H. Kagi et al., PROPER UNDERSTANDING OF DOWN-SHIFTED RAMAN-SPECTRA OF NATURAL GRAPHITE - DIRECT ESTIMATION OF LASER-INDUCED RISE IN SAMPLE TEMPERATURE, Geochimica et cosmochimica acta, 58(16), 1994, pp. 3527-3530
Laser-induced heating during micro-Raman spectroscopy, in some cases,
gives rise to considerable down-shifts in Raman spectra of powder grap
hite samples. Employing the most typical experimental condition for gr
aphitic matter of geochemical and/or cosmochemical interest, 10 mW las
er power at the sample surface and approximately 2 mum laser beam, pow
dered artificial graphite samples revealed the E2g mode in low Raman p
eak position ranging from 1567 to 1576 cm-1. The energy range agrees v
ery well with the previously reported frequency range for anomalously
down-shifted Raman spectra of natural graphites. In order to extract s
pectroscopic information inherent in samples from the E2g frequency, i
t is essential to remove the laser-induced down-shift and to know the
temperature of sample surface. We showed that the intensity ratio of t
he Stokes line to the anti-Stokes line for the E2g Raman band of graph
ite is potential to monitor the temperature of graphite surface. Furth
ermore, our measurement employing enough low laser power without laser
-heating showed that natural graphite sample containing the rhombohedr
al polymorph exhibited the same G-band frequency as the E2g (1582 cm-1
) frequency of hexagonal graphite. Thus, stacking sequences of graphit
e did not affect the intralayer vibrational energy.