unknown protein. Also, if a protein has more than one chromophore, different chromophores can be studied individually if their CT bands differ in energy. In addition to identifying compounds, RR spectroscopy can also supply structural identification about chromophores in some cases. The main advantage of RR spectroscopy over non-resonant Raman spectroscopy is the large increase in intensity of the bands in question (by as much as a factor of 106). This allows RR spectra to be obtained with sample concentrations as low as 10−8 M. It also makes it possible to record Raman spectra of short-lived excited state species when pulsed lasers are used. This is in stark contrast to non-resonant Raman spectra, which usually requires concentrations greater than 0.01 M. RR spectra usually exhibit fewer bands than the non resonant Raman spectrum of a compound, and the enhancement seen for each band can vary depending on the electronic transitions with which the laser is resonant. Since typically, RR spectroscopy are obtained with lasers at visible and near-UV wavelengths, spectra are more likely to be affected by fluorescence. Furthermore, photodegradation (photobleaching) and heating of the sample can occur as the sample also absorbs the excitation light, dissipating the energy as heat. Instrumentation The instrumentation used for resonance Raman spectroscopy is identical to that used for Raman spectroscopy; specifically, a highly monochromatic light source (a laser), with an emission wavelength in either the near-infrared, visible, or near-ultraviolet region of the spectrum. Since the energy of electronic transitions (i.e. the color) varies widely from compound to compound, wavelength-tunable lasers, which appeared in the early 1970s, are useful as they can be tuned to coincide with an electronic transition (resonance). However, the broadness of electronic transitions means that many laser wavelengths may be necessary and multi-line lasers (Argon and Krypton ion) are commonly used. The essential point is that the wavelength of the laser emission is coincident with an electronic absorption band of the compound of interest. The spectra obtained contain non-resonant Raman scattering of the matrix (e.g., solvent) also. Sample handling in Raman spectroscopy offers considerable advantages over FTIR spectroscopy in that glass can be used for windows, lenses, and other optical components. A further advantage is that whereas water absorbs strongly in the infrared region, which limits the pathlengths that can be used and masking large region of the spectrum, the intensity of Raman scattering from water is usually weak and