UV Resonance Raman Spectroscopy Using a New cw Laser Source: Convenience and Experimental Simplicity

A new laser has been developed which generates hundreds of milliWatts of cw UV power below 260 nm. The laser consists of a small-frame Ar + -ion laser which is intracavity doubled with the use of BBO nonlinear optical crystals. More than 300 mW are available at 244 and 257 nm, while 180, 100, and 30 mW are available at 248, 238, and 228.9 nm, respectively. This laser is an ideal source for UV Raman spectroscopy since it avoids the nonlinear and saturation problems common with the typical pulsed laser excitation sources. It also minimizes thermal sample degradation. We demonstrate the increased spectral signal-to-noise ratios possible due to the ability to focus the cw laser into a small-volume element that can be efficiently imaged into the spectrometer. We demonstrate the ability of this laser to excite Raman spectra of solid samples such as coal-liquid residuals, and point out the utility of the 228.9-nm line for studying aromatic amino acids in proteins. We also demonstrate the ability to selectively study pyrene intercalated into calf thymus DNA.