PROPER UNDERSTANDING OF DOWN-SHIFTED RAMAN-SPECTRA OF NATURAL GRAPHITE - DIRECT ESTIMATION OF LASER-INDUCED RISE IN SAMPLE TEMPERATURE

Laser-induced heating during micro-Raman spectroscopy, in some cases, gives rise to considerable down-shifts in Raman spectra of powder graphite samples. Employing the most typical experimental condition for graphitic matter of geochemical and/or cosmochemical interest, 10 mW laser power at the sample surface and approximately 2 mum laser beam, powdered artificial graphite samples revealed the E2g mode in low Raman peak position ranging from 1567 to 1576 cm-1. The energy range agrees very well with the previously reported frequency range for anomalously down-shifted Raman spectra of natural graphites. In order to extract spectroscopic information inherent in samples from the E2g frequency, it is essential to remove the laser-induced down-shift and to know the temperature of sample surface. We showed that the intensity ratio of the Stokes line to the anti-Stokes line for the E2g Raman band of graphite is potential to monitor the temperature of graphite surface. Furthermore, our measurement employing enough low laser power without laser-heating showed that natural graphite sample containing the rhombohedral polymorph exhibited the same G-band frequency as the E2g (1582 cm-1) frequency of hexagonal graphite. Thus, stacking sequences of graphite did not affect the intralayer vibrational energy.