Use of an unintensified charge-coupled device detector for low-light-level Raman spectroscopy

Spontaneous Raman scattering from adsorbates on surfaces is a powerful vibrational spectroscopy technique that is due to the very large frequency range and high resolution currently possible with visible lasers and spectrometers. Because of the extremely low Raman cross sections of nonresonant molecules, one must use either some sort of signal-enhancement technique or long integrations using multichannel detection in order to have monolayer sensitivity. We have used a backthinned, unintensified, liquid-nitrogen-cooled charge-coupled device (CCD) detector to obtain high-quality multichannel unenhanced Raman scattering spectra from organic monolayers on surfaces. This system has roughly a factor-of-10 greater sensitivity for very low-light-level visible excitation Raman spectra than intensified multichannel detectors currently on the market. A description of our detection system is given along with a comparison with other commercially available multichannel detectors suitable for low-light-level visible Raman spectroscopy. We discuss the advantages and disadvantages of using CCD’s, the special problems that one encounters in their use as Raman detectors, and our methods for overcoming these problems. Prospects for the future use of CCD’s for low-light-level Raman spectroscopy will also be discussed.