Monolayer films of several primary substituted hydrocarbons CH3(CH2) n CH2X (X = CH3, OH, NH2, SH, Cl, Br, I; n = 16−30) have been imaged on graphite at the solution−substrate interface using a scanning tunneling microscope (STM). The straight chain hydrocarbons form well-ordered 2-D films on graphite and physisorb with their molecular axes parallel to the surface. The NH2, SH, Br, and I end groups are observed as bright spots in the STM image corresponding to an enhancement in the tunnel current in the vicinity of the functional group, relative to the remainder of the carbon chain. On the other hand, the OH and Cl substituents were not distinguishable from the alkyl chain in the STM images. Comparison of the relative “brightness” of the functional groups with respect to the carbon chain reveals an empirical relationship between increasing relative brightness and increasing molecular polarizability. A model is proposed to describe the STM imaging mechanism for these insulating, physisorbed films in which the role of the adsorbate's polarizability, electronic structure, and orientation with respect to the surface are considered.

Solutions of triacontane/triacontanol in different solvents have been investigated to determine the importance of the long chain hydrocarbon functional group and the solvent in mediating the probability of sticking to a graphite surface. The scanning tunneling microscopy images obtained on graphite for these mixtures suggest that the triacontane and triacontanol molecules phase separate on the surface into "pure" alcohol and alkane regions with a preferential deposition of alcohol molecules over alkane molecules. The nature of the solvent has a dramatic effect on the mobility and sticking probability of the molecules on the surface.

Scanning tunneling microscopy (STM) images of l-docosanol, 1-docosanethiol, didocosyl disulfide, and 1-chlorooctadecane on graphite are compared. The images of the I -docosanethiol and didocosyl disulfide show bright spots which are attributed to the positions of the S-H and S-S functional groups. The STM images of the l-docosanol and l-chlorooctadecane do not show such bright spots. Thb fact that both the S-H and S-S groups appear bright in the STM images indicates that the presence of an S atom on graphite results in a higher tunneling current when the tip scans over it compared to the tunneling current over a C, O. or Cl atom. The diff'erent behavior of the S atoms conrpiired to the O. C. and Cl atoms is discussed in terrns of the interactions between these atoms and the underlving graphite substrate. The persistent brightness of S atoms in the images of molecular adsorbates sugsests that sult'ur nrav serve as a useful "chromophore" for molecules imased bv STM.

Photodissociation of the group VIB (Cr,Mo,W) hexacarbonyls has been studied at 248 nm using molecular beam photofragment spectroscopy. One, two and three photon processes have been observed. Analysis of the product velocity distributions shows that the photodissociation mechanism consists of sequential CO eliminations with the nth photon channel best described as the single photon photodissociation of the stable products of the n-1st photon channel. The product translational energy distribution for the first CO elimination step is quantitatively similar for all three hexacarbonyls and characteristic of a repulsive translational energy release. The product translational energy distributions of all subsequent CO elimination steps are accurately described by a simple, microcanonical model. Qualitative molecular orbital considerations suggest that the large product translational energy observed in the first CO elimination step results from a repulsive σ interaction between the closed shell CO ligand and an excited molecular orbital which has a significant admixture of metal (n+1)pz, (n+1)s and ndz2 orbitals. This repulsive interaction is absent in the remaining CO elimination steps because there are vacancies in the coordination shell along the z axis.

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