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Grain size effects on Raman spectral intensity

Most targets of a flight Raman system in future explorations on planetary surfaces for in situ mineral characterization are likely to be surfaces of rocks and soils scanned without sample preparation. The surface roughness of these targets and the deployment of the instrument by a robotic arm require a simple flight Raman system to gather data from locations a few millimeters on either side of the focal plane of the excitation laser beam. Therefore, the factors affect Raman signal production in rocks and the factors affect Raman signal collection need to be investigated.

Factors affecting the strength of Raman photon production Factors affecting the collection of Raman photons

a. Raman-cross-section s -- intrinsic strength of the oscillating dipole of the target mineral

b. the number of molecules within the volume irradiated by the laser beam --volume to surface ratio,  porosity, grain size, and mixed grain sizes

c. internal heterogeneity within a mineral grain, fractures, chemical zoning, and vitrification

a. Multiple reflections of the excitation laser beam -- penetration depth

b. Multiple internal reflections of the scattered Raman signal -- collection volume

Causes are target surface relief, grain boundaries within the irradiated volume, and changes of index of refraction caused by chemical or structural heterogeneity within a mineral grain.

Experimental tests of grain-size effects--Pure crystals of calcite and olivine were ground, then sieved wet into the following ranges of grain sizes: >250m m, 250-150m m, 150-75m m, 75-37.5m m, <37.5m m, and <<37.5m m. The grains for <<37.5m m category were obtained from the decantate of the suspending liquid.

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Measurement results on single grains: the sum of Raman peak areas (normalized using the data from bulk crystal) decreases as a function of grain volume (as estimated from microscopic images of the grains).

Measurement results on multigrain samples: the sum of Raman peak areas (normalized to the strongest signal obtained in the series) shown as a function of grain size

A simple model for understanding the grain size effect — As a contribution toward understanding the reasons for the difference in grain size effect for different minerals, we have considered a simple model based on the concept of an effective sampling volume (ESV). The ESV is defined as that part of the laser-irradiated volume from which all Raman photons generated penetrate back to the sample surface and fall within the collecting solid angle of the Raman system. The relative ESV of a mineral is estimated as a function of the density of internal boundaries (related to grain thickness along the path of the laser beam), the index of refraction of the mineral (n), and the absorption coefficient (a). Calculations based on this model suggest a strong grain-size effect for calcite but a weak one for olivine, qualitatively consistent with our experimental observations.

Reference: Wang A., "Some grain size effects on Raman scattering intensity for in situ measurements on rocks and soils experimental tests and modeling", Thirty Lunar and Planetary Science Conference, (1999) Huston, Texas, USA.


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