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Reactions on the surface of a variety of transition metal clusters have been studied in the gas phase at near room temperature using a newly developed fast-flow reaction device. Initial examples of the use of this device are provided by survey studies of the reactivity of iron, cobalt, nickel, copper, and niobium clusters in contact with low concentrations of D2, N2 and CO. Dissociative chemisorption of D2 is found to occur with dramatic sensitivity to cluster size in the cases of iron, cobalt, and niobium clusters, the detailed pattern of reactivity differing markedly for each metal. The corresponding reaction is also observed with nickel clusters, but here the reactivity shows only a slow, steady increase with cluster size. Copper clusters are found to be completely unreactive to H2 chemisorption under these conditions. Molecular nitrogen is found to chemisorb readily to clusters of cobalt and niobium, with a reactivity pattern very similar to that observed with D2. Iron clusters are found to show slight reactivity with N2; only a small amount of chemisorption is observed on the most reactive clusters at high N2 concentration, but the pattern of this reactivity with cluster size is consistent with that observed in D2 chemisorption. In contrast to these highly structured reactivity patterns of D2 and N2, carbon monoxide is found to show only a slow, monotonic increase in reactivity with cluster size. It is suggested that these dramatic reactivity patterns for chemisorption on metal clusters provide stringent tests for future theories as to the nature of chemisorption on metal surfaces at a detailed, molecular level.

Resonant two-photon ionization spectroscopy has been applied to the jet-cooled Au, and AgAu molecules. Three new band systems of Au, and two new systems of the poorly characterized AgAu molecule have been observed. Excited state lifetime measurements have been made, and assignments of the excited states are suggested. For Au, the a 'Z;t +X '2: transition has been detected, and vibrational levels of the a "2;: state have been observed up to v' = 33, which lies only 120 cm-' below the convergence limit of the system. This allows a precise confirmation of previous high temperature Knudsen effusion measurements of the bond strength of Au, as D z (Au,) = 2.290 f 0.008 eV. In addition, the excited states of Au, of 0: symmetry are shown to have significantly shorter fluorescence lifetimes than the 1, states, and this is explained as resulting from an admixture of Au + Au-ion pair character in these a = 0: states. The ionization potential of Au, has been bracketed as IP(Au,) = 9.20 f 0.2 1 eV, which may be combined with the values of D g (Au,) and IP(Au) to provide the dissociation energy of the Au: ion as D g (Au,+) = 2.32 f 0.2 1 eV. Detailed comparisons with theoretical results are made for Au,, and assignments of the A and B states of AgAu to a = 0 + and a' = 1, respectively, are proposed.

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A new fast flow device for the study of metal cluster reactions in the gas phase is described and characterized. The new device utilizes metal clusters made by laser vaporization of an appropriate metal target mounted in the throat of a supersonic nozzle which exhausts into a fast-flow reaction tube. Reactants are injected into the flowing helium–metal cluster mixture at a point in the flow tube where shock waves have reheated the gas to roughly 320 K. Turbulence in the wake of these shock waves produces efficient mixing of the reactants. Measurement of the flow properties of this reaction tube indicate a residence time of 150–200 μs with an average density of helium buffer gas equivalent to 50–100 Torr at room temperature. Subsequent free expansion of this reaction mixture into a large vacuum chamber produces a supersonic beam with extensive cooling of the various constituents in the mixture (pyrazine was measured to be rotationally cooled to 10 K). The new cluster reaction device is, therefore, an excellent source for future studies of the jet-cooled metal cluster reaction products themselves.

Iron monocarbide has been investigated between 12 000 and 18 100 cm Ϫ1 in a supersonic expansion by resonant two-photon ionization spectroscopy. Six new electronic states have been identified for which origins relative to the ground state have been determined. Three of these possess ⍀Јϭ3, one possesses ⍀Јϭ4, and two possess ⍀Јϭ2. The ⍀Јϭ3 state with an origin near 13 168 cm Ϫ1 is likely a 3 ⌬ 3 state and has been assigned as the analog of the ͓14.0͔ 2 ⌺ ϩ ←X 2 ⌺ ϩ charge transfer transition in CoC. The ⍀Јϭ4 state is most likely a 3 ⌽ 4 state. Additionally, seven bands with ⍀Јϭ2 have been observed that have proven impossible to systematically group by electronic state. Because every transition rotationally resolved in this study possesses a lower state with ⍀ϭ3, the ground state has been confirmed as arising from an ⍀ϭ3 state that is most likely the ⍀ϭ3 spin orbit component of a 3 ⌬ i term derived from a 1␦ 3 9 1 configuration. The ionization energy ͑IE͒ of FeC has been determined as 7.74Ϯ0.09 eV by varying the wavelength of the ionization photon. When combined with the known IE of Fe and the bond energy of FeC ϩ , the bond energy of FeC is calculated to be 3.9Ϯ0.3 eV. Presentation of the results is accompanied by an analysis of the bonding in FeC from a molecular orbital standpoint.

Resonant two-photon ionization spectroscopy was used to study jet-cooled Ni2 produced by pulsed laser ablation of a nickel target in the throat of a supersonic nozzle using argon as the carrier gas. Spectral regions previously investigated using helium as the carrier gas were reinvestigated, and the improved cooling achieved was found to suppress transitions arising from an Ω=4 state that had been thought to be the ground state. Seven new vibronic progressions were assigned, with spectroscopic constants determined for the excited states. The predissociation threshold in Ni2 was reinvestigated, and a revised value for the binding energy is given as D○0(Ni2)=2.042±0.002 eV. The ionization energy of Ni2 was found to be 7.430±0.025 eV, and from this result and the revised bond dissociation energy of the neutral, the binding energy of the cation was calculated to be D○0(Ni+2)=2.245±0.025 eV. Similarly, D○0(Ni−2)=1.812±0.014 eV is obtained using D○0(Ni2) and the electron affinities of Ni and Ni2. Twenty bands were rotationally resolved, all originating from a lower state of Ω″=0+g or 0−u which we argue is the true ground state, in agreement with ligand field and ab initio theoretical studies. The rotational analysis also yielded a ground state bond length of 2.1545±0.0004 Å for 58Ni2.

Resonant two-photon ionization spectroscopy has been used to study the jet-cooled Al2 molecule. The ground state has been conclusively demonstrated to be of 3Πu symmetry, deriving from the σ1gπ1u electronic configuration. High resolution studies have established the bond length of the X3Πu state as re(X3Πu) =2.701±0.002 Å. The third-law estimate of the Al2 bond strength has been reevaluated using the observed and calculated properties of the low-lying electronic states to give D00 (Al2)=1.34±0.06 eV. In addition to the previously reported E 2 3Σ−g←X3Πu and F 33Σ−g←X3Πu band systems, the E′ 33Πg←X 3Πu, F″–X, F′–X, G 3Πg←X 3Πu, H′ 3Σ−g←X 3Πu, and H3Δg←X3Πu band systems have been observed for the first time. Bands of the G–X, H′–X, and H–X systems have been rotationally resolved and analyzed, providing rotational constants and electronic state symmetries for the upper states of these systems. A discussion of all of the experimentally known states of Al2 is presented, along with comparisons to previous experimental and theoretical work.