P. A. Hackett

The structure of Nb3O and Nb3O+ determined by pulsed field ionization–zero electron kinetic energy photoelectron spectroscopy and density functional theory

The geometrical structures of the ground states of triniobium monoxide, Nb3O, and its cation, Nb3O+, have been determined by an experimental and theoretical study. Vibrationally resolved photoelectron spectra of an Nb3O cluster beam were obtained at 100 and 300 K using the pulsed field ionization‐zero electron kinetic energy technique. The spectra were simulated by calculating multidimensional Franck–Condon factors using the geometries and harmonic vibrational frequencies obtained from density functional theory for the minimum energy structures of the ion and neutral molecule. The rather remarkable agreement between the experiment and the simulated spectra establishes that Nb3O and Nb3O+ have planar C2v structures with the oxygen atom bridging two niobium atoms. These are the most complex transition metal cluster structures to date to be characterized by gas phase spectroscopic techniques.

The bond length of silver dimer

Abstract The spectroscopy of the A–X system of disilver in a supersonic jet has been studied at 120 MHz resolution. The lowest rotational levels were observed and an unequivocal J-numbering was established for the first time. The following bond lengths were derived: ro(X, 107Ag109Ag)=2.53350 (48)A, re(A). The ground state bond length is compared with previous experimental and ab initio determinations.

High resolution study of the (0, 0) and (1, 1) bands of the A0u+-X0g+ system of Au2

Abstract The spectra of the (0, 0) and (1, 1) bands of the A 0 u + - X 0 g + system of digold have been recorded at sub-Doppler resoltuion in a supersonic molecular beam apparatus employing laser vaporization. The rotational structures have been fully resolved from the origin and up, making the J -numbering unequivocal. The analysis confirms the results of a previous rotational analysis of Doppler-limited high temperature spectra of the (3, 0) and (2, 0) bands. The combination of data from both studies leads to improved molecular constants.

Chemical reactivity of iron atoms near room temperature

The reactivity of ground state iron atoms with respect to atom transfer and adduct formation reactions with a variety of simple molecules in Ar buffer gas near room temperature has been investigated. Iron atoms are produced by visible multiphoton dissociation of iron pentacarbonyl or ferrocene, and their removal by added gases under pseudo‐first‐order conditions is monitored by resonance fluorescence excitation at variable time delay following the photolysis pulse. Upper limits for second‐order rate constants for reaction of ground state iron atoms with O2, CO, H2O, (CH3)2O, C2H2, C2H4, N2O, C2H4O, and CF3Cl at room temperature and 100 Torr total pressure are estimated to be in the range (2–10)× 10−15 cm3 molecule−1 s−1, which corresponds to reaction probabilities of less than ≂10−5 per hard sphere collision. Pseudo‐second‐order rate constants in the range (2–60)× 10−14 cm3 molecule−1 s−1 are found for 1:1 adduct formation reactions of ground state iron atoms with C6H6, 1,3‐butadiene, NH3, and NO. The for...

OPTICAL ABSORPTION SPECTRA OF AU7, AU9, AU11, AND AU13, AND THEIR CATIONS : GOLD CLUSTERS WITH 6, 7, 8, 9, 10, 11, 12, AND 13 S-ELECTRONS

The optical absorption spectra of a series of small gold clusters and their cations have been measured, between 1.9 and 5.6 eV, using a method based upon the photodepletion of a molecular beam of their van der Waals complexes containing one and two xenon atoms. This method provides size‐specific information even though the molecular beam contains a wide range of cluster sizes. There is little difference between the spectra of complexes containing one or two xenon atoms. However there is a pronounced odd–even alternation in the spectra of gold clusters with differing numbers of valence s electrons. This alternation is described in terms of a simple electron pairing scheme. The spectrum for Au13 is in reasonable agreement with Dirac scattered‐wave molecular orbital considerations for icosahedral Au13 [A. F. Ramos, R. Arratia‐Perez, and G. L. Malli, Phys. Rev. B 35, 3790 (1987)]. This description of the molecular and electronic structure of small gold clusters in terms of localized molecular orbitals is cont...

REACTIVITY OF NIOBIUM CLUSTERS WITH NITROGEN AND DEUTERIUM

Absolute rate coefficients are reported for reactions of Nbn clusters (n=2–20) with D2 and N2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D2 and N2. The origin of this trend in reactivity with cluster size and the reason why D2 and N2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb2 and Nb8 with H2 and N2. The steric and electronic requirements for dissociation of H2 and N2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H2 or N2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ioniz...

Efficient production of13C2F4 in the infrared laser photolysis of CHClF2

We report the isotopically selective decomposition of chlorodifluoromethane. Chlorodifluoromethane is used industrially in high volume for the production of tetrafluoroethylene and its polymers; thereby it is an attractive working substrate for a medium scale isotope separation process, both in terms of its price and availability.We have studied the infrared multiphoton decomposition of carbon-13 substituted chlorodifluoromethane molecules present at their natural abundance (1.11%). A well defined CO2 laser pulse (80 ns FWHM) was used and both the yield of carbon-13 enriched product and the net absorption of laser radiation were measured. These measurements were made as a function of substrate pressure (10-800 Torr), CO2 laser line (9P 12–9P 32) and fluence (2–8 J cm−2) and were used to determine the energy expenditure per carbon atom produced (ε) at specified product carbon-13 content in the range 30%–96%. The results of these parametric studies were interpreted in terms of the kinetics of multiphoton absorption and dissociation, and allowed an initial optimization of the experimental conditions to minimize ε.Optimum results were obtained at 1046.9 cm−1, 69 cm−1 to the red of the12CHClF2v9 band center. Irradiation of 100 Torr of chlorodifluoromethane at 3.5 J cm−2 gave tetrafluoroethylene containing 50% carbon-13 for an absorption of 140 photons (0.017 keV) per carbon atom produced. This efficiency compares favourably with existing carbon-13 enrichment technologies and would require an absorption pathlength of only 2 m to absorb half the incident photons.

On the vibrational temperature of metal cluster beams : a time-resolved thermionic emission study

Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. It is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the cluster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge the initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and ‘‘Arrhenius factors’’ are presented for delayed ionization of a series of niobium clusters Nbn, n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization potentials by ∼1 eV. The ‘‘Arrhenius factors’’ are all in the range 1011 s−1. These parameters highlight the differences between delayed ionization and photoionization of clusters and draw attention to the need for an adequate theory of unimolecular processes in clusters taking into account both nonadiabatic effects and the important effects arising from the fluxionality of the cluster at high internal energies.Delayed ionization rates for small niobium clusters are measured as a function of the cluster size, the laser wavelength, the number of photons absorbed, and the initial internal energy content of the cluster. It is shown, for the first time, that vibrational excitation of the clusters modulates their rate of delayed ionization. An analysis of the rate of ionization in terms of the total energy content of the cluster establishes unequivocally that delayed ionization is a statistically determined, unimolecular, activated process. It is shown that the rate of delayed ionization can be used to gauge the initial vibrational energy content of the cluster. Quantitative analysis of this effect establishes, for the first time, a thermometry for metal cluster beams. Thermal rate parameters, activation energies, and ‘‘Arrhenius factors’’ are presented for delayed ionization of a series of niobium clusters Nbn, n=5, 6, 7, 8, 9, 11, 12, and 13. The activation energies are lower than the corresponding ionization poten...

Vibrational and geometric structures of Nb3C2 and Nb3C+2 from pulsed field ionization‐zero electron kinetic energy photoelectron spectra and density functional calculations

Vibrational frequencies of three niobium normal modes of triniobium dicarbide neutral and cation have been determined from pulsed field ionization‐zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm−1 in the neutral and 339 cm−1 in the ion. The two deformation modes have frequencies of 238 and 82 cm−1 in the neutral and a degenerate frequency of 258 cm−1 in the ion. The geometry of the triniobium dicarbide has been established by comparing the experimental spectra with theoretical calculations. The cluster has a trigonal bipyramid geometry with carbon atoms capping on each face of the metal frame. The cation cluster has D3h symmetry whereas the neutral cluster has lower symmetry resulting from a Jahn–Teller distortion. A second low‐lying structure with doubly bridging carbon atoms has been identified by the calculations but has not yet been observed.

Interatomic potentials for van der Waals complexes of group 13 metal atoms: AlAr, AlKr, and AlXe

Fluorescence excitation and emission spectra of the van der Waals molecules InAr, InKr, and InXe, formed by laser vaporization of a metal target followed by supersonic expansion, have been recorded. Observed vibrational progressions indicate that the interatomic potentials for the X 2Π1/2,3/2 and B 2Σ+ states can be approximated by Morse functions. Isotopic splittings are observed in the excitation spectra and allow the vibrational numbering in the excited B 2Σ+ state to be assigned. The dissociation energies of the three states are estimated for each molecule and are found to correlate well with the rare gas polarizabilities.