V.C. Analytical Spectroscopy

V. APPLICATIONS

C. Analytical Spectroscopy: A resurgence of interest in spectroscopic sensors has been fueled by increases in performance made possible by advances in laser systems and applications in medicine, environmental monitoring and national security [1-8]. Most of these new approaches make use of the optical/infrared (Op/IR) spectral regions [9-12]. The submillimeter spectral region, while much less well known, has also seen significant technological advances [13]. In the SMM it is possible to use frequency agile, synthesized electronic sources that can be multiplied into the SMM, with power outputs on the order of 1 mW, a power capable of saturating the probed molecules and which corresponds to 10¹⁴ K in a 1 MHz Doppler linewidth. In the SMM the thermal excitation of many rotational energy levels provides a fingerprint that is complexly redundant and resolvable, for even moderately large molecules, because of the small Doppler limit. These fingerprints, contained in more than 10⁵resolvable spectral channels, lead to ‘absolute’ specificity, even in complex mixtures [14-16].

SMM sensors based on electronic frequency synthesis and frequency multiplication are comparable to their optical counterparts [1, 2, 4, 7, 8] in terms of fractional concentration (ppx, x = m, b, t, . .) sensitivity and are orders of magnitude more favorable than time domain THz (TDS-THz) sensors [17, 18] or THz systems based on non-linear optical sources [19]. Systems based on pulsed lasers and difference frequency mixing have also been reported. If the sensitivity figure of merit is amount of sample, the lower optimum pressure (~10 mTorr) of SMM sensors increases their sensitivity by two to five orders of magnitude relative to Op/IR sensors or sensors based on SMM system with lower resolution.