Abstract

Currently available detector technology is beginning to make possible high resolution spontaneous Raman spectroscopy of surfaces, metals, and other systems containing as few as 10¹¹ molecules. Raman scattering is an inelastic process in which an incident photon either gives up or creates an intrinsic quantum of energy in matter. We are interested in the use of Raman scattering as a probe of fingerprint vibrational levels of molecules, for which the change of energy of the incident light is about 0.01 - 0.1 electron volts or 80 - 800 cm⁻¹. The advantages of Raman scattering as a vibrational spectroscopy are its high resolution coupled with a large frequency range. Typically 1 cm⁻¹ resolution is all that is needed for a tenth the linewidth of a vibrational mode of an adsorbate on a metal surface, for example; and a range of 20 to 4000 cm⁻¹ spans a reasonable vibrational range for most adsorbates. Typical surface bonds for small molecules are in the 100 to 500 cm⁻¹ range. Both the resolution and spectral range are quite easy to achieve with visible laser collection optics, detectors and spectrometers. Spontaneous Raman scattering is a process that can be described by second order perterbation theory involving two electric dipole matrix elements and a resonant denominator Raman scattering has different quantum mechanical selection rules for allowed vibrational modes than do infrared absorption (IR) or high resolution electron energy loss (HREELS), and complements these other more traditional surface vibrational spectroscopies. At the same time, Raman scattering has about an order of magnitude better freqency resolution than HREELS and considerably better frequency range than IR, which is currently limited to above about 1000 cm⁻¹ in surface experiments.

© 1986 Optical Society of America