Determination of the laser-induced damage threshold for graphite and coal with deep-UV micro-Raman spectroscopy

A new type of compact deep-UV micro-Raman spectroscopy system was developed with a single monochromator, front-illuminated cooled charge-coupled device, and 266 nm nanosecond pulsed laser to overcome laser-induced fluorescence from surrounding minerals and organic material. Deep-UV micro-Raman spectroscopy is particularly useful in analyzing the fluorescence-free Raman spectra of dispersed low-maturity carbonaceous material and coal, although deep-UV excitation lasers may cause serious degradation and laser-induced heating of the sample surface, especially in microanalysis. The laser-induced damage threshold for fully ordered graphite and coal (VRr = similar to 0.5%) was assessed to facilitate the acquisition of accurate Raman spectra with a spot size of similar to 1 mu m. For fully ordered graphite, there was no serious degradation of the sample surface with an energy fluence of 0.10-2.50 J cm(-2). Some sample surfaces became black at higher fluences of 1.96-2.50 J cm(-2), suggesting irreversible damage by deep-UV lasers. The Raman shift of the G band after measurement involves a downshift of 1.7-7.4 cm(-1) relative to other spectra obtained at low laser fluences of <0.34 J cm(-2). The G band full width at half maximum (FWHM) also increased with increasing laser fluence. Serious degradation of polished coal surfaces occurs at even lower laser fluences of 0.34-2.50 J cm(-2). The degree of change in Raman parameters such as the D and G band FWHM depends on the laser fluence during measurements. Heating and damage by a deep-UV laser is greater than that by visible lasers. Laser fluences of <0.16 and 0.34 J cm(-2) are required for accurate Raman analyses of dispersed carbonaceous material in sedimentary rocks and fully ordered graphite in metasediment, respectively.