Continuously Tuneable, Quasi-Continuous-Wave Source for Ultraviolet Resonance Raman Spectroscopy

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Abstract
A continuously tuneable, quasi-continuous-wave (cw), ultraviolet source for resonance Raman spectroscopy has been developed. The third and fourth harmonics of a picosecond mode-locked titanium: sapphire laser were generated in nonlinear crystals. The pulses which resulted from these processes were spectrally narrow (10–15 cm−1) and had low peak powers (<1 kW), while the system as a whole provides high average powers. Fourth-harmonic wavelengths were generated from 208 to 230 nm with average powers from 15 to 40 mW, while the third harmonic ranged from 255 to 305 nm with over 90 mW of power. The UV light was well collimated and focused well, and the power was stable for hours. Resonance Raman spectra of tryptophan and phenylalanine were recorded to demonstrate the practical application of this system. Spectra with excellent signal-to-noise ratios were recorded in 6 min with no deleterious effects from nonlinear processes. The selective enhancement of tryptophan scattering in a 1:10 molar solution of tryptophan to phenylalanine makes clear the advantage of a continuously tuneable UV source. Elaboration of this system to the full tuning range of titanium: sapphire lasers will create a laser source suitable for resonance Raman spectroscopy throughout the ultraviolet region (205–400 nm).