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

Abstract

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 (VR_r = ~ 0.5%) was assessed to facilitate the acquisition of accurate Raman spectra with a spot size of ~ 1 µm. 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.