Abstract: A microwave sweep oscillator is used to apply an AC signal from the probe tip of a scanning tunneling microscope to a sample, and the reflected signal from the sample is measured by a microwave spectrum/network analyzer, or another device capable of measuring amplitude versus frequency. The frequency of the signal applied by the oscillator may be swept across a spectrum and the optimum frequency of the spectrum is determined so that an improved image of the surface of a sample may be obtained. The spectrum of a known substance may also be recorded and used as a signature for identifying components of an unknown substance by comparison. When the amplitude of the AC signal applied is increased, a sudden change in the reThis invention was made with support from the National Science Foundation, United States Government, under Grant No. CHE-9158375. The government has rights in this invention.