Ultraviolet Resonant Raman Enhancements in the Detection of ...

as the internal standard. For solids, the introduction of an internal standard is more
difficult since heterogeneities in terms of particle sizes and distribution are present. This
results in irreproducible measurements and large error especially when one considers that
the typical laser beam spot size is on the order of tens of microns in diameter.
The effects of laser attenuation and self-absorption of the Raman signal were
derived in the Methods section. The laser penetration depth and return Raman signal are
functions of the optical (absorption and scattering coefficients) and physical (particle size
and sample thickness) properties of the sample. The wavelength dependent laser
penetration depths, evaluated using the integral inside Equation 5.14, are shown in figure
27 for 2,4,6-TNT and PETN. Since the wavelength of the Raman signal of the analyte
varies with the excitation wavelength, the value of the molar absorptivity, , at each
desired excitation wavelength must first be determined. Note: The biggest unknown
material property, and the one most difficult to characterize, is the light scattering
coefficient, which was not included in the model of Equation 5.14. It is recognized that
future model improvements should include the scattering coefficient.
Figure 27. Calculated penetration depth of the UV Raman signal for the phenyl ring
stretch of 2,4,6-TNT (left) and symmetric NO2 stretch of PETN (right).
From Figure 27, it is clear that the deep-UV excited Raman scattering from
explosives will come from very near the sample surface. In general, UV excitation
provides a higher Raman cross section for a sample, because of the dependence of the
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