Ultraviolet Resonant Raman Enhancements in the Detection of ...

I = Klα 2 v 4
= K 'lα 2 ⎛ 1 ⎞
⎝
⎜
λ ⎠
⎟
20
4
,
(3.1)
where I is the scattered signal intensity, K and K’ are composite constants, l is the laser
power, is the polarizability of the electrons in the analyte, and and are the
frequency and wavelength of the incident light. For normal (non-resonance) Raman, the
polarizability of the molecule is constant, but this term becomes very important for the
resonance Raman processes.
The first standoff Raman measurement of high explosives was obtained using a
532 nm laser source [19]. It follows from taking the ratio of Equation 3.1 at two different
wavelengths that shifting to a 229 nm laser source should achieve an immediate 29-fold
improvement in the signal intensity. Raman spectroscopic measurements, acquired with
excitation wavelengths in the near-infrared (NIR), are commonly performed to reduce
contributions from sample fluorescence, which is more likely to occur when using
excitation wavelengths in the visible portion of the electromagnetic spectrum. But a
Raman measurement made at 229 nm should theoretically have a 138-fold advantage to
that made using a 785 nm source, assuming all other factors are the same. However,
Raman spectroscopy is not one-dimensional; other notable factors to consider include
sample fluorescence, absorption, potential resonance enhancements, matrix interferences,
and particularly laser-induced sample degradation.
2. Resonance Raman
In addition to frequency enhancements gained from using a UV laser to make
Raman spectroscopy measurements, additional gains are achieved when the energy of the
laser is close to the energy of a strong electronic transition of the analyte molecule. This
enhancement is known as resonance Raman scattering and can provide enhancements of
three to four orders of magnitude [18].