THE EFFECT OF LASER-INDUCED HEATING UPON THE VIBRATIONAL RAMAN-SPECTRA OF GRAPHITES AND CARBON-FIBERS

Laser-induced sample heating has been shown to significantly shift the E2g vibrational Raman mode (1,580 cm-1) of graphite to lower frequency, with typical bandshifts of approximately 10 cm-1-20 cm-1, depending upon laser power. These shifts were only observed when small samples (particles or fibres) were examined using a Raman microprobe. The temperature sensitivity of the band position was determined to be approximately -0.03 cm-1 K-1 for a Lonza KS-5-75 graphite. A variety of carbons were studied, from amorphous carbons through graphites to carbon fibres, and all showed the same heating effect with similar bandshifts. Frequency shifts were also noted for the 1,355 cm-1 A1g mode, but the magnitude of these shifts was generally smaller than those of the E2g mode. These results have important ramifications for measurements of strain in carbon fibres, in that a Hercules HMS4 fibre showed an E2g mode frequency shift of -0.7 cm-1 per mW of laser beam power incident on the fibre surface with a 5-mu-m spot size. Laser power fluctuations can therefore cause bandshifts of magnitude comparable to those induced by fibre strain if they are not carefully controlled.