| Interest in SMM/THz spectroscopy is partly driven by its potential as an analytic method. Briefly, rotational spectroscopy in the SMM/THz is a high resolution technique capable of absolute specificity due to the fact that each molecule has a unique rotational spectrum (fingerprint) that for most molecules of interest fills only a small portion of the available resolution elements (< 0.1%). Further, because the field has traditionally used low-power devices, very sensitive detector systems are available for making trace measurements. We have designed and built a solid-state Multi-Purpose Spectrometer based on recently available broadband amplifier/multiplier chains and new X-Band electronic synthesizers. The frequency precision and agility of the synthesizers allow the Multi-Purpose Spectrometer to implement many analytic strategies, including fast-scanning, frequency modulation, Stark modulation and cavity absorption without changes to the spectrometer hardware.;The Multi-Purpose Spectrometer was built to investigate the effects of making analytical measurements on larger molecules that contain highly congested spectra (> 1000 lines=GHz). At question is to what extent increasing spectral congestion will impact the ability to identify trace elements of a gas mixture in an otherwise congested spectrum. We report our tests of the effectiveness of the frequency modulation technique in congested environments and whether Stark modulation and cavity absorption techniques are effective alternatives to frequency modulation in these environments. To this end, the frequency modulation spectrum of a series of large, planar molecules based on the benzonitrile backbone (benzonitrile, all six di uorobenzonitriles and penta uorobenzonitrile) have been measured between 180--270 GHz with an FM spectrometer and the ground-state spectra of all but penta uorobenzonitrile were assigned, followed by companion measurements of some of the molecules with the Stark modulation and cavity absorption techniques.;We found that the "Confusion Limit" was reached as the rotational constant C approached 1000MHz and the percentage of resolution elements occupied by lines exceeded 20%. In this "limit", the baseline was indistinguishable from weak, over-lapping absorption features, limiting the sensitivity to trace gas absorption in an otherwise pure sample. Further, as the rotational constant C fell below 800 MHz, over 50% of the resolution elements were filled, leading to significant feature overlap and a breakdown in the FM technique across small portions of the spectrum.;Stark modulation is capable of reducing spectral congestion by selectively modulating only the so-called "prolate-degenerate" transitions in a molecule with no c-type dipole moment. Reduction for even moderately oblate asymmetric rotors is well above 50%. Also, we have identified a new fingerprint based on the response of near-prolate asymmetry doublets to varying electric field strength. Cavity absorption proved capable of making measurements of the broad continuum absorption features of molecules at higher pressures without losing the spectral fingerprints of the strong ground-state spectral lines. In addition, it provides a convenient method for measuring the spectra of molecules limited by the vibrational partition function, as the cavity is sensitive to the integrated absorption of many weak lines from excited vibrational states. |
