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 fl uorescence from surrounding minerals and organic material. Deep – UV micro – Raman spectroscopy is particularly useful in analyzing the fl uorescence – 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 fl uence of 0.10 – 2.50 J cm − 2 . Some sample surfaces became black at higher fl uences 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 fl uences of <0.34 J cm − 2 . The G band full width at half maximum (FWHM) also increased with increasing laser fl uence. Serious degradation of polished coal surfaces occurs at even lower laser fl uences 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 fl uence during measurements. Heating and damage by a deep – UV laser is greater than that by visible lasers. Laser fl uences 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