Intensification of Spontaneous Raman Spectra by Use of Liquid Core Optical Fibers

Intense spontaneous Raman radiation has been obtained from C₆H₆ and C₂CI₄ by passing the focused beam from an argon ion laser (4880 Å, 5 to 250 mW) through filled hollow fused quartz optical fibers having core diameters of ∼75 µm and lengths from 10 to 25 meters. Spectral intensifications by factors of ∼10² to 10³ compared to conventional sample techniques have been obtained with the fiber optics method. Raman spectra were recorded with a Cary model 81 spectrophotometer by collecting the divergent radiation emitted from the end of the optical fiber at the focus of the “image slicer.” However, the intensification method is general and may be employed with any double or triple monochromator by placing the fiber end near the entrance slit. For C₆H₆ at least 33 Raman components were observed in the 3300 > A Δ ν̄ > 1650 cm⁻¹ region including shifts due to overtones and combinations, by using a slit-width of 2 cm⁻¹ with a 15 m fiber length and a laser power of ∼130 mW. The Raman spectra from C₆H₆ were found to be in good agreement with those recently reported by Schrötter and Bofilias. For C₂CI₄ intense spectra were also obtained using 2 cm⁻¹ slit-widths with fiber lengths of 25 m and power levels to 250 mW. These spectra augment those obtained earlier by Wittek and indicate five newly observed Raman and infrared coincidences that in several cases may result from the breakdown of D_2h selection rules. Details of the fiber optics Raman technique are described.