SERS spectra of several aromatic molecules adsorbed on vacuum-deposited silver are reported, as are the spectroscopic changes observed following laser heating. The observed changes are interpreted as being due to molecular reorientation upon the surface. The appearance of normally Raman-forbidden bands in the spectra is attributed to the rapid spatial rate of change in the radiative electric field strength occurring near sharp surface features, causing vibrational modes belonging to the same representations as components of the dipole-quadrupole tensor to become Raman active. This model together with the application of appropriate surface selection rules accounts for the observed intensity distribution of SERS bands, from which the orientation of the adsorbed molecules with respect to the surface normal can be determined.

The sharpness of tips used in scanning tunneling microscopy (STM) is one factor which affects the resolution of the STM image. In this paper, we report on a direct-current (dc) drop-off electrochemical etching procedure used to sharpen tips for STM. The shape of the tip is dependent on the meniscus which surrounds the wire at the air–electrolyte interface. The sharpness of the tip is related to the tensile strength of the wire and how quickly the electrochemical reaction can be stopped once the wire breaks. We have found that the cutoff time of the etch circuit has a significant effect on the radius of curvature and cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. We have constructed an etching circuit with a minimum cut-off time of 500 ns which uses two fast metal–oxide semiconductor field effect transistors (MOSFET) and a high-speed comparator. The radius of curvature of the tips can be varied from approximately 20 to greater than 300 nm by increasing the cutoff time of the circuit.

The surface-enhanced Raman spectrum of benzene and benzene-d6 adsorbed on a silver film vapor-deposited onto a low-temperature substrate is reported. Many lines which correlate with Raman-inactive and even infrared and Raman-silent modes of the free molecule are observed with high intensity in the spectrum. The local symmetry which would cause those particular, otherwise inactive, vibrations to appear is C3v(σd), which corresponds to a benzene molecule lying flat atop an equilateral triangle of silver atoms so that the normals to three alternate edges of benzene point to the centers of the three atoms. An alternative explanation for the activity of these normally silent modes is presented which does not depend on the adsorption site symmetry but which postulates instead that an unusally steep electric field gradient exists near the surface of the metal which causes modes spanned by the same representations which span the elements of a third rank tensor A to become active. These, together with the vibrations which are normally Raman-active in benzene, are the same vibrations which are Raman-active in C3v(σd). An argument in support of the latter explanation is presented based upon the different response to excitation wavelength of the intensities of those modes which are active as a result of the polarizability tensor, α, as contrasted with those which become active because of A. An unequivocal choice between the two models cannot be made, however, on the basis of the data available.

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