The growth of Fe clusters by collisions of Fe atoms with Ar atoms flowing in a supersonic beam was investigated by Fe mass flux measurements and transmission electron (TEM) microscopy. Moderate Ar densities of the order of 1×10 20 m −3 were sufficient to cause cluster growth which was attributed to the low temperature of the Ar beam. TEM imaging of deposited clusters revealed diameter distributions from 2 to 10 nm depending on the deposition time. Extrapolation to zero deposition time revealed a cluster size of 2.4 nm grown in the gas phase. Growth on the surface was attributed to diffusion of single Fe atoms which are co-deposited with the clusters in the process and which agglomerate when they hit a cluster.

Fluorescence excitation spectra of a series of alkylbenzenes cooled in a supersonic free jet have been obtained for the first 1000 cm−1 region of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. The series includes all n-alkylbenzenes up to n-hexyl together with isopropyl- and tert-butylbenzene. As with toluene, the spectra in this region for all alkylbenzenes is found to be dominated by vibrations of five ring modes: 6a, 6b, 1, 12, and 18a. Three of these—the ’’system modes’’ (6b, 12, 18a)—are found to be largely invariant to changes in the alkyl chain length and type— the ’’bath.’’ For n-alkylbenzenes with chain length of three or higher, spectra of two distinct conformations are observed with roughly equal intensity. These conformations are distinguished by configuration about the 1–2 carbon–carbon bond of the alkyl chain. When this configuration is trans [n-(t)-alkylbenzene], the alkyl chain extends away from the phenyl ring leaving the ring free to van der Waals complex binding on both sides of the ring. The He2–n-(t)-alkylbenzene van der Waals complex is thus observed. When the conformation about the 1–2 bond is gauche [n-(g)-alkylbenzene] the alkyl chain partially covers one side of the phenyl ring. This produces a small red-shift of the spectrum (a self-induced solvent shift) and prohibits the formation of the ring-centered dihelium van der Waals complex.

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Copper clusters ranging in size from 1 to 29 atoms have been prepared in a supersonic beam by laser vaporization of a rotating copper target rod within the throat of a pulsed supersonic nozzle using helium for the carrier gas. The clusters were cooled extensively in the supersonic expansion [T(translational) 1 to 4 K, T(rotational)=4 K, T(vibrational)=20 to 70 K]. These clusters were detected in the supersonic beam by laser photoionization with time-of-flight mass analysis. Using a number of fixed frequency outputs of an exciplex laser, the threshold behavior of the photoionization cross section was monitored as a function of cluster size. The 7.9 eV photon energy of the F2 excimer laser was found to be above the ionization potential of all clusters, and the photoion mass spectrum thus produced showed the copper cluster concentration in the beam to follow a monotonically decreasing function of cluster size. The 6.4 eV ArF exciplex laser photon energy was found to be above the photoionization threshold of clusters with three or more atoms in the case of odd-numbered clusters, but only for clusters with eight or more atoms for even-numbered clusters. Extending out to clusters as large as 29 atoms, laser photoionization at 6.4 eV produced a time-of-flight mass distribution with a pronounced even/odd alternation in cluster photoion intensity. This alternation in ionization threshold behavior was attributed to an even/odd alternation in the electronic structure of the copper clusters with the highest occupied molecular orbital (HOMO) of the even clusters being considerably more strongly bonding than it is in the clusters with an odd number of copper atoms. The 4.98 eV photon energy of the KrF exciplex laser was found to lie below the ionization threshold of all clusters in the 1 to 29 atom range. An extensive survey of the ultraviolet absorption spectrum of the copper dimer was also performed with this supersonic beam source. Resonance two-photon ionization (R2PI) with mass selective detection allowed the detection of five new electronic band systems in the region between 2690 and 3200 Å, for each of the three naturally occurring isotopic forms of Cu2. In the process of scanning the R2PI spectrum of these new electronic states, the ionization potential of the copper dimer was determined to be 7.894±0.015 eV.

The laser induced fluoresence excitation spectrum for the A 2 A 1 ↔ X 2 E transition of the methoxy radical has been reinvestigated. An extensive set of vibrational levels has been assigned with the aid of increased vibrational and rotational cooling. Many of these vibrational assignments are confirmed by rotational analysis of bands involving both the symmetric and asymmetric fundamentals of the A state as well as vibrations containing two quanta of the e modes. Although parts of the vibrational structure have been assigned previously, several discrepancies are identified and corrected. Vibrational frequencies have been obtained for all the modes in the A 2 A 1 state of the molecule. The Fermi resonance that exists between ν 3 and ν 2 has been investigated and interaction constants describing it have been obtained.

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Dispe~ fluorescence spectra of a series of laser-excited n-alkylbenzenes cooled in a supersonic free jet have been obtained under effectively collision-free conditions. The series includes all members of the nalkylbenzene family from methyl· through n -hexylbenzene. Pulsed laser excitation was cleanly made into the og, 6bJ, 12b, and ISa b vibronic bands of the IB2(1T1T*)<-IAI ultraviolet absorption spectrum. Resonance fluorescence from the og excitation experiments permitted clear assignment of the six ring modes (6a, 6b, 1, 12, 18a, 9a) which in combinations and short progressions dominate the fluorescence spectra of these molecules. Except for 6a and 1, the frequency of these ground state vibrations is found to be constant throughout the alkylbenzene series to better than 1 part in 100. Fluorescence from the 6bb, 12b, and 18% excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) occurring in the IB 2 (1T1T*) state prior to emission. By comparing the relative intensity of the resonance and relaxed fluorescence, a lower limit to the rate of the IVR process has been measured as a function of the type of system mode excited (6b l , 12 1 , or 18a l ) and as a function of the size and nature of the bath (the alkyl chain in its various conformations). As expected this IVR rate increases with increasing alkyl chain length. For 6bb excitation (vibrational energy, E, = 530 cm-I ) the IVR process is first observable in n-(t)-butylbenzene where the IVR rate. k vr • is ~5 >10 6 sec-I. This rate increases to ~4X107 sec-I and> 10 9 sec-I in 6bb excited n-(t)pentyl-and n-(t)-hexylbenzene, respectively. For 12A excitation (E, = 933 cm-I ) k,,~ 10 8 sec-I for n-(t)propylbenzene and k"X 10 9 sec-I for all higher n-(t)-alkylbenzenes. Vibrational relaxation is found to be substantially slower for alkylbenzenes in the gauche conformation: kvr < 6 X 10' sec -I for 6bb-excited n -(g )pentylbenzene. With increasing alkyl chain length the spectrum of the relaxed fluorescence becomes sharper and closer in frequency and pattern to the resonance fluorescence of the Og-excited molecule. This indicates an increasing involvement in the IVR process of bath modes in the alkyl chain which are far removed from the benzene ring.

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