one highly reactive, one relatively unreactive for clusters in the size range Nb9–Nb12, with the reactive forms predominant in each case. PIE spectra for Nb9–Nb12 were recorded using sufficient D2 added to the cluster source to selectively titrate the reactive forms of these clusters, leaving the unreactive forms as the predominant isomers. It is found that the unreactive forms of Nb9 and Nb12 possess higher IPs than those of the reactive forms, while both forms of Nb10 possess approximately the same IP. [ABSTRACT FROM AUTHOR]

The photoionization efficiency (PIE) spectra for cold iron clusters (Fe6–Fe90) and cobalt clusters (Co3, Co7–Co92) have been measured using a tunable ultraviolet laser combined with time-of-flight mass spectrometry. Vertical ionization potentials (IPs) assigned from the observed photoionization thresholds decrease rapidly but nonmonotonically for both Fen and Con up to n≅20, beyond which IPs evolve more slowly and smoothly. The measured IPs for both iron and cobalt clusters are lower than those predicted by the conducting spherical droplet model. The evolution of IP with increasing cluster radius R is observed to occur more gradually than the R-1dependence predicted by this model. This suggests that the Fermi energy for clusters in this size range decreases with increasing cluster size. The shapes of the PIE spectra near threshold are observed to continuously evolve with increasing cluster size, with the smallest clusters displaying photoionization efficiencies which rapidly increase with energy above threshold, and larger clusters displaying spectra with more gradual slopes. It is proposed that this trend reflects the evolution in the density and distribution of low-lying electronic states which occurs with increasing cluster size. [ABSTRACT FROM AUTHOR]

The threshold photoionization efficiency (PIE) curves for nickel clusters in the size range Ni3 to Ni90 have been measured by laser photoionization with detection by time-of-flight mass spectrometry. Both warm (≤298 K) and cold (≤77 K) clusters have been studied. The PIE curves for 298 K clusters display thermal tails, while these tails are smaller for cold clusters. Cluster ionization potentials (I.P.s) have been determined by two methods: the Watanabe procedure and linear extrapolation of the PIE curves. Dramatic dependence of I.P. on cluster size is found for clusters smaller than 11 atoms, while the I.P.s of larger clusters decrease relatively smoothly and nearly monotonically from 5.84 eV for Ni11 to 5.56 eV for Ni90. The I.P.s for clusters larger than Ni40 show the linear dependence on reciprocal radius (R-1) predicted by the conducting spherical drop model of small particle I.P.s, but do not fit the model quantitatively unless the limiting (R-1→ 0) work function is reduced by 0.46 eV from the bulk polycrystalline value. The differences between the thermal tails of the room temperature and 77 K PIE curves diminish with increasing cluster size, suggesting a reduced difference between neutral and ionic structures for larger clusters. In general, there is poor agreement between our experimental results and theoretically calculated I.P.s for small nickel clusters, with the exception of the recently reported tight-binding theory results of Pastor et al. [Chem. Phys. Lett. 148, 459 (1988)]. [ABSTRACT FROM AUTHOR]

Investigates the infrared spectra of hydrogenated and deuterated iron clusters. Application of density functional calculations on model complexes; Components of the infrared-active vibrational frequencies; Measurement of complexes by depletion of time-of-flight mass peaks.

Studies the reaction of silver clusters with ethylene and ethylene oxide using the infrared photodissociation and photoionization spectroscopy. Ionization potential bracketing; Infrared photodissociation spectra; Role of silver particles in catalyzing the epoxidation of ethylene to ethylene oxide.

Measures the photoionization spectra of manganese clusters consisting of between seven and sixty-four atoms. Decrease of ionization potentials (IP); Reactivity toward molecular hydrogen; Absence of any discontinuity in IP.

Intramolecular spin relaxation may occur in isolated molecules or clusters provided that the density of rovibrational eigenstates is sufficiently high to serve as an energy bath and angular momentum is conserved. In the coupled, zero-field limit, total angular momentum (J) is the sum of spin (S) and rotational (N) momenta such that J and MJ are good angular momentum quantum numbers. In the coupled limit, transitions between Zeeman levels (ΔMJ≠0) cannot occur in the absence of an external torque. However, in the high-field limit, J and MJ are no longer good quantum numbers, as N and S are decoupled and only their projections on the z axis defined by the external field are invariant. In this case MN and MS remain as good quantum numbers so that angular momentum conserving transitions can occur subject to the selection rule ΔMN=-ΔMS. Determination of the magnetic moments of isolated molecules and clusters via a thermodynamics-based analysis requires that their magnetizations are measured at sufficiently large fields that spin-rotation effects become negligible and the Zeeman level structure approaches the free-spin case. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation spectra of Cu3 and Cu3Ar have been recorded from 500 nm to 550 nm. The spectrum of Cu3 shows a number of resolved vibronic bands in 522–538 nm range, in accord with the previous observations of Morse et al. [J. Chem. Phys. 79, 5316 (1983)]. By contrast, the spectrum of Cu3Ar shows no resolved vibronic structure and is displaced over 20 nm to the blue of the Cu3 A←X origin at 539.8 nm. In addition, the oscillator strength of the Cu3Ar absorption band is a factor of six times larger than that of Cu3. These results are discussed in light of the known photophysical and spectroscopic properties of Cu3. [ABSTRACT FROM AUTHOR]

We have reproduced kinetics results indicating that niobium clusters in the size range Nb9–Nb12 are produced in two isomeric forms which possess widely different rates reactivity toward D2. We have exploited this difference in reactivity to record photoionization efficiency spectra of the unreactive forms of Nb9, Nb10, and Nb12 as well of a mixture containing predominantly the reactive forms. We find that the photoionization thresholds measured for Nb9 and Nb12 with the reactive forms of these species depleted via reaction with D2 are shifted to higher energies compared those recorded without D2 present. The photoionization threshold energies for both unreactive and reactive forms of Nb10 are approximately equal. [ABSTRACT FROM AUTHOR]

A tunable alexandrite laser was used to produce O2(b 1∑+g) in the gas phase, with quantitative measurement of O2(b 1∑+g) and O2(a 1Δg) densities achieved by detection of the b→X fluorescence and dimol emission signals, respectively. A simple kinetics analysis indicates that collisional deactivation of O2(b 1∑+g) by ground state oxygen molecules occurs with unit production of O2(a 1Δg). These results are in accord with previous assumptions regarding collisional deactivation of O2(b 1∑+b) based on spin conservation. [ABSTRACT FROM AUTHOR]

The electric dipole polarizabilities of Ni[sub 12]–Ni[sub 58] have been measured via a molecular beam deflection experiment. The majority of the clusters display polarizabilities α between 4 and 6 Å[sup 3] per atom, below the atomic value of 6.8 Å[sup 3] and above that predicted for ideal conducting spheres. Local extrema in α/atom are observed for certain Ni[sub n]. In particular, those clusters that adopt closely packed or quasispherical structures (e.g., Ni[sub 38], Ni[sub 55]) display local minima, while Ni[sub 21], Ni[sub 22], Ni[sub 25], and Ni[sub 49–54], each an icosahedron or polyicosahedron with “missing” atoms, display anomalously high values. The relationship between the polarizabilities of nickel clusters and their shapes and structures is discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]