We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first-order rate constants (larger in Ne than in He) increase sharply with Z of the rare gas, roughly in a 0:1:2:4 progression for He, Ne, Ar, and Kr, suggesting that the relaxation involves relatively long-range polarization effects. Similar behavior was found in the formation/relaxation of the weakly bonded dimer pairs: linear OCO–HCN, T-shaped HCN–CO2; linear FH–NNO and bent NNO–HF; and bent HF–DF and DF–HF. The case of the HCN/CO2 dimers is particularly striking. The T-shaped dimer was found first, using Ar as the carrier gas. Five years later the linear form was found with first run neon as carrier, but it could not be detected at all with Ar as the carrier. These results show that in favorable cases high energy species can be studied in supersonic expansions by freezing out a ‘‘high-temperature’’ concentration with a nonrelaxing carrier gas.

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Rotational spectra were observed for dimers of argon and krypton with pyridine (Pyr) by using a Flygare-Balle Fourier transform microwave spectrometer. The dimers are prolate near symmetric tops. Rotational constants were determined for several isotopic species of each. For Ar-Pyr, we found A, B, and C to be 2990.327 (7), 1207.862 (1), and 1199.335 (2) MHz, and D¡, D]K, and 0K to be 3.58 (5), 19.6 (1), and -23.5 (15) kHz. The corresponding values for 84Kr-Pyr are 2986.703 (1) , 806.9294 (1), and 803.0235 (2) MHz and 1.370 (5), 8.74 (7), and -9.4 (2) kHz. Nuclear quadrupole coupling constants were determined from the hyperfine structure of the rotational transitions for dimers with one or more of the quadrupole nuclei 14N, D, or 83Kr. Analysis of the data shows that the Ar/Kr is above the pyridine ring on a plane of symmetry containing the pyridine center of mass (cm) and the nitrogen. The pyridine cm to rare gas vector R is 3.545 Á for the dimer with Ar and 3.648 Á for Kr. R is rotated from the vertical by ~3.5°toward the nitrogen (-3.5°). The pyridine axis perpendicular to its plane oscillates about the equilibrium position with an average displacement of ~7°. A simple psepdodiatomic model for the interaction potential gives the stretching force constant to be 0.0270 and 0.0351 mdyn/A for the Ar-Pyr and Kr-Pyr dimers, with well depths of 232 and 322 cm"1. In 83Kr-Pyr the electric field gradient (EFG) at the Kr has near-axial symmetry with , = -4.722 MHz in the ab plane and the z axis rotated away from the nitrogen by 26.7°from the vertical (+26.7°). Calculation of the electric field gradient with an electric, distributed multipole expansion model gives results agreeing semiquantitatively with experiment.

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The reactions of nickel clusters with ammonia and with water are used to probe cluster geometrical structure. Ammonia uptake experiments allow the determination of the number of preferred binding sites on cluster surfaces. This number shows pronounced minima in the 50- to 116- atom size range for many of the cluster sizes that appear as magic numbers in mass spectra of rare gas clusters. Since these magic numbers arise from closings of shells and subshells of the Mackay icosahedra, the correlation suggests that ammoniated nickel clusters in this size region also have icosahedral structure. Similar structure is found for ammoniated clusters smaller than ∼30 atoms, but is not seen for room temperature clusters in the vicinity of the third shell closing at 147 atoms. Icosahedral features do appear for the larger clusters at elevated temperatures. For many clusters above 50 atoms, prolonged exposure to ammonia causes a conversion from the icosahedral structure to some other structure that binds more ammonia molecules, and often the two structures are seen together. The equilibrium reaction of a single water molecule with the bare clusters probes the strength of the cluster–water bond. Enhanced water adsorption is often seen for clusters one atom larger than those showing minima in ammonia uptake, suggesting that these bare clusters likewise have icosahedral structure. The reasons for minima in ammonia uptake and maxima in water binding are discussed.

A procedure is described for the determination of metal cluster ionization potentials (IPs) using available excimer laser lines that gives error limits substantially smaller than traditional bracketing experiments. It is based on the observation that the adsorption of ammonia on cluster surfaces lowers cluster IPs, and that the IP lowering is linear in the number of adsorbed NH3 molecules. By determining the minimum number of NH3 molecules needed for ionization by the various excimer lasers, an approximation to the dependence of IP on coverage can be deduced. Extrapolation of this dependence to zero coverage gives the bare cluster IPs. Results are presented for clusters of iron, cobalt, and nickel having from 4 to 100 atoms. The effect of molecular adsorption on cluster IPs is analyzed theoretically, and the comparison with experimental results used to estimate the effective dipole moment of NH3 molecules adsorbed on these clusters. Comparison of the bare cluster IPs with the simple spherical drop model suggests that for transition metal clusters the Fermi level can be a significant function of cluster size.

Experimental evidence is presented that small (19- to 34-atom) clusters of iron, cobalt, and nickel tend to adopt polyicosahedral structure when saturated with ammonia. Clusters in the polyicosahedral series (having 19, 23, 26, 29, 32, and 34 atoms) often saturate with 12 NH3 molecules, the expected number for polyicosahedral structure. Other clusters in this size range, which do not have an obvious number of NH3 binding sites, generally do not show a clear saturation. Adsorbate-induced structural changes are also seen, and suggest that for larger clusters in the series there is a transformation to an alternate structure that binds more than 12 NH3 molecules.

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