The effect of excitation laser power in Raman spectroscopic measurements of the degree of conversion of resin composites.

Objectives: To evaluate the effect of excitation laser power in Raman spectrometry by comparing the spectra and the degree of conversion (DC) values obtained using excitation powers between 300 and 1000mW.

Methods: Five commercial and three experimental resin composites were light cured at 1200mW/cm for 10-20s from a commercial blue-violet LED dental curing unit. Raman spectra were collected from composite specimens within 9min after light-curing. The excitation laser (1064nm) was focused on the spot of 0.4mm in diameter. The following powers were used for specimen excitation (mW): 300, 400, 600, 800, and 1000. From Raman spectra, the DC values were calculated and compared among different laser powers. Also, vector-normalized Raman spectra collected using the lowest excitation power (300mW) were compared to those collected using the maximum excitation power (1000mW).

Results: Varying the excitation laser power between 300 and 1000mW resulted in statistically significant differences in both the DC values and the intensity of particular spectral features. The effect of varying laser power on Raman spectra and obtained DC values was material-dependent. The DC values measured within an individual material using different laser powers varied between 3.2 and 7.2% (absolute DC difference). The spectral bands affected by variations in laser power were assigned to symmetric and asymmetric stretching of -CH (2900-3100cm), symmetric stretching of aliphatic CC (1640cm) and scissoring of C-H (1458cm).

Significance: The DC can be artificially elevated through increasing excitation laser power. This effect should be considered in Raman spectroscopic evaluations of DC in specimens during ongoing post-cure polymerization.