Effects of Particle Size and Laser-Induced Heating on the Raman Spectra of Alpha Quartz Grains

Abstract

Raman spectra of alpha-quartz (Qz) grains of various size (250 microm to < 11 microm) and arrangement (individual and aggregated) have been investigated with a combination of confocal Raman and micro-Raman systems. Frequency downshift and line broadening of the 464 cm(-1), v,(Si-O-Si) band are observed in the smallest size group (< 11 microm, both individual grains and aggregates) because of laser-induced heating and are used to estimate the temperature of the sampled region. The intensity ratio of the anti-Stokes to Stokes Raman lines is also used to estimate the vibrational temperature of the samples under different excitation power. The degree of laser-induced heating is more noticeable in the aggregates than in the individual grains with the use of medium-level laser excitation (< or = 150 mW). Heating diminishes with increasing grain size, and it can only be detected in grain aggregates between 11 and 20 microm in diameter using 150 mW excitation. Intensity studies of the v(s)(Si-O-Si) band using individual grains show no noticeable signs of grain size effects. However, grain size effects become an important factor in the study of aggregates in which spectral intensity diminishes with respect to decreasing grain size.

Stand-off Raman spectroscopic detection of minerals on planetary surfaces

“…The position of the characteristic Raman bands of olivine, however, remains the same for all the samples. Similar Raman measurements were carried out for a-quartz grains with grain size in the range of sub-micron to 250 mm [47]. In the Raman spectrum of the quartz grains of 11 Á/20 mm diameter, the strongest symmetric stretching mode of bridging oxygen (n S (Si Ã/O Ã/Si)) of six-member ring of SiO 4 tetrahedra appears at 464 cm (1 and has the same line width (7.5 cm (1 ) as the bulk material.…”

“…The drop in Raman intensity with decreasing grain size will make it difficult to make quantitative intensity measurements. The fingerprint frequency of minerals remains distinct even in sub-micron grains and can be used for mineral identification in finegrained rocks [47,48].…”

A remote Raman system for planetary exploration: evaluating remote Raman efficiency

“…The Raman features of alpha-quartz have been described by Chio et al 16 (among others 26 ). Symmetric stretching of SiO 2 bonds causes the large Raman band at 463 cm -1 (see Figure 6).…”

“…In cases where remote Raman signal could be identified, the peaks are much more distinct than the absorption bands in VIS-NIR spectra. In cases where remote Raman shift could not be identified, we acquired micro-Raman spectra using a system described by Chio et al 16 . These micro-Raman spectra show distinct features that should be observable with a remote Raman system under the right conditions.…”

“…on received Raman signal. Sharma et al 15 investigated the effects of grain size and orientation, while Chio et al 16 experimentally determined the effects of particle size and laser-induced heating (for quartz). More recently we have made changes in equipment and refined our analytical techniques 17 .…”