The spectroscopic investigation of molecular ions is key to facilitating a better understanding of chemical and physical processes that occur both here on Earth and among distant stars. However, numerous challenges are encountered when attempting to study these highly reactive species in the laboratory that can only be overcome by the most sensitive spectroscopic techniques. This thesis discusses the first implementation of concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (cm-NICE-OHMS) on a continuous gas-flow pinhole supersonic expansion discharge source for the study of cooled molecular ions. The instrument, which began as a difference-frequency generation (DFG) based system, is upgraded to include a continuous-wave optical parametric oscillator (OPO) easily tunable from 2.5-4.6 microns. With theimplementation of the OPO the system demonstrates a noise equivalent absorption of ~1 x 10^(-9) cm^(-1). The effectiveness of cm-NICE-OHMS is tested through the acquisition of transitions of H3+ and HN2+. This study provides confirmation that the source produces H3+ rotationally cooled to 80-120 K, while the more efficiently cooling HN2+ ion demonstrates a temperature of~29 K for low rotational states. Further improvements are discussed that will enable cm-NICE-OHMS to reach its full potential for the detection of molecular ions formed in supersonic expansion discharges. Additionally, the current and future status of molecular ion spectroscopy is discussed, and potential molecular ion targets are highlighted to provide direction for investigations of ions of fundamental and chemical importance.
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Progress in sensitive mid-infrared spectroscopy of cooled molecular ions
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