| A mid-infrared tunable diode laser molecular beam spectrometer for the purpose of trace gas sensing and the study of van der Waals complexes is described. The spectrometer employs a Herriott multipass cell with up to 72 passes. The sample gas is injected parallel to the optical axis through a hole at the center of the far mirror. The molecular absorption transitions are Doppler split, resulting from the laser beam propagating parallel and antiparallel to the molecular beam expansion. The axial expansion leads to narrower line widths and increased sensitivity compared to the traditional vertical injection method as a result of selective sampling of the central part of the molecular expansion with reduced Doppler broadening and longer residence time of the molecular sample in the laser beam. The molecular expansion leads also to selective signal enhancement of low J transitions, as demonstrated for the nu 3 asymmetric stretch vibration of CO2 and 18OCO. The spectrometer performance was evaluated by recording spectra of the CO2-Ar, (CO2)2, CO2-He, CO2-SO2 and CO2-CH4 van der Waals complexes near the R(0) transition of the nu3 band of CO 2 around 2349 cm-1. CO2-CH4 was also studied by Fourier transform microwave spectroscopy, and its equilibrium structure was determined to be T-shaped. A microwave horn antenna was implemented into the spectrometer to enable microwave-infrared double resonance experiments on CO2-SO2 and CO2-CH4, which greatly simplified the spectral assignment. In addition, the previously unknown spectra of 18OCO-N2, 18OCO-Ne, and 18OCO-He were recorded and analyzed. Digital filtering techniques, including the Wiener and Kalman filters, were implemented and evaluated. The feasibility of using a pulsed molecular expansion for trace gas sensing is explored. |
