A Raman spectrometer employs two or more UV (ultraviolet) laser wavelengths to generate UV resonant Raman (UVRR) spectra in organic samples. Resonant Raman scattering results when the laser excitation is near an electronic transition of a molecule, and the enhancement of Raman signals can be several orders of magnitude. In addition, the Raman cross-section is inversely proportional to the fourth power of the wavelength, so the UV Raman emission is increased by another factor of 16, or greater, over visible Raman emissions. The Raman-scattered light is collected using a high-resolution broadband spectrograph. Further suppression of the Rayleigh-scattered laser light is provided by custom UV notch filters.
The complete Raman instrument is compact and robust, and suitable for in-situ chemical analysis. By employing multiple UV lasers at a suitable wavelength spacing, a matrix of resonant Raman bands can be generated for organic compounds in the UV that are distinct and easily resolvable from the fluorescence emission in the visible-wavelength region. The multiple excitation laser wavelengths produce a repeated series of Raman bands, each with the same frequency shifts from the corresponding excitation laser. UV laser excitation, in addition, allows a resonant enhancement of the Raman scattering in organic compounds such as aromatic hydrocarbons, nucleic acids, and proteins. The multiple excitation wavelengths can be generated from a single UV laser by using stimulated Raman scattering in a hydrogen gas cell. This coherent, multi-wavelength light source has the ideal frequency spacing to maximize spectral coverage and to avoid overlap of adjacent Raman spectra.